The Shear, Convergence, and Thermohaline Structure of a Front*

Abstract

With the objective of measuring convergence directly to confirm previous observations of frontal subduction, a seaward upwelling jet off central California was studied using satellite infrared images, hydrographic sections, ship drift, and clusters of surface drifters. The cyclonic front of the jet was sharper than 1 km, resulting in a shear several times larger than f. A cross-frontal convergence of 7 cm s−1 over the width of the front (equivalent to 0.8f) was visible as a 20-m-wide accumulation of debris. The sharpness of the front lasted at least for a day. Away from the cyclonic front, the divergence of the flow was small and the shear was less than 0.6f. Thermohaline layers, originating at the front, were interleaving along isopycnals, suggesting water subduction. It is proposed that the asymetry between anticyclonic and cyclonic sides of the jet, and the strong convergence at the cyclonicfront, resulted from a frictionally driven ageostrophic secondary circulation superimposed on the geostrophic flow.