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Ren-Chieh Lien

Senior Principal Oceanographer

Affiliate Professor, Oceanography





Research Interests

Turbulence, Internal waves, Vortical motions, Surface mixed layer and bottom boundary layer dynamics, Internal solitary waves, Small-scale vorticity, Inertial waves


Dr. Lien is a physical oceanographer specializing in internal waves, vortical motions, and turbulence mixing in the upper ocean and their effects on upper ocean heat, salinity, momentum, and energy budgets. His primary scientific research interests include: (1) upper ocean internal waves and turbulence, especially in tropical Pacific and Indian oceans, (2) strongly nonlinear internal solitary wave energetics and breaking mechanisms, (3) small-scale vortical motions, and (4) bottom boundary layer turbulence. He is especially interested in understanding the modulation of internal waves and turbulence mixing by large-scale processes, as well as the effects of small-scale processes and large-scale flows.

One of Dr. Lien most important findings is the strong modulation of turbulence mixing by large-scale equatorial processes, such as tropical instability waves and Kelvin waves, in the eastern equatorial Pacific. He is especially interested in small-scale, potential vorticity motions %u2014 the vortical mode, which operates on the same scale as internal waves %u2014 and their effects on turbulence mixing and stirring. Lien has led sea-going experiments in the Pacific and Indian oceans and the South China Sea, using a variety of instruments including microstructure profilers, Lagrangian floats, EM-APEX floats, and moorings. He also developed a real-time towed CTD chain system, designed to study small-scale water mass variability in the upper ocean at a vertical and horizontal resolution of O(1 m).

Lien mentors and supervises masters and doctoral students and postdocs. His research and experiments have been funded primarily by the National Science Foundation, the Office of Naval Research, and National Oceanic and Atmospheric Administration.

Department Affiliation

Ocean Physics


B.S. Marine Science, Chinese Culture University, 1978

M.S. Physical Oceanography, University of Hawaii, 1986

Ph.D. Physical Oceanography, University of Hawaii, 1990


Lateral Mixing

Small scale eddies and internal waves in the ocean mix water masses laterally, as well as vertically. This multi-investigator project aims to study the physics of this mixing by combining dye dispersion studies with detailed measurements of the velocity, temperature and salinity field during field experiments in 2011 and 2012.

1 Sep 2012


2000-present and while at APL-UW

Estimates of surface wind stress and drag coefficients in Typhoon Megi

Hsu, J.-Y., R.-C. Lien, E.A. D'asare, and T.B. Sanford, "Estimates of surface wind stress and drag coefficients in Typhoon Megi," J. Phys. Oceanogr., 47, 545-565, doi:10.1175/JPO-D-16-0069.1, 2017.

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1 Mar 2017

Estimates of drag coefficients beneath Typhoon Megi (2010) are calculated from roughly hourly velocity profiles of three EM-APEX floats, air launched ahead of the storm, and from air-deployed dropsondes measurements and microwave estimates of the 10-m wind field. The profiles are corrected to minimize contributions from tides and low-frequency motions and thus isolate the current induced by Typhoon Megi. Surface wind stress is computed from the linear momentum budget in the upper 150 m. Three-dimensional numerical simulations of the oceanic response to Typhoon Megi indicate that with small corrections, the linear momentum budget is accurate to 15% before the passage of the eye but cannot be applied reliably thereafter. Monte Carlo error estimates indicate that stress estimates can be made for wind speeds greater than 25 m s-1; the error decreases with greater wind speeds. Downwind and crosswind drag coefficients are computed from the computed stress and the mapped wind data. Downwind drag coefficients increase to 3.5 ± 0.7 x 10-3 at 31 m s-1, a value greater than most previous estimates, but decrease to 2.0 ± 0.4 x 10-3 for wind speeds > 45 m s-1, in agreement with previous estimates. The crosswind drag coefficient of 1.6 ± 0.5 x 10-3 at wind speeds 30–45 m s-1 implies that the wind stress is about 20° clockwise from the 10-m wind vector and thus not directly downwind, as is often assumed.

Autonomous microstructure EM-APEX floats

Lien, R.-C., T.B. Sanford, J.A. Carlson, and J.H. Dunlap, "Autonomous microstructure EM-APEX floats," Methods Oceanogr., 17, 282-295, doi:10.1016/j.mio.2016.09.003, 2016.

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1 Dec 2016


• The EM-APEXX float measures U, V, T, S, and turbulence.
• First deployment of synchronized autonomous vertical profilers in a swarm.
• Slow profiling speed captures entire turbulence temperature spectrum.
• Turbulent temperature variance dissipation rate and diffusivity are estimated.
• Provides observations to relate turbulence mixing to N, shear, and Ri.

Ocean feedback to pulses of the Madden–Julian Oscillation in the equatorial Indian Ocean

Moum, J.N., K. Pujiana, R.-C. Lien, and W.D. Smyth, "Ocean feedback to pulses of the Madden–Julian Oscillation in the equatorial Indian Ocean," Nat. Comm., 7, 13203, doi:10.1038/ncomms13203, 2016.

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19 Oct 2016

Dynamical understanding of the Madden–Julian Oscillation (MJO) has been elusive, and predictive capabilities therefore limited. New measurements of the ocean’s response to the intense surface winds and cooling by two successive MJO pulses, separated by several weeks, show persistent ocean currents and subsurface mixing after pulse passage, thereby reducing ocean heat energy available for later pulses by an amount significantly greater than via atmospheric surface cooling alone. This suggests that thermal mixing in the upper ocean from a particular pulse might affect the amplitude of the following pulse. Here we test this hypothesis by comparing 18 pulse pairs, each separated by <55 days, measured over a 33-year period. We find a significant tendency for weak (strong) pulses, associated with low (high) cooling rates, to be followed by stronger (weaker) pulses. We therefore propose that the ocean introduces a memory effect into the MJO, whereby each event is governed in part by the previous event.

More Publications

Impact of the Madden–Julian oscillation on the Indonesian throughflow in the Makassar Strait during the CINDY/DYNAMO field campaign

Shinoda, T., W. Han, T.G. Jensen, L. Zamudio, E. Joseph Metzger, and R.-C. Lien, "Impact of the Madden–Julian oscillation on the Indonesian throughflow in the Makassar Strait during the CINDY/DYNAMO field campaign," J. Climate, 29, 6085-6108, doi:10.1175/JCLI-D-15-0711.1, 2016.

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1 Sep 2016

Previous studies indicate that equatorial zonal winds in the Indian Ocean can significantly influence the Indonesian Throughflow (ITF). During the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/Dynamics of the Madden–Julian Oscillation (DYNAMO) field campaign, two strong MJO events were observed within a month without a clear suppressed phase between them, and these events generated exceptionally strong ocean responses. Strong eastward currents along the equator in the Indian Ocean lasted more than one month from late November 2011 to early January 2012. The influence of these unique MJO events during the field campaign on ITF variability is investigated using a high-resolution (1/25°) global ocean general circulation model, the Hybrid Coordinate Ocean Model (HYCOM). The strong westerlies associated with these MJO events, which exceed 10 m s−1, generate strong equatorial eastward jets and downwelling near the eastern boundary. The equatorial jets are realistically simulated by the global HYCOM based on the comparison with the data collected during the field campaign. The analysis demonstrates that sea surface height (SSH) and alongshore velocity anomalies at the eastern boundary propagate along the coast of Sumatra and Java as coastal Kelvin waves, significantly reducing the ITF transport at the Makassar Strait during January–early February. The alongshore velocity anomalies associated with the Kelvin wave significantly leads SSH anomalies. The magnitude of the anomalous currents at the Makassar Strait is exceptionally large because of the unique feature of the MJO events, and thus the typical seasonal cycle of ITF could be significantly altered by strong MJO events such as those observed during the CINDY/DYNAMO field campaign.

Trains of large Kelvin–Helmholtz billows observed in the Kuroshio above a seamount

Chang, M.-H., S.-Y. Jheng, and R.-C. Lien, "Trains of large Kelvin–Helmholtz billows observed in the Kuroshio above a seamount," Geophys. Res. Lett., 43, 8654-8661, doi:10.1002/2016GL069462, 2016.

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28 Aug 2016

Trains of large Kelvin–Helmholtz (KH) billows within the Kuroshio current at ~230 m depth off southeastern Taiwan and above a seamount were observed by shipboard instruments. The trains of large KH billows were present in a strong shear band along the 0.55 m s−1 isotach within the Kuroshio core; they are presumably produced by flow interactions with the rapidly changing topography. Each individual billow, resembling a cat's eye, had a horizontal length scale of 200 m, a vertical scale of 100 m, and a timescale of 7 min, near the local buoyancy frequency. Overturns were observed frequently in the billow cores and the upper eyelids. The turbulent kinetic energy dissipation rates estimated using the Thorpe scale had an average value of O(10−4) W kg−1 and a maximum value of O(10−3) W kg−1. The turbulence mixing induced by the KH billows may exchange Kuroshio water with the surrounding water masses.

Intensification of the subpolar front in the Sea of Japan during winter cyclones

Zhao, N. S. Iwasaki, A. Isobe, R.-C. Lien, and B. Wang, "Intensification of the subpolar front in the Sea of Japan during winter cyclones," J. Geophys. Res., 121, 2253-2267, doi:10.1002/2015JC011565, 2016.

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1 Apr 2016

The response of the subpolar front in the Sea of Japan (also known as the East Sea) to winter cyclones is investigated based on quantitative analyses of gridded and satellite data sets. Cyclone passages affecting the sea are detected using time series of spatially averaged surface turbulent heat fluxes. As the cyclones develop, there are strong cold-air outbreaks that produce twice the normal heat loss over the sea. After removal of sea surface temperature (SST) seasonal trends, we found that cyclone passage (hence, cooling) mainly occurred over 3 days, with maximum SST reduction of –0.4°C. The greatest reduction was found along the subpolar front, where frontal sharpness (i.e., SST gradient) increased by 0.1°C (100 km)−1. Results of a mixed-layer model were consistent with both temperature and frontal sharpness, and localized surface cooling along the subpolar front resulted from both horizontal heat advection and turbulent heat fluxes at the sea surface. Further analyses show that this localized cooling from horizontal heat advection is caused by the cross-frontal Ekman flow (vertically averaged over the mixed layer) and strong northwesterly winds associated with the cold-air outbreak during cyclone passage.

Eddy-Kuroshio interaction processes revealed by mooring observations off Taiwan and Luzon

Tsai, C.-J., M. Andres, S. Jan, V. Mensah, T.B. Sanford, R.-C. Lien, and C.M. Lee, "Eddy-Kuroshio interaction processes revealed by mooring observations off Taiwan and Luzon," Geophys. Res. Letts., 42, 8090-8105, doi:10.1002/2015GL065814, 2015.

