SEASONAL VARIABILITY OF ALONGSHORE GEOSTROPHIC VELOCITY OFF CENTRAL CALIFORNIA

Seasonal variability of alongshore geostrophic velocity relative to 500 dbar is examined from 23 years of hydrographic data along two sections off central California. The seasonal cycles are determined by least square fits of the gappy data records to harmonics with annual and semiannual periods. Attention is focused on variability over the outer continental slope. The seasonal variations over the continental slope compare favorably with existing models of eastern boundary current systems in two respects, namely: the surface flow leads the predominantly annual wind forcing in this region by about one month; and the deep poleward counterflow is coherent with the local poleward barotropic pressure gradient at both locations, with a phase lag of approximately 2 months. However, contrary to the results of existing dynamical models, the semiannual poleward pressure gradient and undercurrent off Point Conception appear to be unrelated to the wind forcing, which is predominantly annual in this region. These semiannual variations are highly coupled to the semiannual flow throughout the water column inside the Southern California Bight, suggesting that they may be topographically generated.