Forecast

Issue Notes - Issued Sep 9

ENSO status: La Niña Watch. IOD status: Neutral. SAM status: Neutral. Favouring 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. Overall, models are now starting to look more like La Niña, with more widespread ocean cooling occurring in the South Pacific, and the 30-day SOI now above +7. While these are good signs that La Nina is starting to develop, a declaration would likely happen no earlier than November, meaning this is more likely to be a late and weaker event if it does eventuate. A La Niña typically increases rainfall over northern and southeastern Australia during spring, but has little effect for southwestern WA. The Indian Ocean Dipole (IOD) is in a neutral phase and is now unlikely to develop into either phase this spring. Conditions are closer to a positive IOD than to a negative phase, so there may be some minor reductions in the frequency of northwest cloudbands impacting Australia. A neutral IOD tends to bring a small increase in spring rainfall for southwest WA, but has little effect elsewhere. The Southern Annular Mode (SAM) is now neutral, having recovered from the initial sudden Stratospheric Warming (SW) event over Antarctica in mid-July. However, another SW event is occurring in mid September that may further dampen the polar vortex. These events increase the chances of negative SAM events during September and October. A negative SAM increases the frequency of cold fronts. During spring, this increases rainfall over western Tas and southern Vic, but decreases rainfall over eastern Australia. Rainfall outlooks are showing close to average for most of the country during September and October, with a higher chance of above average rainfall for northern and eastern Australia from November. Models are also hinting that rainfall is more likely to arrive in the form of showers and thunderstorms, rather than broad rainbands, meaning rainfall in practice will likely be hit-and-missfor most of Australia this spring.