Mean structure and seasonal variability of the poleward undercurrent off southern California

The magnitude, location, and extent of the California Undercurrent (CUC) off southern California is investigated using cruises of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) from 1993 to 2003 which provide hydrographic, biochemical, and acoustic Doppler current profiler (ADCP) velocity data on a quarterly basis. This study is the first use of the decade‐long ADCP time series; it improves on prior geostrophic calculations by providing an absolute velocity reference for estimates of currents and transport. The long‐term mean reveals two undercurrent cores in the region south of Point Conception and north of Baja, California: one in the region of the continental slope within the Southern California Bight (SCB) and a second off the Santa Rosa Ridge (SRR). A single core is observed off Point Conception. Spiciness is found to be a good indicator of the presence of the CUC; however, direct velocity observations or a deep reference level are required to resolve the full strength of the CUC cores. In particular, the core off the SRR would be almost entirely missed by geostrophic calculations relative to 500 m, the maximum depth sampled by CalCOFI. The undercurrent transport off Point Conception is estimated to be about 1.7 ± 0.1 Sv, slightly less than the sum of that estimated for the SCB (1.0 ± 0.1 Sv) and that estimated off the SRR (1.1 ± 0.1 Sv) and consistent with some of the flow turning offshore at Point Conception. The CUC in the SCB is strongest in summer, while that off the SRR is strongest in fall. The CUC off Point Conception is strong in both summer and fall, reflecting the confluence of these two branches. Interannual variability is also present, and the velocity and spiciness of the CUC appear to peak during El Niño periods.