Ventilated Thermocline Strongly Affected by a Deep Mixed Layer: A Theory for Subtropical Countercurrent

Abstract

The mixed layer thickness generally increases in the northward direction, but its change seems to occur rather sharply in a narrow transition zone, referred to as a mixed layer front here. The author investigates the effects of the mixed layer front on the ventilated thermocline structure, especially focusing on the generation mechanism of the subtropical countercurrent. If the mixed layer front is not parallel to the surface density contour, the fluid with minimum isopycnal potential vorticity is formed around the intersection of the mixed layer front and outcrop line. If the mixed layer front slants northeastward and surface density is zonally uniform, as seen in a numerical experiment carried out by Kubokawa and Inui in which the subtropical countercurrent was reproduced, the minimum potential vorticity on a high-density isopycnal occurs in the east, while that in a low-density isopycnal occurs in the west. In the present study, analytic solutions for a simple three-layer model are presented first to demonstrate that such an inhomogeneous distribution of isopycnal potential vorticity can generate a subtropical countercurrent. Then, a multilayer ventilated thermocline model coupled with the mixed layer is solved numerically. For the northeastward slanting mixed layer front, as advected southward, the low potential vorticity fluids in different layers converge in the horizontal plane, forming a thick ventilated layer in the central western region of the subtropical gyre. This thick ventilated layer lifts the base of the surface layer and generates a subtropical countercurrent along the southeastern edge of this region. On the other hand, a southeastward slanting mixed layer front can also generate a countercurrent.