12-Month Rainfall Forecast

Issue Notes

ENSO status: Weak La Niña possible. IOD status: Strongly Negative. SAM status: Negative. The El Niño Southern Oscillation (ENSO) is leaning towards a weak La Niña pattern. Both the atmosphere (SOI) and ocean (RNino3.4) indices have recently both tipped into La Niña zones, but need to be sustained for at least three months for an official declaration. If a weak La Niña does develop, it is expected to slowly strengthen into December, before starting to weaken in January. 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 in a strong negative phase. Weekly values have now fallen to their lowest on record (data since 2008), with the IOD West showing further cooling and IOD East continuing to warm. This event may become the strongest event since 1906 (based on the monthly IOD). 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, but possibly earlier with the development of tropical cyclones). 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 negative phase, following a Sudden Stratospheric Warming (SSW) event over Antartica in September. While that event is now weakening, another SSW is forecast to occur early in November, potentially contradicting forecasts that a positive SAM should be developing in Novemeber. A negative SAM during spring typically reduces rainfall over NSW and southern Qld, and increases 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, drierconditions over southern WA, and near normal rainfall elsewhere. Note that this forecast does not factor in another SSW event. In that event, similar conditions to October, with wetter conditions across the south and drier conditions in the east would dominate.