The role of ocean currents in modulating coral bleaching susceptibility

Mass coral bleaching events are degrading coral reefs, impacting the livelihoods of the hundreds of millions of people who depend on these ecosystems for employment and sustenance. Nearly all studies on coral bleaching implicate temperature as the sole or primary driver of bleaching, and it is often suggested that global warming is entirely responsible for the increasing frequency of bleaching in recent decades. However, based on a global bleaching database from 1982 to 2016, temperature alone captures less than half of observed bleaching events and leads to more false alarms than correct predictions. Furthermore, the number of bleaching events per reef is not correlated to the rate of local warming since pre-industrial times. Clearly, other factors in addition to temperature are involved. Here, we used a combination of coral skeletal cores and ecological surveys to build timelines of bleaching events over the past several decades on the Great Barrier Reef (GBR) and in the Red Sea. We then investigated the causes of bleaching with remote sensing and ocean reanalysis products. In both reef systems, severe bleaching occurred when anomalously high temperatures coincided with currents conducive to upwelling of nutrient-rich waters. Similarly, the hottest summer on record at each of the GBR and Red Sea caused only minor bleaching because anomalous currents dampened upwelling at these times. Our results indicate that excess nutrients can exacerbate the effect of heat stress in causing coral bleaching, consistent with the findings of multiple laboratory studies. Changes to ocean currents in response to climate change could therefore play a key role in modulating the sensitivity of corals to continued global warming. Incorporating oceanographic processes, such as current-driven upwelling, into coral bleaching forecasts is likely to improve our predictive ability beyond existing systems based solely on temperature.