One of Australia’s main climate drivers – the Indian Ocean Dipole (IOD) – may have just entered a phase that typically brings warm and dry weather to Australia.

What is the Indian Ocean Dipole?

Australia's weather is shaped by the seasons – cooler winters, warmer summers, and rainfall patterns that vary significantly depending on the time of year and location. But layered on top of this natural seasonality are larger, slower-moving climate drivers that can push conditions above or below average for that time of year.

The most well-known climate driver that influences Australia’s weather is the El Niño-Southern Oscillation (ENSO), which alternates between El Niño, neutral and La Niña phases.

On the other side of Australia is a similar climate driver called the Indian Ocean Dipole (IOD).

The IOD is an index that measures the difference in sea surface temperature anomalies across the tropical Indian Ocean. It is calculated by comparing the anomalies in two defined regions:

The IOD East region, which sits to the southwest of Indonesia

The IOD West region, which lies off the east coast of Africa, near the Horn of Africa

The IOD fluctuates between positive, neutral and negative phases.

A positive IOD occurs when sea surface temperatures are cooler than average in the IOD East region and warmer than average in the IOD West region. This phase is typically associated with abnormally warm and dry weather in Australia, particularly in winter and early summer.

A negative IOD occurs when cooler-than-average water sits in the IOD West region and abnormally warm water lies in the IOD East region. This phase often causes above average rain and below average maximum temperatures in Australia.

What is the IOD doing now?

The weekly IOD index rose to +0.44°C during the seven days ending on July 26, which is the first time it has exceeded the positive IOD threshold of +0.4°C since March this year.

Image: Weekly IOD index values. Source: Weatherzone.

This is a sign that a positive IOD may be developing, however, the index will need to remain above +0.4°C for several consecutive weeks for this to be declared as a positive IOD event.

Forecast models suggest that this could be the start of a positive IOD event. According to the Bureau of Meteorology, “models suggest a positive IOD may develop during the southern hemisphere winter and persist into spring, although there remains some variation in the timing and strength of this potential event.”

In addition to the potential positive IOD, the coming months will also be influenced by an established and strengthening El Niño in the Pacific Ocean. This climate driver will also promote abnormally dry and warm weather in Australia through late winter and spring.

Seasonal forecast models are responding to the influence of these two climate drivers, hinting at drier-than-average weather over large parts of Australia in the coming months.

Image: Chance of exceeding median rainfall in September, according to the ECMWF SEAS5 model. Source: Weatherzone.

Meteorologists will be watching the IOD closely in the coming weeks to see if these early signs of a positive IOD mature into a fully fledged event. Keep an eye on the Weatherzone news feed for regular IOD updates in the weeks and months ahead.