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16 Oct 2015

The influence and fate of westward propagating eddies that impinge on the Kuroshio were observed with pressure sensor-equipped inverted echo sounders (PIESs) deployed east of Taiwan and northeast of Luzon. Zero lag correlations between PIES-measured acoustic travel times and satellite-measured sea surface height anomalies (SSHa), which are normally negative, have lower magnitude toward the west, suggesting the eddy-influence is weakened across the Kuroshio. The observational data reveal that impinging eddies lead to seesaw-like SSHa and pycnocline depth changes across the Kuroshio east of Taiwan, whereas analogous responses are not found in the Kuroshio northeast of Luzon. Anticyclones intensify sea surface and pycnocline slopes across the Kuroshio, while cyclones weaken these slopes, particularly east of Taiwan. During the 6%u2009month period of overlap between the two PIES arrays, only one anticyclone affected the pycnocline depth first at the array northeast of Luzon and 21%u2009days later in the downstream Kuroshio east of Taiwan.

The LatMix summer campaign: Submesoscale stirring in the upper ocean

Shcherbina, A.Y., et al., including E. D'Asaro, R.R. Harcourt, C.M. Lee, R.-C. Lien, and T.B. Sanford, "The LatMix summer campaign: Submesoscale stirring in the upper ocean," Bull. Am. Meteor. Soc., 96, 1257-1279, doi:10.1175/BAMS-D-14-00015.1, 2015.

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1 Aug 2015

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s-1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.

The formation and fate of internal waves in the South China Sea

Alford, M.H., et al., including R.-C. Lien and L. Rainville, "The formation and fate of internal waves in the South China Sea," Nature, 521, 65-69, doi:10.1038/nature14399, 2015.

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29 Apr 2015

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6, 7. For over a decade, studies8, 9, 10, 11 have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

The surface mixed layer heat budget from mooring observations in the central Indian Ocean during Madden–Julian Oscillation events

Chi, N.-H., R.-C. Lien, E.A. D'Asaro, and B.B. Ma, "The surface mixed layer heat budget from mooring observations in the central Indian Ocean during Madden–Julian Oscillation events," J. Geophys. Res., 119, 4638-4652, doi:10.1002/2014JC010192, 2014.

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1 Jul 2014

The oceanic surface mixed layer heat budget in the central equatorial Indian Ocean is calculated from observations at two mooring sites (0°S 79°E and 1.5°S 79°E) during three active and calm phases of Madden–Julian Oscillation (MJO) events between September 2011 and January 2012. At both mooring locations, the surface mixed layer is generally heated during MJO calm phases. During MJO active phases at both mooring locations, the surface mixed layer is always cooled by the net surface heat flux and also sometimes by the turbulent heat flux at the bottom of the surface mixed layer. The turbulent heat flux at the bottom of the surface mixed layer, however, varies greatly among different MJO active phases and between the two mooring locations. A barrier layer exerts control on the turbulent heat flux at the base of the surface mixed layer; we quantify this barrier layer strength by a "barrier layer potential energy," which depends on the thickness of the barrier layer, the thickness of the surface mixed layer, and the density stratification across the isothermal layer. During one observed MJO active phase, a strong turbulent heat flux into the mixed layer was diagnosed, despite the presence of a 10–20 m thick barrier layer. This was due to the strong shear across the barrier layer driven by the westerly winds, which provided sufficient available kinetic energy to erode the barrier layer. To better simulate and predict net surface heat fluxes and the MJO, models must estimate the oceanic barrier layer potential energy, background shear, stratification, and surface forcing accurately.

Transition from partly standing to progressive internal tides in Monterey Submarine Canyon

Hall, R.A., M.H. Alford, G.S. Carter, M.C. Gregg, R.-C. Lien, D.J. Wain, and Z. Zhao, "Transition from partly standing to progressive internal tides in Monterey Submarine Canyon," Deep Sea Res. II, 104, 164-173, doi:10.1016/j.dsr2.2013.05.039, 2014.

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1 Jun 2014

Monterey Submarine Canyon is a large, sinuous canyon off the coast of California, the upper reaches of which were the subject of an internal tide observational program using moored profilers and upward-looking moored ADCPs. The mooring observations measured a near-surface stratification change in the upper canyon, likely caused by a seasonal shift in the prevailing wind that favoured coastal upwelling. This change in near-surface stratification caused a transition in the behaviour of the internal tide in the upper canyon from a partly standing wave during pre-upwelling conditions to a progressive wave during upwelling conditions. Using a numerical model, we present evidence that either a partly standing or a progressive internal tide can be simulated in the canyon, simply by changing the initial stratification conditions in accordance with the observations. The mechanism driving the transition is a dependence of down-canyon (supercritical) internal tide reflection from the canyon floor and walls on the depth of maximum stratification. During pre-upwelling conditions, the main pycnocline extends down to 200 m (below the canyon rim) resulting in increased supercritical reflection of the up-canyon propagating internal tide back down the canyon. The large up-canyon and smaller down-canyon progressive waves are the two components of the partly standing wave. During upwelling conditions, the pycnocline shallows to the upper 50 m of the watercolumn (above the canyon rim) resulting in decreased supercritical reflection and allowing the up-canyon progressive wave to dominate.

Evolution of the Kuroshio Tropical Water from the Luzon Strait to the east of Taiwan

Mensah, V., S. Jan, M.-D. Chiou, T.H. Kuo, and R.-C. Lien, "Evolution of the Kuroshio Tropical Water from the Luzon Strait to the east of Taiwan," Deep-Sea Res. I, 86, 68-81, doi:10.1016/j.dsr.2014.01.005, 2014.

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1 Apr 2014

This study examines the evolution of the Kuroshio Tropical Water (KTW) from the Luzon Strait to the I-Lan Ridge northeast of Taiwan. Historical conductivity temperature depth (CTD) profiles are analyzed using a method based on the calculation of the root mean square (rms) difference of the salinity along isopycnals. In combination with analysis of the distribution of the salinity maximum, this method enables water masses in the Kuroshio and the vicinity, to be tracked and distinguished as well as the detection of the areas where water masses are modified. Vertical and horizontal eddy diffusivities are then calculated from hydrographic and current velocity data to elucidate the dynamics underlying the KTW interactions with the surrounding water masses. Changes in KTW properties mainly occur in the southern half of the Luzon Strait, while moderate variations are observed east of Taiwan on the right flank of the Kuroshio. In spite of a front dividing the KTW from the South China Sea Tropical Water (SCSTW) on Kuroshio%u05F3s western side, mixing between these two water masses seemingly occurs in the Luzon Strait. These water masses%u05F3 interaction is not evident east of Taiwan. The estimation of eddy diffusivities yields high horizontal diffusivities (Kh~102 m2 s%u22121) all along the Kuroshio path, due to the high current shear along the Kuroshio%u05F3s flanks. The vertical diffusivity approaches 10%u22123 m2 s%u22121, with the highest values in the southern Luzon Strait. Instabilities generated when the Kuroshio encounters the rough topography of this region may enhance both vertical and horizontal diffusivities there.

Large-amplitude internal solitary waves observed in the northern South China Sea: Properties and energetics

Lien, R.-C., F. Henyey, B. Ma, and Y.J. Yang, "Large-amplitude internal solitary waves observed in the northern South China Sea: Properties and energetics," J. Phys. Oceanogr., 44, 1095-1115, doi:10.1175/JPO-D-13-088.1, 2014.

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1 Apr 2014

Five large-amplitude internal solitary waves (ISWs) propagating westward on the upper continental slope in the northern South China Sea were observed in May–June 2011 with nearly full-depth measurements of velocity, temperature, salinity, and density. As they shoaled, at least three waves reached the convective breaking limit: along-wave current velocity exceeded the wave propagation speed C. Vertical overturns of ~100 m were observed within the wave cores; estimated turbulent kinetic energy was up to 1.5 x 10-4 W kg-1. In the cores and at the pycnocline, the gradient Richardson number was mostly <0.25. The maximum ISW vertical displacement was 173 m, 38% of the water depth. The normalized maximum vertical displacement was ~0.4 for three convective breaking ISWs, in agreement with laboratory results for shoaling ISWs. Observed ISWs had greater available potential energy (APE) than kinetic energy (KE). For one of the largest observed ISWs, the total wave energy per unit meter along the wave crest E was 553 MJ m-1, more than three orders of magnitude greater than that observed on the Oregon Shelf. Pressure work contributed 77% and advection contributed 23% of the energy flux. The energy flux nearly equaled CE. The Dubriel–Jacotin–Long model with and without a background shear predicts neither the observed APE > KE nor the subsurface maximum of the along-wave velocity for shoaling ISWs, but does simulate the total energy and the wave shape. Including the background shear in the model results in the formation of a surface trapped core.

Modulation of Kuroshio transport by mesoscale eddies at the Luzon Strait entrance

Lien, R.-C., B. Ma, Y.-H. Cheng, C.-R. Ho, B. Qiu, C.M. Lee, and M.-H. Chang, "Modulation of Kuroshio transport by mesoscale eddies at the Luzon Strait entrance," J. Geophys. Res., 119, 2129-2142, doi:10.1002/2013JC009548, 2014.

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1 Apr 2014

Measurements of Kuroshio Current velocity at the entrance to Luzon Strait along 18.75°N were made with an array of six moorings during June 2012 to June 2013. Strong positive relative vorticity of the order of the planetary vorticity f was observed on the western flank of the Kuroshio in the upper 150 m. On the eastern flank, the negative vorticity observed was about an order of magnitude smaller than f. Kuroshio transport near its origin is computed from direct measurements for the first time. Kuroshio transport has an annual mean of 15 Sv with a standard deviation of 3 Sv. It is modulated strongly by impinging westward propagating eddies, which are identified by an improved eddy detection method and tracked back to the interior ocean. Eight Kuroshio transport anomalies >5 Sv are identified; seven are explained by the westward propagating eddies. Cyclonic (anticyclonic) eddies decrease (increase) the zonal sea level anomaly (SLA) slope and reduce (enhance) Kuroshio transport. Large transport anomalies of >10 Sv within O(10 days) are associated with the pairs of cyclonic and anticyclonic eddies. The observed Kuroshio transport was strongly correlated with the SLA slope (correlation = 0.9). Analysis of SLA slope data at the entrance to Luzon Strait over the period 1992–2013 reveals a seasonal cycle with a positive anomaly (i.e., an enhanced Kuroshio transport) in winter and spring and a negative anomaly in summer and fall. Eddy induced vorticity near the Kuroshio has a similar seasonal cycle, suggesting that seasonal variation of the Kuroshio transport near its origin is modulated by the seasonal variation of the impinging mesoscale eddies.

Wind stress and near-surface shear in the equatorial Atlantic Ocean

Wenegrat, J.O., M.J. McPhaden, and R.-C. Lien, "Wind stress and near-surface shear in the equatorial Atlantic Ocean," Geophys. Res. Lett., 41, 1226-1231, doi:10.1002/2013GL059149, 2014.

