IOD's new (dry) heights | Weather In Focus

The past three months have collectively been the driest on record for this time of year in some areas of northern and western NSW.

While many factors influence our day-to-day weather, this prolonged dry period has been underpinned by something called a positive Indian Ocean Dipole (IOD).

A positive IOD occurs when warmer-than-usual water sits at the surface of the western tropical Indian Ocean, near Africa, and relatively cool water lies on the eastern side of the Indian Ocean, near Indonesia.

This pattern limits the amount of moisture being transported across Australia from the north west, which typically causes below average rainfall and above average daytime temperatures in large parts of the country during winter and spring.

This year's positive IOD event is one of the strongest in recent decades.

The IOD is measured using an index that shows the difference is sea surface temperatures on either side of the Indian Ocean.

A positive IOD is declared when this IOD index exceeds +0.4 degrees Celsius for a number of weeks.

This threshold has now been exceeded for the past 11 consecutive weeks and the latest value of +2.15C was the highest since the Bureau of Meteorology started publishing weekly data weekly in 2001.

It's also likely to be the highest IOD index value since at least 1997.

Unsurprisingly, rainfall has been sparse across large areas of the Murray Darling Basin during the past few months, coinciding with the progressively strengthening positive IOD.

August and September both ranked among the 10 driest respective months on record for the Murray Darling Basin, with data going back to 1900.

Looking ahead, the positive IOD is likely to further limit rainfall in parts of the Murray Darling Basin during the rest of spring, before rapidly losing its influence on our weather as we head into summer.