El Nino

Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 4 "Climate"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

El Nino is the warm phase of a repeating climate swing in the equatorial Pacific Ocean. Every 3–7 years, warm water from the central Pacific moves east to the coast of Peru and Ecuador and replaces the cold Peru current. It usually appears around Christmas. Its ocean part, the change in sea-surface temperature, pairs with an air-pressure swing called the Southern Oscillation, and together they are called ENSO (El Nino–Southern Oscillation) [3].

It matters because it changes rain and drought patterns far beyond the Pacific. It often weakens the Indian south-west monsoon, so IMD watches it closely when it makes its seasonal forecast.

Explanation

Normal year: the setting El Nino disturbs

• Trade winds push warm water west. Trade winds are steady easterly winds that blow from the subtropical high-pressure belts towards the equator.
• They drag warm surface water from east to west across the equatorial Pacific.

• This water piles up near Indonesia and northern Australia as the Western Pacific Warm Pool. Here the sea level is slightly higher and the warm layer of water is deeper.

• Cold water rises off Peru.

• Warm surface water moves away from the South American coast, and cold deep water rises to take its place. This rising of deep water is called upwelling.

• This cold water is part of the Humboldt (Peru) current. It is full of nutrients, which feed plankton, and the plankton feed huge shoals of anchovy.

• Walker circulation. This is an east–west loop of air along the equatorial Pacific.

• Air rises over the warm west Pacific and Indonesia. This gives low pressure at the surface and heavy rain.
• High in the sky the air flows east. Over the cool east Pacific off Peru it sinks, giving high pressure and dry weather.
• Near the surface it flows back west as the trade winds.
• Jacob Bjerknes named it in 1969 after Sir Gilbert Walker of the India Meteorological Department (IMD).

How El Nino forms (cause → process → result)

• Cause: the trade winds weaken.
• Less warm water is held back in the west.

• The warm water slides east towards South America.

• Process: the air loop shifts.

• The Walker circulation weakens or moves east.

• The zone of rising air and heavy rain moves from Indonesia towards the central and eastern Pacific.

• Result: the ocean off Peru changes.

• Warm water covers the Peru coast, so upwelling stops.
• Nutrients run out, plankton decline and the anchovy fisheries collapse.
• The normally dry Peru coast gets heavy rain and floods. Indonesia and Australia get sinking air, drought and bushfires.

The air side: Southern Oscillation and SOI

• Southern Oscillation: a see-saw of surface air pressure between the eastern Pacific and the Indian Ocean–Australia region. When pressure rises on one side, it falls on the other.
• Southern Oscillation Index (SOI) = pressure at Tahiti (central-south Pacific) minus pressure at Darwin (northern Australia).

• During El Nino, pressure falls at Tahiti and rises at Darwin, so the SOI becomes strongly negative.

• Bjerknes (1969) showed that El Nino (ocean) and the Southern Oscillation (air) are two sides of one event, called ENSO.

• Gilbert Walker found the Southern Oscillation in the 1920s while looking for ways to forecast the Indian monsoon.

How an event is declared, and its types

• Nino 3.4 region: a box in the central-east equatorial Pacific at 5°N–5°S, 120°–170°W [2].
• Oceanic Nino Index (ONI): NOAA's main measure. It is the three-month running average of the sea-surface temperature anomaly (how far the temperature is from normal) in Nino 3.4.
• An anomaly of +0.5 °C or more means El Nino conditions [2].
• NOAA counts it as an official event only if the anomaly stays past this limit for five overlapping three-month periods in a row [2].

• Other agencies, such as the Australian Bureau of Meteorology and IMD, use their own rules. So they may declare the same event at different times.

• El Nino Modoki ("similar but different" in Japanese), described by Ashok and others (2007):

• The warmest water sits in the central Pacific, with cooler water on both the west and east sides.
• This makes two Walker loops instead of one.
• It can hurt the Indian monsoon even when it is "weak" by the Nino 3.4 measure.

