Correlation scales, objective mapping, and absolute geostrophic flow in the California Current

Abstract

The spatial covariances of the time‐dependent density and geostrophic velocity fields off southern California are determined from a unique set of repeated hydrographic observations (44 cruises) made by the California Cooperative Oceanic Fisheries Investigations from 1984 to 1994. The covariances and objective analysis are used to combine direct velocity observations, from shipboard acoustic Doppler current profiler (ADCP) measurements made on a recent survey (October 1993), with hydrographic observations. The analysis reduces ageostrophic noise in the ADCP velocities by smoothing and enforcing horizontal nondivergence; additionally, the velocities are mapped over scales that are dynamically consistent with the hydrography. Maximum surface geostrophic flow in the California Current in October 1993 is about 35 cm s−1, 50% larger than that estimated assuming a 500‐m level of no motion. Absolute flow at 500 m is O(10 cm s−1) and indicates that the surface eddy field penetrates through the thermocline. Uncertainty in the geostrophic reference calculated from the ADCP measurements is of O(4 cm s−1). The velocity residual (objectively analyzed minus raw ADCP estimates) exhibits smaller correlation scales than the geostrophic flow.