Interannual Variability of the Australian Summer Monsoon Onset: Possible Influence of Indian Summer Monsoon and El Niño

Abstract

The date of Australian summer monsoon onset (ASMO) is found to be well correlated with the monsoon rainfall of India during the preceding June to September. Years of below (above) normal Indian summer monsoon rainfall (ISMR) are followed by delayed (early) ASMO. Sea surface temperature (SST) anomalies during the September to November season over the tropical Indian Ocean, the equatorial eastern Pacific Ocean, and the ocean north of Australia also correlate significantly with the date of the following ASMO. Delays in ASMO are associated with cold SST north of Australia and warm SST in the tropical Indian and equatorial cast Pacific oceans. Previous studies have shown that a warm SST is created over the tropical Indian Ocean in years of poor ISMR. We hypothesize that a warm SST anomaly over the Indian Ocean delays the seasonal southward and eastward migration of the cloudiness maximum. A delay in the southeastward movement of cloudiness results in a delayed ASMO. A similar hypothesis already has been suggested to explain the variability of the date of monsoon onset over India. Weak ISMR often is associated with the contemporaneous presence of El Niño, although many weak monsoons occur without El Niño. Thus warm SSTs in the eastern equatorial Pacific are related to a delayed ASMO through the Indian monsoon. Another signature of El Niño is the presence of negative SST anomalies north of Australia, adding to the delay in ASMO. Warm SSTs in the central and eastern Pacific may also act directly to delay ASMO by causing convection near and east of the date line and subsidence near Australia.