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28 Feb 2014

The upper ocean response to wind stress is examined using 8 months of unique near-surface moored velocity, temperature, and salinity data at 0°N, 23°W in the equatorial Atlantic. The effects of wind stress and shear on the time-varying eddy viscosity are inferred using the surface shear-stress boundary condition. Parameterizations of eddy viscosity as a function of wind stress and shear versus wind stress alone are then examined. In principle, eddy viscosity should be proportional to the inverse shear, but how it is represented implicitly or explicitly can affect estimates of the near-surface flow field. This result may explain some discrepancies that have arisen from using parameterizations based only on wind stress to characterize the effects of turbulent momentum mixing.

The variability of internal tides in the Northern South China Sea

Ma, B.B., R.-C. Lien, and D.S. Ko, "The variability of internal tides in the Northern South China Sea," J. Oceanogr., 69, 619-630, doi:10.1007/s10872-013-0198-0, 2013.

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1 Oct 2013

An array of three bottom-mounted ADCP moorings was deployed on the prevailing propagation path of strong internal tides for nearly 1 year across the continental slope in the northern South China Sea. These velocity measurements are used to study the intra-annual variability of diurnal and semidiurnal internal tidal energy in the region. A numerical model, the Luzon Strait Ocean Nowcast/Forecast System developed at the U.S. Naval Research Laboratory that covers the northern South China Sea and the Kuroshio, is used to interpret the observed variation of internal tidal energy on the Dongsha slope. Internal tides are generated primarily at the two submarine ridges in the Luzon Strait. At the western ridge generation site, the westward energy flux of the diurnal internal tide is sensitive to the stratification and isopycnal slope associated with the Kuroshio. The horizontal shear at the Kuroshio front does not modify the propagation path of either diurnal or semidiurnal tides because the relative vorticity of the Kuroshio in Luzon Strait is not strong enough to increase the effective inertial frequency to the intrinsic frequency of the internal tides. The variation of internal tidal energy on the continental slope and Dongsha plateau can be attributed to the variation in tidal beam propagation in the northern South China Sea.

Observations of the cold wake of Typhoon Fanapi (2010)

Mrvaljevic, R.K., P.G. Black, L.R. Centurioni, Y.-T. Chang, E.A. D'Asaro, S.R. Jayne, C.M. Lee, R.-C. Lien, I.-I. Lin, J. Morzel, P.P. Niiler, L. Rainville, and T.B. Sanford, "Observations of the cold wake of Typhoon Fanapi (2010)," Geophys. Res. Lett., 40, 316-321, doi:10.1002/grl.50096, 2013.

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28 Jan 2013

Several tens of thousands of temperature profiles are used to investigate the thermal evolution of the cold wake of Typhoon Fanapi, 2010. Typhoon Fanapi formed a cold wake in the Western North Pacific Ocean on 18 September characterized by a mixed layer that was >2.5°C cooler than surrounding water, and extending to >80 m, twice as deep as the pre-existing mixed layer. The initial cold wake became capped after 4 days as a warm, thin surface layer formed. The thickness of the capped wake, defined as the 26°C to 27°C layer, decreased, approaching the background thickness of this layer with an e-folding time of 23 days, almost twice the e-folding lifetime of the Sea Surface Temperature (SST) cold wake (12 days). The wake was advected several hundreds of kilometers from the storm track by a pre-existing mesoscale eddy. The observations reveal new intricacies of cold wake evolution and demonstrate the challenges of describing the thermal structure of the upper ocean using sea surface information alone.

Internal tides on the East China Sea continental slope

Lien, R.-C., T.B. Sanford, S. Jan, M.-H. Chang, and B.B. Ma, "Internal tides on the East China Sea continental slope," J. Mar. Res., 71, 151-186, doi:10.1357/002224013807343461, 2013.

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1 Jan 2013

Strong semidiurnal internal tides are observed on the continental slope of the East China Sea (ECS) using an array of subsurface moorings and EM-APEX floats. A Princeton Ocean Model (POM) is used to simulate the effects of stratification profiles on the generation and propagation of M2 internal tides; model simulations are compared with observations. On the ECS continental slope northeast of Taiwan, the semidiurnal barotropic tidal current flows nearly perpendicular to the shelf break and continental slope, favoring the generation of internal tides. Both the critical slope analysis and numerical model results suggest multiple generation sites on the continental slope, shelf break and around North MienHua Canyon. Unique high-wavenumber semidiurnal internal tides with a dominant vertical scale of ~100 m are found on the continental slope. The high-wavenumber semidiurnal internal tides appear in a form of spatially coherent shear layers across the ECS slope. They propagate vertically both upward and downward. Patches of strong energy and shear at a typical vertical scale of O(50 m) are present at the intersections of the upward and downward propagating high-wavenumber internal tides. The strong shear of high-wavenumber semidiurnal tides could play an important role in triggering shear instability on the ECS slope. The semidiurnal internal tidal energy flux, primarily in low wavenumbers, on the ECS slope, exhibits strong temporal and spatial variations. The observed depth integrated energy flux is 3.0–10.7 kW m-1, mostly seaward from the continental shelf/slope. The POM model predicts similar seaward energy fluxes at a slightly weaker magnitude, 1.0–7.2 kW m-1. The difference may be due to the absence of mesoscale processes in the model, e.g., the Kuroshio Current and eddies, the assumed horizontally uniform stratification profiles, insufficient model resolution for the abrupt canyon bathymetry, and the lack of the other major semidiurnal tidal constituent, S2, in the model. On the ECS slope, the total energy in the internal wave continuum, between 0.3 cph (beyond semidiurnal tidal harmonics) and the buoyancy frequency, is 6-13 times that of the Garrett–Munk model, presumably as a result of the energy cascade from strong inertial waves and internal tides in the region.

Observations of a freshwater pulse induced by Typhoon Morakot off the northern coast of Taiwan in August 2009

Jan, S., J. Wang, Y.J. Yang, C.-C. Hung, C.-S. Chern, G. Gawarkiewicz, R.-C. Lien, L. Centurioni, J.-Y. Kuo, and B. Wang, "Observations of a freshwater pulse induced by Typhoon Morakot off the northern coast of Taiwan in August 2009," J. Mar. Res., 71, 18-46, doi:10.1357/002224013807343452, 2013.

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1 Jan 2013

In this paper we describe large-scale impacts from a typhoon on the circulation over the continental shelf and slope north of Taiwan. Typhoon Morakot was a category 2 tropical storm that landed in central Taiwan, but caused destruction primarily in southern Taiwan from Aug. 8–10, 2009. The typhoon brought record-breaking rainfall; approximately 3 m accumulated over four days in southern Taiwan. River discharge on the west coast of Taiwan increased rapidly from Aug. 6–7 and peaked on Aug. 8, yielding a total volume 27.2 km3 of freshwater discharged off the west coast of Taiwan over five days (Aug. 6–10). The freshwater mixed with ambient seawater, and was carried primarily by the northeastward-flowing Taiwan Strait current to the sea off the northern coast of Taiwan. Two joint surveys each measured the hydrography and current velocity in the Taiwan Strait and off the northeastern coast of Taiwan roughly one week and two and a half weeks after Morakot. The first survey observed an Ω-shaped freshwater pulse off the northern tip of Taiwan, in which the salinity was ~1 lower than the climatological mean salinity. The freshwater pulse met the Kuroshio and formed a density front off the northeastern coast of Taiwan. The hydrographic data obtained in the second survey suggested that the major freshwater pulse left the sea off the northern and northeastern coasts of Taiwan, which may have been carried by the Kuroshio to the northeast. Biogeochemical sampling conducted after Morakot suggested that the concentrations of nutrients in the upper ocean off the northern coast of Taiwan increased remarkably compared with their normal values. A typhoon-induced biological bloom is attributed to the inputs both from the nutrient-rich river runoff and upwelling of the subsurface Kuroshio water.

Internal tides and mixing in a submarine canyon with time-varying stratification

Zhao, Z., M.H. Alford, R.-C. Lien, M.C. Gregg, and G.S. Carter, "Internal tides and mixing in a submarine canyon with time-varying stratification," J. Phys. Oceanogr., 42, 2121-2142, doi:10.1175/JPO-D-12-045.1, 2012.

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1 Dec 2012

The time variability of the energetics and turbulent dissipation of internal tides in the upper Monterey Submarine Canyon (MSC) is examined with three moored profilers and five ADCP moorings spanning February–April 2009. Highly resolved time series of velocity, energy, and energy flux are all dominated by the semidiurnal internal tide and show pronounced spring-neap cycles. However, the onset of springtime upwelling winds significantly alters the stratification during the record, causing the thermocline depth to shoal from about 100 to 40 m. The time-variable stratification must be accounted for because it significantly affects the energy, energy flux, the vertical modal structures, and the energy distribution among the modes. The internal tide changes from a partly horizontally standing wave to a more freely propagating wave when the thermocline shoals, suggesting more reflection from up canyon early in the observational record. Turbulence, computed from Thorpe scales, is greatest in the bottom 50–150 m and shows a spring-neap cycle. Depth-integrated dissipation is 3 times greater toward the end of the record, reaching 60 mW m-2 during the last spring tide. Dissipation near a submarine ridge is strongly tidally modulated, reaching 10-5 W kg-1 (10–15-m overturns) during spring tide and appears to be due to breaking lee waves. However, the phasing of the breaking is also affected by the changing stratification, occurring when isopycnals are deflected downward early in the record and upward toward the end.

Modulation of equatorial turbulence by a tropical instability wave

Inoue, R., R.-C. Lien, and J.N. Moum, "Modulation of equatorial turbulence by a tropical instability wave," J. Geophys. Res., 117, doi:10.1029/2011JC007767, 2012.

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11 Oct 2012

Strong modulation of turbulent mixing by a westward-propagating tropical instability wave (TIW) was observed in the stratified shear layer between the equatorial undercurrent (EUC) and the surface mixed layer during October and November 2008 at 0°N 140°W. The unique deep diurnal-cycle mixing in the stratified layer beneath the equatorial cold tongue was observed where nighttime turbulent mixing was a factor of 10 greater than during daytime. The turbulent kinetic energy dissipation rate, ε, was O(10-6) W kg-1, and the turbulent heat flux was ~ –500 W m-2, at least 5–10 times greater than observed previously in the central equatorial Pacific. Turbulence mixing varied significantly during the four distinct phases of the meridional flow associated with the TIW. Observations during the northward-to-southward transition recorded the largest values of reduced shear squared, the thickest nighttime surface mixed layer, the deepest penetration of the deep-cycle turbulence, and the largest turbulent heat flux and largest integrated εin the deep-cycle layer (DCL). During steady southward flow, the depth of the bases of the nighttime surface mixed layer and of the DCL were the shallowest. A 50-m-thick layer of strong turbulence was observed immediately above the EUC core during the northward-to-southward and steady southward phases. Here, the average ε exceeded 10-6 W kg-1, the eddy diffusivity exceeded 10-3 m2 s-1, and the turbulent heat flux was ~ –500 W m-2. To parameterize mixing in the central equatorial Pacific accurately, numerical models must simulate the enhancement of mixing associated with TIWs and also the variability of mixing in different TIW phases.

