Wind-Modulated Buoyancy Current Pulses Associated with Outflow from San Francisco Bay

John L Largier, University of California Davis, Coastal & Marine Sciences Institute, Davis, CA, United States
Abstract:
If the flux is large enough, freshwater runoff to the ocean forms a plume and buoyancy current that spreads outward and along-coast under its own buoyant forcing. However, smaller outflows form plumes that are advected by flows and forcing in the receiving environment (i.e., alongshore flows, winds, and waves). The low-salinity outflow from San Francisco Bay is tidally pulsed and shows marked seasonal and synoptic variation. In dry summer months with strong winds, the outflow plume is subject to shelf currents and winds and advected southward. In contrast, during high-flow events in winter, the outflow is driven by buoyancy with little influence from shelf currents and winds, spreading northward. However, in both seasons buoyant water may be trapped and thus accumulate in the Gulf of Farallones in a way that results in northward buoyant flow pulses with an origin at Point Reyes (the northern headland defining the Gulf), rather than at the mouth of the Bay. Further, wind-driven southward flow that separates at the headland can arrest a buoyant plume at Point Reyes. Northward pulses of buoyant water can thus occur following a weakening of wind forcing or a strengthening of buoyancy forcing. Further, control of flows past Point Reyes are influenced by the sea-level gradient across the headland, which varies in response to both wind forcing and Bay outflow. North of Point Reyes, poleward buoyancy currents are important for harmful algal blooms, larval dispersal, and pollution events. High-frequency data from nearshore moorings, is combined with HF-radar data on surface currents, satellite data, and ancillary data to resolve flow patterns and forcing dynamics. Further, long-term salinity monitoring at Bodega Marine Laboratory reveals interannual variability that appears to be a key factor for the regional ecology.