The shear, convergence, and thermohaline structure of a front

With the objective of measuring convergence directly to confirm previous ob servations of frontal subduction, a seaward upwelling jet off central Calif ornia was studied using satellite infrared images, hydrographic sections, s hip 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 (equiva lent to 0.8f) was visible as a 20-m-wide accumulation of debris. The sharpn ess of the front lasted at least for a day. Away from the cyclonic front, t he divergence of the flow was small and the shear was less than 0.6f Thermo haline layers, originating at the front, were interleaving along isopycnals , suggesting water subduction, It is proposed that the asymetry between ant icyclonic and cyclonic sides of the jet, and the strong convergence at the cyclonic front, resulted from a frictionally driven ageostrophic secondary circulation superimposed on the geostrophic Row.