Forecast

Issue Notes - Issued Jul 25

ENSO status: La Niña Watch. IOD status: Neutral. Negative possible. SAM status: Negative. The El Niño Southern Oscillation (ENSO) is in a La Niña Watch, meaning there is approximately a 50% chance of a La Niña developing this year. Some ocean cooling is occurring in the eastern Pacific, with 3 out of 7 global models forecasting a La Niña to develop in 2024. However, the Nino3.4 region is still warm from last years El Niño, meaning if La Niña were to develop, it would likely be declared no earlier than October. Warmer than normal waters are situated in the Coral Sea, providing moisture and humidity to the east coast. The atmosphere continues to be neutral. A La Niña typically increases rainfall over central Australia, Qld and NSW, but has little effect on southern WA, SA, Vic and Tas during winter. The Indian Ocean Dipole (IOD) is in a neutral phase. Ocean warming has been occurring near Indonesia, with far more modest warming near Africa, lowering the IOD index. Northerly winds have weakened off theAustralian continent, but cloud patterns are more inline with a positive phase. All this indicates that a negative IOD could be developing, but the atmosphere and oceans are currently decoupled. 4 out of 5 of the international models (all except BoMs model) are forecasting a negative IOD by October. A negative IOD increases the number of northwest cloudbands that cross Australia during winter and spring, increasing rainfall across central and southeastern parts of Australia. The Southern Annular Mode (SAM) is negative after a Sudden Stratospheric Warming event over Antartica in mid-July. This event has significantly altered the shape of the polar vortex, and directly leads to a negative SAM phase. Given the magnitude of this change, weather in Australia (and the rest of the southern hemisphere) will likely go through periods of wetter (trough) and drier (ridge) patterns every 2-4 weeks during winter and spring. A negative SAM increases the frequency of cold fronts. During the cool months, this increasesrainfall through southwest WA, SA, inland and southern NSW, Vic and Tas, and dries out eastern Qld and NSW. Rainfall outlooks are showing above average rainfall for southwest WA, southern SA, inland and southern NSW, Vic and Tas in August and September. From October, wetter than average conditions are expected for eastern Australia, and drier for inland parts of WA.