Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning
Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning
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DOI：
10.1038/ngeo2749
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发表时间：
2016-08-01
影响因子：
18.3
通讯作者：
Talley, Lynne D.
中科院分区：
文献类型：
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作者：
Abernathey, Ryan P.;Cerovecki, Ivana;Talley, Lynne D.
海洋翻转环流要求海水浮力发生持续的热力学变化。南大洋陡峭倾斜的等密度线为环极深水提供了一条从中间深度上升的通道，而没有强烈的贯叶混合（1-3），在那里它直接被热量和淡水的表面通量转化，并分裂成上下分支（4-6）。虽然海冰对盐水的排斥被认为有助于形成下分支（7），但海冰在上分支中的作用还不太清楚，部分原因是缺乏对海冰厚度和输送的观测（8，9）。在这里，我们使用南大洋状态估计来量化海冰淡水通量，这是一种最先进的数据同化方法，融合了数百万次海洋和冰的观测。然后，我们使用水团转换框架（10）来比较大气、海冰和冰川淡水通量、热通量和上层海洋混合在转换上分支内浮力方面的相对作用。我们发现，海冰是一个占主导地位的术语，与差异盐水排斥和冰融化转化上升的绕极深水在类似的速度为22 × 10（6）米（3）秒（-1）。这些结果意味着南极海冰在上分支中的重要作用，并表明剩余翻转和风驱动的海冰输送是紧密耦合的。
Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing(1-3), where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch(4-6). While brine rejection from sea ice is thought to contribute to the lower branch(7), the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport(8,9). Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework(10) to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with differential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of similar to 22 x 10(6) m(3) s(-1). These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.