Within coastal environments, multiple physical processes influence the concentration of nutrients, oxygen, salinity, and other important ecological variables. Many of these physical mechanisms, which include surface and internal waves, tides, winds, and daily heating and cooling, produce considerable nearshore environmental variability on diurnal time scales. In the case of tropical coral reefs, diurnal temperature variability has been previously demonstrated to enhance coral resistance to thermal stress, and therefore reduce the prevalence of bleaching. Here, we ascertain the importance of diurnal temperature variability over other thermal stress metrics for predicting coral bleaching, using a collection of over 100 in situ temperature time series from 5 global reef regions matched with 46 bleaching observations. Results indicate that high-frequency temperature variability is the most influential metric in explaining bleaching outcomes, offering a mitigating effect such that a 1 °C increase in daily temperature range would reduce the odds of more severe bleaching by a factor of 33. Implications for prioritizing reefs for conservation are noted.
As there are multiple physical processes driving diurnal temperature variability in the nearshore and within coral reefs, we are further concerned with the role of strong atmosphere-ocean heat fluxes over nonuniform bathymetry, generic features of tropical reef environments, in shaping the nearshore thermal environment. In such regions, surface heat fluxes lead to greater volumetric heating and cooling in shallower waters than adjacent deeper ones, establishing a horizontal temperature gradient that subsequently drives a vertically sheared exchange flow. The importance of this thermally-driven exchange to cross-shore volume and heat transport is apparent in multiple field observations, whereby modulations from alongshore currents are considerable. Using the Regional Ocean Modeling System (ROMS), we investigate the role of steady, upwelling- and downwelling-favorable alongshore currents of varying magnitudes in altering the structure of thermal exchange. Circulation in a base-case simulation with no alongshore forcing demonstrates a robust diurnal pattern consisting of a downslope flow from convective cooling and a buoyant warm front from surface heating, with cross-shore velocities of O(1) cm/s. Mild upwelling- and downwelling-favorable alongshore currents enhance the nearshore temperature gradient, thereby strengthening the thermal exchange. However, as the alongshore current is strengthened, the resulting near-bed shear-generated turbulence induces substantial vertical mixing, dampening temperature gradients and weakening thermally-driven exchange.
When the alongshore forcing is further increased such that the associated turbulent mixing homogenizes the water column, the thermally-driven exchange vanishes, yet there remains nontrivial baroclinic cross-shore exchange, which we investigate by invoking bottom Ekman theory. A theoretical model derived from central differences of the classical Ekman balances is used to compute the horizontal velocity profiles resulting from 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 of ROMS, which includes nonlinear advection terms in the governing equations. The resulting dynamics and consequences for baroclinic cross-shore exchange of these additional terms are discussed in the context of the alongshore momentum budget.
