APL Home

Campus Map

David Winkel

Senior Oceanographer - Retiree





Research Interests

Physical Oceanography, Mixing and Internal Waves, Data Analysis


Dr. Winkel studies internal waves and turbulent mixing by analyzing observations collected from vertical profilers. His dissertation work focussed on measurements taken with the Multi-Scale Profiler in the vertically sheared Florida Current. He is responsible for managing data taken on several research cruises and for assisting in data analysis and collection. Dr. Winkel joined the Laboratory as a graduate student in 1986 and as a staff member in 1998.

Department Affiliation

Ocean Physics


B.S. Mathematics, University of Washington, 1976

M.S. Oceanography, University of Washington, 1990

Ph.D. Oceanography, University of Washington, 1998


2000-present and while at APL-UW

Mixing over the steep side of the Cycladic Plateau in the Aegean Sea

Gregg, M.C., M.H. Alford, H. Kontoyiannis, V. Zervakis, and D. Winkel, "Mixing over the steep side of the Cycladic Plateau in the Aegean Sea," J. Mar. Syst., 89, 30-47, doi:10.1016/j.marsys.2011.07.009, 2012.

More Info

1 Jan 2012

Intensive microstructure sampling over the southern slope of the Cycladic Plateau found very weak mixing in the pycnocline, centered on a thin minimum of diapycnal diffusivity with Kρ=1.5 x 10-6 m2 s-1. Below the pycnocline, Kρ increased exponentially in the bottom 200 m, reaching 1 x 10-4 m2 s-1 a few meters above the bottom. Near-bottom mixing was most intense where the bottom slope equaled the characteristic slope of the semi-diurnal internal tide. This suggests internal wave scattering and/or generation at the bottom, a conclusion supported by near-bottom dissipation rates increasing following rising winds and with intensifying internal waves. Several pinnacles on the slope were local mixing hotspots. Signatures included a vertical line of strong mixing in a pinnacle's wake, an hydraulic jump or lee wave over a downstream side of the summit, and a 'beam' sloping upward at the near-inertial characteristic slope. Because dissipation rate averages were dominated by strong turbulence, ε/vN2 > 100, the effect on Kρ of alternate mixing efficiencies proposed for this range of turbulent intensity is explored.

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.

More Info

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.

Reduced mixing from the breaking of internal waves in equatorial waters

Gregg, M.C., T.B. Sanford, and D.P. Winkel, "Reduced mixing from the breaking of internal waves in equatorial waters," Nature, 422, 513-515, doi:10.1038/nature01507, 2003.

More Info

3 Apr 2003

In the oceans, heat, salt and nutrients are redistributed much more easily within water masses of uniform density than across surfaces separating waters of different densities. But the magnitude and distribution of mixing across density surfaces are also important for the Earth's climate as well as the concentrations of organisms. Most of this mixing occurs where internal waves break, overturning the density stratification of the ocean and creating patches of turbulence. Predictions of the rate at which internal waves dissipate were confirmed earlier at mid-latitudes. Here we present observations of temperature and velocity fluctuations in the Pacific and Atlantic oceans between 42° N and 2° S to extend that result to equatorial regions. We find a strong latitude dependence of dissipation in accordance with the predictions. In our observations, dissipation rates and accompanying mixing across density surfaces near the Equator are less than 10% of those at mid-latitudes for a similar background of internal waves. Reduced mixing close to the Equator will have to be taken into account in numerical simulations of ocean dynamics—for example, in climate change experiments.

More Publications

Patterns of shear and turbulence across the Florida Current

Winkel, D.P., M.C. Gregg, and T.B. Sanford, "Patterns of shear and turbulence across the Florida Current," J. Phys. Oceanogr., 32, 3269-3285, doi:10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2, 2002.

More Info

1 Nov 2002

Measurements by the Multi-Scale Profiler (MSP) at seven stations spanning the Straits of Florida characterize levels and patterns of internal wave activity and mixing in this vertically sheared environment. Contrasting properties suggest five mixing regimes. The largest and most vast is the interior regime, where the background flow has an inverse Richardson number (Ri1) ranging up to 0.55, shear is dominated by fluctuations that are 1–4 times stronger than in the open ocean, and turbulent diffusivities are similarly moderate at (1–4) x 10-5 m2 s-1. The high-velocity core of the current, near the surface at midchannel, is associated with weak mixing. To its west is a zone of high mean shear, where strong stratification results in background Ri-1 of only 0.4, fluctuations are weak, and diffusivity is moderate. Intermittent shear features beneath the core have mean Ri-1 > 1 and strong turbulence. Two regimes are related to channel topography. Adjacent to the steep eastern slope, finescale shear is predominately cross-channel, and turbulence varies from nearly the weakest to nearly the strongest. Within 100 m of the channel floor, turbulent stratified boundary layers are mixing at (2–6) x 10-4 m2 s-1 to account for one-half of the section-averaged diffusivity. Using existing finescale parameterizations, observed dissipation rates can be predicted within a factor of 2 for most of this dataset, despite significantly strong mean shear and generally anisotropic and asymmetric fluctuations. The exceptions are in the high mean shear zones, where total rather than fluctuating shear yields reasonable estimates, and in some of the more turbulent regions, where shear underestimates mixing. Given its overall moderate levels of turbulence and finescale shear, the Florida Current is not a hot spot for oceanic mixing.

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center