Why global models went wrong with their La Niña predictions — and what its delay means

Occurring in irregular cycles of anywhere between two to seven years, ENSO has three phases — warm (El Niño or ‘The Little Boy’ in Spanish), cool (La Niña), and neutral.

During the neutral phase, the eastern Pacific (off the northwestern coast of South America) is cooler than the western Pacific (around Philippines and Indonesia). This is because prevailing trade winds — caused by Earth’s rotation, between 30 degrees north and south of the equator — move east to west, sweeping warmer surface water along with them. The relatively cool waters from below rise to the surface to replace the displaced water.

Phases of ENSO.

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

In India, El Niño is associated with suppressed rainfall during the monsoon season, while La Niña is known to help monsoon activity. The previous El Niño event occurred between June 2023 and May 2024. Prior to that, one of the longest recorded La Niña episodes lasted from 2020 to 2023.

Anthropogenic climate change has exacerbated the impact of both El Niño and La Niña-related hazards, including high temperatures, heavy precipitation and droughts

What did global weather models predict this year?

One of the strongest El Niño events of all time ended this June, following which ENSO entered the neutral phase. Initial projections made by multiple global weather models suggested that the onset of La Niña conditions would occur around July. But by mid-July, it was clear that La Niña would be delayed.

At the time, the US-based National Oceanic and Atmospheric Administration (NOAA) had said that the transition from neutral to positive sea surface temperature, suggesting ENSO neutral giving way for La Niña, would take place between August and October. The Bureau of Meteorology (BoM), Australia, too, maintained the La Niña ‘watch’ stage in July, predicting the emergence of cooler-than-usual sea surface conditions in the latter half of this year.

The India Meteorological Department (IMD) had, since the first stage Long Range Forecast issued in mid-April, maintained that La Niña conditions would emerge during the later half of the June – September monsoon season. And most importantly, La Niña could enhance rainfall in the months of August and September. This year’s seasonal prediction was dependent on the emergence of La Niña, resulting in ‘above’ normal rainfall in the final two monsoon months (more on that later).

Why were initial predictions off the mark?

There is no single reason behind the predictions being off the mark. Moreover, while almost all predictions got the onset timing wrong, they were largely accurate about the severity of the upcoming La Niña, expected to be fairly weak this time around.

In fact, a primary reason cited by weather modellers and meteorologists for the error in predicting La Niña’s onset is its severity. Weather models are likely to better pick up signals in cases of strong La Niña (or El Niño) phases, rather than weak ones.

Moreover, there are other factors that impact surface and subsurface conditions across the Pacific Ocean. In particular, these include the inter-seasonal variability in the atmosphere, wind and pressure, directly linked to the movement of the Madden Julian Oscillation (MJO), an eastward moving band of rain-bearing winds and clouds. The interplay of different weather systems makes predictions harder.

Currently, ENSO-neutral conditions continue to prevail in the Pacific Ocean, as per NOAA. The latest predictions suggest that the earliest signs of the onset of La Niña will likely emerge by the end of September or in early October. La Niña would peak in November, and is expected to last throughout the winter in the northern hemisphere.

How has the delay in La Niña’s onset impacted India’s monsoon?

La Niña is known to enhance the rainfall during the southwest monsoon, India’s chief rainy season which lasts from June to September. Since the monsoon has already reached its fag end, and La Niña conditions are yet to emerge in the equatorial Pacific Ocean, the climatic phenomenon will play no direct role in the country’s rainfall at the moment.

But, there are many other factors which affect the rainfall during the monsoon season, and a delayed La Niña onset does not definitely mean an adverse monsoon performance. This has largely been the case so far.

In August, India as a whole recorded nearly 16 per cent surplus rainfall. The forecast for September too is promising, with the IMD predicting a 109 per cent ‘above’ normal rainfall over most geographical areas (where 100 denotes normal rainfall).

Overall in this season, starting from June, India has received rainfall that is 8 per cent more than normal. But, as is expected, there are wide variations in precipitation at the regional level. Almost every state in the east and the northeastern region has seen deficient rainfall, and so have many states in the north and the northwest. Central and south India have, however, received more than normal rainfall till now.

What does the La Niña outlook look like in the coming months?

Supposing La Niña’s onset begins by late September-October, it could still influence India’s weather later in the year.

Notably, it could influence rainfall during the northeast monsoon season (October – December). Also known as the winter monsoon, this is limited to Tamil Nadu, coastal Andhra Pradesh, Rayalaseema, southern interior Karnataka and Kerala. Climatologically, La Niña does not favour northeast monsoon rainfall. That said, there have been exceptions in the past. Moreover, since all these meteorological subdivisions have received either normal or surplus rainfall during the ongoing southwest monsoon, the possibility of lower rainfall later in the year should not be a worry at present.

The north Indian Ocean basin — which covers the Bay of Bengal and the Arabian Sea — sees cyclone development during March to May, and October to December, with peak activity during May and November. During La Niña years, climatologically there are high chances of frequent cyclogenesis, with storms having above-average intensity, and longer-than-average life. This will be something that Met officials and administrators will be watching out for.

Lastly, La Niña years in the past have typically seen harsher, colder winters.

Anjali Marar is a science communicator at Raman Research Institute.