The temperatures in the equatorial Pacific Ocean have just turned in a direction that suggests that the winter is likely to be colder than usual. Here is how.
We are now past the September or autumnal equinox, which fell on September 22 this year. This means that the sun is now south of the equator and temperatures in the northern hemisphere will now fall. How will temperatures behave as we move toward winter? There is no forecast for winter temperatures yet, which begins officially in India only in December. However, there is a forecast for another factor that has a big impact on winter temperatures in India: the temperatures in the equatorial Pacific Ocean. They have just turned in a direction that suggests that the winter is likely to be colder than usual, but the answer is not definitive yet. Here is how.
New Delhi, India-January 19, 2024: People are seen wearing warm clothes while going to work on a cold winter morning at South West Delhi near Kapashera, in New Delhi, India, on Friday, 19 January 2024. (Photo by Parveen Kumar/Hindustan Times)
While several factors affect the winter season in India, temperature deviations in the equatorial Pacific Ocean are often correlated with winter temperatures. A negative deviation of 0.5°C or more – called a La Nina – generally leads to a colder winter. The opposite of this – a positive deviation of 0.5°C or more, which is called El Nino – leads to a warmer winter than usual. When the deviation is under 0.5°C, the Pacific’s condition is considered neutral, which does not have a strong impact on winter temperatures.
So, what is the status of the Pacific right now? The El Nino-La Nina phenomena are usually tracked by deviations in a particular part of the Pacific: the Nino 3.4 region (5°N-5°S, 120°W-170°W). After staying in the neutral state since May, the deviation in this region reached -0.5°C for the first time in the week ending September 18, the latest week for which this data is available from the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration (NOAA), a US government agency. However, a single week’s deviation is not enough to declare a La Nina. The phenomenon is declared only when the deviations hold for a consistent period, such that the three-month running mean of this deviation is -0.5°C for five consecutive months.
The trends seen so far mean that La Nina may be developing now, but one must wait for the trend to hold for a definitive answer. However, probabilities of its development are available even now. For example, the September-November season has an equal chance (50% probability) of both neutral and La Nina conditions, according to the model-based forecast of the International Research Institute for Climate and Society (IRI) at the Columbia Climate School issued on September 19. La Nina is, however, the more probable event as we move towards winter. The probability of its occurrence in the IRI forecast is 60% for October-December, 62% for November-January, and 57% for December-February, the official winter season in India.
What do the IRI forecasts mean for winter in India? Since it takes time for sea surface temperatures to impact the temperature of the air (or what we commonly know as just temperature), it appears that the probability of a colder-than-usual winter is more than that for a normal winter. This is because La Nina would already be underway for a while when the peak winter season begins in India.
The IRI’s forecast, however, needs to be read with caution. It is a forecast based completely on multiple models, which are run at different times and have their own biases. The IRI model-based forecast does not adjust for these biases or the fact that they took inputs of current atmospheric conditions at different times. The CPC official forecast corrects for these biases. As the accompanying chart shows, the CPC forecast issued on September 12 has a higher probability of a La Nina. It also predicts the event will last a little longer, although much shorter than the triple-year La Nina that lasted from 2020 to 2022.
The higher probability of La Nina in the CPC forecast, however, does not guarantee a necessarily harsh winter. La Nina’s impact on winter also depends on its strength, with stronger La Nina events making winter colder. This is why one must watch the trends in the Nino 3.4 region for some time to get a fair idea of how the winter may turn out.
Abhishek Jha, HT’s assistant editor-data, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.
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