Dynamics of cross-shore thermal exchange on a tropical fore-reef

Abstract

The dynamics of cross-shelf circulation influence the exchange of water masses and the distribution of heat, salt, nutrients, contaminants, sediment, and planktonic organisms like larvae or phytoplankton in the nearshore coastal ocean. In addition, in the case of coral reefs, the horizontal redistribution of heat by cross-shelf circulation can moderate the daily variations in temperature experienced by polyps, thus potentially reducing thermal stress. Observations of the velocity structure at the Kilo Nalu Observatory on the south shore of Oahu, Hawaii show that thermally driven baroclinic exchange is the dominant mechanism for cross-shore transport for this tropical forereef environment. Estimates of the exchange and net fluxes show that the average residence time for the zone shoreward of the 12m isobath is generally less than one day. The thermal forcing is identified from the relationship between the magnitude of the exchange and the surface heat flux. Temporal variability of the exchange is examined and compared with meteorological data and water properties. Dynamic flow regimes are characterized following the framework presented by Monismith et al. 2006, and the observations at Kilo Nalu are shown to be in the unsteady temperature regime. Spatial observations are presented which support our observations of the thermally driven exchange, but also illustrate the complexity associated with alongshore variability.