Convectively Coupled Tropical Waves and Their Influence on Rainfall in Tropical Australia: Observations and Predictability
Fadhlil Rizki Muhammad
Convectively coupled tropical waves (CCTWs) are modes of intra-seasonal variability that affect tropical circulation and rainfall. We find that the CCTWs with an off-equatorial convective centre, such as equatorial Rossby waves (ER), mixed Rossby-gravity waves (MRG), and tropical depression-type waves (TD-type) increase the chance of extreme rainfall (above the 90th percentile) by around 1.5 – 2.4 times for ER waves and 1.4 – 1.6 times for MRG and TD-type waves compared to the seasonal probability during austral summer. These values are comparable with the impact of the Madden-Julian Oscillation (MJO), which increases the probability of extreme rainfall by around 1.3 – 2.7 times compared to the seasonal probability. The representation of CCTWs is assessed in the ACCESS-S2 hindcast. We show that the predictability of ER waves and MJO in the filtered outgoing longwave radiation (OLR) during austral summer extends out to around 9 and 16 days, respectively (r > 0.5). Space-time spectral analysis also shows that the representation of CCTWs in the OLR field is underestimated. Further cross-spectral analysis demonstrates that there is a weak convection-circulation coupling bias in the model. This result shows that these waves can be leveraged in S2S predictions to increase forecasting skill for extreme events in the tropics.
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