Numerical study of the Subtropical Front and the Subtropical Countercurrent

Using a multi-level numerical model, it is shown that the Subtropical Front and the Subtropical Countercurrent can be reproduced realistically in a highly idealized model, as a consequence of the coupling effect of wind driven gyre circulation and differential heating. In the model, the North Pacific Ocean is idealized as a rectangular flat-bottomed model ocean, and is driven by wind stress, which features the Westerlies and the Trades, and by heat flux through the sea surface formulated after Haney (1971).In the model ocean, a shallow front and an eastward current associated with the front are formed around the central latitude of the Subtropical Gyre, which show close similarities to the Subtropical Front and the Subtropical Countercurrent in the real ocean.Although the detailed mechanism of formation of the Subtropical Front and the Subtropical Countercurrent is not clarified in the present study, two factors are found inessential for the formation of the Subtropical Front and the Subtropical Countercurrent. First, the results of the model indicate that a small trough of wind stress curl in the lower latitudes of the Subtropical Gyre, which Yoshida and Kidokoro (1967a, b) attributed to the Subtropical Countercurrent, is not necessary for the formation of the Subtropical Front and the Subtropical Countercurrent, since they are reproduced well in the model without the trough. Second, using a model driven by meridional wind stress, it is shown that the meridional Ekman convergence, which many authors related to the Subtropical Front, is not essential for the formation of the Subtropical Front and the Subtropical Countercurrent.