Regional variation of water temperature around Okinawa coasts and its relationship to offshore thermal environments and coral bleaching

To clarify the relationship between the regional variability of water temperatures and the spatially non-uniform appearances of coral mass bleaching and consequent mortality, we have investigated the near-shore temperatures continuously monitored around the Ryukyu Islands in 1999 and NOAA-SST image data in 1998 and 1999. They indicate appreciable regional differences in temperature (e.g., at the Kerama Islands located southwest of Okinawa Island the temperature in summer was generally lower than that at the Okinawa Island coasts). Comparison of the solar radiation distribution and NOAA-SST images with bottom bathymetry and a numerical experiment suggest that the presence of the shelf around the Kerama Islands caused the relatively lower temperature (i.e., the warm water mass from the Kuroshio currents may be propagated toward Okinawa Island but may be blocked at the shelf edge, resulting in relatively lower temperature at the Kerama Islands). Further comparison with the reported changes in the coral coverage in the Ryukyu Islands after the mass coral bleaching event in summer 1998 suggests that one of the primary causes of the regional variability of the coral bleaching and consequent mortality is the regional variation of water temperature.