Dynamics of barotropic low-frequency fluctuations in San Francisco Bay during upwelling

Abstract

Observations of wind, surface elevations, and currents in San Francisco Bay during the 1999 upwelling season are analyzed to understand the dynamics of low-frequency currents in upwelling-dominated estuaries. Principal component analysis is carried out to distinguish the different uncorrelated components of the low-frequency fluctuations in the observations. Analyses of ADCP observations at two locations in the Bay show that barotropic currents flow in the direction of winds in the shallow parts of the cross section and flow against the wind in the deeper parts of the cross-section. We ran the SUNTANS model with three forcing functions: (i) winds, (ii) low-frequency surface elevations, and (iii) winds and low-frequency surface elevations, to determine the forcing functions that best reproduce the observed low-frequency fluctuations. Analyses of observations and model simulations show that wind-driven flow in the shallow areas and upwind in the deeper areas, consistent with linear theory. Model simulations also show that the low-frequency currents in the Bay generated due to local winds capture the mean low-frequency barotropic fluctuations seen in the observations during the upwelling season. Model simulations showed that the current generated due to the coastal sea level forcing at the mouth of the Bay is small because the coastal sea levels inside the Bay are in phase with that at the mouth and thus generate weak or negligible pressure gradients. We conclude that forcing of low-frequency sea level fluctuations along the offshore boundaries in the model simulations does not lead to improvement in the prediction of low-frequency currents in San Francisco Bay.