Theoretical and numerical study on shocks and subtropical front in a wind-driven oceanic gyre

本文利用一个几何结构简单的海洋环流模式(GCM)，研究了副热带逆流及其伴生锋的形成机制。在大洋气旋中，深层混合层出现在副热带气旋的北部，而在副热带气旋的中部则突然变浅。混合层锋面、混合层深度的狭窄过渡带、中西部副热带气旋向东北副热带气旋倾斜、混合层锋面与露头线交汇处产生低位涡度流体。由于地面密度在纬向上基本均匀，而混合层锋面向东北倾斜，因此密度较大的等压面上的最小位涡流体产生于东北部，而密度较轻的等压面上的最小位涡流体则产生在西部地区。各等压线上的低位涡度流体向西南俯冲平流，垂直堆积，在中西部副热带环流中形成一个厚厚的低位涡度池。利用通风温跃层模式，研究了混合层深度分布对环流结构的影响，证明了数值实验所提出的机制可以产生这种逆流。另一方面，向东南倾斜的混合锋面也会产生逆流。在后一种情况下，杜瓦(1992)讨论的类似机制是可行的。特征方程表明，在这个系统中，非线性陡化也可能是重要的。

In the present study, the generation mechanism of the subtropical countercurrent and associated front is investigated by using an ocean general circulation model (GCM) with a simple geometry. A theory for the countercurrent is also presented.In the ocean GCM,the deep mixed layr occurs in the northern subtropical gyre and is shallows abruptly in the central subtropical gyre. The mixed-layr front, the narrow transition zone of the mixed-layr depth, slants from western central subtropical gyre to northeastern subtropical gyre, and the low potential vorticity fluid is produced at the intersection between the mixed-layr front and the outcrop line. Since the surface density is almost zonally uniform while the mixed-layr front slants northeastward, the minimum potential vorticity fluid in a dense isopycnal is produced in the northeastern region, while that in lighter density isopycnal in the western region. Subducted and advected southwestward, the low potential vorticity fluid in each isopycnal is vertically piled up and makes a thick low potential vorticity pool in the western central subtropical gyre. It is found that the model subtropical countercurrent occurs along the southwestern edge of this pool.Using a ventilated thermocline model, we investigate the effects of the mixed-layr depth distribution on the gyre structure, and it is demonstrated that the mechanism suggested by the numerical experiment can generate the countercurrent. On the other hand, southeastward slanting mixed-layr front can also generate a countercurrent. In the later case, a similar mechanism discussed by Dewar (1992) works. The characteristic equations suggest that the nonlinear steepening may also be important in this system.