Trapped core formation within a shoaling nonlinear internal wave

Lien, R.-C., E.A. D'Asaro, F. Henyey, M.-H. Chang, T.-T. Tang, and Y.-J. Yang, "Trapped core formation within a shoaling nonlinear internal wave," J. Phys. Oceanogr., 42, 511-525, doi:10.1175/2011JPO4578.1, 2012.

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1 Apr 2012

Large-amplitude (100–200 m) nonlinear internal waves (NLIWs) were observed on the continental slope in the northern South China Sea nearly diurnally during the spring tide. The evolution of one NLIW as it propagated up the continental slope is described. The NLIW arrived at the slope as a nearly steady-state solitary depression wave. As it propagated up the slope, the wave propagation speed C decreased dramatically from 2 to 1.3 m s-1, while the maximum along-wave current speed Umax remained constant at 2 m s-1. As Umax exceeded C, the NLIW reached its breaking limit and formed a subsurface trapped core with closed streamlines in the coordinate frame of the propagating wave. The trapped core consisted of two counter-rotating vortices feeding a jet within the core. It was highly turbulent with 10–50-m density overturnings caused by the vortices acting on the background stratification, with an estimated turbulent kinetic energy dissipation rate of O(10-4) W kg-1 and an eddy diffusivity of O(10-1) m2 s-1. The core mixed continually with the surrounding water and created a wake of mixed water, observed as an isopycnal salinity anomaly. As the trapped core formed, the NLIW became unsteady and dissipative and broke into a large primary wave and a smaller wave. Although shoaling alone can lead to wave fission, the authors hypothesize that the wave breaking and the trapped core evolution may further trigger the fission process. These processes of wave fission and dissipation continued so that the NLIW evolved from a single deep-water solitary wave as it approached the continental slope into a train of smaller waves on the Dongsha Plateau. Observed properties, including wave width, amplitude, and propagation speed, are reasonably predicted by a fully nonlinear steady-state internal wave model, with better agreement in the deeper water. The agreement of observed and modeled propagation speed is improved when a reasonable vertical profile of background current is included in the model.

Circulation and intrusions northeast of Taiwan: Chasing and predicting uncertainty in the cold dome

Gawarkiewicz, G., S. Jan, P.F.J. Lermusiaux, J.L. McClean, L. Centurioni, K. Taylor, B. Cornuelle, T.F. Duda, J. Wang, Y.J. Yang, T. Sanford, R.-C. Lien, C. Lee, M.-A. Lee, W. Leslie, P.J. Haley Jr., P.P. Niiler, G. Gopalakrishnan, P. Velez-Belchi, D.-K. Lee, and Y.Y. Kim, "Circulation and intrusions northeast of Taiwan: Chasing and predicting uncertainty in the cold dome," Oceanography 24, 110-121, doi:10.5670/oceanog.2011.99, 2011.

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5 Dec 2011

An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the "cold dome" frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome's dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broad-scale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7%u20138, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.

From Luzon Strait to Dongsha Plateau: Stages in the life of an internal wave

Farmer, D.M., M.H. Alford, R.-C. Lien, Y.J. Yang, M.-H. Chang, and Q. Li, "From Luzon Strait to Dongsha Plateau: Stages in the life of an internal wave," Oceanography 24, 64-77, doi:10.5670/oceanog.2011.95, 2011.

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5 Dec 2011

Tidal currents in Luzon Strait south of Taiwan generate some of the largest internal waves anywhere in the ocean. Recent collaborative efforts between oceanographers from the United States and Taiwan explored the generation, evolution, and characteristics of these waves from their formation in the strait to their scattering and dissipation on Dongsha Plateau and the continental slope of mainland China. Nonlinear internal waves affect offshore engineering, navigation, biological productivity, and sediment resuspension. Observations within Luzon Strait identified exceptionally large vertical excursions of density (as expressed primarily in temperature profiles) and intense turbulence as tidal currents interact with submarine ridges. In the northern part of the strait, the ridge spacing is close to the internal semidiurnal tidal wavelength, allowing wave generation at both ridges to contribute to amplification of the internal tide. Westward radiation of semidiurnal internal tidal energy is predominant in the north, diurnal energy in the south. The competing effects of nonlinearity, which tends to steepen the stratification, and rotational dispersion, which tends to disperse energy into inertial waves, transform waves traveling across the deep basin of the South China Sea. Rotation inhibits steepening, especially for the internal diurnal tide, but despite the rotational effect, the semidiurnal tide steepens sufficiently so that nonhydrostatic effects become important, leading to the formation of a nonlinear internal wave train. As the waves encounter the continental slope and Dongsha Plateau, they slow down, steepen further, and are modified and scattered into extended wave trains. At this stage, the waves can "break," forming trapped cores. They have the potential to trap prey, which may account for their attraction to pilot whales, which are often seen following the waves as they advance toward the coast. Interesting problems remain to be explored and are the subjects of continuing investigations.

Mean structure and variability of the cold dome northeast of Taiwan

Jan, S., C.-C. Chen, Y.-L. Tsai, Y.J. Yang, J. Wang, C.-S. Chern, G. Gawarkiewicz, R.-C. Lien, L. Centurioni, and J.-Y. Kuo, "Mean structure and variability of the cold dome northeast of Taiwan," Oceanography 24, 100-109, doi:10.5670/oceanog.2011.98, 2011.

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5 Dec 2011

The "cold dome" off northeastern Taiwan is one of the distinctive oceanic features in the seas surrounding Taiwan. The cold dome is important because persistent upwelling makes the region highly biologically productive. This article uses historical data, recent observations, and satellite-observed sea surface temperatures (SST) to describe the mean structure and variability of the cold dome. The long-term mean position of the cold dome, using the temperature at 50 m depth as a reference, is centered at 25.625N, 122.125E. The cold dome has a diameter of approximately 100 km, and is maintained by cold (< 21C) and salty (> 34.5) waters upwelled along the continental slope. The ocean currents around the cold dome, although weak, flow counterclockwise. The monsoon-driven winter intrusion of the Kuroshio current onto the East China Sea shelf intensifies the upwelling and carries more subsurface water up to the cold dome than during the summer monsoon season. On a shorter timescale, the cold dome's properties can be significantly modified by the passage of typhoons, which creates favorable physical conditions for short-term Kuroshio intrusions in summer. The surface expression of the cold dome viewed from satellite SST images is often not domelike but instead is an irregular shape with numerous filaments, and thus may contribute substantially to shelf/slope exchange. As a result of persistent upwelling, typhoon passage, and monsoon forcing, higher chlorophyll a concentrations, and thus higher primary productivity, are frequently observed in the vicinity of the cold dome.

Seasonal and mesoscale variability of the Kuroshio near its origin

Rudnick, D.L., S. Jan, L. Centurioni, C.M. Lee, R.-C. Lien, J. Wang, D.-K. Lee, R.-S. Tseng, Y.Y. Kim, and C.-S. Chern, "Seasonal and mesoscale variability of the Kuroshio near its origin," Oceanography 24, 52-63, http://dx.doi.org/10.5670/oceanog.2011.94, 2011

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5 Dec 2011

The Kuroshio is the most important current in the North Pacific. Here, we present historical data and recent observations of the Kuroshio off the coasts of Taiwan and the Philippine Archipelago, with a focus on its origins. Seasonal climatologies from shipboard hydrographic and velocity measurements, and from surface drifters, demonstrate changes in the Kuroshio caused by the monsoon. In particular, seasonal monsoon forcing affects the degree of penetration of the Kuroshio through Luzon Strait. Data from surface drifters and underwater gliders describe its mesoscale variability. Velocities derived from drifters make clear the mesoscale variability associated with the Subtropical Countercurrent east of the Kuroshio. Underwater gliders document mesoscale structure prominent in salinity extrema associated with water masses. The evolution of these water masses as they progress northward near the Kuroshio indicates strong mixing in the region.

Typhoon-ocean interaction in the western North Pacific: Part 2

Pun, I.F., Y.-T. Chang, I.-I. Lin, T.Y. Tang, and R.-C. Lien, "Typhoon-ocean interaction in the western North Pacific: Part 2," Oceanography 24, 32-41, doi:10.5670/oceanog.2011.92, 2011.

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5 Dec 2011

During summer 2010, the Taiwan National Science Council and the US Office of Naval Research conducted a large typhoon-ocean field experiment named Impact of Typhoons on the Ocean in the Pacific (ITOP). The goals were to investigate the highly complex physical processes associated with typhoon-ocean interactions. This article highlights the Taiwanese efforts during the ITOP experiments, including work on operational satellite-derived upper-ocean thermal structure and in situ observations from moorings. A brief review of typhoon-ocean interaction research in Taiwan is also provided.

Flow and mixing in Ascension, a steep, narrow canyon

Gregg, M.C., R.A. Hall, G.S. Carter, M.H. Alford, R.-C. Lien, D.P. Winkel, and D.J. Wain, "Flow and mixing in Ascension, a steep, narrow canyon," J. Geophys. Res., 116, doi: 10.1029/2010JC006610, 2011.

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20 Jul 2011

A thin gash in the continental slope northwest of Monterey Bay, Ascension Canyon, is steep, with sides and axis both strongly supercritical to M2 internal tides. A hydrostatic model forced with eight tidal constituents shows no major sources feeding energy into the canyon, but significant energy is exchanged between barotropic and baroclinic flows along the tops of the sides, where slopes are critical. Average turbulent dissipation rates observed near spring tide during April are half as large as a two week average measured during August in Monterey Canyon. Owing to Ascension's weaker stratification, however, its average diapycnal diffusivity, 3.9 x 10^-3 m^2 s^-1, exceeded the 2.5 x 10^-3 m^2 s^-1 found in Monterey. Most of the dissipation occurred near the bottom, apparently associated with an internal bore, and just below the rim, where sustained cross-canyon flow may have been generating lee waves or rotors. The near-bottom mixing decreased sharply around Ascension's one bend, as did vertically integrated baroclinic energy fluxes. Dissipation had a minor effect on energetics, which were controlled by flux divergences and convergences and temporal changes in energy density. In Ascension, the observed dissipation rate near spring tide was 2.1 times that predicted from a simulation using eight tidal constituents averaged over a fortnightly period. The same observation was 1.5 times the average of an M2-only prediction. In Monterey, the previous observed average was 4.9 times the average of an M2-only prediction.

Nonlinear internal wave properties estimated with moored ADCP measurements

Chang, M.-H., R.-C. Lien, Y.J. Yang, and T.Y. Tang, "Nonlinear internal wave properties estimated with moored ADCP measurements," J. Atmos. Ocean. Technol., 28, 802-815, doi:10.1175/2010JTECHO814.1 , 2011.

