InMotion: Influx of Momentum into the Arctic Ocean - Changes Associated with Sea Ice Reduction

The current reduction of sea ice has a major impact on the Arctic Ocean because it promotes the ocean’s 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.当前海冰消退正对北冰洋造成重大影响，因其加剧了海洋与大气的直接交换过程。在我们受美国国家科学基金会（National Science Foundation, NSF）资助的‘InMotion：北冰洋动量输入——与海冰消退相关的变化’项目中，我们研究了观测到的海冰覆盖变薄与退缩现象对输入北冰洋的风能总量的影响。本研究首次针对1979-2012年期间，大气向海洋输送的动量通量（momentum flux）的全北冰洋估算结果展开分析，该通量又称海面应力（ocean surface stress），重点考察其季节变化特征与长期趋势。海面应力受四项核心因素调控：(i) 风速、(ii) 近地面层大气稳定度、(iii) 海面粗糙度、(iv) 海冰状况。尽管作为动量源的风强迫呈现出清晰的季节循环特征，但研究期内风速并未出现全海盆尺度的显著趋势，无法解释年平均海面应力在观测期内的上升现象。进一步分析显示，近地面层大气稳定度的变化幅度远小于风强迫的变化幅度。正是海冰条件的改变，驱动了动量输送过程的变化，进而对海面应力的季节特征与长期趋势均产生显著影响。持续的海冰变薄已削弱了海冰覆盖的整体强度，使得秋季、冬季与春季输入北冰洋的动量通量有所增加；但在夏季，海冰退缩会导致海面粗糙度降低，反而减少了动量输送。大气向海洋输入的动量通量增加，预计将引发表层海流加速与更强的垂直混合过程。这两种过程均会改变海洋中的热量与营养盐分布，进而可能带来海冰消融加速、季节变化加剧以及海洋生物生产力提升等一系列连锁效应。本研究将提升我们理解与预测该动量通量未来变化的能力。因此，本研究有助于增进我们对北极地区资源开采、航运及生态系统变化的认知与管理——而所有这些活动均依赖于海冰覆盖的演化进程。