Diurnal Thermally Driven Cross-Shore Exchange in Steady Alongshore Currents

Abstract

Idealized numerical modeling of thermally driven baroclinic exchange is performed to understand how cross-shore flow is modulated by steady alongshore currents and associated shear-generated turbulence. In general, we find that shear-driven vertical mixing reduces the temperature gradients responsible for establishing the baroclinic flow, such that cross-shore thermal exchange diminishes with alongshore current speed. Circulation in a base-case simulation of thermal exchange with no alongshore forcing contains a cooling response consisting of a midday flow in the form of a downslope current with a compensating onshore near-surface flow driving cross-shore exchange, followed by an afternoon warming response flow via an offshore-directed surface warm front, with a compensating return flow at the bottom. Nighttime convective cooling enhances vertical mixing and decelerates the warming response, and the diurnal cycle is renewed. In this base-case scenario, representative of tropical reef environments with optically clear water and weak alongshore flow, surface heating and cooling can drive cross-shore circulation with O(1) cm s−1 velocities. Alongshore flow forcing is implemented to induce upwelling- and downwelling-favorable cross-shore circulation. For mild alongshore forcing, the baroclinic cross-shore exchange flow is enhanced due to an increase in the horizontal temperature gradient. Stronger alongshore flow leads to diminished thermally driven exchange, ultimately reaching a regime where the cross-shore exchange is due predominantly to Ekman dynamics. Though exchange velocities are relatively small (O(1) cm s−1), these persistent exchange flows are capable of flushing the nearshore region multiple times per day, with important implications for water properties of nearshore ecosystems.