The role of Rossby wave breaking in coral bleaching on the Great Barrier Reef

The frequencies of thermal coral bleaching events (CBEs) over the Great Barrier Reef (GBR) continue to increase with five mass CBEs reported since 2016. While changes in the local meteorology, such as reduced wind speeds and decreased cloud cover, are known to heat the shallow reef waters, little consideration has been given to the overriding synoptic meteorology. The 2022 CBE, occurring under La Niña conditions, saw ocean temperatures at Davies Reef increase 1.9℃ over 19-days and subsequently cool 2.1℃ back to seasonal norms over eight days. 

This event was found to be triggered by repeated Rossby wave breaking disrupting the local trade winds. As the trades broke down, calm winds and clear skies covered the reef, thus inhibiting the latent heat flux, allowing for the build-up of ocean heat to at least 18m. Following the re-establishment of the trade winds via coastal ridging, latent heat fluxes, the primary driver of the event, tripled allowing the ocean to rapidly cool. 

Concurrent to the mass bleaching event, these are the same Rossby wave breaking events found to cause the historic Lismore flooding located hundreds of kilometres south of the GBR. This case study notes the first reported linkage between mass coral bleaching and a severe flooding event.