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1 Jun 2011

A method is developed to estimate nonlinear internal wave (NLIW) vertical displacement, propagation direction, and propagation speed from single moored acoustic Doppler current profiler (ADCP) velocity observations. The method is applied to three sets of bottom-mounted ADCP measurements taken on the continental slope in the South China Sea in 2006-07. NLIW vertical displacement is computed as the time integration of ADCP vertical velocity observations corrected with the vertical advection of the background flow by the NLIW. NLIW vertical currents displace the background horizontal current and shear by ~150 m. NLIW propagation direction is estimated as the principal direction of the wave-induced horizontal velocity vector, and propagation speed is estimated using the continuity equation in the direction of wave propagation, assuming the wave's horizontal spatial structure and propagation speed remain constant as the NLIW passes the mooring, typically O(10 min). These NLIW properties are estimated simultaneously and iteratively using the ADCP velocity measurements, corrected for their beam-spreading effect. In most cases, estimates converge to within 3% after four iterations. The proposed method of extracting NLIW properties from velocity measurements is confirmed using NLIWs simulated by the fully nonlinear Dubreil-Jacotin-Long model. Estimates of propagation speed using the ADCP velocity measurements are also in good agreement with those calculated from NLIW arrival times at successive moorings. This study concludes that velocity measurements taken from a single moored ADCP can provide useful estimates of vertical displacement, propagation direction, and propagation speed of large-amplitude NLIWs.

The breaking and scattering of the internal tide on a continental slope.

Klymak, J.M., M.H. Alford, R. Pinkel, R.-C. Lien, Y.J. Yang, and T.Y. Tang, "The breaking and scattering of the internal tide on a continental slope." J. Phys. Oceanogr., 41, 926-945, doi:10.1175/2010JPO4500.1, 2011.

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1 May 2011

A strong internal tide is generated in the Luzon Strait that radiates westward to impact the continental shelf of the South China Sea. Mooring data in 1500-m depth on the continental slope show a fortnightly averaged incoming tidal flux of 12 kW m-1, and a mooring on a broad plateau on the slope finds a similar flux as an upper bound. Of this, 5.5 kW m-1 is in the diurnal tide and 3.5 kW m-1 is in the semidiurnal tide, with the remainder in higher-frequency motions. Turbulence dissipation may be as high as 3 kW m-1. Local generation is estimated from a linear model to be less than 1 kW m-1. The continental slope is supercritical with respect to the diurnal tide, implying that there may be significant back reflection into the basin. Comparing the low-mode energy of a horizontal standing wave at the mooring to the energy flux indicates that perhaps one-third of the incoming diurnal tidal energy is reflected. Conversely, the slope is subcritical with respect to the semidiurnal tide, and the observed reflection is very weak. A surprising observation is that, despite significant diurnal vertical-mode-2 incident energy flux, this energy did not reflect; most of the reflection was in mode 1.

The observations are consistent with a linear scattering model for supercritical topography. Large fractions of incoming energy can reflect depending on both the geometry of the shelfbreak and the phase between the modal components of the incoming flux. If the incident mode-1 and mode-2 waves are in phase at the shelf break, there is substantial transmission onto the shelf; if they are out of phase, there is almost 100% reflection. The observations of the diurnal tide at the site are consistent with the first case: weak reflection, with most of it in mode 1 and almost no reflection in mode 2. The sensitivity of the reflection on the phase between incident components significantly complicates the prediction of reflections from continental shelves.

Finally, a somewhat incidental observation is that the shape of the continental slope has large regions that are near critical to the dominant diurnal tide. This implicates the internal tide in shaping of the continental slope.

Speed and evolution of nonlinear internal waves transiting the South China Sea

Alford, M.H., R.-C. Lien, H. Simmons, J. Klymak, S. Ramp, Y.J. Yang, D. Tang, and M.-H. Chang, "Speed and evolution of nonlinear internal waves transiting the South China Sea," J. Phys. Oceanogr., 40, 1338-1355, 2010.

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6 Jan 2010

In the South China Sea (SCS), 14 nonlinear internal waves are detected as they transit a synchronous array of 10 moorings spanning the waves' generation site at Luzon Strait, through the deep basin, and onto the upper continental slope 560 km to the west. Their arrival time, speed, width, energy, amplitude, and number of trailing waves are monitored. Waves occur twice daily in a particular pattern where larger, narrower "A" waves alternate with wider, smaller "B" waves. Waves begin as broad internal tides close to Luzon Strait's two ridges, steepening to O(3–10 km) wide in the deep basin and O(200–300 m) on the upper slope.

Nearly all waves eventually develop wave trains, with larger/steeper waves developing them earlier and in greater numbers. The B waves in the deep basin begin at a mean speed of ~5% greater than the linear mode-1 phase speed for semidiurnal internal waves (computed using climatological and in situ stratification). The A waves travel ~5–10% faster than B waves until they reach the continental slope, presumably because of their greater amplitude. On the upper continental slope, all waves speed up relative to linear values, but B waves now travel 8–12% faster than A waves, in spite of being smaller.

Solutions of the Taylor–Goldstein equation with observed currents demonstrate that the B waves' faster speed is a result of modulation of the background currents by an energetic diurnal internal tide on the upper slope. Attempts to ascertain the phase of the barotropic tide at which the waves were generated yielded inconsistent results, possibly partly because of contamination at the easternmost mooring by eastward signals generated at Luzon Strait's western ridge. These results present a coherent picture of the transbasin evolution of the waves but underscore the need to better understand their generation, the nature of their nonlinearity, and propagation through a time-variable background flow, which includes the internal tides.

Sea surface cooling at the equator by subsurface mixing in tropical instability waves

Moum, J.N., R.-C. Lien, A. Perlin, J.D. Nash, M.C. Gregg, and P.J. Wiles, "Sea surface cooling at the equator by subsurface mixing in tropical instability waves," Nature Geosci., 2, 761-765, 2009.

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11 Oct 2009

Changes in sea surface temperature of equatorial waters have critical effects on the large-scale atmospheric circulation. So far, large-scale, energetic tropical instability waves in equatorial waters have been thought to warm the sea surface through both meridional and zonal advection. Here, we present shipboard profiling measurements of turbulence kinetic-energy dissipation rate that reveal unanticipated vigorous mixing associated with tropical instability waves. The meridional tropical instability-wave shear increases the shear above the core of the Equatorial Undercurrent, which is already large, nudging the flow toward instability. As a consequence, turbulence dissipation rates and heat fluxes are many times greater than previous measurements at the same location but in the absence of tropical instability waves. The vertical divergence of turbulence heat flux is sufficient to cool the upper layer by 2 K per month, and heat the core of the Equatorial Undercurrent by 10 K per month. Long-term records at 140°W further reveal that cooling of the sea surface is significantly correlated to tropical-instability-wave kinetic energy. Thus, seasonal surface cooling in the central equatorial Pacific may be largely caused by mixing induced by tropical instability waves.

Vorticity and turbulence in the wake of a bridge pier

Lien, R.-C., T.B. Sanford, "Vorticity and turbulence in the wake of a bridge pier," IEEE J. Ocean. Eng., 34, 307-314, doi:10.1109/JOE.2009.2019383, 2009.

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1 Jul 2009

Dissipation rates of turbulence kinetic energy (TKE) epsiv and enstrophy zeta2 are reported in a high Reynolds number turbulent wake. Previous turbulent wake observations have been made in laboratory experiments with relatively low Reynolds number flows O (103). Results presented here are from a set of rare field observations of vorticity and turbulence in a turbulent wake with a high Reynolds number O (107). The turbulent wake was formed by an unsteady strong tidal current interacting with a bridge pier. Measurements were taken mostly in the intermediate wake, i.e., 10 les x/d les 60, where x is the downstream distance and d is the width of the bridge pier. Both epsiv and zeta2 show a similar downstream decay rate that is faster than that predicted by the self-preservation similarity in the far wake. The theoretical relation epsiv = nuzeta2 for high Reynolds number flow is confirmed by field observations. The magnitudes of the vertical and horizontal components of enstrophy do not differ significantly. The turbulence internal intermittency is ~ 0.2, estimated from autocorrelation coefficients of enstrophy; this value is close to that reported previously in turbulent wakes and jets.

Turbulent mixing and internal tides in Gaoping (Kaoping) Submarine Canyon, Taiwan

Lee, I.-H., R.-C. Lien, J.T. Liu, W.-S. Chuang, "Turbulent mixing and internal tides in Gaoping (Kaoping) Submarine Canyon, Taiwan," J. Mar. Syst., 76, 383-396, doi:10.1016/j.jmarsys.2007.08.005, 2009.

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20 Mar 2009

Turbulent overturning on scales greater than 10 m is observed near the bottom and in mid-depth layers within the Gaoping (formerly spelled Kaoping) Submarine Canyon (KPSC) in southern Taiwan. Bursts of strong turbulence coexist with bursts of strong sediment concentrations in mid-depth layers. The turbulence kinetic energy dissipation rate in some turbulence bursts exceeds 10-4 W kg-1, and the eddy diffusivity exceeds 10-1 m2 s-1. Within the canyon, the depth averaged turbulence kinetic energy dissipation rate is ~ 7 x 10-6 W kg-1, and the depth averaged eddy diffusivity is ~ 10-2 m2 s-1. These are more than two orders of magnitude greater than typical values in the open ocean, and are much larger than those found in the Monterey Canyon where the strong turbulent mixing has also been.

The interaction of tidal currents with the complex topography in Gaoping Submarine Canyon is presumably responsible for the observed turbulent overturning via shear instability and the breaking of internal tides and internal waves at critical frequencies. Strong 1st-mode internal tides exist in KPSC. The depth averaged internal tidal energy near the canyon mouth is ~ 0.17 m2 s-2. The depth integrated internal tidal energy flux at the mouth of the canyon is ~ 14 kW m-1, propagating along the axis of the canyon toward the canyon head. The internal tidal energy flux in the canyon is 3–7 times greater than that found in Monterey Canyon, presumably due to the more than 10 times larger barotropic tide in the canyon. Simple energy budget calculations conclude that internal tides alone may provide energy sufficient to explain the turbulent mixing estimated within the canyon. Further experiments are needed in order to quantify the seasonal and geographical distributions of internal tides in Gaoping Submarine Canyon and their effects on the sediment flux in the canyon.

Modulation of equatorial turbulence by tropical instability waves

Lien, R.-C., E.A. D'Asaro, and C. Menkes, "Modulation of equatorial turbulence by tropical instability waves," J. Geophys. Lett., 35, doi:10.1029/2008GL035860, 2008.

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24 Dec 2008

The sea surface temperature in the Pacific equatorial cold tongue is influenced strongly by the turbulent entrainment flux. A numerical model using a level-1.5 turbulence closure scheme suggests strong modulation of the entrainment flux by tropical instability waves (TIWs). Turbulence observations taken by a Lagrangian float encountering a TIW confirm the spatial pattern of turbulent flux variation predicted by the model.

