Copyright The Oceanographic Society of Japan. A Two-Level Model of Subtropical Gyre and Subtropical

One of the most striking features of the North Pacific upper circulation is the Subtropical Front, which lies across the central subtropical gyre and is accompanied by an eastward current called the Subtropical Countercurrent. Motivated by this structure, this article develops a two-level model for a subtropical gyre under differential heating, and inves-tigates the possibility of generating the gyre-scale front. The purely wind-driven circu-lation solution is derived, in which the fluid at the lower level is at rest, and the anomaly from this solution represents a stationary Rossby wave. This stationary Rossby wave has a tendency to steepen at the eastern side of the high density region. It is shown that, when the north-south gradient of surface density is smaller than a certain critical value, the stratification becomes unstable near the northwestern boundary and the stationary Rossby wave is inevitably generated, although the Rossby wave generated in this manner is too weak to generate any strong current. If the vertical shear at the northwestern corner of Sverdrup region is assumed to be weak, however, the Rossby wave signal can be strong enough to produce a frontlike structure similar to the Subtropical Front. The density and current distributions obtained under such conditions resemble well the upper layer ones in the OGCM experiment carried out by Takeuchi (1984a). The effect of the vertical diffusion on the gyre structure is also briefly discussed. Keywords: ⋅Wind-driven ocean circulation, ⋅ subtropical countercurrent, ⋅ subtropical gyre, ⋅ subtropical front, ⋅ two-level model. 1