California Sea Grant College Program

Residual flow and exchange along a channel connecting an embayment with the coastal ocean are examined experimentally, using direct observations of currents and scalar concentrations across the mouth of San Francisco Bay. The study encompasses separate experiments during each of three "seasons": winter/spring runoff (March 2002), summer upwelling (July 2003), and fall relaxation (October 2002). Within each experiment, transects across the channel were repeated approximately every 12 minutes for 25 hours during both sprilng and neap tides. Velocity was measured from a boat-mounted acoustic Doppler current profiler. Scalar concentrations were measured at the surface and from a tow-yoed SeaSciences Inc. Acrobat.

Several sources of residual circulation were isolated: baroclinic flow, tidal pumping, and frictional phasing. We further isolated a portion of tidal pumping as tidal trapping of a headland eddy during flood tide. Density-driven flow is complicated by a dramatic growth in the cross-channel density gradient during the second half of ebb tide, whichdrives along-chmmel shear at the beginning flood tide, creating an asy1mnetry in frictional phasing.

Velocity fields formed by residual circulation mechanisms combine with scalar concentration fiellds to defme scalar exchange processes. Net salinity exchange rates for each season are quantified with hmmonic analysis. Harmonic results m·e  then decomposed into flux mechanisms using temporal and spatial correlations. In tllis study, the temporal conelation of cross-sectionally averaged salinity and velocity (tidal pumping flux) is the largest component of the dispersive tlux of salinity into the bay. From the tidal pumping flux pmtion of the dispersive flux, it is shown that there is less exchange than was found in earlier studies. Furthernore, tidal pumping flux scales strongly with freshwater flow because of density-driven movement of a tidally trapped eddy and stratification-induced increases in ebb-flood frictional phasing. Complex bathymetry leads to salinity exchange that scales differently with flow than would be expected from simple tidal asymmetry and gravitational circulation models.