Combined role of ENSO and IOD on compound drought and heatwaves in Australia using two CMIP6 large ensembles

Compound drought and heatwaves (CDHWs) can cause significant socio-economic and ecological impacts. A better understanding of historical spatiotemporal changes of Australian CDHWs and their underlying physical mechanisms can help improve their predictability. We analyse changes in Australian CDHW metrics in the extended summer season (November to March) during the period 1958-2020. Our results suggest that CDHWs increased significantly in terms of their frequency, duration, amplitude, and severity in the recent period (1989/90-2019/20) relative to a historical period (1958/59-1988/89), particularly in eastern Australia. We further analysed the influence of co-occurring modes of El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on the frequency and severity of Australian CDHWs using two Coupled Model Intercomparison Project phase 6 (CMIP6) single model initial-condition large ensembles. We found that frequency, duration, and severity of CDHWs are significantly increased during strong El Nino phases across northeast Australia compared to neutral ENSO and IOD conditions. This increase is widespread over northeast and southeast Australia during the concurrence of strong El Ninos and moderate-strong positive IOD events. Our results show that an extreme CDHW season in terms of their frequency, duration, and severity occurs one out of every two seasons of strong El Nino over the northeast Australia. Moreover, the same applies for the co-occurring strong El Nino and positive IOD over the northeast and southeast Australia as well. Our results highlight the importance of ENSO and IOD combinations for Australian CDHW events. Our findings provide insights into the importance of climate variability in driving Australian CDHWs.

Automated summary

This study analyzed the historical spatiotemporal changes of compound drought and heatwaves (CDHWs) in Australia and explored the influence of co-occurring El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on their frequency and severity. The results showed a significant increase in the frequency, duration, amplitude, and severity of CDHWs in recent years, particularly in eastern Australia. It was also found that the frequency, duration, and severity of CDHWs are significantly increased during strong El Nino phases and moderate-strong positive IOD events, with widespread impacts on northeast and southeast Australia.