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Mike Gregg

Professor Emeritus, Oceanography

Professor, Oceanography

Email

gregg@apl.washington.edu

Phone

206-543-1353

Biosketch

The idea that the cumulative action of centimeter-scale mixing affects the ocean's largest scales guides Mike Gregg's research. Evolving technology now enables the mixing to be put into the context of the meter-to-kilometer-scale processes directly producing it, such as internal waves, bottom and surface boundary layers, thermohaline staircases and intrusions, and hydraulic responses to flow constrictions. Because large-scale models, particularly coupled climate models, have grid scales vastly larger than those of the mixing and even of the intermediate-scale processes producing it, it is a goal to always try to work toward parameterizations that can be used in these models.

Department Affiliation

Ocean Physics

Education

B.S. Physics, Yale University, 1961

Ph.D. Physical Oceanography, Scripps Institution of Oceanography, 1971

Publications

2000-present and while at APL-UW

Using an ADCP to estimate turbulent kinetic energy dissipation rate in sheltered coastal water

Greene, A.D., P.J. Hendricks, and M.C. Gregg, "Using an ADCP to estimate turbulent kinetic energy dissipation rate in sheltered coastal water," J. Atmos. Ocean. Techno., 32, 318-333, doi:10.1175/JTECH-D-13-00207.1, 2015.

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1 Feb 2015

Turbulent microstructure and acoustic Doppler current profiler (ADCP) data were collected near Tacoma Narrows in Puget Sound, Washington. Over 100 coincident microstructure profiles have been compared to ADCP estimates of turbulent kinetic energy dissipation rate (ε). ADCP dissipation rates were calculated using the large-eddy method with theoretically determined corrections for sensor noise on rms velocity and integral-scale calculations. This work is an extension of Ann Gargett's approach, which used a narrowband ADCP in regions with intense turbulence and strong vertical velocities. Here, a broadband ADCP is used to measure weaker turbulence and achieve greater horizontal and vertical resolution relative to the narrowband ADCP. Estimates of ε from the Modular Microstructure Profiler (MMP) and broadband ADCP show good quantitative agreement over nearly three decades of dissipation rate, 3 x 10-8 – 10-5 m2 s-3. This technique is most readily applied when the turbulent velocity is greater than the ADCP velocity uncertainty (σ) and the ADCP cell size is within a factor of 2 of the Thorpe scale. The 600-kHz broadband ADCP used in this experiment yielded a noise floor of 3 mm s-1 for 3-m vertical bins and 2-m along-track average (~four pings), which resulted in turbulence levels measureable with the ADCP as weak as 3 x 10-8 m2 s-3. The value and trade-off of changing the ADCP cell size, which reduces noise but also changes the ratio of the Thorpe scale to the cell size, are discussed as well.

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.

Wind-driven submesoscale subduction at the north Pacific subtropical front

Hosegood, P.J., M.C. Gregg, and M.H. Alford, "Wind-driven submesoscale subduction at the north Pacific subtropical front," J. Geophys. Res., 118, 5333-5352, doi:10.1002/jgrc.20385, 2013.

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1 Oct 2013

Upper ocean observations from the north Pacific subtropical front during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss. Prior to generation, a sharp thermohaline front was intensified by confluent flow of 1–2 x 10-5 s-1. Relative vertical vorticity across a surface-intensified, along-front jet on the warm side of a frontal trough was 0.5 f. During the storm, buoyancy loss arose due to cooling of ~650 W m-2 and down-front wind stress <0.5 N m-2 that generated a southward, cross-front Ekman transport of dense water over light. The resulting wind-driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude. The intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline. They were approximately 20 m thick and horizontally elongated in the cross-frontal direction. The near-surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline, whereas the subsurface intrusions were composed of warm and saline water from the surface. The apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterized by O(1) Rossby and Richardson numbers, pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer, and sloping isopycnals. The intrusion properties, background environmental context, and forcing history are consistent with prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss, possibly resulting in symmetric instability.

More Publications

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
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