Diagnosis of the Three-Dimensional Circulation Associated with Mesoscale Motion in the California Current

Abstract

A high-resolution upper-ocean survey of a cyclonic jet meander and an adjacent cyclonic eddy in the California Current region near 38°N, 126°W was conducted as part of the summer of 1993 Eastern Boundary Currents program. Temperature and salinity were measured from a SeaSoar vehicle, and velocity was measured by shipboard acoustic Doppler current profiler (ADCP). SeaSoar data show a density front at a depth of 70?100 m with strong cyclonic curvature. The geostrophic velocity fields, referenced to the ADCP data at 200 m, show a strong surface-intensified jet (maximum speed of 0.9 m s?1) that follows the density front along a cyclonic meander. Relative vorticities within the jet are large, ranging from ?0.8f to +1.2f, where f is the local Coriolis parameter. The SeaSoar density and ADCP velocity data are used to diagnose the vertical velocity via the Q-vector form of the quasigeostrophic omega equation. The diagnosed vertical velocity field shows a maximum speed of 40?45 m d?1. The lateral distribution of vertical velocity is characterized by two length scales: a large (?75 km) pattern where there is downwelling upstream and upwelling downstream of the cyclonic bend, and smaller patches arrayed along the jet core with diameters of 20?30 km. Geostrophic streamline analysis of vertical velocity indicates that water parcels make net vertical excursions of 20?30 m over 2?3 days, resulting in net vertical velocities of 7?15 m d?1. Water parcels moving along geostrophic streamlines experience maximum vertical velocities in the regions of maximum alongstream change in relative vorticity, an indication of potential vorticity conservation.