Tidal and Wind-Forced Near-Inertial Current and Shear Variability in the Upper Ocean of the Eastern Arctic: Sensitivity to Changing Ocean and Sea Ice State

Till Baumann, University of Alaska Fairbanks, Fairbanks, AK, United States, Laurence Padman, Earth & Space Research, Corvallis, OR, United States, Igor V Polyakov, University of Alaska Fairbanks, International Arctic Research Center, Fairbanks, United States, Ilker Fer, University of Bergen, Bergen, Norway, An T Nguyen, University of Texas at Austin, Oden Institute for Computational Engineering and Sciences, United States and Susan L. Howard, Earth and Space Research, Seattle, WA, United States
Abstract:
Long-term (2004-present) mooring measurements in the Eastern Arctic Ocean demonstrate a trend towards increasing tidal-band kinetic energy and vertical shear in the upper ocean, concurrent with observed increases in the upward flux of Atlantic Water heat to the surface mixed layer (SML) and sea ice. This raises the possibility that increasing shear in tidal-band currents is contributing to increased mixing. Here, we report analyses of two years (2013-2015) of mooring data along 126oE from the Laptev Sea continental shelf to the deep Eurasian Basin. Both semidiurnal baroclinic tides and wind-forced near-inertial waves contribute to a strong seasonal cycle of semidiurnal-band currents (SBCs) and shear in the upper ocean; however, the proximity of inertial and tidal frequencies in this region complicates separation of the two energy sources. Peak SBCs, exceeding 30 cm/s at sites in the deep Eurasian Basin, occur in summer during periods of low ice concentration; in winter, the depth of significant SBCs increases following the deepening of the mixed layer. Numerical models of varying complexity demonstrate that both the ocean background state (hydrographic structure and mean flow) and ice pack influence the magnitude and spatial patterns of SBCs. The strong wind-forced contribution to SBCs highlights the need for accurate models of wind stress, at high spatial and temporal resolution, capable of exciting inertial oscillations. The dependence of both tidal and wind-forced SBCs on ocean and sea ice background, and the inferred ability of SBCs to modify their background through shear instabilities, suggest a feedback (of as-yet unknown amplitude and sign) that can affect the evolution of eastern Arctic upper-ocean hydrography and ice pack. In turn, this feedback indicates the need for explicit representation of changing SBCs in predictive models of the evolving Arctic Ocean.