Equilibration of the Antarctic Circumpolar Current by Standing Meanders
Equilibration of the Antarctic Circumpolar Current by Standing Meanders
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DOI：
10.1175/jpo-d-13-0163.1
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发表时间：
2014-07
影响因子：
3.5
通讯作者：
A. Thompson;A. N. Garabato
中科院分区：
文献类型：
--
作者：
A. Thompson;A. N. Garabato
南极绕极流S显著的等轴斜率对风应力变化的不敏感性被认为源于中尺度涡旋的作用，该涡旋抵消了由风驱动的埃克曼倾覆--这一框架在纬向对称的绕极气流中得到了验证。涡旋特征的显著纬向变化表明，局地动力可能改变了ACC路径上的这种平衡。对涡旋分辨海洋GCM的分析表明，ACC可以被分解为广泛的涡旋活动较弱的区域，其中地面风使等轴线陡峭，以及少量站立的弯道，等轴线在其上松弛。曲折的位置符合(I)由强涡引起的平均气流及其垂直结构的改变，通过Eliassen-Palm通量的散度来测量，以及(Ii)在周围地区的深涡动能增加达两个数量级。在曲流内部，涡度收支在相对涡度平流和水平辐散之间表现出平衡，为强垂直速度的产生和层结的快速变化提供了机制。这些诊断中的时间波动与Eliassen-Palm通量和海底速度的变异性相关，这意味着与海底的耗散过程有关。在更大的尺度上，底部压力扭矩与行星涡度的正压平流在空间上相关，而正压平流又与曲流结构的变化有关。根据这些结果，提出了一种替代机制来降低ACC斜压性对强迫变化的敏感性，而不是ACC范围内的瞬变涡旋特征的变化。
The insensitivity of the Antarctic Circumpolar Current (ACC)’s prominent isopycnal slope to changes in wind stress is thought to stem from the action of mesoscale eddies that counterbalance the wind-driven Ekman overturning—a framework verified in zonally symmetric circumpolar flows. Substantial zonal variations in eddy characteristics suggest that local dynamics may modify this balance along the path of the ACC. Analysis of an eddy-resolving ocean GCM shows that the ACC can be broken into broad regions of weak eddy activity, where surface winds steepen isopycnals, and a small number of standing meanders, across which the isopycnals relax. Meanders are coincident with sites of (i) strong eddy-induced modification of the mean flow and its vertical structure as measured by the divergence of the Eliassen–Palm flux and (ii) enhancement of deep eddy kinetic energy by up to two orders of magnitude over surrounding regions. Within meanders, the vorticity budget shows a balance between the advection of relative vorticity and horizontal divergence, providing a mechanism for the generation of strong vertical velocities and rapid changes in stratification. Temporal fluctuations in these diagnostics are correlated with variability in both the Eliassen–Palm flux and bottom speed, implying a link to dissipative processes at the ocean floor. At larger scales, bottom pressure torque is spatially correlated with the barotropic advection of planetary vorticity, which links to variations in meander structure. From these results, it is proposed that the “flexing” of standing meanders provides an alternative mechanism for reducing the sensitivity of the ACC’s baroclinicity to changes in forcing, separate from an ACC-wide change in transient eddy characteristics.