Where is La Niña? And why did global models err in their predictions?

ENSO has three phases – warm (El Niño), cool (La Niña), and neutral — which occur in irregular cycles of two to seven years. La Niña last occurred in 2020-2023, and El Niño in 2023-24.

In the neutral phase, the eastern side of the Pacific Ocean (near the northwestern coast of South America) is cooler than the western side (near the Philippines and Indonesia). This is due to the prevailing wind systems that move from east to west, sweeping the warmer surface waters towards the Indonesian coast. The relatively cooler waters from below come up to replace the displaced water.

In the El Niño phase, these wind systems weaken, leading to lesser displacement of warmer waters off the South American coast. Consequently, the eastern Pacific becomes warmer than usual. The opposite happens in the La Niña phase — the trade winds become stronger than usual, and push larger quantities of water to the western Pacific.

In India, El Niño is associated with decreased rainfall and higher temperatures, while La Niña is associated with increased rainfall and hence lower temperatures.

What is the current status of the ocean? What were ENSO predictions this year?

According to the United States’ National Oceanic and Atmospheric Administration (NOAA), as of December 9, the equatorial sea surface temperatures were near-to-below average along the central and eastern Pacific Ocean. Thus, ENSO neutral conditions continued to prevail, as the La Niña ‘watch’ phase continued during November and early December.

Last week, the Oceanic Nino Index (ONI) in the Nino 3.4 region — one of the four major regions along the equatorial Pacific Ocean which is assessed to declare the ENSO phase — was minus 0.3 degrees Celsius.

The ONI is based on the temperature departures from average calculated of the three-monthly average sea surface temperature in the equatorial Pacific. Typically, a La Niña onset is declared when the ONI in this region touches minus 0.5 degrees or below.

This year, the weather models, based on the collected ocean signals, had suggested the likely emergence of La Niña during August-September. These projections were later updated to predict that La Niña would emerge in October-December. The latest projections, however, now say that a short and weak La Niña would emerge between December and February.

Experts said that even if the ONI values touch the requisite threshold value, the temperatures would return to normal shortly after, making the La Niña episode a brief and mild one. The weak La Niña could transition to ENSO-neutral by March-May 2025, experts say. This means that La Niña is likely to leave a minimal impact on the Indian winter this year.

Why have La Niña predictions missed the mark this year?

Generally, the accuracy of weather models is higher in cases of pronounced changes in the sea surface temperatures, that is, when there is likely to be a strong El Niño or La Niña. This is unlikely to be the case this time around, which has likely led to weather models not getting things right, as they are unable to factor in minute temperature variations in their inputs.

Besides, some other climatological reasons might also be playing a role in the models being off the mark.

1. 01

   Ocean-atmosphere coupling

   This year, the interaction between the ocean and atmosphere was not as expected —  leading to temperatures along the equatorial Pacific Ocean remaining warm or just near normal. This might be because El Niño conditions continued into 2024, meaning that its impact continued to play out in terms of ocean temperatures. The coupled ocean-atmosphere system reflected ENSO-neutral conditions.

2. 02

   Westerly wind anomalies

   Atmosphere has a strong relation in maintaining and controlling sea surface temperatures. During September-October, when there were chances of a transition into La Niña phase, the westerly wind anomalies were dominant. Climatologically, westerly wind anomalies are unfavourable for the development of La Niña.

3. 03

   Monsoons and ENSO

   As the El Niño phase ended and the ENSO neutral phase coincided with the onset of the Indian Summer monsoon, June-September this year saw bountiful and above normal rainfall over India. Monsoon and ENSO interact with each other. A good monsoon can influence the westerly wind anomalies, which in turn can delay/affect the onset of La Niña.