The strongest observed turbulence mixing occurred at the leading edge of the TIW trough; turbulence diffusivity K – 10-2 m2 s-1 and turbulent heat flux Q – 1000 W m-2 at the base of surface mixed layer. The weakest observed turbulence occurred at –2° south of the TIW trough; K – 10-4 m2 s-1 and Q – 10 W m-2. The TIW caused nearly two decades of turbulence variation within an O(1000 km) zonal scale and O(100 km) meridional scale. Model results suggest that the increased entrainment heat flux at the leading edge of the TIW trough can be explained by the enhancement of shear at the surface mixed layer base modulated by the TIWs.

Numerical study of baroclinic tides in Luzon Strait

Jan, S., R.-C. Lien, and C.-H. Ding, "Numerical study of baroclinic tides in Luzon Strait," J. Oceanogr., 64, 789-802, doi:10.1007/s10872-008-0066-5, 2008.

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1 Oct 2008

The spatial and temporal variations of baroclinic tides in the Luzon Strait (LS) are investigated using a three-dimensional tide model driven by four principal constituents, O1, K1, M2 and S2, individually or together with seasonal mean summer or winter stratifications as the initial field. Barotropic tides propagate predominantly westward from the Pacific Ocean, impinge on two prominent north-south running submarine ridges in LS, and generate strong baroclinic tides propagating into both the South China Sea (SCS) and the Pacific Ocean. Strong baroclinic tides, ~19 GW for diurnal tides and ~11 GW for semidiurnal tides, are excited on both the east ridge (70%) and the west ridge (30%). The barotropic to baroclinic energy conversion rate reaches 30% for diurnal tides and ~20% for semidiurnal tides.

Diurnal (O1 and K1) and semidiurnal (M2) baroclinic tides have a comparable depth-integrated energy flux 10–20 kW m-1 emanating from the LS into the SCS and the Pacific basin. The spring-neap averaged, meridionally integrated baroclinic tidal energy flux is ~7 GW into the SCS and ~6 GW into the Pacific Ocean, representing one of the strongest baroclinic tidal energy flux regimes in the World Ocean. About 18 GW of baroclinic tidal energy, ~50% of that generated in the LS, is lost locally, which is more than five times that estimated in the vicinity of the Hawaiian ridge. The strong westward-propagating semidiurnal baroclinic tidal energy flux is likely the energy source for the large-amplitude nonlinear internal waves found in the SCS. The baroclinic tidal energy generation, energy fluxes, and energy dissipation rates in the spring tide are about five times those in the neap tide; while there is no significant seasonal variation of energetics, but the propagation speed of baroclinic tide is about 10% faster in summer than in winter. Within the LS, the average turbulence kinetic energy dissipation rate is O(10-7) W kg-1 and the turbulence diffusivity is O(10-3) m2 s-1, a factor of 100 greater than those in the typical open ocean. This strong turbulence mixing induced by the baroclinic tidal energy dissipation exists in the main path of the Kuroshio and is important in mixing the Pacific Ocean, Kuroshio, and the SCS waters.

A composite view of surface signatures and interior properties of nonlinear internal waves: Observations and applications

Chang, M.-H., R.-C. Lien, T.Y. Tang, Y.J. Yang, and J. Wang, "A composite view of surface signatures and interior properties of nonlinear internal waves: Observations and applications," J. Atmos. Ocean. Technol., 25, 1218-1227, 2008.

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30 Jul 2008

Surface signatures and interior properties of large-amplitude nonlinear internal waves (NLIWs) in the South China Sea (SCS) were measured during a period of weak northeast wind (2 m s-1) using shipboard marine radar, an acoustic Doppler current profiler (ADCP), a conductivity-temperature-depth (CTD) profiler, and an echo sounder. In the northern SCS, large-amplitude NLIWs propagating principally westward appear at the tidal periodicity, and their magnitudes are modulated at the spring–neap tidal cycle. The surface scattering strength measured by the marine radar is positively correlated with the local wind speed when NLIWs are absent. When NLIWs approach, the surface scattering strength within the convergence zone is enhanced. The sea surface scattering induced by NLIWs is equivalent to that of a 6 m s-1 surface wind speed (i.e., 3 times greater than the actual surface wind speed). The horizontal spatial structure of the enhanced sea surface scattering strength predicts the horizontal spatial structure of the NLIW.

NLIW horizontal velocity convergence is positively correlated with the enhancement of the surface scattering strength. NLIW amplitude is positively correlated with the spatial integration of the enhancement of the surface scattering strength within the convergence zone of NLIWs. Empirical formulas are obtained for estimating the horizontal velocity convergence and the amplitude of NLIWs using radar measurements of surface scattering strength. The enhancement of the scattering strength exhibits strong asymmetry; the scattering strength observed from behind the propagating NLIW is 24% less than that observed ahead, presumably caused by the skewness and the breaking of surface waves induced by NLIWs. Above the center of NLIWs, the surface scattering strength is enhanced slightly, associated with isotropic surface waves presumably induced or modified by NLIWs. This analysis concludes that in low-wind conditions remote sensing measurements may provide useful predictions of horizontal velocity convergences, amplitudes, and spatial structures of NLIWs. Further applications and modification of the presented empirical formulas in different conditions of wind speed, surface waves, and NLIWs or with other remote sensing methods are encouraged.

Observations of turbulence mixing and vorticity in a littoral surface boundary layer

Lien, R.-C., T.B. Sanford, and W.-T. Tsai, "Observations of turbulence mixing and vorticity in a littoral surface boundary layer," J. Phys. Oceanogr., 38, 648-669, 2008.

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1 Mar 2008

Measurements of small-scale vorticity, turbulence velocity, and dissipation rates of turbulence kinetic energy were taken in a littoral fetch-limited surface wave boundary layer. Drifters deployed on the surface formed convergence streaks with 1-m horizontal spacing within a few minutes. In the interior, however, no organized pattern of velocity, vorticity, or turbulence mixing intensity was found at a similar horizontal spatial scale. The turbulent Langmuir number La was 0.6–1.3, much larger than the 0.3 of the typical open ocean, suggesting comparable importance of wind-driven turbulence and Langmuir circulation. Observed turbulent kinetic energy values are explained by the wind-driven shear turbulence. The production rate of turbulence kinetic energy associated with the vortex force is about 10-7 W kg-1, slightly smaller than that generated by the wind-driven turbulence. The rms values of the streakwise component of vorticity and the vertical component of vorticity have a similar magnitude of ~0.02 s-1. Vertical profiles of turbulent kinetic energy, streakwise and vertical components of vorticity showed a monotonic decrease from the surface. Traditionally, surface convergence streaks are regarded as signatures of Langmuir circulation. Two large-eddy simulations with and without Stokes drift were performed. Both simulations produced surface convergence streaks and vertical profiles of turbulent kinetic energy, vorticity, and velocity consistent with observations. The observations and model results suggest that the presence of surface convergence streaks does not necessarily imply the existence of Langmuir circulation. In a littoral surface boundary layer where surface waves are young, fetch-limited, and weak, and La = O(1), the turbulence mixing in the surface mixed layer is primarily due to the wind-driven shear turbulence, and convergence streaks exist with or without surface waves.

Assessing the west ridge of Luzon Strait as an internal wave mediator

Chao, S.-Y., D.-S. Ko, R.-C. Lien, and P.-T. Shaw, "Assessing the west ridge of Luzon Strait as an internal wave mediator," J. Oceanogr., 63, 897-911, 2007.

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7 Nov 2007

The Luzon Strait is blocked by two meridional ridges at depths, with the east ridge somewhat higher than the west ridge in the middle reaches of the Strait. Previous numerical models identified the Luzon Strait as the primary generation site of internal M2 tides entering the northern South China Sea (Niwa and Hibiya, 2004), but the role of the west-versus-east ridge was uncertain. We used a hydrostatic model for the northern South China Sea and a nonhydrostatic, process-oriented model to evaluate how the west ridge of Luzon Strait modifies westward propagation of internal tides, internal bores and internal solitary waves. The dynamic role of the west ridge depends strongly on the characteristics of internal waves and is spatially inhomogeneous. For M2 tides, both models identify the west ridge in the middle reaches of Luzon Strait as a dampener of incoming internal waves from the east ridge. In the northern Luzon Strait, the west ridge is quite imposing in height and becomes a secondary generation site for M2 internal tides. If the incoming wave is an internal tide, previous models suggested that wave attenuation depends crucially on how supercritical the west ridge slope is. If the incoming wave is an internal bore or internal solitary wave, our investigation suggests a loss of sensitivity to the supercritical slope for internal tides, leaving ridge height as the dominant factor regulating the wave attenuation. Mechanisms responsible for the ridge-induced attenuation are discussed.

High-frequency internal waves on the Oregon continental shelf

D'Asaro, E.A., R.-C. Lien, and F. Henyey, "High-frequency internal waves on the Oregon continental shelf," J. Phys. Oceanogr., 37, 195601976, doi:10.1175/JPO3096.1, 2007.

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1 Jul 2007

Measurements of vertical velocity by isopycnal-following, neutrally buoyant floats deployed on the Oregon shelf during the summers of 2000 and 2001 were used to characterize internal gravity waves on the shelf using measurements of vertical velocity. The average spectrum of Wentzel–Kramers–Brillouin (WKB)-scaled vertical kinetic energy has the level predicted by the Garrett–Munk model (GM79), plus a narrow M2 tidal peak and a broad high-frequency peak extending from about 0.1N to N and rising a decade above GM79. The high-frequency peak varies in energy coherently with time across its entire bandwidth. Its energy is independent of the tidal energy. The energy in the "continuum" region between the peaks is weakly correlated with the level of the high-frequency peak energy and is independent of the tidal peak energy. The vertical velocity is not Gaussian but is highly intermittent, with a calculated kurtosis of 19. The vertical kinetic energy varies geographically. Low energy is found offshore and nearshore. The highest energy is found near a small seamount. High energy is found over the rough topography of Heceta Bank and near the shelf break. The highest energy occurs as packets of high-frequency waves, often occurring on the sharp downward phase of the M2 internal tide and called "tidal solibores."

A few isolated waves with high energy are also found. Of the 1-h periods with the highest vertical kinetic energy, 31% are tidal solibores, 8% are isolated waves, and the remainder of the periods appear unorganized. The two most energetic tidal solibores were examined in detail. As compared with the steady, propagating, two-dimensional, inviscid, internal-wave solutions to the equations of motion with no background shear [i.e., the Dubreil–Jacotin–Long (DJL) equation], all but the most energetic observed waveforms are too narrow for their height to be solitary waves. Despite the large near-N peak in vertical kinetic energy, the M2 internal tide contributes over 80% of the energy, ignoring near-inertial waves. The tidal solibores make a very small contribution, 0.5%, to the overall internal-wave energy.

Measurement of scalar variance dissipation from Lagrangian floats

D'Asaro, E.A., and R.-C. Lien, "Measurement of scalar variance dissipation from Lagrangian floats," J. Atmos. Ocean. Technol., 24, 1066-1077, doi:10.1175/JTECH2031.1, 2007.

1 Jun 2007

Pilot whales follow internal solitary waves in the South China Sea

Moore, S.E., and R.-C. Lien, "Pilot whales follow internal solitary waves in the South China Sea," Mar. Mammal Sci., 23, 193-196, doi:10.1111/j.1748-7692.2006.00086.x, 2007.

