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

Detail

1. Why the ocean and the air are linked

• The ocean and the atmosphere are not separate. Winds move the surface water. The warmth of that water then decides where air rises and where it sinks.
• The clearest example of this link is in the 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 South America in the east.
• ENSO (El Nino–Southern Oscillation) is a repeating swing in two things across the equatorial Pacific. The ocean part is a change in sea-surface temperature, which is El Nino. The air part is a change in air pressure, which is the Southern Oscillation [3].

2. The normal (neutral) year

Trade winds push warm water west

• The trade winds (steady easterly winds that blow from the subtropical high-pressure belts towards the equator) blow from east to west across the equatorial Pacific.
• They drag the warm surface water westward. This water piles up near Indonesia and northern Australia.
• This patch of very warm water is 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. 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, a cold current that flows north along the coast of Peru and Chile.
• Upwelled water is full of nutrients. It feeds plankton, and the plankton feed huge shoals of anchovy. This makes the Peru coast one of the world's richest fishing grounds.

The Walker circulation

• Over the warm west Pacific and Indonesia, air is heated. It rises, forms clouds and gives heavy rain. This creates low pressure at the surface.
• High in the sky, this air flows east.
• Over the cool east Pacific (off Peru), the air cools and sinks. This creates high pressure and dry weather. The Peru–Chile coast is a desert partly for this reason.
• At the surface, the air flows back west as the trade winds. The loop is now complete.
• Walker circulation: this east–west loop of air along the equatorial Pacific. Air rises in the west, flows east aloft, sinks in the east and returns west near the surface.
• In 1969 Jacob Bjerknes gave it this name, after Sir Gilbert Walker, Director-General of the India Meteorological Department (IMD). Walker found the Southern Oscillation in the 1920s while he was looking for ways to forecast the Indian monsoon.

3. El Nino: the warm phase

What happens

• Every 3–7 years, warm water from the central Pacific drifts east to the South American coast. It replaces the cold Peru current.
• It shows up around Christmas. Peruvian fishermen named it El Nino, Spanish for "the Christ child".

The chain of events

• The trade winds weaken. Less warm water is held back in the west, so it slides east.
• The Walker circulation weakens or moves east. The zone of rising air and heavy rain moves 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 fisheries collapse.

4. 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.
• It is measured at two stations:
• Tahiti (French Polynesia, central-south Pacific) stands for the east.

• Darwin (northern Australia) stands for the west.

• 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.

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

5. Effects of a strong ENSO event (El Nino phase)

Region	Normal year	Strong El Nino
West coast of South America (Peru, Ecuador)	Dry desert coast; cold water; rich fishery	Heavy rain and floods on the dry coast; 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

• ENSO is watched closely because it helps long-range forecasting, for example IMD's seasonal monsoon forecast.

6. La Nina: the cold phase

• La Nina ("the little girl") is the opposite of El Nino.
• Stronger trades push even more warm water west. The east Pacific becomes cooler than normal and upwelling off Peru becomes stronger.
• The Walker circulation becomes stronger. The SOI is positive.
• Rising air and rain are stronger over Indonesia and the region around it. This usually helps a good Indian monsoon.

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

7. How events are declared: Nino 3.4 and the ONI

• Nino 3.4 region: a box in the central-east equatorial Pacific at 5°N–5°S, 120°–170°W. It overlaps parts of the Nino 3 and Nino 4 regions [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.
• Anomaly ≥ +0.5 °C means El Nino (warm phase) conditions. Anomaly ≤ –0.5 °C means La Nina (cool phase) conditions [2].

• The anomaly must stay past the limit for five overlapping three-month periods in a row before NOAA counts it as an official El Nino or La Nina event [2].

• Other agencies use their own rules, for example the Australian Bureau of Meteorology and IMD. So agencies can declare the same event at different times.

8. El Nino Modoki

• El Nino Modoki ("Modoki" is Japanese for "similar but different"), described by Ashok and others (2007).
• In this type, the warmest water sits in the central Pacific, with cooler water on both sides (west and east).

• This makes two Walker loops instead of one, and the rising air sits over the central Pacific.

• Its global effects are different from those of a normal (east Pacific) El Nino.

