LA NINA
1. Context
While the La Niña was expected to emerge by July this year, it is yet to. The India Meteorological Department now expects a La Niña to set in by late 2024 or early 2025, plus a milder winter due to this delay.
2. What is La Nina?
- La Niña, one phase of the El Niño Southern Oscillation (ENSO), occurs when the Pacific Ocean between Indonesia and South America experiences cooler-than-normal temperatures. In contrast, El Niño marks the warming of this same region.
- Both phases have a profound effect on global atmospheric circulation and weather conditions.
- During La Niña years, India typically experiences normal or above-average monsoon rainfall, while Africa often faces droughts, and hurricane activity in the Atlantic intensifies.
- On the other hand, El Niño tends to bring extreme heat and droughts to India, while increasing rainfall in the southern United States.
- The current decade began with an unusual sequence of three consecutive La Niña events from 2020 to 2022, known as a Triple Dip La Niña, which was followed by an El Niño in 2023.
- Climate change could lead to more frequent and intense La Niña and El Niño events as warming sea and land temperatures destabilize the Pacific's balance.
- This may also lead to more severe La Niña occurrences, which are often associated with harsh winters in India
3. La Nina and Winter
- The year 2024 stands out as La Niña has not developed as anticipated. Typically, La Niña forms during the monsoon or pre-monsoon seasons, and since 1950, it has only occurred twice between October and December.
- While global forecasts had predicted its emergence during this monsoon season, as of December, there is only a 57% chance of it forming in 2024. Even if it does develop, it is expected to remain weak, though it could still impact global weather patterns.
- The declaration of a La Niña or El Niño event relies on several indices. One key measure is the Oceanic Niño Index (ONI), which compares the three-month average sea surface temperatures in the East-Central Tropical Pacific with the 30-year average.
- If the difference is +0.5ºC or higher, it signifies El Niño, while a difference of –0.5ºC or lower indicates La Niña. At present, the value is approximately –0.3ºC.
- For an event to be officially classified as a full-fledged La Niña or El Niño, the ONI values must cross the respective thresholds for at least five consecutive months
4. What is the meteorology?
- Southern Indian cities like Bengaluru and Hyderabad are experiencing an unusually cold winter this year, whereas northern India is seeing a delayed winter with above-average temperatures. Some reports attribute the colder conditions in the south to La Niña; however, current ONI values do not support this claim. If La Niña had already developed, northern India would likely be experiencing a colder-than-usual winter as well.
- A 35-year analysis of meteorological data conducted by researchers at the Council on Energy, Environment, and Water in New Delhi shows that while La Niña winters typically bring colder nights compared to El Niño, daytime temperatures tend to be higher.
- Key meteorological factors such as wind speed and the planetary boundary layer height (PBLH) — the lowest part of the atmosphere influenced by interactions between land and air — also change during ENSO phases, which can affect air quality.
- The study found that during La Niña winters, average wind speeds are higher throughout the day. Stronger winds help reduce air pollution by dispersing pollutants. Additionally, the researchers observed that PBLH tends to be slightly lower during La Niña winters.
- If La Niña develops, colder temperatures in northern India could lead to increased biomass burning for heating, exacerbating air pollution.
- However, while a lower PBLH might trap pollutants closer to the ground, stronger winds could help disperse them, potentially improving air quality
5. La Nina and Monsoon
- Summers during El Niño years tend to be particularly severe, as seen in April this year when India experienced intense and record-breaking heat waves. If La Niña develops and persists into the summer of 2025, it could provide some relief from extreme heat.
- El Niño is also known to disrupt monsoons, with India recording below-average rainfall in at least half of all El Niño years since 1871. However, data since 1980 suggests some changing patterns.
- For instance, both northern and southern India have experienced reduced rainfall during stronger El Niño events, while central India has remained relatively unaffected.
- In contrast, La Niña supports stronger monsoons, as demonstrated by the “normal” to “above-normal” rainfall recorded in the La Niña years of 2020, 2021, and 2022. By comparison, the El Niño year of 2023 saw “below-normal” rainfall.
- Therefore, the formation of a La Niña now or in early 2025, persisting through the monsoon season, would be a positive development
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For Prelims: El Niño, La Niña, and ENSO
For Mains: GS I - World Geography
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Source: The Hindu
