Negative Indian Ocean Dipole (IOD) increases rain

The IOD was negative from late August to late November 2010 encouraging clouds to form over the eastern Indian Ocean and enhancing the flow of moisture over Australia from the northwest.

October 2010 monthly sea surface temperature anomalies (°C) in the Indian Ocean show much warmer than usual waters in the eastern end of the dipole near Australia. (Anomalies are calculated with respect to the Reynolds climatology for 1971–2000 from the National Climatic Data Center.)

The 2010 IOD event was characterised by much warmer than normal ocean surface temperatures in the eastern Indian Ocean, and slightly above-average ocean surface temperatures in the west. This differed from the classical pattern for a negative IOD, which has warmer than usual water in the east and cooler than usual water in the west. At least part of the unusual behaviour of the IOD can be linked to global warming which is leading to a rapid warming of the Indian Ocean both in the east and west.

The combination of a negative IOD and La Niña conditions has historically increased the likelihood of heavy rainfall across Australia which may partially explain the strong rainfall response. However, during the 2010–11 La Niña a strong positive Southern Annular Mode (SAM) partly offset at least some of the effect of the negative IOD across southern, and particularly southwestern, Australia (see SAM below).

There was a weakly positive IOD event during spring 2011, coinciding with the 2011–12 La Niña. The positive IOD may have partially moderated the effect of the La Niña in southeastern Australia during these months, reducing rainfall from what might otherwise have been observed.