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

9. Notable events and the India link

Event	Type	Key point
1982–83	Strong El Nino	One of the strongest of the 20th century
1997–98	Strong El Nino	India's monsoon was normal, helped by a positive Indian Ocean Dipole (IOD)
2015–16	Strong El Nino	Among the strongest on record
2020–23	"Triple-dip" La Nina	La Nina for three winters in a row
2023–24	Strong El Nino	Followed the triple-dip La Nina
Winter 2025–26	Weak La Nina	Fading towards ENSO-neutral by mid-February 2026 [4]
2026	El Nino	"Firmly established" by mid-August 2026; WMO gave a nearly 100% chance that it lasts through December 2026–February 2027 and expected it to become very strong before peaking near the end of 2026 [5]

• Indian Ocean Dipole (IOD): a difference in sea-surface temperature between the western and eastern Indian Ocean. A positive IOD (warmer west, near Africa) can offset El Nino and support the Indian monsoon. More on this in indian-climate-monsoon.
• 2026 example: El Nino with no IOD support
• April 2026: IMD forecast June–September monsoon rain at 92% of the Long Period Average (LPA), with a model error of ±5%, which is "below normal". It expected El Nino to develop during the season [6].
• LPA (the average all-India monsoon rainfall for 1971–2020) is about 87 cm [6].
• May 2026: IMD lowered the forecast to 90% of LPA (±4%) [7].
• 30 September 2026: IMD reported strong El Nino conditions, with sea surface much warmer than normal in the east-central and eastern equatorial Pacific, still strengthening towards the end of the year [8].
• At the same time the IOD was neutral and expected to stay neutral through the northeast monsoon season [8].
• October 2026 all-India rainfall was forecast to be below normal (<85% of LPA) [8].

Prelims Hooks

• Walker circulation: air rises over the warm west Pacific and Indonesia and sinks over the cool east Pacific (Peru). Bjerknes named it in 1969 after Sir Gilbert Walker of IMD.
• SOI = Tahiti pressure – Darwin pressure. It is negative in El Nino and positive in La Nina. A common trap is to reverse 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].
• El Nino weakens the cold Humboldt (Peru) current and stops upwelling, which leads to the collapse of the anchovy fishery.
• During El Nino the arid Peru coast gets heavy rain, while Australia and Indonesia face drought. A common trap is to say El Nino brings drought to Peru.
• La Nina brings stronger trades, a cooler east Pacific and a positive SOI. It usually means a good Indian monsoon.
• El Nino Modoki (Ashok et al., 2007): the warmest water is in the central Pacific with cooler water on both sides. It can hurt the Indian monsoon even when weak.
• Strong El Ninos: 1982–83, 1997–98, 2015–16, 2023–24. The triple-dip La Nina lasted from 2020 to 2023.
• In 1997 India's monsoon was normal despite a strong El Nino because of a positive IOD. So El Nino does not always mean Indian drought.
• Gilbert Walker found the Southern Oscillation while looking for monsoon predictors for IMD in the 1920s.

Mains Points

• How a Pacific event reaches India (GS-I)
• Weak trade winds make the Walker circulation weaken or move east.
• The rising air moves away from the Indonesia–Indian Ocean side, and sinking air becomes stronger over South Asia.
• The moisture supply to the south-west monsoon weakens, which raises the risk of drought.

• Other systems can change the result: a positive IOD (1997) or the location of warming (Modoki) can soften or worsen the effect. Pacific warming is only one factor in the outcome.

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

• A failed monsoon reduces kharif sowing, reservoir storage and hydropower, and raises food inflation.
• El Nino also raises global temperatures, so heatwaves are more likely. In mid-2026 WMO warned of heat stress and faster droughts and urged preparedness [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.

• The global pattern shows that the ocean and atmosphere act as one system

• El Nino brings floods to Peru and Ecuador, drought to Australia and Indonesia, floods in China and a fisheries collapse off Peru.

• Monitoring through Nino 3.4, the ONI and the SOI allows warnings months in advance. This supports climate-risk planning and links to the Sendai Framework.

• Changing ENSO behaviour

• Recent years have seen three strong El Ninos since 1997, a three-year La Nina (2020–23) and a quick switch from La Nina in early 2026 to a very strong El Nino by late 2026 [4][5].
• This makes it harder to forecast the monsoon from the Pacific alone. It strengthens the case for watching the Pacific, the Indian Ocean (IOD) and the central-Pacific (Modoki) patterns together.