Marine heatwaves in the Great Barrier Reef and Coral Sea, their mechanisms and impacts on shallow and mesophotic coral ecosystems

<div>The iconic Great Barrier Reef (GBR) World Heritage Area and adjacent Coral Sea Marine Park are under serious threat from global climate change. Given the increase in the frequency, intensity and severity of mass coral bleaching events associated with marine heatwaves (MHWs) in this region it is essential that we improve our understanding of the drivers and mechanisms underlying&nbsp;MHWs and the extent to which they impact both shallow and deeper coral reef ecosystems. This study used coarse-resolution and high-resolution sea surface temperature (SST) data to identify all major MHWs occurring in the GBR and Coral Sea region over the last three decades (1992-2022) and map significant MHW events over the past seven years (2015-2022), respectively. We then investigated the mechanisms of these MHWs in relation to both remote and local drivers through statistical and heat budget analyses. Finally, we identified potential coral reef refugia in this region using aerial-survey coral bleaching data and Autonomous Underwater Vehicle (AUV) images, and examined their underlying mechanisms using ocean model and <em>in-situ</em> oceanographic data. The results confirmed that MHWs in this region indeed increased in frequency, intensity and extent over the past three decades. El Niño, especially when it is in phase with positive Indian Ocean Dipole, was found to be the key remote driver leading to significant MHWs. However, the more recent strong MHWs also tend to occur without these climatic events, signifying the impacts of long-term climate change. We also found that reduced wind speed and shoaling mixed layer depth, often together with reduced cloudiness, which can occur with or without the influence of remote drivers, were the main local drivers pre-conditioning these MHWs.&nbsp;Anomalous air-sea heat flux into the ocean, which is mainly controlled by shortwave solar radiation (cloudiness) and latent heat flux (wind), was the most constant contributor to the 2015-16 and 2019-20 MHW events. However, local oceanographic dynamics, especially horizontal advection and turbulent mixing, played important roles in local MHW heat budgets. Importantly, this study confirms that shallow-water coral bleaching severity was indeed positively related to the cumulative MHW intensity in the 2015-16 and 2019-20 MHWs. We identified the shallow reefs in the northern GBR along the path of the North Queensland Current as potential coral reef refugia from bleaching because of the up to 2 oC thermal relief that the ocean current provides. We also found that, except during abnormal weather events such as tropical cyclones, the mesophotic reefs in the Coral Sea Marine Park may also act as potential coral reef refugia from bleaching because of the thermal protection provided by the shallow mixed layer depth.</div><div> <b>Citation:</b> Zhi Huang, Ming Feng, Steven J. Dalton, Andrew G. Carroll, Marine heatwaves in the Great Barrier Reef and Coral Sea: their mechanisms and impacts on shallow and mesophotic coral ecosystems, <i>Science of The Total Environment</i>, Volume 908, 2024, 168063, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2023.168063.