Regionally different marine heatwave ocean carbon sink responses are consistent with carbonate understanding

Abstract The occurrence of marine heatwaves, sustained periods of anomalously elevated temperatures in the marine environment, appear to be increasing with time which is a symptom of anthropogenic climate change. These extreme temperatures impact large oceanic areas, and they can have catastrophic impacts on marine ecosystems. However, the impact on the marine carbonate system, air-sea carbon dioxide (CO 2 ) exchange and the resulting ocean carbon sink before and after the heatwave appears unclear. Here we investigate the impact on the ocean carbonate system of five documented marine heatwaves in Western Australia (2011), the North Pacific ‘blob’ (2015), the Southern Ocean (2016), South Pacific (2016) and Equatorial Indian Ocean (2016). The changes in the carbonate system and the air-sea CO 2 exchange 12 months before, during and 12 months after each heatwave were evaluated using a well characterised and spatially complete surface observation-based carbonate system dataset. The signature of each heatwave was clearly identified within the data, but the carbonate-system specific signature was regionally different. All the heatwaves studied showed a significant reduction in dissolved inorganic carbon (DIC), where negative anomalies increased in magnitude before the heatwave peak, followed by a decrease afterwards. The carbonate chemistry changes mean that fugacity of CO 2 (fCO 2 (sw) ) and pH anomalies in all the heatwaves were controlled by the variations in DIC and temperature, with anomalies occurring both before and after the heatwaves, but the specifics varied across regions. The effect of these heatwaves on the air-sea exchange of CO 2 varied during the heatwave lifetime, which was driven by a combination of the carbonate system state and meteorological condition. These results identify that the impact of the heatwaves on the carbonate system and ocean uptake cannot be easily generalised, and it is important to assess the heatwave impacts before, during and after the severe heatwave conditions.