Possible roles of regional SST anomalies in long-term changes in the relationship between the Indian and Australian summer monsoon rainfall

The present study investigates long-term changes in the relationship between interannual variations of the Indian monsoon rainfall (IMR) and the Australian monsoon rainfall (AMR) during the period 1900-2010. It is found that the relationship between austral summer AMR and the following boreal summer IMR (denoted as the AMR-IMR relationship) is negative during 1900-1930 and 1960-1990, but weak positive during 1930-1960. This long-term change in the AMR-IMR relationship is more prominent than the change in the relationship between boreal summer IMR and the succeeding austral summer AMR (denoted as the IMR-AMR relationship). The impact of regional sea surface temperature (SST) anomalies north of Australia on the IMR to AMR transition has experienced obvious changes during the analysis period. The long-term change in the IMR-AMR relationship is attributed to the change in impacts of regional SST anomalies north of Australia and their coherence with equatorial Pacific and tropical Indian Ocean SST anomalies. It is found that the east-west SST anomaly gradient in the North Indian Ocean is an important factor in the IMR variations. The impact of the North Indian Ocean zonal SST anomaly gradient on the AMR to IMR transition has changed largely during the analysis period. This appears to be a major reason for the long-term change in the AMR-IMR relationship. The changes in the impacts of regional SST anomaly or SST anomaly gradient are associated with changes in the amplitude of regional SST anomalies that are likely due to the change in the El Nio-Southern Oscillation (ENSO)-related SST anomaly pattern.