ENSO Weather and Coral Bleaching on the Great Barrier Reef, Australia

The most devastating mass coral bleaching has occurred during El Niño events, with bleaching reported to be a direct result of increased sea surface temperatures (SSTs). However, El Niño itself does not cause SSTs to rise in all regions that experience bleaching. Nor is the upper ocean warming trend of 0.11°C per decade since 1971, attributed to global warming, sufficient alone to exceed the thermal tolerance of corals. Here we show that weather patterns during El Niño that result in reduced cloud cover, higher than average air temperatures and higher than average atmospheric pressures, play a crucial role in determining the extent and location of coral bleaching on the world's largest coral reef system, the World Heritage Great Barrier Reef (GBR), Australia. Accordingly, synoptic‐scale weather patterns and local atmosphere‐ocean feedbacks related to El Niño–Southern Oscillation (ENSO) and not large‐scale SST warming due to El Niño alone and/or global warming are often the cause of coral bleaching on the GBR. Plain Language Summary Historically, the cause of the most devastating mass coral bleaching events on the World Heritage Great Barrier Reef (GBR)—the world's largest coral reef system—was believed to be El Niño‐driven change in sea surface temperatures. However, El Niño itself does not cause SSTs to rise in all regions that experience bleaching including the GBR, Australia. Here we show that changes in weather patterns during El Niño events, that result in light winds, high surface air temperatures and humidity, and clear skies over the GBR are linked to anomalously warm waters over individual coral reefs that experience bleaching. Accordingly, we concluded that local meteorology has been the primary cause of coral bleaching on the GBR during El Niño events over at least the past 34 years. Understanding the energy balance of individual coral reefs at scales of tens of square kilometers under different climate states such as El Niño and La Niña in a warming climate is therefore essential to informed prediction of coral bleaching, and the development of environmental management policy to ensure coral reefs such as on the GBR are protected for future generations. Key Points We apply an atmospheric circulation typing to historical ENSO coral bleaching events reported on the Great Barrier Reef (GBR), Australia We find local meteorology has a causal link to coral bleaching at individual reef scale rather than El Niño‐forced change in SSTs We conclude that w