Dark clouds gather in the skies of Valair near the Kerala-Tamil Nadu border in Palakkad in July. | Photo credit: KK Mustafa
La Niña, a phase of the El Niño Southern Oscillation (ENSO) occurs when the region of the Pacific Ocean between Indonesia and South America is cooler than normal. Its counterpart, El Niño, represents warming in the same region. These two phases significantly influence global atmospheric circulation and weather patterns. During La Nina years, India receives normal or above-normal rainfall during the monsoon season. Yet the same phenomenon causes drought in Africa and intensifying storms in the Atlantic Ocean. Conversely, El Niño brings extreme heat and drought to India while increasing rainfall in the southern United States.
La Niña was expected to appear in July this year, but it has not happened yet. The Indian Meteorological Department now expects La Nina to begin in late 2024 or early 2025, with mild winters due to this delay. The decade began with three consecutive La Niña events (2020–2022), a rare occurrence known as a Triple Deep La Niña, followed by an El Niño in 2023. Climate change may increase the frequency and intensity of La Niña and El Niño events, with rising ocean and land temperatures disrupting the balance of the Pacific Ocean. It can also amplify extreme La Nina events, which usually cause harsh winters in India.
Will La Niña occur this winter?
2024 is different; The US National Oceanic and Atmospheric Administration actually called it “frightening”. La Niña did not appear as expected. Historically, La Niña has usually formed during monsoon or pre-monsoon periods, and it has formed only twice between October and December since the 1950s. Global forecasts had also predicted the onset of this monsoon. But as of December, it remains only 57% likely to happen in 2024. If it still happens it will be weak but it could affect the global climate.
The onset of La Nina or El Niño can be announced based on several indicators. For example, the Oceanic Nino Index (ONI) compares the three-month average sea surface temperature in the east-central tropical Pacific with the 30-year average. When the difference between the two is 0.5°C or higher, it’s an El Niño, and when it’s -0.5°C or less, it’s a La Nina. Currently, it is around -0.3 degrees Celsius. To be classified as a full La Niña or El Niño, ONI values need to exceed the thresholds at least five times consecutively.
La Niña Winter Meteorology
Cities in southern India such as Bengaluru and Hyderabad are experiencing colder than normal winters this year, while north India is experiencing a late winter with above-normal temperatures. Some reports have linked southern Chile to La Niña, but current ONI values suggest otherwise. If La Nina had developed earlier, North India might have experienced colder than normal winters.
An analysis of more than 35 years of climate data by researchers at the Energy Environment and Water Council, New Delhi, found that La Nina winters have colder nights, warmer daytime temperatures than El Niños. Meteorological parameters such as wind speed and planetary boundary layer height (PBLH)—the lowest atmospheric layer directly affected by Earth-atmosphere interactions—also vary during ENSO phases, affecting air quality.
The team found that during La Niña winters, average wind speeds were higher throughout the day. Faster winds help reduce air pollution by blowing pollutants away. They also found that the average PBLH is slightly lower during La Niña winters.
If La Niña sets in, lower temperatures in northern India could cause people to burn more biomass to heat, worsening air pollution. A low PBLH can also pass more pollutants near the ground. But high wind speeds can disperse pollutants, potentially leading to better air quality.
La Niña and Monsoon
El Niño summers are relatively harsh, as was the case in April this year, when India experienced intense, record-breaking heat waves. If La Niña arrives and continues through the summer of 2025, it could provide relief from high temperatures. In addition, an El Niño often disrupts the monsoon, with India historically receiving below average rainfall in at least half of all El Niño years since 1871. But the same statistics also indicate patterns that have developed since the 1980s.
Both North and South India, for example, receive less rainfall during more intense El Niño events, while Central India is barely affected. On the other hand, a La Nina promotes a stronger monsoon as evidenced by “normal” or “above normal” rainfall in the La Nina years of 2020, 2021 and 2022. El Niño had “below normal” rainfall. The year of 2023.
So it would be a welcome development if La Nina forms now or early next year and continues till monsoon season. This means less intense heat and more rain for India.
Mohammad Rafiuddin is a program associate, and Shikhar Tiwari and Rishikesh P are consultants – all at the Council on Energy, Environment and Water (CEEW).
published – December 17, 2024 at 08:30 pm IST