Tidal exchange at the bay‐ocean boundary

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Abstract
For bays and inlets with narrow entrances, tidal exchange with the ocean is often characterized by an ebb‐flood asymmetry in which the ebb flow exits the bay as a jet, while the flood flow is drawn in approximately as a radial sink, a process often termed “tidal pumping.” In this paper, the importance of tidal pumping in controlling the exchange of water at the bay‐ocean boundary is examined based on observations at the mouth of San Diego Bay. The observations are novel in combining the use of shipboard acoustic Doppler current measurements with a unique bay‐water tracer technique to provide visualization of the exchange as well as exchange budgets. Visualization of the tidal evolution of flow and tracers suggests that, in accordance with the classical tidal pumping model, the horizontal dynamics at the mouth of San Diego Bay are governed by an asymmetry between the jet‐like nature of the ebb flow and the sink‐like nature of the flood flow. This asymmetry results in strong tidal velocity and phase gradients similar to those observed in previous numerical modeling studies. A one‐dimensional trajectory model based on the observations indicates that the combined influence of the residual circulation and offshore gradients in the tidal velocity and phase provides the mechanism by which water from the bay is exchanged with water from the ocean. Both bottom friction and thermal gradients affect the dynamics of the vertical exchange. The resulting asymmetry in the offshore flow leads to a vertical mode of “pumping” similar to that observed in the horizontal view. This horizontal and vertical pumping of the offshore flow is shown to produce a strong correlation between the sectionally averaged velocity and bay‐water concentration, resulting in a significant net tidal exchange between the bay and ocean.

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