A deeper look into marine heatwaves off the coast of Western Australia

Marine heatwaves (MHWs) have significantly impacted the southwest region of Western Australia (WA), particularly shelf regions, which are vital for commercial and recreational fisheries; however, sub-surface temperature anomalies remain understudied. We analysed 14 years of temperature data from moorings off Perth, WA, to investigate surface and sub-surface MHWs. Additionally, we contrasted the oceanic and atmospheric conditions during the austral summers of 2010/2011 and 2020/2021, both La Niña years, to analyse the spatial and depth extent of MHWs and the factors that contributed to or inhibited MHW development. Surface MHWs were found to extend to depths of up to 175 m on the continental shelf. Several sub-surface MHWs, undetectable at the surface, were observed. These were shorter in duration but more intense. In 2010/2011, strong negative wind anomalies enhanced the Leeuwin Current, reduced wind-driven cooling, and increased atmospheric heat input, leading to widespread surface warming and a severe MHW. In contrast, 2020/2021 lacked anomalous winds, and no surface MHW developed despite the La Niña conditions. Eddies, observed by sea level anomaly (SLA), contributed to localised sub-surface warming through downwelling of warm surface waters during both events. Our findings show that MHWs can have a time variable depth extent and, even if undetectable at the surface, can persist at sub-surface levels. These findings highlight that sea surface temperature alone cannot be used to detect sub-surface MHWs, but SLA can be a good indicator. This emphasises the need to view MHWs as dynamic 4D events—spanning longitude, latitude, depth, and time—to better understand their development, progression, decay, and ecosystem impacts. • MHWs are dynamic 4D events evolving with longitude, latitude, depth, and time. • Sub-surface MHWs off Western Australia (not detectable at surface) are more intense than surface MHWs. • The Leeuwin Current affects the temperature at shallower depths over a broad area. • Downwelling due to anticyclonic eddies can lead to localised sub-surface MHWs off WA. • Negative wind anomalies reduce cooling of the ocean surface. • Large scale oceanic pre-conditioning is critical to the development of MHWs during La Niña.