Why does the Ballenas Channel have the coldest SST in th Gulf of California?

Abstract

Moored and hydrographic observations at the three most important sills in the northern Gulf of California (NGC) are used to describe the circulation. At the deepest San Esteban (SE) sill (600 m), the mean along-gulf flow is weak and outward (toward the mouth of the gulf) in the entire water column, whereas the mean deep flow is inward and bottom-intensified at the San Lorenzo (SL) sill (400 m), which controls the southern entrance to the Ballenas Channel (BC). However, large tidal currents (>1 m/s during spring tides) in the SE sill drive a net inward bottom transport of 0.09 Sv (1 Sv = 1 × 106 m3/S), due to a tidal pumping process. At the SL sill the net transport is also about 0.09 Sv, but here the mean flow contributes more than the tides and both are into the gulf. At the northern BC sill, which controls the northward entrance to the BC, the mean near-bottom flow is southward, implying that the bottom water of this deep basin is renewed at both of its ends. Moreover, the mean surface flow at both ends of the BC is out of the channel and, hence, the convergence at the bottom is compensated by a divergence at the surface, generating a persistent upwelling (∼5 m/day) within the channel. This circulation pattern primarily explains why the BC has the coldest SST and is one of the most biologically productive basins in the Gulf of California. Copyright 2006 by the American Geophysical Union.