Can vertical mixing from turbulent kinetic energy mitigate coral bleaching? An application of high frequency ocean radar

Coral bleaching is an ecological response to stressful physical conditions observed to occur when strong insolation coupled with stratification of the water column leads to anomalous warming of the surface water. Stratfication requires calm winds, the absence of waves, and an absence of currents: conditions which result in limited mixing of the water column and thus confine heat due to insolation at the ocean surface. There is a strong need to identify which of the physical parameters are more significant at any given time and, more importantly, to monitor the physical parameters in near realtime to serve as a tool for long-term planning and management for marine parks and coastal waters. This paper reviews the contribution that currents make to mixing in the water column through the dissipation of turbulent kinetic energy and takes a further step to evaluate the use of surface current data to provide an index of vertical mixing. In this work, when the surface current speed is greater than a critical value, the water column is found to be vertically mixed even in the absence of wind or waves. A phased array HF Ocean Radar deployed in the southern part of the Great Barrier Reef provides a map of surface currents with high spatial resolution (4km) every 10 minutes over the grid. These surface currents are used to predict vertical stratification and mixing which can then be used as an indication for conditions under which bleaching might occur.