BOM: Positive Indian Ocean Dipole continues to strengthen

BOM, 15 October 2019

A strong positive Indian Ocean Dipole (IOD) continues to influence Australian and global climate. The El Niño–Southern Oscillation (ENSO) remains neutral.

The current positive Indian Ocean Dipole event has strengthened significantly over the past month. The latest weekly value of +2.15 °C is the strongest positive weekly value since at least 2001 (when the Bureau’s weekly dataset commenced), and possibly since 1997, when strong monthly values were recorded. Over the past month, strong easterly trade winds across the tropical Indian Ocean aided upwelling of cooler water in the eastern Indian Ocean. At the same time, very warm waters off the Horn of Africa have caused an even greater temperature gradient across the basin.

Given the strength of the trade winds, the IOD may strengthen further over the next fortnight. However, international climate models surveyed by the Bureau indicate the positive IOD is unlikely to persist far into summer. IOD breakdown occurs when the monsoon trough moves into the southern hemisphere in early December. With the monsoon trough having a record-late retreat from India this year, the shift into the southern hemisphere may also be later than usual.

Typically, a positive IOD brings below average winter–spring rainfall to southern and central Australia, with warmer days for the southern two-thirds of the country. Positive IOD events are often associated with a more severe fire season for southeast Australia. Learn more about the Indian Ocean Dipole.

In the tropical Pacific Ocean, the El Niño–Southern Oscillation (ENSO) remains neutral. Most indicators of ENSO are near-average, although the Southern Oscillation Index (SOI) is negative (El Niño-like) due to very high atmospheric pressure at Darwin. The corresponding pressure in Tahiti is largely within normal bounds. This suggests the negative SOI is not related to a developing El Niño, but rather is likely related to the strong positive Indian Ocean Dipole and the cooler waters between Australia and Indonesia.

Climate models forecast neutral ENSO for the remainder of 2019 and into the first quarter of 2020. When ENSO is neutral, it has little effect on Australian and global climate, meaning other influences are more likely to dominate.

Indian Ocean Dipole outlooks

The Indian Ocean Dipole (IOD) remains strongly positive, with the latest weekly index value to 13 October at +2.15 °C. This is the highest weekly value observed in the Bureau’s dataset which extends from 2001 to present, and possibly the highest since 1997. Values for each of the last four weeks have been above the old record set before the current event. The previous record was +1.48 °C for the week ending 5 November 2006.

The very strong positive values of the IOD are due to the strong temperature gradient across the Indian Ocean (see weekly SST map for the Indian Ocean). SSTs to the south of the Indonesian islands of Sumatra and Java reach more than 2 degrees cooler than average for the week ending 13 October, while warm anomalies close to the Horn of Africa reach more than 2 degrees warmer than average.

The overall pattern of sea surface temperatures has remained generally consistent with a positive IOD pattern since late May, with warmer than average sea surface temperatures in the central and western tropical Indian Ocean, and average to cooler than average waters in the eastern tropical Indian Ocean, to the north of Australia and south of Indonesia.

All six international climate models surveyed by the Bureau indicate the IOD will remain positive into December.

The retreat of the Southwest Indian Monsoon has been very slow this year, and models are indicating a slower break down of the IOD than usual. However, it remains unlikely that the positive IOD will persist far into summer. IOD events dissipate as the monsoon trough moves into the southern hemisphere, which changes the broadscale wind patterns over the IOD region and returns sea surface temperatures to near average.