The index used to monitor the Indian Ocean Dipole has just reached its lowest value since 2016, bolstering the likelihood of a wet and cool spring in Australia.

As we pointed out last week, a negative Indian Ocean Dipole (IOD) event is currently underway. It's the first proper negative IOD in five years and has already produced above average rain and decent snow in Australia during the last two months.

Now, the index used to monitor the IOD has dipped to its lowest value since 2016 and it could go even lower in the coming weeks.

Monitoring the IOD

The IOD is measured using a weekly index that compares sea surface temperatures on either side of the tropical Indian Ocean.

When the ocean's surface is unusually cold in the west, near Africa, and abnormally warm in the east, near Indonesia, the IOD index is in a negative phase.

Image: Sea surface temperature anomalies in the Indian Ocean on July 26, showing cooler-than-average water near the Horn of Africa and unusually warm water to the northwest of Australia, near Indonesia. This is a negative IOD pattern. Source: NOAA Coral Reef Watch

When the IOD index stays below a value of minus 0.4ºC for around eight weeks, a negative IOD event is declared, which is what happened last week.

The latest weekly IOD index value, measured in the seven days ending on July 25, was minus 0.88ºC. This is well below the negative IOD threshold of minus 0.4ºC and the lowest weekly value since 2016.

Image: Weekly IOD index values from the last four-and-a-half years. The lowest value on the far right is the latest weekly value of minus 0.88ºC. Source: Bureau of Meteorology

What does a negative IOD do?

The IOD is a climate driver, which means it has a broadscale influence on Australia's weather between about May and November.

A negative IOD typically has a range of impacts in Australia during winter and spring, including:

• Above average rain and increased cloud cover, particularly the south and southeast
• Below average daytime temperatures across Australia's southeast mainland
• Above average snowfall in the Alps

Looking ahead, forecast models suggest that the IOD index will continue to drop to even lower values in the coming weeks. This outlook increases the likelihood of heavy rain, cold days and thick snow cover as we head into the start of spring.

We will continue to bring you updates on the development of the IOD during the coming months.