Revisiting internal waves and mixing in the Arctic Ocean

Abstract

[1] To determine whether deep background mixing has increased with the diminishment of the Arctic sea ice, we compare recent internal wave energy and mixing observations with historical measurements. Since 2007, the North Pole Environmental Observatory has launched expendable current probes (XCPs) as a part of annual airborne hydrographic surveys in the central Arctic Ocean. Mixing in the upper 500 m is estimated from XCP shear variance and Conductivity-Temperature-Depth (CTD) derived Brunt-V€ ais€ al€ a frequency. Internal wave energy levels vary by an order of magnitude between surveys, although all surveys are less energetic and show more vertical modes than typical midlatitude Garrett-Munk (GM) model spectra. Survey-averaged mixing estimates also vary by an order of magnitude among recent surveys. Comparisons between modern and historical data, reanalyzed in identical fashion, reveal no trend evident over the 30 year period in spite of drastic diminution of the sea ice. Turbulent heat fluxes are consistent with recent double-diffusive estimates. Both mixing and internal wave energy in the Beaufort Sea are lower when compared to both the central and eastern Arctic Ocean, and expanding the analysis to mooring data from the Beaufort Sea reveals little change in that area compared to historical results from Arctic Internal Wave Experiment. We hypothesize that internal wave energy remains lowest in the Beaufort Sea in spite of dramatic declines in sea ice there, because increased stratification amplifies the negative effect of boundary layer dissipation on internal wave energy.

Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review

“…The effect of decreasing ice cover on the internal wave energetics, however, is not well established. Comparisons of internal wave energy between modern and historical data, reanalysed in identical fashion, reveal no trend evident over the 30-year period in spite of drastic diminution of sea ice (Guthrie et al, 2013). The possible increase in internal wave forcing due to reduced sea ice cover may be offset by increased stratification by meltwater, which amplifies the dissipation of internal wave energy in the under-ice boundary layer.…”

“…Reduction of the sea ice cover is, however, expected to increase the background mixing levels . Although the measurements in the MIZ are in support of this hypothesis (Fer et al, 2010), the effect of decreasing ice cover on the internal wave energetics is not yet well established (Guthrie et al, 2013).…”

Near-Inertial Mixing in the Central Arctic Ocean

“…The water is shallow (70 m), and the observations may not be representative of the internal wave propagation or sea ice dynamics in the deep Arctic. Comparisons of internal wave energy between modern and historical data, reanalyzed in identical fashion, reveal no evident trend over a 30-yr period in spite of drastic diminution of the sea ice (Guthrie et al 2013). Guthrie et al (2013) attribute this to the increased upper-layer stratification by meltwater that increases the boundary layer dissipation of internal wave energy.…”

“…The vertical resolution of a raw profile is about 0.4 m. XCP velocity measurement depends on the nonzero amplitude of the vertical component of the magnetic field, which is large at high latitudes; however, compass errors due to the decreasing size of the horizontal component of the magnetic field result in increasing errors in velocity direction as the magnetic pole is approached. Nevertheless, they have been used successfully at Arctic latitudes (e.g., D'Asaro and Morison 1992;Guthrie et al 2013). The profiles are processed using 2-m-long moving segments and averaged 1 m vertically, using the data processing package developed at the Applied Physical Laboratory, University of Washington.…”

Structure and dynamics of mesoscale eddies over the Laptev Sea continental slope in the Arctic Ocean

“…by Guthrie et al (2013), who utilized a collection of Expendable Current Profiler measurements to estimate diapycnal mixing for several parts of the Arctic Ocean. 15…”