Variability of Marine Heatwave Events in the Southeast Indian Ocean using an Ocean General Circulation Model

An increasing number of Marine Heatwaves (MHWs), i.e. extended periods of anomalously warm temperatures, has been observed in the global ocean during the last decade. Often these extreme events are reported to have major impacts on marine ecosystems and regional fisheries. Notable events have occurred off the coast of Western Australia - the strongest one showing mean sea surface temperatures at $3^{\circ }$C above the long-term mean in 2011 and leading to extensive loss of kelp forest and a regime shift from subtropical to tropical communities. This event was attributed to a combination of an anomalous strong Leeuwin Current and high local air-sea heat fluxes. However in general, physical drivers of MHWs, as well as their depth-structure are still poorly understood. Using a suite of simulations with a global Ocean General Circulation Model at ${0.25}^{\circ }$ resolution from 1958-2016, we investigate Eastern Indian Ocean variability with focus on MHWs, including their depth-structure, along the coast of Western Australia. In particular, we are interested in the impacts of large-scale ocean and climate variability, such as the Indonesian Throughflow and El Ni\~{n}o - Southern Oscillation, on the study region. Initial results show pronounced fresh anomalies associated with major heat wave events, e.g. in 2000 and 2011, coinciding with a peak in the Leeuwin Current strength. Temperature and salinity anomalies of these large events extend to over $200$m depth. A unique set of sensitivity experiments allows us to distinguish the contribution of wind vs. buoyancy forcing to individual events and assess instances of reinforcing vs. compensating behaviour. Different classes of events and their forcing mechanisms are characterized. Because MHWs are by definition rare, most studies to date are based on satellite products or regional modelling. Our work can provide valuable contributions for advancing the understanding of MHW drivers from surface to depth and regional to large scales.