1 Jan 2007

Measurement of turbulent kinetic energy dissipation rate with a Lagrangian float

Lien, R.-C., and E.A. D'Asaro, "Measurement of turbulent kinetic energy dissipation rate with a Lagrangian float," J. Atmos. Ocean. Technol., 23, 964-976, 2006.

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1 Jul 2006

This study tests the ability of a neutrally buoyant float to estimate the dissipation rate of turbulent kinetic energy ε from its vertical acceleration spectrum using an inertial subrange method. A Lagrangian float was equipped with a SonTek acoustic Doppler velocimeter (ADV), which measured the vector velocity 1 m below the float's center, and a pressure sensor, which measured the float's depth. Measurements were taken in flows where estimates of ε varied from 10-8 to 10-3 W kg-1. Previous observational and theoretical studies conclude that the Lagrangian acceleration spectrum is white within the inertial subrange with a level proportional to ε. The size of the Lagrangian float introduces a highly reproducible spectral attenuation at high frequencies. Estimates of the dissipation rate of turbulent kinetic energy using float measurements εfloat were obtained by fitting the observed spectra to a model spectrum that included the attenuation effect. The ADV velocity measurements were converted to a wavenumber spectrum using a variant of Taylor's hypothesis. The spectrum exhibited the expected –5/3 slope within an inertial subrange. The turbulent kinetic energy dissipation rate εADV was computed from the level of this spectrum. These two independent estimates, εADV and εfloat, were highly correlated. The ratio εfloatADV deviated from one by less than a factor of 2 over the five decades of ε measured. This analysis confirms that ε can be estimated reliably from Lagrangian float acceleration spectra in turbulent flows. For the meter-sized floats used here, the size of the float and the noise level of the pressure measurements sets a lower limit of εfloat > 10-8 W kg-1.

Energy flux of nonlinear internal waves in northern South China Sea

Chang, M.H., R.-C. Lien, T.Y. Tang, E.A. D'Asaro, and Y.J. Yang, "Energy flux of nonlinear internal waves in northern South China Sea," Geophys. Res. Lett., 33, doi:10.1029/2005GL025196, 2006.

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4 Feb 2006

We analyze three sets of ADCP measurements taken on the Dongsha plateau, on the shallow continental shelf, and on the steep continental slope in the northern South China Sea (SCS). The data show strong divergences of energy and energy flux of nonlinear internal waves (NLIW) along and across waves' prevailing westward propagation path. The NLIW energy flux is 8.5 kW m-1 on the plateau, only 0.25 kW m-1 on the continental shelf 220 km westward along the propagation path, and only 1 kW m-1 on the continental slope 120 km northward across the propagation path. Along the wave path on the plateau, the average energy flux divergence of NLIW is ~0.04 W m-2, which corresponds to a dissipation rate of O(10-7 – 10-6) W kg-1. Combining the present with previous observations and model results, a scenario of NLIW energy flux in the SCS emerges. NLIWs are generated east of the plateau, propagate predominantly westward across the plateau along a beam of ~100 km width that is centered at ~21°N, and dissipate nearly all their energy before reaching the continental shelf.

Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf

Carter, G.S., M.C. Gregg, and R.-C. Lien, "Internal waves, solitary-like waves, and mixing on the Monterey Bay shelf," Continent. Shelf Res., 25, 1499-1520, DOI: 10.1016/j.csr.2005.04.011, 2005

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24 Jun 2005

Microstructure measurements taken on the Monterey Bay continental shelf, within 4 km of the shelf break, reveal a complex mixing environment. Depth- and time-averaged dissipation rates and diapycnal diffusivities were elevated above observations made over other continental shelves with no significant topography, but were below those influenced by topographic features. The close proximity of the shelf break/canyon rim, locally generated internal tides, and nonlinear internal waves all contributed to the elevated turbulence. The complex bathymetry associated with Monterey Submarine Canyon allowed an internal tide to be generated at depths greater than 1500 m, as well as at the shelf break. The observed velocity field was normally dominated by upward energy propagation from the local shelf break generated internal tide, but near low tide downward energy propagation from a surface reflection of the internal tide generated below 1500 m was observed. Turbulent dissipation rates were not well parameterized by either the open-ocean Gregg–Henyey model or the recently developed MacKinnon–Gregg shelf model. Like its application on the New England shelf, the MacKinnon–Gregg model had the correct functional dependence on shear and stratification (dissipation increasing with increasing shear and increasing stratification), however, the magnitude and range of values were too small.

The most surprising finding was the presence of what we believe to be large, high-aspect-ratio, downslope-propagating nonlinear internal solitary-like waves of elevation. Upon reaching the canyon rim, these waves propagated into deep water and transformed into waves of depression. On the shelf south of the canyon, the waves of elevation accounted for 20% of the observed turbulent kinetic energy dissipation. Off the shelf, where the solitary-like waves changed to downward displacement, their average dissipation increased 10-fold, and accounted for nearly half the dissipation in the upper 150 m.

Prediction of underwater sound levels from rain and wind

Ma, B.B., J.A. Nystuen, and R.-C. Lien, "Prediction of underwater sound levels from rain and wind," J. Acoust. Soc. Am., 117, 3555-3565, 2005.

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1 Jun 2005

Wind and rain generated ambient sound from the ocean surface represents the background baseline of ocean noise. Understanding these ambient sounds under different conditions will facilitate other scientific studies. For example, measurement of the processes producing the sound, assessment of sonar performance, and helping to understand the influence of anthropogenic generated noise on marine mammals. About 90 buoy-months of ocean ambient sound data have been collected using Acoustic Rain Gauges in different open-ocean locations in the Tropical Pacific Ocean. Distinct ambient sound spectra for various rainfall rates and wind speeds are identified through a series of discrimination processes. Five divisions of the sound spectra associated with different sound generating mechanisms can be predicted using wind speed and rainfall rate as input variables. The ambient sound data collected from the Intertropical Convergence Zone are used to construct the prediction algorithms, and are tested on the data from the Western Pacific Warm Pool. This physically based semi-empirical model predicts the ambient sound spectra (0.5–50 kHz) at rainfall rates from 2–200 mm/h and wind speeds from 2 to 14 m/s.

Energy of nonlinear internal waves in the South China Sea

Lien, R.-C., T.Y. Tang, M.H. Chang, and E.A. D'Asaro, "Energy of nonlinear internal waves in the South China Sea," Geophys. Res. Lett., 32, 10.1029/2004GL022012, 2005.

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12 Mar 2005

Four sets of ADCP measurements were taken in the South China Sea (SCS); these results were combined with previous satellite observations and internal-tide numerical model results. Analysis suggests that strong internal tides are generated in Luzon Strait, propagate as a narrow tidal beam into the SCS, are amplified by the shoaling continental slope near TungSha Island, become nonlinear, and evolve into high-frequency nonlinear internal waves (NIW). Internal waves in the SCS have geographically distinct characteristics. (1) West of Luzon Strait the total internal wave energy (Eiw ) is 10 x that predicted by Garrett-Munk spectra (EGM) (Levine, 2002). There is no sign of NIW. (2) Near TungSha Island Eiw = 13 x EGM. Strong nonlinear and high-harmonic tides are present. Repetitive trains of large-amplitude NIW appear primarily at a semidiurnal periodicity with their amplitudes modulated at a fortnightly tidal cycle. The rms vertical velocity of NIW shows a clear spring-neap tidal cycle and is linearly proportional to the barotropic tidal height in Luzon Strait with a 1.85-day time lag, consistent with the travel time of internal tides from Luzon Strait to TungSha Island. (3) At the northern SCS shelfbreak Eiw = 4 x EGM. Single depression waves are found, but no multiple-waves packets are evident. (4) On the continental shelf Eiw = 2 x EGM . Both depression and elevation NIW exist.

Lagrangian estimates of diapycnal mixing in a simulated K–H instability

D'Asaro, E.A., K.B. Winters, and R.-C. Lien, "Lagrangian estimates of diapycnal mixing in a simulated K–H instability," J. Atmos. Ocean. Technol., 21, 799-809, 2004.

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1 May 2004

The Lagrangian properties of a high-resolution, three-dimensional, direct numerical simulation of Kelvin–Helmholtz (K–H) instability are examined with the goal of assessing the ability of Lagrangian measurements to determine rates and properties of ocean mixing events. The size and rotation rates of the two-dimensional K–H vortices are easily determined even by individual trajectories. Changes in density along individual trajectories unambiguously show diapycnal mixing. These changes are highly structured during the early phases of the instability but become more random once the flow becomes turbulent. Only 36 particles were tracked, which is not enough to usefully estimate volume-averaged fluxes from the average rates of temperature change. Similarly, time- and volume-averaged vertical advective flux can be estimated to only 20% accuracy. Despite the relatively low Reynolds number of the flow, the dissipation rates of energy and density variance are correlated with the spectral levels of transverse velocity and density in an inertial subrange, as expected for high-Reynolds-number turbulence. The Kolmogorov constants are consistent with previous studies. This suggests that these inertial dissipation methods are the most promising techniques for making useful measurements of diapycnal mixing rates from practical Lagrangian floats because they converge rapidly and have a clear theoretical basis.

Lagrangian spectra and diapycnal mixing in stratified flow

Lien, R.-C., and E.A. D'Asaro, "Lagrangian spectra and diapycnal mixing in stratified flow," J. Phys. Oceanogr., 34, 978-984, 2004.

1 Apr 2004

Turbulence spectra and local similarity scaling in a strongly stratified oceanic bottom boundary layer

Lien, R.-C., and T.B. Sanford, "Turbulence spectra and local similarity scaling in a strongly stratified oceanic bottom boundary layer," Cont. Shelf Res., 24, 375-392, doi:10.1016/j.csr.2003.10.007, 2004.

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1 Feb 2004

In the turbulence inertial subrange, wavenumber spectra of vertical velocity and streamwise velocity in a strongly stratified oceanic bottom boundary layer agree with the local similarity scaling laws found previously in the stable atmospheric boundary layer. At scales greater than the turbulence inertial subrange, oceanic velocity spectra exceed the universal spectra. This additional energy at large scales could be due either to internal waves, inappropriate estimates of turbulence parameters, or non-stationarity of the data. The strong stratification in the observed tidal bottom boundary layer is maintained by the advective density gradient. Results reported here include the effects of horizontal advection.

The Kolmogorov constant for the Lagrangian velocity spectrum and structure function

Lien, R.-C., and E.A. D'Asaro, "The Kolmogorov constant for the Lagrangian velocity spectrum and structure function," Phys. Fluids, 14, 4456-4459.

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1 Dec 2002

The inertial subrange Kolmogorov constant for the Lagrangian velocity structure function C0 is related to the inertial subrange constant for the Lagrangian acceleration spectrum β by C0 = πβ. However, Rλ must be greater than about 105 for the inertial subrange of the structure function to be sufficiently wide to accurately determine C0, while values of Rλ greater than 102 are sufficient to determine π. Taking these Rλ limitations into account, the only two known high-quality independent measurements of C0 are 5.5 and 6.4.

