Double-celled circulation in coastal upwelling

Simple numerical experiments on two-dimensional coastal upwelling are made with emphasis on the role of non-geostrophic solenoidal field of density in the formation of double-celled circulation and multi-celled density front. Geometry of shelf and slope is not taken into account. Existence of poleward undercurrent presumably caused by the longshore variation of the large scale pressure field is also suppressed for the sake of simplicity.The results are, (1) double-celled circulation revealed in the present experiment is closely related with the internal frictional layer, where the horizontal density gradient balances with the vertical gradient of the longshore velocity and the vertical diffusion of the vorticity. (2) density front formed by the emergence of the pycnocline to the sea surface is successively advected offshoreward by the Ekman transport. (3) the pycnocline intersecting the sea surface forms the density front which is nearly vertical on account of the small scale convection. The surface currents converge at the front and construct an anti-clockwise circulation (viewed from the lee side). (4) small coefficient of eddy viscosity and strong wind stress lead the Ekman transport unstable and form a multi-celled structure in the frontal region.