Contrasting heat budget dynamics during two La Niña marine heat wave events along northwestern Australia

Over a three year period, two marine heat waves occurred along the coast of Western Australia (WA) during the alternate austral summer periods of 2010/11 and 2012/13, both linked to the strong La Niña conditions. This led to severe impacts to marine ecosystems over more than 12 degrees of latitude, including unprecedented bleaching of many coral reefs, with sea surface temperature (SST) anomalies reaching up to 5o C above climatological averages. However, despite occurring during similar climate (La Niña), the observed warming patterns differed substantially between the two heat waves, with the central coast of WA (south of 22o S) experiencing its greatest warming in 2010/11; whereas, the northwestern coast (north of 22o S) experiencing higher warming in 2012/13. To investigate how oceanic and atmospheric processes drove these very different spatial patterns, we conducted an ocean heat budget analysis of the region using a combination of remote sensing observation, in situ data, and a high resolution (~1 km) numerical ocean model (Regional Ocean Modeling System). The results revealed a substantial spatial differences in the contributions made by along-shelf heat advection and air-sea heat exchange between the two heat wave events. During 2010/11, anomalous heat advection was present across the entire region, but was much stronger south of 22o S where the poleward flowing Leeuwin Current consolidates. During 2012/13, air-sea heat exchange had a much more positive (warming) influence on the SSTs (especially in the northwest), and when combined with the more positive contribution also made by heat advection north of 22o S, can explain the regional differences in warming between the two events. Overall, the results highlight how subtle differences in regional oceanic and atmospheric heat fluxes can lead to regional heterogeneity in warming patterns during marine heat waves, even when under the influence of similar large-scale climate drivers.