High-resolution marine heatwave mapping in Australasian waters using Himawari-8 SST and SSTAARS data

Abstract

Marine heatwaves (MHWs) have significant ecological and economic impact at local and regional scales. Consequently, there is a pressing need to map the fine-scale 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 Himawari-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 identified two broad areas of MHW hotspots that occurred between August 2015 and February 2019. The Tropical Warm Pool region (including the Great Barrier Reef and part of the Coral Sea) 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 underlying driver for the MHWs, and the air-sea heat flux was likely the key local process controlling the heat budget. 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. The 2015–16 summer Bass Strait and East Tasmania MHW event was likely due to the anomalous heat transported by the East Australia Current. The air-sea heat flux, however, was likely an important local driver for the MHW events in the Tasman Sea during the 2016 autumn, the 2017–18 summer and the 2018–19 summer. The anomalous heat flux into ocean was attributed to potential underlying climate drivers including ENSO teleconnections, positive SAM and their interaction. 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. Two case studies in the Beagle Marine Park located in north-eastern Bass Strait and the Elizabeth and Middleton Reefs situated ~550 km east of the Australian coast within the Lord Howe Marine Park proved the value of Himawari-8 SST and SSTAARS data in mapping fine details of MHWs in a small area, which could not be captured by the broad-scale DOISST data. During the 2017–18 summer event, the case studies revealed much stronger MHW influence in the shallow waters east of the Beagle Marine Park where most of the important rocky reef habitats exist. During the same event, the case studies also confirmed more severe MHW impact on the Elizabeth Reef than the Middleton Reef which resulted in bleaching to selective coral species on the Elizabeth Reef. The near-real time MHWs mapping showed 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.