Cole, ST, et al 2017 Ice and ocean velocity in the Arctic marginal ice zone: Ice roughness and momentum transfer. Elem Sci Anth, 5: 55, DOI: https://doi.org/10.1525/elementa.241 IntroductionThe canonical view of subinertial ocean currents immediately beneath sea ice consists of a logarithmic boundary layer, within which the stress is independent of depth, and an Ekman layer, where the influence of the Earth's rotation becomes important (Figure 1; McPhee, 2008). Together these layers are termed the ice-ocean boundary layer (IOBL), and encompass the upper tens of meters of the Arctic Ocean. The logarithmic boundary layer is typically a few meters thick at most, but within it currents vary logarithmically with depth. The Ekman layer under sea ice has currents that decay and rotate with depth. The specific details RESEARCH ARTICLEIce and ocean velocity in the Arctic marginal ice zone: Ice roughness and momentum transfer The interplay between sea ice concentration, sea ice roughness, ocean stratification, and momentum transfer to the ice and ocean is subject to seasonal and decadal variations that are crucial to understanding the present and future air-ice-ocean system in the Arctic. In this study, continuous observations in the Canada Basin from March through December 2014 were used to investigate spatial differences and temporal changes in under-ice roughness and momentum transfer as the ice cover evolved seasonally. Observations of wind, ice, and ocean properties from four clusters of drifting instrument systems were complemented by direct drill-hole measurements and instrumented overhead flights by NASA operation IceBridge in March, as well as satellite remote sensing imagery about the instrument clusters. Spatially, directly estimated ice-ocean drag coefficients varied by a factor of three with rougher ice associated with smaller multi-year ice floe sizes embedded within the first-year-ice/multi-year-ice conglomerate.Temporal differences in the ice-ocean drag coefficient of 20-30% were observed prior to the mixed layer shoaling in summer and were associated with ice concentrations falling below 100%. The ice-ocean drag coefficient parameterization was found to be invalid in September with low ice concentrations and small ice floe sizes. Maximum momentum transfer to the ice occurred for moderate ice concentrations, and transfer to the ocean for the lowest ice concentrations and shallowest stratification. Wind work and ocean work on the ice were the dominant terms in the kinetic energy budget of the ice throughout the melt season, consistent with free drift conditions. Overall, ice topography, ice concentration, and the shallow summer mixed layer all influenced mixed layer currents and the transfer of momentum within the air-ice-ocean system. The observed changes in momentum transfer show that care must be taken to determine appropriate parameterizations of momentum transfer, and imply that the future Arctic system could become increasingly seasonal.
The abyssal Southwest Pacific Basin has warmed significantly between 1992-2017, consistent with warming along the bottom limb of the meridional overturning circulation seen throughout the global oceans. Here we present a framework for assessing the abyssal heat budget that includes the time-dependent unsteady effects of decadal warming and direct and indirect estimates of diapycnal mixing from microscale temperature measurements and finescale parameterizations. The unsteady terms estimated from the decadalwarming rate are shown to be within a factor of 3 of the steady state terms in the abyssal heat budget for the coldest portion of the water column and therefore, cannot be ignored. We show that a reduction in the lateral heat flux for the coldest temperature classes compensated by an increase in warmer waters advected into the basin has important implications for the heat balance and diffusive heat fluxes in the basin. Finally, vertical diffusive heat fluxes are estimated in different ways: using the newly available CTD-mounted microscale temperature measurements, a finescale strain parameterization, and a vertical kinetic energy parameterization from data along the P06 transect along 32.5°S. The unsteady-state abyssal heat budget for the basin shows closure within error estimates, demonstrating that (i) unsteady terms have become consequential for the heat balance in the isotherms closest to the ocean bottom and (ii) direct and indirect estimates from full depth GO-SHIP hydrographic transects averaged over similarly large spatial and temporal scales can capture the basin-averaged abyssal mixing needed to close the deep overturning circulation.
Turbulent mixing in the ocean is often parameterized in terms of the downscale energy transfer by internal waves. Expressed in terms of the vertical wavenumber spectrum of velocity shear and isopycnal strain, the “finescale parameterization” relies on several parameters that set the shape of the spectrum. Here, we use an unsupervised learning model to identify dominant spectral shapes and their geographical dependencies in data from 1875 global hydrographic profiles. The model reveals clustered regions with underlying spectra deviating significantly from the canonical Garrett-Munk (GM) spectrum, suggesting potential departures from the underlying assumption about the downscale energy cascade. These clusters are further found to be associated with regions where parameterized estimates of mixing differ significantly from temperature microstructure observations. Extending the methodology to other hydrographic datasets has the potential to reveal reasons for parameterization bias and to identify the dynamical underpinnings leading to more robust parameterizations of oceanic turbulent mixing.
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