ENSO

Also called: El Nino Southern Oscillation · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

ENSO (El Nino–Southern Oscillation) is a repeating swing across the equatorial Pacific Ocean in two linked things: sea-surface temperature, which is the ocean part (El Nino and its opposite, La Nina), and surface air pressure, which is the air part (the Southern Oscillation) [3]. It matters because one change in the Pacific shifts rain and drought around the world, including India's monsoon. Forecasters such as IMD therefore use it for long-range (seasonal) forecasting.

Explanation

The normal (neutral) year: the Walker circulation

• The trade winds push warm water west
• The trade winds (steady easterly winds blowing from the subtropical high-pressure belts towards the equator) blow from east to west across the equatorial Pacific.

• They drag warm surface water west. It piles up near Indonesia and northern Australia and forms 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 water moves away from South America. Cold, deep water rises to take its place. This is upwelling.

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

• The Walker circulation (an east–west loop of air along the equatorial Pacific)

• Over the warm west Pacific, air heats up and rises. Clouds form and heavy rain falls, and low pressure forms at the surface.
• High in the sky, this air flows east.
• Over the cool east Pacific, the air sinks. This gives high pressure and dry weather. It is one reason the Peru–Chile coast is a desert.
• Near the surface, the air flows back west as the trade winds, and the loop is complete.
• Jacob Bjerknes gave the loop this name in 1969, after Sir Gilbert Walker of the India Meteorological Department (IMD).

The warm phase: El Nino

• Every 3–7 years, warm water drifts east to the South American coast and replaces the cold Peru current. It shows up around Christmas, so Peruvian fishermen called it El Nino ("the Christ child").
• The chain of events
• The trade winds weaken, so less warm water is held in the west and it slides east.
• The Walker circulation weakens or moves east. Rising air and heavy rain move from Indonesia towards the central and eastern Pacific.
• Warm water covers the Peru coast, so upwelling stops. Nutrients run out, plankton decline and the anchovy fishery collapses.

The air side: the Southern Oscillation and the 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.
• Gilbert Walker found it in the 1920s while he was looking for ways to forecast the Indian monsoon.
• Southern Oscillation Index (SOI) = Tahiti pressure minus Darwin pressure
• El Nino: pressure falls at Tahiti and rises at Darwin, so the SOI becomes strongly negative.

• La Nina: pressure is high at Tahiti and low at Darwin, so the SOI becomes positive.

• In 1969 Bjerknes showed that El Nino (ocean) and the Southern Oscillation (air) are two sides of one event. That joined event is ENSO.

Phases, types and how events are declared

Feature	El Nino	Neutral	La Nina
Trade winds	Weak	Normal	Strong
East Pacific sea surface	Warm	Cool	Very cool
Walker circulation	Weak or moved east	Normal	Strong
SOI (Tahiti – Darwin)	Strongly negative	Near zero	Positive
Peru upwelling and fishery	Stops; fishery collapses	Normal	Stronger
Usual effect on Indian monsoon	Weaker; drought risk	Normal	Good monsoon

• La Nina ("the little girl") is the cold phase. Stronger trade winds push even more warm water west. Rising air and rain grow stronger over Indonesia and the nearby region.
• 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 sides.
• This makes two Walker loops instead of one.

• It can hurt the Indian monsoon even when the event is "weak" by the Nino 3.4 measure.

• Declaring an event (NOAA)

• Nino 3.4 region: a box at 5°N–5°S, 120°–170°W in the central-east equatorial Pacific [2].
• Oceanic Nino Index (ONI): the three-month running average of the sea-surface temperature anomaly (how far the temperature is from normal) in Nino 3.4 [2].
• ≥ +0.5 °C means El Nino conditions. ≤ –0.5 °C means La Nina conditions [2].
• The anomaly must stay past this limit for five overlapping three-month periods in a row before NOAA calls it an official event [2].
• Other agencies, such as the Australian Bureau of Meteorology and IMD, use their own rules. So they can declare the same event at different times.

