Atmospheric Drivers of Tasman Sea Marine Heatwaves

Abstract

Abstract Marine heatwaves (MHWs) can severely impact marine biodiversity, fisheries, and aquaculture. Consequently, there is an increasing desire to understand the drivers of these events to inform their predictability so that proactive decisions may be made to reduce potential impacts. In the Tasman Sea (TS), several relatively intense and broad-scale MHWs have caused significant damage to marine fisheries and aquaculture industries. To assess the potential predictability of these events, we first determined the main driver of each MHW event in the TS from 1993 to 2021. We found that those MHWs driven by ocean advection—approximately 45% of all events—are generally longer in duration and less intense and affected a smaller area compared with the remaining 55%, which are driven by air–sea heat fluxes, are shorter in duration, and are more surface intense. As ocean advection–driven events in the TS have been closely studied and reported previously, we focus here on atmospherically driven MHWs. The predictability of these events is assessed by identifying the patterns of atmospheric pressure, winds, and air–sea heat fluxes in the Southern Hemisphere that coincide with MHWs in the Tasman Sea. We found that atmospherically driven MHWs in this region are more likely to occur during the positive phase of the asymmetric Southern Annular Mode (A-SAM)—which presents as an atmospheric zonal wave-3 pattern and is more likely to occur during La Niña years. These A-SAM events are linked to low wind speeds and increased downward solar radiation in the TS, which lead to increased surface ocean temperatures through the reduction of mixing. Significance Statement The purpose of this study is to understand factors of the atmosphere that contribute to an accumulation of heat in the upper ocean in the Tasman Sea to better inform predictability. Higher incidences of ocean extreme thermal events (known as marine heatwaves) in this region are becoming increasingly more common and threatening the important marine industries that support the people of both Australia and New Zealand. We need to know the sources of this extra heat to understand whether such events can be predicted. Previous studies have found the East Australian Current to be responsible for around half of these events, and our results show a connection between a known atmospheric pattern and the other half. As we continue to improve our ability to anticipate this pattern, this suggests that we may also be able to predict these extreme heating events.