Reevaluating the Roles of Eddies in Multiple Barotropic Wind-Driven Gyres
Reevaluating the Roles of Eddies in Multiple Barotropic Wind-Driven Gyres
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重新评估涡流在多个正压风驱动环流中的作用
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发表时间：
2005
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通讯作者：
B. Fox‐Kemper
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作者：
B. Fox‐Kemper
在粘性和副热带强迫相同的情况下，多涡旋海洋模式的平均副热带环流比单涡旋计算的要弱。传统上，环流的减少归因于涡旋间涡通量，它抵消了一些风输入，其中一部分不需要拉格朗日质量交换(耗散曲折理论)。在高雷诺数的正压模式中，只有在严格反对称涡旋和滑移边界条件下，涡间涡通量才是控制因素。当使用无滑移边界条件时，几乎不会出现涡间通量，但在多重涡旋计算中，副热带涡旋仍然明显较弱。这里显示了只有在多个涡旋中存在的正弦不稳定模式可以极大地提高涡旋的输送效率。这种效率的提高减少了平衡所需的平均流通量。在滑移边界条件下，涡间输运可能要大得多。然而，由于风强迫与海洋--两个不相等的涡旋--相关，随着雷诺数的增加，相反的风强迫区域之间的平均涡量通量而不是涡流通量变得越来越重要。通过对平衡涡度收支和涡旋输送效率的诊断，给出了不同结果的物理合理性。讨论了改变1)边界条件、2)涡度源和汇、3)涡旋输送效率和4)涡旋对称度的计算。
Multiple-gyre ocean models have a weaker mean subtropical circulation than single-gyre calculations with the same viscosity and subtropical forcing. Traditionally, this reduction in circulation is attributed to an intergyre eddy vorticity flux that cancels some of the wind input, part of which does not require a Lagrangian mass exchange (theory of dissipative meandering). Herein the intergyre eddy vorticity flux is shown to be a controlling factor in barotropic models at high Reynolds number only with exactly antisymmetric gyres and slip boundary conditions. Almost no intergyre flux occurs when no-slip boundary conditions are used, yet the subtropical gyre is still significantly weaker in multiple-gyre calculations. Sinuous modes of instability present only in multiple gyres are shown here to vastly increase the eddy vorticity transport efficiency. This increase in efficiency reduces the mean circulation necessary for equilibrium. With slip boundary conditions, the intergyre eddy transport is possibly much larger. However, with wind forcing relevant for the ocean—two unequal gyres—a mean flow flux of vorticity rather than an eddy flux between the regions of opposing wind forcing is increasingly important with increasing Reynolds number. A physical rationalization of the differing results is provided by diagnosis of the equilibrium vorticity budget and eddy transport efficiency. Calculations varying 1) boundary conditions, 2) sources and sinks of vorticity, 3) eddy transport efficiency, and 4) the degree of symmetry of the gyres are discussed.