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.
• Measuring stations: Tahiti (French Polynesia, central-south Pacific) for the east, and Darwin (northern Australia) for the west.
• Effects of a strong El Nino around the world

Region	Normal year	Strong El Nino
West coast of South America (Peru, Ecuador)	Dry desert coast; cold water; rich fishery	Heavy rain and floods; fisheries collapse
Indonesia and Australia	Rising air; heavy rain	Sinking air; drought, bushfires
India	Normal monsoon support	Monsoon often weak; drought sometimes
China	Normal	Floods

• Notable events: strong El Ninos in 1982–83, 1997–98, 2015–16 and 2023–24. A "triple-dip" La Nina lasted from 2020 to 2023 (La Nina for three winters in a row).
• India in 1997–98: the monsoon stayed normal despite a strong El Nino, because a positive Indian Ocean Dipole (IOD) helped. The IOD is a difference in sea-surface temperature between the western and eastern Indian Ocean. A positive IOD means the west, near Africa, is warmer.
• 2026: El Nino with no IOD support
• A weak La Nina in winter 2025–26 was fading towards ENSO-neutral by mid-February 2026 [4].
• By mid-August 2026 El Nino was "firmly established". WMO expected it to become very strong and peak near the end of 2026 [5].
• In April 2026 IMD forecast June–September rain at 92% of the Long Period Average (LPA), or "below normal" [6]. In May 2026 it lowered this to 90% of LPA [7].
• On 30 September 2026 IMD reported strong El Nino conditions while the IOD was neutral. It forecast October rainfall to be below normal (<85% of LPA) [8].

Don't confuse with

• La Nina: the cold phase of ENSO, with stronger trade winds, a cooler east Pacific and a positive SOI. It usually means a good Indian monsoon. El Nino is the warm phase, with a negative SOI.
• Walker circulation: an east–west loop of air along the equator. ENSO is the swing that weakens it (El Nino) or strengthens it (La Nina). Do not mix it up with the north–south Hadley cell.
• Indian Ocean Dipole (IOD): a temperature difference across the Indian Ocean, not the Pacific. A positive IOD can offset El Nino, as in 1997.
• El Nino Modoki: the warmest water is in the central Pacific, with two Walker loops. A normal El Nino warms the east Pacific near Peru.

Prelims Hooks

• SOI = Tahiti pressure – Darwin pressure. It is negative in El Nino and positive in La Nina. A common trap is to swap the order of the two stations.
• Nino 3.4 region: 5°N–5°S, 120°–170°W. NOAA's ONI limit is ±0.5 °C on three-month running means, held for five overlapping periods in a row [2].
• During El Nino the arid Peru coast gets heavy rain, while Australia and Indonesia get drought. A common trap is to say El Nino brings drought to Peru.
• El Nino weakens the cold Humboldt (Peru) current and stops upwelling, so the anchovy fishery collapses.
• The Walker circulation was named by Bjerknes (1969) after Sir Gilbert Walker of IMD, who found the Southern Oscillation in the 1920s while looking for monsoon predictors.
• El Nino does not always mean drought in India. 1997 had a normal monsoon despite a strong El Nino, thanks to a positive IOD.

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 grows stronger over South Asia.
• Less moisture reaches the south-west monsoon, so the risk of drought rises.

• Other systems can change the result. A positive IOD (1997) can soften the effect, and central-Pacific warming (Modoki) can make it worse even in a "weak" event.

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

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

• Policy tools 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

• Recent decades saw three strong El Ninos after 1997–98, the 2020–23 triple-dip La Nina, and a quick switch from La Nina in early 2026 to a very strong El Nino by late 2026 [4][5].
• The Pacific alone cannot predict the monsoon well. India needs to watch the Pacific (Nino 3.4, ONI, SOI), the Indian Ocean (IOD) and central-Pacific (Modoki) patterns together. Early warnings months ahead support climate-risk planning under the Sendai Framework.

Related concepts

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

Read more

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