Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
02 April 2020

The Decadal Reduction of Southeastern Australian Autumn Rainfall since the Early 1990s: A Response to Sea Surface Temperature Warming in the Subtropical South Pacific

Science

This study highlights the role of sea surface temperature (SST) warming in the subtropical South Pacific (SSP) in the autumn rainfall reduction in SEA since the early 1990s. The warmer SSP SST enhances rainfall to the northwest in the southern South Pacific convergence zone (SPCZ); the latter triggers a divergent overturning circulation with the subsidence branch over the eastern coast of Australia. As such, the subsidence increases the surface pressure over Australia, intensifies the subtropical ridge, and reduces the rainfall in SEA. This mechanism is further confirmed by the result of a sensitivity experiment using an atmospheric general circulation model. Moreover, this study further indicates that global warming and natural multidecadal variability contribute approximately 44% and 56%, respectively, of the SST warming in the SSP since the early 1990s.

Impact

This study proposes a mechanism that the decadal change in the tropical easterly and autumn rainfall reduction in northeastern Australia are due to the SST warming in the SSP. In response to the SST warming, autumn rainfall is suppressed in both northeastern and southeastern Australia, and an easterly anomaly over northern Australia, which is related to the anticyclonic anomaly over Australia, is apparent in the lower troposphere. These responses are attributed to the SST-warming-induced regional divergent overturning circulation between the SSP and eastern Australia.

Summary

The change in the SEA autumn rainfall is attributed to a concurrent SST warming in the SSP. Since the early 1990s, the increasing SST in the SSP has enhanced rainfall to the northwest in the South Pacific convergence zone; the latter triggers a divergent overturning circulation with the subsidence counter over eastern Australia. As such, the subsidence increases surface pressure over Australia, intensifies the subtropical ridge, and thus reduces the rainfall in southeastern and northeastern Australia. The mechanism is further confirmed by the results of the sensitivity experiment with an AGCM. According to the model simulations, the SSP SST warming reduces SEA autumn rainfall by approximately 34 mm, which is 70% of the observed autumn rainfall reduction in SEA after the early 1990s. Moreover, we further estimate the relative contributions of global warming and natural multidecadal variability on the SSP SST warming after the early 1990s based on the observational data. Global warming accounts for 44% of the SSP SST warming between the 1991–2009 and 1972–90 periods, and natural multidecadal variability accounts for 56%, in which the interdecadal Pacific oscillation contributes to 24% of the SSP SST change.

Contact
Aixue Hu
National Center for Atmospheric Research (NCAR)