12-Month Rainfall Forecast

Issue Notes

ENSO status: Neutral. Weak La Niña possible IOD status: Negative. Moderate strength. SAM status: Neutral. Varying between phases. The El Niño Southern Oscillation (ENSO) is in a neutral pattern, leaning slightly towards a La Niña pattern. With the BoM switch to using the Relative-Nino (RNino) index system (subtracts the Nino value from the remaining global equatorial average SST's, therefore accounts for climate change), there is now a higher chance of a La Niña event being declared this year. The overall pattern is very similar to last year (retroactively declared La Niña), and therefore impacts during late spring and summer would be expected to be similar. A La Niña typically increases rainfall over northern and eastern Australia during spring and summer. Weak La Ninas in particular are also correlated with higher tropical cyclone activity. The Indian Ocean Dipole (IOD) is officially in a negative phase. Over the last few weeks, the negative IOD has maintained its intensity as a moderate strengthevent. The atmosphere is showing strong coupling over the equator, but is not necessarily flowing down to Australia. However, tropical activity is expected to help this process move over Australia towards the end of October. This negative event is expected to run throughout spring, and breakdown at the natural end-point of IOD events: The arrival of the Northern Australian Monsoon, most likely in mid-December. A negative IOD typically increases the amount of northwest cloudbands that cross Australia, increasing rainfall from northwest WA through to southeastern Australia during spring. The Southern Annular Mode (SAM) is in a neutral phase, and has been fluctuating between positive and negative phases over the last few weeks. Without significant forcing, the SAM is expected to continue to fluctuate for much of the rest of spring, with a slight favouritism towards a negative event in October and positive in November. A negative SAM during spring typically reduces rainfall over NSW and southern Qld, andincreases rainfall over southwest WA, SA, Vic, and Tas, with positive event providing the opposite effect. Rainfall outlooks indicate wetter than average conditions across the northern and eastern Australia over the next several months, with above average conditions extending to SA during spring. Drier than normal conditions are expected for western and southern Tas during spring, with average conditions elsewhere.