Marufish World of Disaster PreventionBOM: Increasing chances of a negative IOD event in 2021

22 June 2021

The Indian Ocean Dipole (IOD) index has been below the negative IOD threshold for the past five weeks, with no models forecasting a rise in the index any time soon. Large parts of the eastern Indian Ocean are now warmer than average, with cooler than average waters near the Horn of Africa—a pattern typical of a negative IOD. Four of the five climate models surveyed by the Bureau predict a negative IOD for the second half of the southern hemisphere winter and into spring.

A negative IOD event is declared when there have been at least eight weeks below the IOD index threshold of −0.4 °C. Negative IOD conditions increase the chances of above average winter–spring rainfall for parts of southern Australia.

The El Niño–Southern Oscillation (ENSO) remains neutral with all oceanic and atmospheric indicators within the neutral range. Most climate model outlooks indicate this neutral ENSO state is likely to continue for the southern hemisphere winter and early spring, with tropical Pacific sea surface temperatures slowly cooling over the coming six months. This cooling pattern may be partly contributing to the wetter than average climate outlooks for parts of eastern Australia.

Indian Ocean

The Indian Ocean Dipole (IOD) index has been below the negative IOD threshold (−0.4 °C) for five consecutive weeks. The latest weekly value of the IOD index to 20 June was −0.70 °C.

While a negative IOD event is not declared until the index remains below the threshold for at least 8 weeks, this pattern can still influence Australian rainfall as the event develops, with wetter than average conditions in central and southern Australia more likely.

Most of the five surveyed climate models anticipate values of the IOD index will remain within negative thresholds into the middle of spring. Four of five models predict negative IOD will persist until October, with two of these continuing into November.

Weekly sea surface temperatures (SSTs) in the Indian Ocean were warmer than average across much of the southern and eastern halves of the basin, while small areas of cooler than average SSTs around the Horn of Africa and the Arabian Peninsula have strengthened compared to two weeks ago.