Circulation in the Panama Bight

A stationary circulation model for the Panama Bight is developed and used with hydrographic data from four cruises. The horizontal circulation is described by geostrophic and wind-driven currents. Two derived contributions to vertical motion are the horizontal geostrophic divergence and the vorticity equation involving the wind stress. The surface geostrophic circulation is cyclonic during each cruise period with the Colombia Current flowing northward at speeds up to 100 cm sec−1; at no time is the width of this current more than 180 km nor the depth in excess of 100 meters. Farther offshore a previously unreported current flows southward between the surface and 250 meters; its transport across a transect along 4°N and within 200 km of the coast accounts for at least 20% of the net meridional geostrophic transport (northward) of 0.6–8.7×106 m3 sec−1 within 400 km of the coast. Meridional Ekman transport across the same transect is about one-tenth the geostrophic transport. Upwelling due to geostrophic divergence is typically 13×10−4 cm sec−1 across the base of the surface layer (25 meters), whereas upwelling produced by the curl of the wind stress across the same surface is about 50×10−4 cm sec−1. Upwelling caused by the meridional Ekman transport is usually 10% or less of the wind stress curl. The resultant upwelling from geostrophic and wind stress divergence and from the wind stress curl is largest in two locations: in the center of the bight and along the eastern half of the gulf entrance, where both rates are about 45×10−4 cm sec−1.