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Shear Variance and Associated Heat Flux in the Upper Arctic Ocean During SHEBA |
Scripps Institution of Oceanography
challe@opg1.ucsd.edu
The thickness of Arctic sea-ice is sensitive to the value of the vertical heat flux in the upper ocean [Fleming and Semtner, 1991]. This flux is supported by turbulent processes which are presumably driven by breaking internal waves [D'Asaro and Morison, 1992]. Gregg [1989] has demonstrated that the fine scale (10 meter) shear variance, along with the bouyancy frequency, is a useful metric of the mixing rates.
During SHEBA, an 11 month record of upper ocean currents was obtained using repeat sequence coded Doppler sonars. The sonars consistently provided profiles of ice-water relative velocity to depths of 300 meters, with 3 meter vertical resolution. These records were used to examine the shear climatology in the Canadian Basin, over the Northwind Ridge, and near the Chukchi Plateau.
Factors affecting shear variance in SHEBA include the generation of near inertial waves by under-ice relief and seafloor topography, and the refraction of internal waves by eddies. High shear variance is often associated with downward propagating near inertial waves, presumably generated near the base of the ice [D'Asaro and Morison, 1992, Merrifeld and Pinkel, 1996]. Extremely large shear variance is also found over anticyclonic vortices, where the downward propagating wavefield is refracted. Shear inside the eddies is much reduced. As the camp drifts over the Northwind Ridge, shear variance associated with upward propagating near inertial waves becomes significant.
If shear is indeed a metric of mixing, the large scale heat fluxes in the upper Beaufort Sea and near the Northwind Ridge must be sensitive to ice motion (storms). Heat fluxes in the Beaufort Sea will also be affected by the population of eddies, which concentrate near-inertial energy. In addition, heat fluxes over geographic features, such as the Northwind Ridge, will be large. Thus, factors as diverse as atmospheric storm tracks, eddy generation at Barrow Canyon [D'Asaro, 1988], and seafloor topography, as well as the bulk temperature of the warm Atlantic waters, may play a role in establishing the climatological heat flux.
