Cool, Near-Bottom Intrusions in the Central Great Barrier Reef

The Central Great Barrier Reef is characterised by an open reef matrix on the outer continental shelf. Channels between emergent reefs facilitate the exchange of cool, nutrient-rich slope waters into the lagoon. Over the slope, the poleward East Australian Current flows in the opposite direction of southeasterly winds. Shelf and slope moorings are deployed as part of the Queensland node of the Integrated Marine Observing System (Q-IMOS). From 2009 - 2015, observations of ocean thermal structure reveal cool, near-bottom intrusions on the shelf. These observations are compared with output from a regional baroclinic hydrodynamic model developed as part of the eReefs project. A metric is developed and used to identify 64 bottom intrusion events over the six year period. These events occur predominantly during the austral summer wet season. The intrusions are associated with cooling of near-bottom temperatures by 1 to 3 deg. C over approximately one week. Intrusion events tend to occur during weakened southeasterly winds or poleward wind bursts. Over the slope, uplift of sub-surface water provides the source waters for the intrusions. Over the shelf, onshore flow leads to near-bottom cooling. Model analyses quantify the processes involved in driving these intrusions onto the shelf, including the roles of geostrophic flows and Ekman currents.