In a surprise, the month of September and especially October (so far) have averaged drier than expected considering the normally wet bias Australian climate produces when Indian Ocean dipole (IOD) is negative and La Nina is developing. The dry and increasingly warmer than normal pattern, especially in the East (and more recently across the North) is due to negative phase of the southern annular mode (-SAM) or negative Antarctic oscillation (-AAO) in that anomalous high pressure covers Antarctica forcing strong mid-latitude upper troughs, one of which is south of Southeast Australia.

A weak La Nina is forecast for Q4/2025 shifting to neutral phase during early 2026 and possibly to El Nino during the middle of next year. A strong ENSO episode within the next 12 months is not expected although forecast confidence in a phase change is reasonable.

Important as a review, is the influence on summertime rainfall patterns in South America by a semi-permanent upper-level low pressure trough located off the Southeast coast of Brazil for most years in this decade. The just-issued ECMWF “monthlies” are forecasting the presence of this low-pressure system for early-to-middle summer.

Surprising is overall success of the GFS ENS skill score of forecasting the upper air pattern (using 500 MB anomalies) during the 16-30-day period across both North and South America for the past 30-day, 14-day, and 7-day intervals. Traditionally, the ECM “weeklies” are used to make a “first guess” at the expected weather during this timeframe. ECM rivaled GFS ENS for North America forecasts and finished second to GFS ENS in South America.