A heat balance of the Great Barrier Reeef with particular emphasis on recent sea surface temperature trends

This study estimates a surface heat budget of the Great Barrier Reef (GBR) and Coral Sea off the Queensland coast in Australia (10° S to 26° S, 142° E to 155° E). The ocean-atmosphere heat fluxes and ocean advective heat transport are examined for the present and future so as to determine the relative roles of the atmosphere and ocean in creating anomalies in oceanic heat content and sea surface temperature. Schemes are developed to estimate a reliable high-resolution climatology of the total ocean-atmosphere heat exchange, comprising radiation and turbulent heat flux components. These were based on satellite data for the study area over the period 1995 to 2005. The climatology was based on satellite data obtained from the SSM/I, AVHRR and VISSR (GMS and GOES) spacecrafts and applied to selected algorithms and techniques detailed throughout the thesis. These satellite-based ocean-atmosphere heat fluxes were validated with measurements collected over the study area, therefore providing an accurate and reliable heat flux dataset with which to examine recent sea surface temperature changes in the region and its impact on biological processes such as mass coral bleaching. Estimates of monthly average shortwave radiation fluxes had the highest radiation component errors, with root-mean-square differences/errors (RMSE) of approximately 14.8 W m-2. Errors in monthly downward and upward longwave radiation fluxes were smaller, with RMSE differences of 8.5 and 2.2 W m-2, respectively. Monthly averaged latent and sensible heat flux estimates show RMSE of approximately 25.2 W m-2 and 3.4 W m-2, respectively. These improved estimates allow a higher confidence in studies which examine recent sea surface temperature trends and observed mass coral bleaching for the region. It is proposed that the greatest uptake of heat occurs over the austral spring/summer period in the central and southern regions of the GBR. This agrees well with areas where anomalously high sea surface temperatures are observed and where the most significant coral bleaching occurred, and not in the most northern more tropical region as one may expect. The surface heat budget climatology was used to examine the mass bleaching episode that occurred in 2002. Results show that areas of maximum and minimum bleaching are better discriminated by the anomaly from mean seasonal values in the net surface heat flux, with accuracy of 86 and 79 percent, respectively, as apposed to using the absolute net surface heat flux, or absolute or anomalous sea surface temperature. It was postulated that the relationship results from a combination of direct stress due to higher solar radiation, plus indirect stress due to local heat uptake by ocean waters. This study also conducts detailed examinations of the relationships between the net surface heat flux, oceanic heat fluxes, and the changes in sea surface temperature to gain an understanding into the dominant processes of the surface heat budget in influencing the variability of water temperatures in the GBR and Coral Sea region. It is shown that the highest variability of sea surface temperature of the GBR and Coral Sea region is located in the reef system and related to the shallowest mixed layer. In such a region of shallow mixed layers and strong net surface heat fluxes, sea surface temperature is consequently very sensitive to small changes in ocean-atmosphere heat fluxes, ultimately resulting in an environment with high sensitivity to surface temperature. The highest correlation between the net surface heat flux and changes and sea surface temperature is shown to exist in the southern area of the GBR (below about 18° S), suggesting that accurate estimates of net surface heat fluxes and mixed layer depths could be used as a good prediction tool for the estimation of sea surface temperature, which is vital in understanding the processes that initiate localised coral bleaching. Overall, the