Global warming, regional trends and inshore environmental conditions influence coral bleaching in Hawaii

Hawaiian waters show a trend of increasing temperature over the past several decades that are consistent with observations in other coral reef areas of the world. The first documented large-scale coral bleaching occurred in the Hawaii region during late summer of 1996, with a second in 2002. The bleaching events in Hawaii were triggered by a prolonged regional positive oceanic sea surface temperature (SST) anomaly greater than 1degreesC that developed offshore during the time of annual summer temperature maximum. High solar energy input and low winds further elevated inshore water temperature by 1-2degreesC in reef areas with restricted water circulation (bays, reef flats and lagoons) and in areas where mesoscale eddies often retain water masses close to shore for prolonged periods of time. Data and observations taken during these events illustrate problems in predicting the phenomena of large-scale bleaching. Forecasts and hind-casts of these events are based largely on offshore oceanic SST records, which are only a first approximation of inshore reef conditions. The observed oceanic warming trend is the ultimate cause of the increase in the frequency and severity of bleaching events. However, coral reefs occur in shallow inshore areas where conditions are influenced by winds, orographic cloud cover, complex bathymetry, waves and inshore currents. These factors alter local temperature, irradiance, water motion and other physical and biological variables known to influence bleaching.