Dynamics of Thermally-Driven Exchange Modified by Coriolis, Winds, and Currents.

The hydrodynamics of nearshore cross-shelf circulation influence the transport of heat, salt, nutrients, and planktonic organisms, and are therefore important for coastal ecosystems. Specifically, thermally-driven baroclinic exchange flows have been observed to drive leading-order circulation and heat transport over reefs. Yet the interaction of thermally-driven exchange with other forcing mechanisms, such as alongshore currents or winds, remains unclear. This study implements idealized numerical modeling with the Regional Ocean Modeling System (ROMS) to examine the structure of cross-shore thermal exchange in the context of alongshore currents and wind forcing. The dynamics of the cross-shore exchange are examined for a variety of forcing regimes, comparing net exchange, and cross-shore momentum balances. We investigate how variations in surface wind stress, bottom turbulence, and Coriolis force account for modifications in the exchange from the baseline surface-heat-flux-only setup. For a linearly sloped bathymetry under diurnally periodic surface heating and cooling, nearshore exchange is largely driven by a lagged gravity current that propagates downslope during midday heating, while a buoyant offshore-directed surface flow contributes to a local peak in exchange in the afternoon/evening. We find that steady upwelling-favorable alongshore currents increase the exchange associated with the gravity current, but result in weaker exchange in the afternoon. Conversely, steady upwelling-favorable alongshore winds result in diminished exchange from the gravity current and enhanced exchange associated with the warming phase flow. Results are ultimately used to characterize the influence of the various forcing mechanisms on nearshore circulation and heat transport, therefore highlighting the pertinence of thermally-driven exchange to ecological concerns.