Seasonal variability of alongshore geostrophic velocity off central California

Abstract

Seasonal variability of alongshore geostrophic velocity relative to 500 dbar is examined from 23 years of hydrographic data along two sections off central California (one off Point Sur and the other off Point Conception). 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. Geostrophic flow in the upper 100 m along both sections is coherent and predominantly annual with equatorward flow from February to September and poleward flow from October to January. The flow deeper than 100 m is distinctly different along the two sections. The nearshore deep flow is predominantly semiannual off Point Conception (notably different from surface flow) and poleward all year with maxima in December and June. This semiannual variability at depth is a prevalent feature of the California Current system. The nearshore deep flow off Point Sur is unusual in that it is predominantly annual with maximum poleward flow in December and weak equatorward flow from March to May. The seasonal variations over the continental slope compare favorably with existing models of eastern boundary current systems in two respects: (1) The surface flow leads the predominantly annual wind forcing in this region by about one month; and (2) 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.