During austral summer the El Niño-Southern Ocsillation (ENSO) is known to affect rainfall in Australia. Recent studies focusing on austral spring have attributed aspects of this influence to covarying sea surface temperatures associated with the Indian Ocean Dipole (IOD). The present study examines the influence of the Indian Ocean on ENSO’s teleconnection to Australian rainfall in summer. The main mode in the Indian Ocean during this season is the Indian Ocean Basin-wide Mode (IOBM), as the IOD does not last into this season. We find that the IOBM extends the tropical influence of ENSO westwards, by extending the convection anomalies towards the Indian Ocean, affecting rainfall in northern Australia. This extension of convection anomalies to the north of Australia promotes an equivalent-barotropic Rossby wave train, inducing a pressure anomaly centre over the Tasman Sea. This anomaly centre extends ENSO’s Pacific-South American pattern, which is ENSO’s Rossby wave influence in the Pacific, towards Australia’s eastern coast.

The teleconnections of the IOBM also shows a strong asymmetry during this season. A stronger negative IOBM provides is a greater influence on rainfall than with the same magnitude positive IOBM anomaly. This is consistent with the ENSO response on rainfall, where the La Niña phase has a stronger impact on Australian rainfall than the El Niño phase. Consequentially the negative IOBM is highly coherent with a La Niña and a positive IOBM with an El Niño.