The role of thermal stratification in tidal exchange at the mouth of San Diego Bay

We have examined, from an observational viewpoint, the role of thermal stratification in the tidal exchange process at the mouth of San Diego Bay. In this region, we found that both horizontal and vertical exchange processes appear to be active. The vertical exchange in this case was apparently due to the temperature difference between the'bay water and ocean water. We found that the structure of the outflow and the nature of the tidal exchange process both appear to be influenced by thermal stratification. The tidal outflow was found to lift-off tan the bottom during the initial and later stages of the ebb flow when barotropic forcing was weak. During the peak ebb flow, the mouth section was flooded, and the outflow extended to the bottom. As the ebb flow weakened, a period of two-way exchange occurred, with the surface layer flowing seaward, and the deep layer flowing into the bay. The structure of the tidal-residual flow and the residual transport of a measured tracer were strongly influenced by this vertical exchange. Exchange appeared to occur laterally as well, in a manner consistent with the tidal-pumping mechanism described by Stommel and Farmer [1952]. Tidal cycle variations in shear and stratification were characterized by strong vertical shear and breakdown of stratification during the ebb, and weak vertical shear and build-up of stratification on the flood. Evaluation of multiple tidal-cycles from time-series records of flow and temperature indicated that the vertical variations of the flow and stratification observed during the cross-sectional measurements are a general phenomenon during the summer. Together, these observations suggest that thermal stratification can play an important role in regulating the tidal exchange of low-inflow estuaries.