Alongshore sub-thermocline variability in the central California Current system

Abstract

Moored current meter observations were made along the central California continental slope from P. Piedras Blanca to the Farallon Islands. The study area covers an alongshore distance of approximately 290 km and examined the data from off Pt. Piedreas Blancas (P4), Pt. Sur (P2 and P3), Monterey Bay (MB1 and MB@) and the Farallon Islands (D and E). Time and frequency domain analyses were performed on three time segments that included data from combinations of the above mooring locations based on common time periods and depths (350 m - 500 m). Segment 1 revealed a mean poleward flow attributed to the California Undercurrent at all moorings. Segment 2 had a similar mean poleward flow but also exhibited an equatorward reversal at periods between 19.5 and 58.5 days at P2 and MB2 that was less apparent at P3 further offshore and appeared to be coastally trapped. Observed wavelengths were compared to the simple wave theory indicating that the observed signal may have resulted from a combination of two theories i.e. a coastally trapped wave. The complex bottom topography between Pt. Sur and the Monterey Bay prohibited more rigorous comparison between theory and the observations. Satellite sea surface temperature imagery during Segment 3 showed an anti-cyclonic meander with a 65 km radius outside of the Monterey Bay. Current and temperature records at P2 and MB2 indicated that the surface feature's position varied and influenced the currents at depths down to 500 m. Time domain empirical orthogonal functions were calculated for alongshore and across-shore components separately. The first two empirical modes accounted for between 81% and 86% of the alongshore variance in all segments and were attributed to the mean California Undercurrent influence (mode1) and to the deviations from the mean state (mode 2). The first two across-shore modes explained between 66% and 81% of the variance and consistently demonstrated opposing current structures between the Pt. Sur and Monterey Bay moorings, likely due to the Monterey Bay meander influence.