12-Month Rainfall Forecast

Issue Notes

ENSO status: Weak La Niña declared. IOD status: Negative, weakening. SAM status: Persistent Strongly Negative. The El Niño Southern Oscillation (ENSO) is now officially in a weak La Niña pattern. Both the atmosphere (SOI) and ocean (RNino3.4) indices are into La Niña zones, and have been sustained for enough time for a declaration. This La Niña event is expected to strength a little into December, then start to weaken from January, and is predicted to end in March (depending on tropical activity). A La Niña typically increases rainfall over northern and eastern Australia during spring and summer. Weak La Niñas in particular are also correlated with higher tropical cyclone activity. The Indian Ocean Dipole (IOD) is in negative phase, but is starting to weaken. Recent tropical activity over Australia has severely weakened the IOD pattern, and it is possible that this event is nearly over. If it lingers for longer, it is less likely to have an impact moving into December, and will definitely terminatewith the arrival of the first monsoon onset of the season. 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 strong negative phase, following a Sudden Stratospheric Warming (SSW) event over Antartica in September and another in early November. Models are now firmly in favour of keeping a strong negative SAM through December and into January. In line with La Niña and the seasonal disconnect of the stratospheres effect on the troposphere over Antartica, they expect a change into a more positive SAM pattern in February. 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 southern Australia, in northern Australia and Qld (mainly fromJanuary), below average in NSW until February, and average elsewhere.