Nino Indices Tumble Yet Again: SOI Drops Too

In the last weekly post, Skymet deliberated extensively on the La Niña conditions in the tropical Pacific. A consistent drop in the sea surface temperatures in January led to the strengthening of La Niña conditions. Accordingly, in the last article, the Oceanic Nino Index (ONI) for the quarter Nov-Dec-Jan was asserted to have breached the threshold temperature anomaly of -0.5°C and hence the commencement of La Niña, albeit as a borderline case. The ONI for the last quarter was listed as tentative, as it needed statutory confirmation by the Climate Prediction Center (CPC). The forecasting arm of NOAA has now confirmed the ONI reaching the requisite mark of -0.5°C for the first time this season and therefore, kicking off the much-anticipated La Niña event.

The worries are not yet over. The temperature anomaly in the Nino 3.4 region, the marker index for ONI, has shown a consistent drop over the last three weeks. The index had earlier reached a moderate scale of -1.1°C on 30 Dec 2024. However, the latest value for the week ending 10 Feb 2025 was -0.6°C. Sustained and adequate cooling in the Nino region is very essential for the endurance of the cool phase of ENSO. Continuation of borderline temperature anomalies, if any, is not without the fear of early collapse of the event.

Nino indices have been unsteady across the central and eastern tropical Pacific. The eastern flank of the Nino region is more responsive to the cool phase, and Nino 3 & Nino 1+2, closer to the coast, are near neutral. Also, there is a disparity between the dynamical and statistical models. The statistical models are inclined to borderline La Niña, even during the Indian Summer Monsoon 2025, but the dynamical models get terminated and turn ENSO neutral during the spring barrier itself.

MJO: A robust Madden-Julian Oscillation event continues. The MJO pulse depicts a high-amplitude signal with consistent eastward propagation. While the intra-seasonal signal is expected to sail from the Western Pacific to the Western Hemisphere & Africa in Phase 8, it may destructively interfere with the La Niña base state. The propagation may become slow and also shrink in amplitude during the last week of February.

Cooler-than-average sea surface water in the east-central equatorial Pacific exhibits characteristic changes during La Niña events. The atmosphere has been looking La Niña-ish for a few months with stronger-than-average trade winds, more clouds and rain over Indonesia, and drier conditions in the central Pacific. These two features are hallmarks of Walker Circulation. The stronger the circulation, the brighter the chances of evolving La Niña.