| Summary: | The current reduction of sea ice has a major impact on the Arctic Ocean because it promotes the oceans direct exchange with the atmosphere. In our NSF-funded project InMotion: Influx of Momentum into the Arctic Ocean Changes Associated with Sea Ice Reduction we investigated how the observed thinning and retreat of the sea ice cover affects the amount of wind energy entering the Arctic Ocean. For the first time Arctic-wide estimates of the momentum flux from the atmosphere into the ocean (a.k.a. ocean surface stress) are analyzed with respect to seasonality and trends for the period 1979-2012. The ocean surface stress is affected by (i) wind speed, (ii) surface layer stability, (iii) surface roughness, and (iv) sea ice conditions. While the wind forcing, i.e. the momentum source, prescribes a seasonal cycle, there is no basin-wide trend in wind speed that could explain the increase in annual mean ocean surface stress over the study period. Further, variations in atmosphere surface layer stability were found to be small compared to those in the wind forcing. It is the changing sea ice conditions that drive changes in momentum transfer with implications for both seasonality and long-term trends in ocean surface stress. Continuous sea ice thinning has weakened the ice cover so that more momentum is transferred into the Arctic Ocean during fall, winter and spring. In summer, however, the ice retreat results in surface roughness reduction and thus less momentum transfer. An increased momentum flux from the atmosphere into the ocean can be expected to result in accelerated surface currents and more vertical mixing. Both will affect the distribution of heat and nutrients with possible implications regarding accelerated sea ice melt, increased seasonality, and bioproductivity. This study will enhance our ability to understand and predict the future changes in this momentum flux. Consequently, it contributes to our understanding and management of resource extraction, shipping, and ecosystem change in the Arctic, all of which depend on the evolution of the sea ice cover.
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