Cross-Shore Exchange Associated with Baroclinic Thermal and Ekman Dynamics

Speaker: Aryan Safaie
Institution: University of Rhode Island, Graduate School of Oceanography
Location: Geology 3640
Date: February 4, 2025
Time: 12:00 pm to 1:00 pm


Abstract:

Within coastal environments, multiple physical processes influence the concentration of nutrients, salinity, and temperature, and are capable of producing considerable nearshore environmental variability on diurnal time scales. To explore a particular source of nearshore transport generic to tropical environments, idealized numerical modeling using the Regional Ocean Modeling System (ROMS) is performed to assess the role of steady, upwelling- and downwelling-favorable
alongshore currents in altering the structure of baroclinic thermally-driven cross-shore exchange. Circulation in a base-case simulation with no alongshore forcing demonstrates a robust diurnal pattern consisting of downslope flow from convective cooling and a buoyant warm front produced by surface heating, with cross-shore velocities of O(1) cm/s. Mild alongshore currents enhance the nearshore temperature gradient, thereby strengthening the thermal exchange. However, as the alongshore forcing is increased, the resulting near-bed shear-generated turbulence induces substantial vertical mixing, dampening temperature gradients and weakening thermally-driven exchange. For sufficiently strong alongshore currents, the associated turbulent mixing homogenizes the water column, and the thermally-driven exchange vanishes, yet there remains nontrivial cross-shore exchange, which we investigate by invoking bottom Ekman theory. A solution derived from central differences of the classical Ekman balances is used to compute the horizontal velocity profiles forced by an alongshore bottom stress, and we find that the ability of the theoretical solution to reproduce the expected flow features largely depends on the form of the eddy viscosity. Velocities of the theoretical model are then compared with those simulated by ROMS, highlighting the role of nonlinear advection terms in the governing equations.