The main cause of coral bleaching is persistently high sea temperatures. This bleaching event has coincided with a marine heat wave on the GBR where the monthly average sea surface temperature (SST) anomaly for March was greater than 1ºC for much of the Central GBR, and reaching 2ºC for much of the Northern GBR (top figure).
Since October 2015, IMOS & CSIRO Slocum gliders have traversed this region, tracking the seasonal evolution of coastal waters. The intensive missions* were principally planned to help validate the eReefs model but have also provided unprecedented observations of the formation, persistence and now waning of the thermal stress of the GBR waters.
Glider pilots remotely navigate the gliders through complex pathways between reefs, which can be challenging with strong currents and tides. Gliders are capable of making observations in places difficult for ships and can stay at sea for several weeks. By sampling the water in see-saw manner, the gliders can reveal how deep the warming extends. By April 2016, glider observations indicated water throughout the water column on the inner shelf was still warmer than historical observations.
While revealing where the warming occurs, the gliders can also show where cooling occurs. A glider transect in Palm Passage, on the outer Central GBR, detected upwelling, in which cooler water lifts up onto the shelf from offshore. This cool water may provide relief to marine ecosystems sensitive to the marine heatwave. SST from 27 March (bottom figure) indicate the outer reef of the Northern GBR, 2-3 degrees cooler, than the inner reef, may be buffered by a combination of upwelling and tidal mixing.
In May 2016 two gliders identified the existence of dense shelf water cascades for the first time in the central and northern GBR. These cascades can occur when heat loss during autumn causes waters to cool and become more dense near the coast. Both cascades occurred nearly at the same time off Port Douglas and the other off Mission Beach south of Cairns. These cascades flow offshore and can cool the communities living near the sea bed. Ongoing glider deployments into winter will inform how long the warming lasts.