<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.

<div>The Great Barrier Reef (GBR) World Heritage Area and adjacent Coral Sea Marine Park are under serious threat from global climate change. This study used sea surface temperature (SST) data to identify major marine heatwaves (MHWs) occurring in this region over the last three decades (1992–2022) and to map significant MHW events that have occurred between 2015 and2022. We investigated the mechanisms of the MHWs and identified potential coral refugia. MHWs in this region have increased in frequency, intensity and spatial extent. El Niño, especially when it is in phase with positive Indian Ocean Dipole, was the key remote driver leading to intense MHWs. However, the more recent strong MHWs (e.g., 2017 and 2022) occurred without these climatic events, signifying the impacts of long-term climate change and local drivers. We also found that reduced wind speed and shoaling mixed layer depth, often together with reduced cloudiness, were the main local drivers pre-conditioning these MHWs.&nbsp;Anomalous air-sea heat flux into the ocean, 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 MHW heat budgets. This study confirmed that shallow-water coral bleaching severity was positively related to the cumulative MHW intensity in these two MHWs. We identified the shallow reefs along the path of the North Queensland Current as potential coral refugia from bleaching because of the cooler waters upwelled from the ocean current. We also found that, except during abnormal weather events such as tropical cyclones, the mesophotic reefs in the Coral Sea Marine Park may be less susceptible to severe bleaching as the MHWs are more confined within the shallow mixed layer.</div> Presented at the 30th Conference of the Australian Meteorological and Oceanographic Society (AMOS) 2024

Marine heat waves (MHWs) have significant ecological and economic impact. Consequently, there is a pressing need to map the temporal and spatial patterns of MHWs, for both historical and near real-time events. Satellite remote sensing of Sea Surface Temperature (SST) provides fundamental data for the mapping of MHWs. This study used high-resolution Himarwari-8 SST and the Sea Surface Temperature Atlas of the Australian Regional Seas (SSTAARS) data, which have a spatial resolution of ~ 2 km, to map recent and near real-time MHW events in waters around Australasia. The high-resolution MHWs mapping has identified two broad areas of MHW hotspots between August 2015 and February 2019. Firstly, the Tropical Warm Pool region (including the GBR and part of the Coral Sea) between the maritime continent and the Australian continent was affected by strong and prolonged MHW conditions for the greater part of 2016. The unusually strong 2015-16 El Niño event was believed to be the primary driver for the MHWs, and the air-sea heat flux rather than the ocean advection was the main local process controlling the heat budget. Secondly, the south-east of the study area (including Australia’s south-east coast, the Tasman Sea and New Zealand’s east coast) suffered severe MHWs in 2015-16, 2017-18 and 2018-19. ENSO played little role in the generation of the MHWs in this region. Instead, the MHWs in the western part of this region were more likely due to the extensive heat transported by the East Australia Current; while in the eastern part, the MHWs were more likely due to more local climate modes such as SAM. This mapping has not only enhanced our understanding of the spatio-temporal characteristics of several previously documented MHWs but also identified and mapped several previously undocumented MHWs. The case study in the Beagle Marine Park proved the values of Himawari-8 SST and SSTAARS data in mapping fine details of MHWs in a small area, which are not possible for broad-scale SST data such as the Optimal Interpolated SST (OISST) which has a spatial resolution of ~ 25 km. The case study revealed much stronger MHW influence in the shallow waters east of the marine park where most of the important rocky reef habitats exist. The near-real time MHWs mapping shows that both the GBR and the Coral Sea marine parks were experiencing MHW conditions in early March 2020, with most affected areas having strong MHW class.

Himawari-8, a geostationary satellite operated by the Japan Meteorological Agency, became operational in July 2015. The high frequency (10 min) and resolution (~2km) of Himawari-8 data provides an enormous opportunity for the monitoring and investigation of highly dynamic oceanographic phenomena. This presentation aims to demonstrate the value of himawari-8 SST data for studies of the Bonney Coast upwelling, East Australian Current (EAC) and Madden-Julian Oscillation (MJO) diurnal SST (dSST) variations. During the 2016–17 summer, we identified three distinct upwelling events along the Bonney Coast. Each event surpassed its predecessor in terms of area of influence, minimum temperature and duration. All of the three events developed quickly, with a 5-fold increase of spatial extent within the first 48 hours. The EAC’s areal extent mapped between July 2015 and Sept 2017 showed clear seasonal and intra-seasonal variations. During summer, the EAC and its extension frequently encroached into the coastal areas of northern NSW and eastern Tasmania. The encroachment to the coastal area of southern NSW was more sporadic. A composite analysis based on MJO phases during the summer seasons of 2015–16 and 2016–17 showed that the dSST typically peaked during phases 2 and 3 off the northwest shelf, prior to the onset of the active phases of MJO (phase 4). The analysis indicated that dSST is negatively correlated with the surface wind speed but positively correlated with short-wave latent heat flux. In future, these monitoring and analytical capabilities can be effectively implemented in Geoscience Australia’s Digital Earth Australia platform. Presented at the 2018 Australian Coastal and Oceans Modelling and Observations Workshop (ACOMO)

The Digital Earth Australia Hotspots web service has been developed as part of the Digital Earth Australia Hotspots national bushfire monitoring system. The service delivers hotspot data derived from (a growing number of) satellite-born instruments that detect light in the thermal wavelengths. The colour of the spot represents the time the Hotspot was last observed by a passing satellite (e.g. 0-2 hours). The colour does not indicate severity. Typically, the satellite data are processed with a specific algorithm that highlights areas with an unusually high temperature. In principle, however, Hotspots may be sourced from non-satellite sources. Lineage (for eCatID 101800 and 101780): The Sentinel Hotspots system was originally developed in 2010. The Sentinel Hotspots webservice was republished in 2016 as part of a platform upgrade. The Digital Earth Australia Hotspots system and webservices was redeveloped in 2019 as part of a platform upgrade.

The Digital Earth Australia Hotspots web service has been developed as part of the Digital Earth Australia Hotspots national bushfire monitoring system. The service delivers hotspot data derived from (a growing number of) satellite-born instruments that detect light in the thermal wavelengths. The colour of the spot represents the time the Hotspot was last observed by a passing satellite (e.g. 0-2 hours). The colour does not indicate severity. Typically, the satellite data are processed with a specific algorithm that highlights areas with an unusually high temperature. In principle, however, Hotspots may be sourced from non-satellite sources.