Internal waves and turbulence in the upper central equatorial Pacific: Lagrangian and Eulerian observations

Lien, R.-C., E.A. D'Asaro, and M.J. McPhaden, "Internal waves and turbulence in the upper central equatorial Pacific: Lagrangian and Eulerian observations," J. Phys. Oceanogr., 32, 2619-2639, 2002.

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1 Sep 2002

In the shear stratified flow below the surface mixed layer in the central equatorial Pacific, energetic near-N (buoyancy frequency) internal waves and turbulence mixing were observed by the combination of a Lagrangian neutrally buoyant float and Eulerian mooring sensors. The turbulence kinetic energy dissipation rate ε and the thermal variance diffusion rate χ were inferred from Lagrangian frequency spectral levels of vertical acceleration and thermal change rate, respectively, in the turbulence inertial subrange. Variables exhibiting a nighttime enhancement include the vertical velocity variance (dominated by near-N waves), ε, and χ. Observed high levels of turbulence mixing in this low-Ri (Richardson number) layer, the so-called deep-cycle layer, are consistent with previous microstructure measurements. The Lagrangian float encountered a shear instability event. Near-N waves grew exponentially with a 1-h timescale followed by enhanced turbulence kinetic energy and strong dissipation rate. The event supports the scenario that in the deep-cycle layer shear instability may induce growing internal waves that break into turbulence. Superimposed on few large shear-instability events were background westward-propagating near-N waves. The floats' ability to monitor turbulence mixing and internal waves was demonstrated by comparison with previous microstructure measurements and with Eulerian measurements.

Lagrangian analysis of a convective mixed layer

D'Asaro, E.A., K.B. Winters, and R.-C. Lien, "Lagrangian analysis of a convective mixed layer," J. Geophys. Res., 107, doi:10.1029/2000JC000247, 2002.

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14 May 2002

We characterize and quantify the transport of heat (Boussinesq density) in a highly idealized entraining convective mixed layer based on simulations of Lagrangian measurements in a two-dimensional model. The primary objectives are to assess and explore the merits and difficulties in estimating the heat budget from perfect and imperfect Lagrangian floats. A significant advantage of Lagrangian measurements is that the time derivative of temperature along these trajectories gives a direct measure of the diffusive heat flux. Using simulated perfect Lagrangian floats, estimates of the surface buoyancy flux, the depth of the mixed layer, vertical profiles of advective and diffusive heat flux, and the overall rate of cooling are shown to agree accurately with the known results extracted from the Eulerian simulations. The Lagrangian nature of the data is exploited to reveal the structure of the flow within the convective layer and to quantify the heat fluxes associated with the different types of eddies.

Phase plots of Lagrangian trajectories in density-depth space reveal three distinct classes of motions: (1) plumes, which develop in the cold, heavy near-surface thermal boundary layer and plunge into the mixed layer interior carrying heavy water downward; (2) interior turbulence, comprising random motions between the base of the thermal boundary layer and the base of the surface mixed layer; and (3) entrainment of interior water into plumes below the thermal boundary layer, i.e., a transition from class 2 to class 1. Plumes dominate the heat transport. Simulations were also made using slightly buoyant floats; these are not perfectly Lagrangian. Buoyancy concentrates the floats near the surface resulting in an oversampling of the stronger plumes. Making the same heat budget calculations as with the perfect floats results in a nonzero estimated Lagrangian heating rate in the interior and a curved profile of vertical heat flux that is up to 3 times too large. The local time rate change of density is not significantly affected. A correction of the heat transport for oversampling of the plumes removes most of the error. This technique allows the correct heat budget to be measured for this flow using weakly buoyant floats.

Observations of turbulence in a tidal beam and across a coastal ridge

Lien, R.-C., and M.C. Gregg, "Observations of turbulence in a tidal beam and across a coastal ridge," J. Geophys. Res., 106, 4575-4591, doi:10.1029/2000JC000351, 2001.

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15 Mar 2001

During a microstructure survey off California in Monterey Bay, we found a midwater beam of strong turbulence emanating from the shelf break along the ray path of the semidiurnal M2 internal tide. Within the 50-m-thick beam the turbulence kinetic energy dissipation rate ε exceeded 10-6 W kg-1, and the diapycnal eddy diffusivity Kρ was > 0.01 m2 s-1. The beam extended 4 km off the shelf break. Several factors suggest that this beam of strong turbulence resulted from the breaking of semidiurnal internal tides: the beam appeared to originate from the shelf break, which is a potential generation site for semidiurnal internal tides; the beam closely followed the ray path of the semidiurnal internal tide; the averageε off the shelf break varied by a factor of 100 with a semidiurnal tidal periodicity; the isopycnal displacement confirmed the presence of semidiurnal internal tides. Processes associated with the breaking of internal tides are intermittent and sporadic. At the same location we also observed equally intense turbulence in a ~100-m-thick layer of stratified water across the ridge of a sea fan. This layer of strong turbulence was separated from the bottom and was clearly not generated by bottom friction. Although less well resolved in time, the strong turbulence above the bottom seemed to vary with the semidiurnal tide and existed at the lee of the ridge, where the isopycnal surface dipped and rebounded in a pattern resembling that of internal hydraulic jumps. On the basis of the behavior of the density field, we believe that the deep mixing was most likely produced by the across-ridge current of internal tides. The breaking of internal tides at middepth, where the Richardson number is close to the critical value, is likely due to shear instability. The presence of the coastal ridge provides an alternative pathway for converting energy from internal tides to turbulence via internal hydraulics. Multiplying the average ε in the midwater beam by the length of the global coastline gives 31 GW, only a small fraction of the estimated 360 GW dissipated globally by M2 internal tides. Our observations suggest that either most internal tides are generated away from shelf breaks or most internal tides generated at shelf breaks propagate away from their generation sites, rather than dissipate locally, and eventually contribute to pelagic mixing.

The wave-turbulence transition in stratified flows

D'Asaro, E.A., and R.-C. Lien, "The wave-turbulence transition in stratified flows," J. Phys. Oceanogr., 30, 1669-1678, 2000.

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1 Jul 2000

Mixing in a stratified ocean is controlled by different physics, depending on the large-scale Richardson number. At high Richardson numbers, mixing is controlled by interactions between internal wave modes. At Richardson numbers of order 1, mixing is controlled by instabilities of the large-scale wave modes. A "wave–turbulence (W–T) transition separates these two regimes. This paper investigates the W–T transition, using observed oceanic and atmospheric spectra and parameterizations. Viewed in terms of Lagrangian (intrinsic) frequency spectra, the transition occurs when the inertial subrange of turbulence, confined to frequencies greater than the buoyancy frequency N, reaches the level of the internal waves, confined to frequencies less than N. Viewed in terms of vertical wavenumber spectra, the W–T transition occurs when the bandwidth of internal waves becomes small. Both of these singularities occur when the typical internal wave velocity becomes comparable to the phase speed of the lowest internal wave mode. At energies below that of the W–T transition, the dissipation rate varies as the energy squared; above the transition the dependence is linear. The transition occurs at lower shear and dissipation rates where the phase speed of the lowest mode is smaller, that is, in shallower water for the same stratification. Traditional turbulence closure models, which ignore internal waves, can be accurate only at energies above the W–T transition.

Spectral characteristics of velocity and vorticity fluxes in an unstratified turbulent boundary layer

Lien, R.-C., and T.B. Sanford, "Spectral characteristics of velocity and vorticity fluxes in an unstratified turbulent boundary layer," J. Geophys. Res., 105, 8659-8672, 2000.

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15 Apr 2000

Wavenumber spectral characteristics of the velocity and vorticity fluxes in an unstratified turbulent boundary layer are presented. The observed vertical and streamwise velocity spectra agree with empirical forms found in the atmospheric boundary layer. Spectral ratios of 4/3 between the vertical and streamwise velocity spectra and the agreement between the observed vorticity flux quad spectrum and that of isotropic turbulence suggest local isotropy at scales smaller than Z. The normalized cospectrum of the momentum flux agrees remarkably well with the empirical form found in the atmospheric boundary layer. In the inertial subrange the momentum flux cospectrum shows a clear spectral slope of –7/3. The observed composite vorticity flux cospectrum has most of its variance at the streamwise wavenumber kx =(1–10) Z-1 and has a spectral slope of –7/3 in the inertial subrange. The –7/3 spectral slope is consistent with a dimensional argument, assuming that the vorticity flux cospectrum is proportional to the gradient of the mean vorticity, and depends on the turbulence kinetic energy dissipation rate ε and the wavenumber. A model turbulent vorticity flux cospectrum is constructed based on the shape of observed spectra, a –7/3 spectral slope in the inertial subrange, and the similarity scaling of the vorticity flux in an unstratified turbulent boundary layer. The turbulence vorticity flux is directly related to the divergence of turbulence momentum flux, the force exerted by turbulence on the mean flow. Therefore our proposed empirical cospectral form of the vorticity fluxes might be useful for turbulence parameterization in numerical models.

Lagrangian measurements of waves and turbulence in stratified flows

D'Asaro, E.A. and R.-C. Lien, "Lagrangian measurements of waves and turbulence in stratified flows," J. Phys. Oceanogr., 30, 641-655, 2000.

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1 Mar 2000

Stratified flows are often a mixture of waves and turbulence. Here, Lagrangian frequency is used to distinguish these two types of motion.

A set of 52 Lagrangian float trajectories from Knight Inlet and 10 trajectories from below the mixed layer in the wintertime northeast Pacific were analyzed using frequency spectra. A subset of 28 trajectories transit the Knight Inlet sill where energetic internal waves and strong turbulent mixing coexist.

Vertical velocity spectra show a progression from a nearly Garrett–Munk internal wave spectrum at low energies to a shape characteristic of homogeneous turbulence at high energies. All spectra show a break in slope at a frequency close to the buoyancy frequency N. Spectra from the Knight Inlet sill are analyzed in more detail. For "subbuoyant" frequencies (less than N) all 28 spectra exhibit a ratio of vertical-to-horizontal kinetic energy that varies with frequency as predicted by the linear internal wave equations. All spectra have a shape similar to that of the Garrett–Munk internal wave spectrum at subbuoyant frequencies. These motions are much more like waves than turbulence. For "superbuoyant" frequencies (greater than N) all 28 spectra are isotropic and exhibit the –2 spectral slope of inertial subrange homogeneous turbulence. These motions appear to be turbulent.

These data suggest that stratified flows may be modeled as the sum of nearly isotropic turbulence with superbuoyant Lagrangian frequencies and anisotropic internal waves with subbuoyant Lagrangian frequencies. The horizontal velocities are larger than the vertical velocities for the internal wave component but approximately equal for the turbulent component. Vertical kinetic energy is therefore a better indicator of turbulent kinetic energy than is horizontal or total kinetic energy.

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