Walker circulation

Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Beyond NCERT

Meaning

Walker circulation is the east–west loop of air along the equatorial Pacific Ocean. Air rises over the warm western Pacific and Indonesia, flows east high in the sky, sinks over the cool eastern Pacific off Peru, and flows back west near the surface as the trade winds.

It is the air half of the ocean–air link behind El Nino and La Nina. When this loop weakens or moves, rainfall changes far away, including the Indian monsoon.

Explanation

How the loop forms (normal or neutral year)

Trade winds pile up warm water in the west

• The trade winds blow from east to west across the equatorial Pacific. These are steady easterly winds that blow from the subtropical high-pressure belts towards the equator.
• They drag warm surface water west, and it collects 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 in the east

• Warm water moves away from the South American coast, and cold deep water rises to take its place. This is upwelling (deep water rising to the surface).
• This cold water is part of the cold Humboldt (Peru) current.

The four limbs of the loop

• Rising limb (west): warm sea heats the air over Indonesia and the west Pacific. The air rises, forms clouds and gives heavy rain. This leaves low pressure at the surface.
• Upper limb: high in the sky, the air flows east.
• Sinking limb (east): over the cool east Pacific off Peru, the air cools and sinks. This makes high pressure and dry weather. This is one reason the Peru–Chile coast is a desert.
• Surface limb: near the surface the air flows back west as the trade winds, and the loop is complete.
• The loop feeds itself:
• Strong trades keep the west warm and the east cool.
• That temperature difference keeps the loop going.
• The loop keeps the trades blowing.

How it changes: El Nino, La Nina and Modoki

El Nino (warm phase) weakens the loop

• The trade winds weaken. Warm water that was held back in the west slides east to the South American coast.
• The Walker circulation weakens or moves east. The zone of rising air and heavy rain shifts from Indonesia to the central and eastern Pacific.
• Off Peru, upwelling stops and the anchovy fishery collapses.

La Nina (cold phase) strengthens the loop

• 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, and rain over Indonesia and the region around it increases.

El Nino Modoki splits the loop in two

• "Modoki" is Japanese for "similar but different". Ashok and others described this type in 2007.
• The warmest water sits in the central Pacific, with cooler water on both sides.
• This makes two Walker loops instead of one, with the rising air over the central Pacific.

How it is measured: the Southern Oscillation

• The Southern Oscillation is 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 the surface pressure signal of the Walker loop.
• Southern Oscillation Index (SOI) = Tahiti pressure minus Darwin pressure.
• Strong Walker loop (La Nina): high pressure at Tahiti, low pressure at Darwin, so the SOI is positive.

• Weak Walker loop (El Nino): pressure falls at Tahiti and rises at Darwin, so the SOI is strongly negative.

• ENSO (El Nino–Southern Oscillation) is the combined swing. The ocean part is the change in sea-surface temperature (El Nino), and the air part is the change in air pressure (Southern Oscillation) [2].

• In 1969, Jacob Bjerknes showed that the two are one event. He named the loop 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.

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
Usual effect on Indian monsoon	Weaker; drought risk	Normal	Good monsoon

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.
• Rising limb: the Western Pacific Warm Pool, near Indonesia and northern Australia. It has heavy rain in normal years and drought and bushfires during strong El Nino.
• Sinking limb: the cool east Pacific off Peru. Here the dry Peru–Chile coastal desert and the anchovy-rich upwelling zone of the Humboldt current lie under sinking air.
• Pressure stations:
• Tahiti (French Polynesia, central-south Pacific) stands for the east.

• Darwin (northern Australia) stands for the west.

• Nino 3.4 region (5°N–5°S, 120°–170°W) is the central-east equatorial Pacific box. NOAA tracks warming here to declare El Nino or La Nina [1].

• Central Pacific: during El Nino Modoki, the rising air sits here and the single loop splits into two.
• India: a weak or east-shifted loop moves rising air away from the Indonesia–Indian Ocean side. This can weaken the south-west monsoon.
• 2026 example: IMD reported strong El Nino conditions on 30 September 2026, with a neutral Indian Ocean Dipole (IOD). It forecast October rainfall to be below normal (<85% of LPA) [5].

Don't confuse with

• Hadley cell: a north–south loop. Air rises near the equator and sinks over the subtropical high-pressure belts. The Walker circulation is an east–west loop along the equator in the Pacific.
• Southern Oscillation: this is the surface pressure see-saw between Tahiti and Darwin, measured by the SOI. The Walker circulation is the actual loop of moving air that causes this see-saw.
• El Nino: this is the ocean event, the warming of the east Pacific sea surface. The Walker circulation is the air loop that weakens or moves east because of it.
• Indian Ocean Dipole (IOD): this is a sea-surface temperature difference between the west and east Indian Ocean, not the Pacific. A positive IOD can offset a weak Walker loop and support the Indian monsoon, as in 1997.

Prelims Hooks

• Walker circulation: air rises over the warm west Pacific and Indonesia (low pressure, rain) and sinks over the cool east Pacific off Peru (high pressure, dry). Bjerknes named it in 1969 after Sir Gilbert Walker of IMD.
• SOI = Tahiti pressure – Darwin pressure. It is positive when the Walker loop is strong (La Nina) and strongly negative when it is weak (El Nino). A common trap is to reverse the order of the two stations.
• During El Nino the loop weakens or moves east. The arid Peru coast gets heavy rain, while Indonesia and Australia face drought. A common trap is to say El Nino brings drought to Peru.
• During La Nina the trades are stronger and the Walker loop is stronger. This usually means a good Indian monsoon.
• El Nino Modoki (Ashok et al., 2007): the warmest water is in the central Pacific, which creates two Walker loops. It can hurt the Indian monsoon even when it is "weak" by the Nino 3.4 measure.
• The surface limb of the Walker circulation is the trade winds. Its sinking limb over the east Pacific is one reason for the Peru–Chile coastal desert.

Mains Points

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

• Other systems can change the result. A positive IOD (1997) or warming in the central Pacific (Modoki) can soften or worsen the effect, so the Pacific is only one factor.

• Economy and disaster management (GS-III)

• A weak Walker loop and a failed monsoon reduce kharif sowing, reservoir storage and hydropower, and they raise food inflation.
• Because Walker and ENSO signals show up months in advance through Nino 3.4, the ONI and the SOI, they support IMD's long-range forecasts (MME and MMCFS models [5]). They also help contingency crop plans, drought declaration and heat action plans.

• In mid-2026, WMO warned of heat stress and faster droughts linked to El Nino and urged preparedness [4].

• Changing behaviour of the loop

• Recent years show large swings:
  • a triple-dip La Nina (a strong loop for three winters, 2020–23)
  • a weak La Nina in winter 2025–26 that was fading towards ENSO-neutral by mid-February 2026 [3]
  • an El Nino that was firmly established by mid-August 2026 and expected to become very strong [4]
• 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.

Related concepts

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

Read more

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