Upwelling front and two-cell circulation

A two-cell circulation associated with a front observed in coastal upwelling regions is studied numerically in a three-dimensional level model. An ocean with a flat bottom is forced by the wind stress with a longshore variation. Upwelling is induced in the region next to the coast. In association with the upwelling, the pycnocline slopes up toward the coast and intersects the sea surface forming a front. After that, downwelling is induced just inshore-side of the front and upwelling offshore-side. The transverse circulation in the present model seems to reproduce the observed two-cell circulation. It is found that the generation of the two-cell circulation is due to deviations of the longshore flow from the thermal-wind relation (geostrophy). The deviations are caused by the onshore-offshore movements of the front. Although no vorticity input through the wind stress is assumed, several barotropic vortices are induced by the effect of the inclination of the pycnocline and grow as long as the winds continue to blow. The observed poleward undercurrent may be interpreted as a combination of motions of the internal mode associated with the front and a barotropic flow associated with a cyclonic vortex.