Cross-shore thermally-driven exchange on two coral reef shorelines

The dynamics of cross-shelf circulation influence the distribution of heat, salt, nutrients, contaminants, sediment, and planktonic organisms in the nearshore coastal ocean. For tropical reef coastlines, the horizontal redistribution of heat by cross-shelf circulation moderates the daily variations in temperature experienced by coral polyps, potentially reducing thermal stress. A key mechanism for this exchange is thermally driven baroclinic circulation. Here, observations from two reefs, at Eilat, Israel and Oahu, Hawaii, are presented that highlight the role of thermally forced exchange in cross-shore transport for distinct dynamic regimes. At each site, daytime conditions are characterized by offshore flow at the surface in response to increased temperatures in shallower water near shore. Nighttime cooling results in offshore flow near the bed. While an advection-dominated thermal balance provides a good description of the flows observed off Eilat by Monismith et al. (2006), phase differences between the heat flux, the thermal response and the cross-shore flows indicate that the Oahu flow is in an unsteady regime. Estimates of momentum and thermal balance terms from field data further confirm differences in flow regimes.