Where it occurs

• Equatorial Pacific Ocean: the widest stretch of ocean along the equator. It runs from Indonesia and Australia in the west to Peru and Ecuador in the east.
• Peru and Ecuador (west coast of South America): warm water arrives, the anchovy fishery collapses, and the desert coast gets heavy rain and floods.
• Indonesia and Australia: drought and bushfires.
• China: floods.
• India: the monsoon is often weak, and there is sometimes drought.
• 1997–98: India's monsoon stayed normal despite a strong El Nino, helped by a positive Indian Ocean Dipole (IOD). The IOD is a difference in sea-surface temperature between the western and eastern Indian Ocean. A positive IOD (warmer west, near Africa) supports the monsoon.
• 2026: In April, IMD forecast June–September monsoon rain at 92% of the Long Period Average (LPA) (±5%), which is "below normal". It expected El Nino to develop during the season [6]. In May it lowered the forecast to 90% of LPA (±4%) [7]. The LPA is about 87 cm, the average all-India monsoon rain for 1971–2020 [6].

• On 30 September 2026, IMD reported strong El Nino conditions that were still strengthening, while the IOD was neutral. October rainfall was forecast to be below normal (<85% of LPA) [8].

• Notable El Nino events: 1982–83 (one of the strongest of the 20th century), 1997–98, 2015–16 (among the strongest on record) and 2023–24. The 2026 El Nino was "firmly established" by mid-August 2026. WMO expected it to become very strong before peaking near the end of 2026 [5].

Don't confuse with

• La Nina ("the little girl"): the cold phase and the opposite of El Nino. It brings stronger trade winds, a cooler east Pacific, stronger upwelling, a positive SOI, and usually a good Indian monsoon.
• Southern Oscillation: the air-pressure see-saw between Tahiti and Darwin. El Nino is the ocean (sea-surface temperature) part. ENSO is the two together.
• El Nino Modoki: the warmest water is in the central Pacific with cooler water on both sides. In a normal El Nino the warming spreads to the east Pacific off Peru.
• Indian Ocean Dipole (IOD): a temperature difference across the Indian Ocean, not the Pacific. A positive IOD can cancel out El Nino's effect on India, as in 1997.

Prelims Hooks

• SOI = Tahiti pressure minus Darwin pressure. It is strongly negative in El Nino. A common trap is to reverse the order of the two stations.
• During El Nino the arid Peru coast gets heavy rain, while Australia and Indonesia face drought. Saying "El Nino brings drought to Peru" is a trap.
• El Nino weakens the cold Humboldt (Peru) current and stops upwelling. This causes the collapse of the anchovy fishery.
• Nino 3.4 region: 5°N–5°S, 120°–170°W. NOAA's ONI limit is +0.5 °C, held for five overlapping three-month periods in a row [2].
• Walker circulation: air rises over Indonesia and sinks off Peru. It was named by Bjerknes (1969) after Sir Gilbert Walker of IMD.
• Strong El Ninos: 1982–83, 1997–98, 2015–16, 2023–24. In 1997 a positive IOD kept India's monsoon normal, so El Nino does not always mean drought in India.

Mains Points

• How a Pacific event reaches India (GS-I)
• Weak trade winds make the Walker circulation weaken or move east.
• Rising air moves away from the Indonesia–Indian Ocean side, and sinking air becomes stronger over South Asia.

• Less moisture reaches the south-west monsoon, which raises the risk of drought. The result also depends on the IOD (1997) and on where the warming sits (Modoki).

• Effects on the economy and disaster management (GS-III)

• A weak monsoon reduces kharif sowing, reservoir storage and hydropower, and pushes up food prices.
• El Nino also raises global temperatures. In mid-2026 WMO warned of heat stress and faster-developing droughts and urged countries to prepare [5].

• Policy responses include ENSO-linked long-range forecasts (IMD's MME and MMCFS models [8]), contingency crop plans, drought declaration rules and heat action plans.

• Changing ENSO behaviour makes forecasting harder

• The triple-dip La Nina (2020–23) gave way to a strong El Nino in 2023–24. Then a weak La Nina in early 2026 [4] quickly turned into a very strong El Nino by late 2026 [5].
• So India cannot forecast its monsoon from the Pacific alone. The Pacific, the IOD and Modoki patterns need to be watched together.

Related concepts

• Walker circulation
• Southern Oscillation
• ENSO
• Nino 3.4 region
• El Nino Modoki

Read more

• The circulation and the oceans: Walker circulation, El Nino and ENSO