In contrast to well-studied continental shelves, relatively little is known about cross-shelf circulation on island coastlines typically characterized by steep bathymetry and narrow shelves. Cross-shelf circulation results in the exchange of water masses and controls the distribution of heat, salt, nutrients, contaminants, sediment, and planktonic organisms like larvae or phytoplankton in the coastal ocean. In addition, in the case of coral reefs, the horizontal redistribution of heat by cross-shelf circulation moderates the daily variations in temperature experienced by coral polyps, thus potentially reducing thermal stress. This study will use existing data sets from two locations and high resolution numerical modeling to study the thermally driven exchange flows and how they vary with cross-shore distance and are modulated by swell energy, shelf slope, nearshore morphology, wind regimes and seasonal changes
Observations at two reefs at Eilat, Israel and Oahu, Hawaii have highlighted the role of thermally forced baroclinic exchange in cross-shore transport. At each site, daytime conditions were characterized by offshore flow at the surface in response to increased temperatures in shallower water nearshore. Nighttime cooling resulted in offshore flow near the bottom. Significant differences in flow response at the two sites indicate distinct dynamic regimes, however. Time series data from Oahu further indicates that the exchange provides a first order contribution to the overall cross-shore exchange. This study will examine the dynamical structure of thermally driven flows and establish the extent of their influence and generality for tropical coasts by leveraging existing data sets in a comprehensive historical analysis coupled with high-resolution numerical modeling spanning a wide parameter space ranging from a relatively simple wedge case with generally two-dimensional bathymetry (similar to Eilat) to a complex (more typical) forereef-lagoon environment with significant wave forcing (Kilo Nalu Observatory, Oahu). It will provide fundamental understanding of cross-shore transport processes in reef environments and should reveal the extent to which thermally driven baroclinic exchange contributes to cross-shore circulation for tropical coastlines in general. Improved understanding should lead to the construction of models capable of making accurate predictions of these flows. The results of the proposed research will have particular applicability to ecologically rich and economically important coral reefs and other steep, tropical, coastal environments, such as understanding how reef ecosystems will respond to environmental changes like ocean acidification or rising ocean temperatures, as well as to efforts to protect them like the establishment of marine reserves.
