If you reside in southeastern Australia, you've likely observed a noticeable lack of rainfall. This isn't merely a perception; the rainfall in August fell below the usual averages, mainly affecting regions such as Victoria, New South Wales, most of Tasmania, South Australia, and Queensland. 

Turning our attention to September, it has received considerably less rainfall—between 50 and 100mm below the typical amounts for this time of year in Victoria, New South Wales, and Tasmania. In some areas, this deficit ranges from 100 to 200mm, as depicted by the dark orange hues on the map. 

Image: Rainfall Anomalies (mm) 1st to 22nd September 2023, Source: BOM

But why is this occurring?

Currently, an El Niño event is in progress, and model forecasts suggest a sustained warming of the central to eastern Pacific. Typically, El Niño leads to reduced spring rainfall in eastern Australia. Simultaneously, a positive Indian Ocean Dipole (IOD) is also in effect, contributing to decreased spring rainfall in central and southeast Australia.

When a positive IOD and El Niño coincide, their drying influence tends to be more pronounced and widespread throughout Australia.

Adding to these factors, the Southern Annular Mode (SAM) index is presently in a negative phase. During spring, a negative SAM is associated with reduced rainfall in parts of eastern New South Wales (NSW) and Victoria.

What is expected for the next few months?

Climate models suggest that this El Niño is likely to persist, with sea surface temperatures expected to remain above El Niño thresholds until early 2024. The positive IOD pattern is anticipated to persist until the end of spring. Only the SAM is expected to return to a neutral state by late September, before returning to a negative phase in October.

Image: Average of international model forecasts for NINO3.4 and IOD, Source: BOM 

Image: Southern Annular Mode (SAM) daily index, Source: BOM

In summary, southeastern Australia's rainfall deficit is expected to linger, with El Niño and a positive IOD combined. Their drying impact will intensify and extend across much of southern and eastern Australia from October to December.