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

本文采用简单几何的海洋环流模式（GCM）研究了副热带环流及其相关锋的产生机制。本文还提出了一种逆流理论。在海洋环流中，深层混合层出现在北副热带环流中，浅层出现在中副热带环流中。混合层锋是混合层深度的狭窄过渡带，从副热带中部环流西部向副热带东北环流倾斜，在混合层锋与露头线相交处产生低位涡度流体。由于混合层锋向东北倾斜时，地表密度几乎是纬向均匀的，因此东北地区产生密度较大的等压线中最小位涡量流体，而西部地区产生密度较小的等压线中最小位涡量流体。各等压线内的低位涡流体在西南俯冲平流作用下垂直堆积，在副热带中部环流西部形成一个较厚的低位涡池。研究发现，典型副热带逆流发生在该池的西南边缘。利用一个通风的温跃层模型，我们研究了混合层深度分布对环流结构的影响，并证明了数值实验提出的机制可以产生逆流。另一方面，东南倾斜的混合层锋也会产生逆流。在后一种情况下，Dewar（1992）讨论的类似机制起作用。特征方程表明，非线性陡化在该系统中也起着重要作用。

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.