Resurgence of IOD means more rain

Parts of inland NSW were once again placed on flood watch amid an outbreak of rain and thunderstorms, caused by a cold front and associated low. Some rivers experienced major flooding in the middle of the week. 

This latest rainfall comes off the back of a notably wet winter and early spring period.

NSW experienced its third wettest winter on record and some towns have already doubled their monthly average rainfall for September, including Cobar, Narrabri and Brewarrina.

The Indian Ocean is largely to blame for this string of wet months, more specifically a negative phase of the Indian Ocean Dipole (IOD). 

The IOD is an index that represents a pattern of sea surface temperatures between Africa and Indonesia.

A negative IOD occurs when warmer-than-usual waters pool to the north-west of Australia and cooler waters sit off the east coast of Africa.

This temperature contrast enhances the amount of moisture and rainfall over parts of Australia at this time of year.

Conversely, a positive IOD represents the opposite pattern of sea surface temperatures and usually causes drier weather in Australia.

The current negative IOD event peaked in early July. This was also one of the main reasons Australia had its second wettest winter on record.

After weakening during late winter, the negative IOD has re-strengthened quite rapidly in recent weeks. It is currently at its strongest negative value since mid-July and once again close to the events record-breaking peak.

During the next week, another bout of heavy spring rainfall could cause flooding across parts of inland NSW. This is due largely to the re-strengthening of the negative IOD. 

Despite the recent spike, forecast models indicate the resurgence of the negative IOD was a blip in the overall weakening trend.

The IOD is expected to decline throughout the remainder of the spring, however it should continue to enhance rainfall across eastern Australia for the rest of the season.