Forecast

Issue Notes - Issued Nov 18

ENSO status: La Niña Watch. IOD status: Neutral. SAM status: Positive. Trending positive. 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. The current technical definition of a La Niña is only a small chance to be met, although ocean and atmospheric conditions do resemble a La Niña pattern. Trade winds are still stronger than average along the western equatorial Pacific, and the Nino3.4 is low, but is above the threshold of -0.8C. However, the Relative-Nino Index (factors in equatorial ocean temperatures elsewhere around the world, compensating for the background warming signal) is below the threshold. A La Niña typically increases rainfall over northern and eastern Australia (especially eastern Qld) during summer, but has little effect for WA and SA. The Indian Ocean Dipole (IOD) is in a neutral phase. Recent conditions have been similar to a negative IOD, but this has likely been heavily affected by veryactive bursts of tropical weather in mid-October and mid-late November. Regardless if these conditions continue, the arrival of the monsoon over northern Australia overwrites any IOD effects, so this driver is unlikely to significantly affect Australia this year. The Southern Annular Mode (SAM) is in a positive phase, having switched from the negative phase that persisted from July to October. This positive event is expected to persist through summer as a result of stratospheric moisture from the 2022 Tongan volcanic eruption (similar to the past two summers). A positive SAM decreases the frequency of cold fronts, with more regular high pressure systems and easterly winds. During summer, this increases rainfall over eastern Australia, and reduces rainfall over southeastern SA, western Vic and western Tas. Rainfall outlooks are showing above average rainfall over western, northern and eastern Australia, with strong signals over the Kimberley, Top End and Cape York Peninsula (likely related to forecastmonsoon activity). Drier conditions are expected for western Tas, with average conditions for SA and western Vic.