The Impacts of Convectively Coupled Equatorial Waves on Extreme Rainfall in Northern Australia

Abstract

Abstract Convectively coupled equatorial waves (CCEWs) with an off-equatorial convective center, such as equatorial Rossby (ER) waves, mixed Rossby–gravity (MRG) waves, and tropical depression (TD)-type waves, can be the potential sources of predictability for subseasonal to seasonal prediction over northern Australia. To establish the statistical relationship of the wave–rainfall interaction, we investigate the influences of CCEWs on rainfall means and extremes during the austral summer (December–February) and autumn (March–May) from 1981 to 2018. The results show that ER waves increase the average daily rainfall by up to 7 mm day−1 (4 mm day−1) during the austral summer (autumn) and increase the probability of extreme rainfall (above the 90th percentile) by around 1.5–2.4 times (summer) and 1.1–1.8 times (autumn) relative to climatology. MRG and TD-type waves are shown to have a smaller impact, increasing rainfall by around 1–4 mm day−1 (1–1.5 and 1–3 mm day−1) during the summer (autumn) and extreme probability by 1.4–1.6 times and 1.25–1.9 times (1.3–1.8 times and 1.27–1.7 times), respectively. The increase in rainfall can be attributed to the enhancement of moisture convergence during the time of the rainfall. Furthermore, moisture gain and enhancement of moisture advection were found ahead of the convective center. Additionally, we find that interactions between multiple waves can act to amplify or suppress the mean daily and probability of extreme rainfall. This research highlights the important role of CCEWs on northern Australian precipitation variability. Significance Statement This research is the first to delve into the significant role of convectively coupled equatorial waves (CCEWs) on subseasonal rainfall variability over northern Australia, particularly equatorial Rossby (ER) waves, mixed Rossby–gravity (MRG) waves, and tropical-depression-type (TD-type) waves. A robust statistical relationship is found between these waves and rainfall, suggesting these waves are major contributors to increased daily rainfall and extreme rainfall probabilities during austral summer and autumn. These findings emphasize the significance of CCEWs on northern Australian rainfall variability and highlight the potential of CCEWs for subseasonal to seasonal forecasts in Australia.