Abstract:  Using data from current meter sites and tide gauges and the SLIM finite element unstructured numerical model, I modeled the water circulation in the Great Barrier Reef with a horizontal resolution that can capture variability of currents at a spatial scale relevant to coral reefs in the Great Barrier Reef (GBR). The model uses grid cells in the range of 150 m near reefs to nearly 20 km in open waters far from reefs. The age of waters and the flushing time in the GBR were investigated under different conditions for the wind and inflow from the Coral Sea by the North Caledonia Jet (NCJ). On the GBR shelf, the wind increases the flushing time because the reversing wind-driven currents transport some water back to the source; this process is comparable to that in an estuary where water that leaves the estuary at ebb tide may return at flood tide, a process parameterized by the return coefficient. The age of NCJ waters leaving the GBR may reach 0.6 to 1 year. In the south and central regions of the GBR, the southeasterly wind was observed to deflect seaward away from the inner shelf and towards the outer shelf the southward flowing NCJ inflow, making room for a wind-driven current of opposite direction on the inner shelf. When the wind relaxes, coastal waters is advected seaward offshore between Cape Upstart and the Whitsundays and this may be the process that brings coal dust to the GBR. Areas of high reef density (i.e. closely aggregated reefs) are poorly flushed because the prevailing currents are steered around and away from these regions, which is an oceanographic process called the ‘sticky water’ effect. The sticky water effect leads to decreased flushing and a high exposure time in high reef density areas in the southern and central regions of the GBR matrix. In turn this generated hotspots of high self-seeding, and these hotspots existed under the weather conditions typical of the coral spawning season. Away from these areas, self-seeding was less likely to occur and larval replenishment would result mainly from connectivity between reefs located kilometers to tens of kilometers apart.  A simple analytical formula is presented that explains about ~70% of the variation in larval retention in both calm weather and windy conditions. Complex reef mosaics and the related sticky water effect may have significant implications on the fate of larvae, and thus on connectivity for coral reefs worldwide. The coastal waters of the Great Barrier Reef (GBR) are hypersaline (salinity ~37) during the dry season as a result of evaporation greatly exceeding rainfall, of shallow waters, and of the presence of numerous bays along the coast preventing rapid flushing. These hypersaline waters are not flushed out by salinity-driven baroclinic currents because these waters are vertically well-mixed. Instead these waters are transported by a longshore residual current and thus form a coastal boundary layer of hypersaline waters. Because every bay supplies hypersaline waters, the width of the coastal hypersaline layer increases southwards. The dynamics of the GBR hypersaline coastal boundary layer thus differ from the classical inverse hypersaline systems where the salinity gradient is mainly 1-D with a dominant along-channel salinity gradient.

Biography:  Fernando is a physical oceanographer and mathematician analyzing the water circulation by combining field data and numerical modeling to quantify estuarine and coastal hydrodynamic processes. He endeavors to collaborate on such studies with marine biologists, chemical and geological oceanographers.