The Southern Annular Mode: its influence on interannual variability of rainfall in North Australia

The present study investigates the lead relationship of the April-May (AM) Southern Annular Mode (SAM) with the northern Australia rainfall (NAR) variability in the subsequent December-February (DJF). The AM SAM was revealed to exhibit a considerable negative relationship with the NAR in the following DJF, which was still significant after removing the possible influence of El Nino-Southern Oscillation (ENSO). Further analysis revealed that the AM SAM can trigger the dipole sea surface temperature anomalies (SSTA) in the southern Indian Ocean through air-sea interactions. The dipole SSTA persists across the following months from June to September, and then it evolves to form the dipole SSTA at locations to the east of previous definitions of the Subtropical Indian Ocean Dipole from October to February of the following year. Based on the circulation anomalies, the AM SAM-related dipole SSTA in DJF is associated with an eastward shift of the Walker circulation between the tropical eastern Indian Ocean and the western Pacific Ocean, resulting in upward (downward) motion over the Indian (Pacific) Ocean and easterly surface wind anomalies across northern Australia. The anomalous easterlies are part of a localized anticyclonic circulation centred over the study region, which indicates diminished moisture influx into the northern parts of the continent, resulting in dry conditions over northern Australia. Besides, model results based on the preceding AM SAM and September ENSO indicate enhanced predictive skill of the NAR associated with combined indices compared to either single index.

Automated summary

This study investigates the relationship between the Southern Annular Mode (SAM) and the rainfall variability in northern Australia. The results show a significant negative correlation between the SAM and the subsequent rainfall in northern Australia. Further analysis reveals that the SAM can trigger sea surface temperature anomalies in the southern Indian Ocean through air-sea interactions, which impact the rainfall patterns in northern Australia.