Impacts from Tropical Variability on Indian and Australian Rainfall: Inter-decadal Variations

Abstract:

Rainfall anomalies during June-September over Indian subcontinent and southern Australia are remotely influenced by diabatic heating anomalies resulting from tropical sea surface temperature variability, including El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Evidence from reanalysis datasets and a set of CMIP5 models pre-industrial outputs, shows considerable interdecadal variations in the relationship between regional rainfall and tropical variability. Those variations can be ascribed to varying coherence of ENSO with the IOD on the interdecadal timescale. In the Indian subcontinent, when ENSO and the IOD are strongly coherent in decadal periods, ENSO dominates an inverse relationship with rainfall and overwhelms the impacts from the IOD through a baroclinic teleconnection induced by ENSO; in decadal periods when the coherence of tropical variability is weak, the positive effect from the IOD dominates rainfall variability through a baroclinic teleconnection induced by the IOD, accompanied by a weakened influence from ENSO. In southern Australia, tropical variability plays a more distinct role in rainfall variability compared to its counterpart in India. Either an El Nino or a positive IOD leads to decreased rainfall, through equivalent barotropic Rossby wave trains emanating from the tropical Indian Ocean. An anomalously strong rainfall response in southern Australia is associated with a strong coherence of ENSO and the IOD in decadal periods. In decadal periods of weak coherence, the effect of ENSO can’t reach southern Australia.