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

本文利用一个简单的海洋环流模式（GCM），对副热带逆流及其锋的形成机制进行了研究。在海洋环流模式中，深混合层出现在副热带环流的北方，而在副热带环流的中部突然变浅。混合层锋是混合层深度的狭窄过渡带，从中副热带涡旋西部向东北部倾斜，在混合层锋与露头线的交汇处产生低位涡流体。由于混合层锋向东北倾斜，而面密度几乎是纬向均匀的，因此，高密度等密度区的最小位涡流产生于东北部，而低密度区的最小位涡流产生于西部。各等密度线上的低位涡流体在俯冲和平流作用下，向西南方向垂直堆积，在副热带环流的中西部形成一个较厚的低位涡池。发现模式副热带逆流沿着该池西南缘出现，利用通气温跃层模式，研究了混合层深度分布对环流结构的影响，并证实了数值试验提出的逆流产生机制。另一方面，向东南倾斜的混合层锋也能产生对流。在后一种情况下，Dewar（1992）讨论的类似机制也起作用。特征方程表明，非线性陡峭化在这个系统中也可能是重要的。

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.