Maarg Manthan · Topic 9.1

Walker Circulation and Normal Conditions in the Pacific

Geography › El Niño and Climate Classification · Topic 9.1

The Walker Circulation is the east-west loop of air over the tropical Pacific, with rising air and heavy rain over the warm western Pacific near Indonesia, sinking air over the cool eastern Pacific off South America, easterly trade winds at the surface and westerly winds aloft. Under normal conditions it keeps a warm pool in the west and upwelling of cold water off Peru.

6 min read · UPSC Prelims · Geography · El Niño and Climate Classification

At a Glance

Discovered by Sir Gilbert Walker, 1920s
Rising air Western Pacific near Indonesia
Sinking air Eastern Pacific off Peru
Surface wind Easterly trade winds
Index Southern Oscillation Index (Tahiti minus Darwin)
Exam link UPSC Prelims, Geography: El Niño and Climate Classification

Where Does It Fit?

This is the first topic of the category on El Niño and climate classification. The previous category ended with the polar vortex, and this topic describes the usual state of the tropical Pacific, which is the baseline for understanding El Niño. The next topic covers El Niño and its effects. Previous topic: Polar Vortex. Next topic: El Niño and Its Effects.

What Is the Walker Circulation?

In short: the Walker Circulation is the loop of air that rises over the warm west Pacific and sinks over the cool east Pacific, driven by the east-west difference in sea temperature.

The circulation is named after Sir Gilbert Walker, a British scientist who was Director General of Observatories in India in the early twentieth century and who studied the failures of the Indian monsoon. In the 1920s he found that the air pressure over the Indian and the west Pacific oceans rises when the pressure over the east Pacific falls, a see-saw that he called the Southern Oscillation. Jacob Bjerknes later explained in 1969 that it is linked to the ocean, and showed that the atmosphere and ocean in the tropical Pacific work as one coupled system. The circulation differs from the north-south Hadley cell, which carries heat to the poles, as in General Circulation of the Atmosphere, since it is an east-west cell along the equator.

What Are the Normal Conditions in the Pacific?

Feature	Western Pacific (Indonesia, Australia)	Eastern Pacific (Peru, Ecuador)
Sea surface	Warm pool of 28 degrees Celsius or more, with sea level about half a metre higher	Cool, about 20 to 24 degrees Celsius, as cold water rises from below
Thermocline	Deep, around 150 to 200 m, because warm water piles up	Shallow, around 50 m or less, so cold water is close to the surface
Air pressure	Low (Darwin, Australia)	High (Tahiti)
Air motion	Rising air, cumulonimbus clouds, heavy rain	Sinking air, dry conditions; the Peruvian coast is a desert
Surface wind	Easterly trade winds blow towards the west, pushing warm surface water along	Winds blow offshore, which moves surface water away from the coast

The trade winds drive warm surface water westward, and the water that leaves the east is replaced by cold water that rises from below, called upwelling. The cold Humboldt Current, also called the Peru Current, as in Pacific and Atlantic Ocean Currents, adds to this. The upwelling brings nutrients to the surface, and the coast of Peru has one of the richest fisheries in the world, especially the anchoveta. Aloft, the air flows eastward from the west Pacific to the east Pacific to complete the loop. The thermocline is the layer where the temperature drops rapidly with depth, between the warm surface layer and the cold deep water.

What Is the Southern Oscillation and How Is It Measured?

The pressure see-saw is measured by the Southern Oscillation Index, SOI, which is the difference between the standardised sea level pressure at Tahiti and at Darwin. In a normal or La Niña phase, Tahiti has high pressure and Darwin low pressure, which gives a positive SOI, with strong trade winds. In an El Niño phase the pressure at Tahiti falls and that at Darwin rises, giving a sustained negative SOI. The ocean-atmosphere system reinforces itself, which is called the Bjerknes feedback: stronger trade winds increase the temperature gradient, which in turn strengthens the winds. The system is held in balance by this feedback, but a slight weakening of the winds can start the opposite cycle, as in El Niño and Its Effects.

Why Does the Walker Circulation Matter for India?

The Walker Circulation connects the Pacific and the Indian monsoon. When it is normal, the rising branch is close to Indonesia and the sinking branch lies to the east, and the monsoon circulation over South Asia is not disturbed. When the circulation weakens or shifts east, the sinking air can move over the Indian region and weaken the monsoon, as in Mechanism of the Indian Monsoon. As an analytical point, Walker’s work was done on a very practical problem, the prediction of the Indian monsoon, and it ultimately created the science of seasonal forecasting. Today the Pacific is watched through a network of moored buoys across the equator, and satellite measurements of sea surface temperature and sea level.

Why Is This Topic Important?

Base concept Needed for El Niño questions
India link Walker studied the monsoon
Fisheries Peruvian upwelling

What Came Next?

The next topic describes El Niño and its effects. Next topic: El Niño and Its Effects.

Key People and Terms

Walker Circulation East-west loop over the Pacific
Southern Oscillation Pressure see-saw between east and west Pacific
SOI Tahiti minus Darwin pressure index
Thermocline Layer of rapid temperature drop with depth
Upwelling Rise of cold deep water
Warm pool Very warm water in the west Pacific

Exam Corner

Points to Remember

Walker Circulation was found by Gilbert Walker.
Normal: low pressure at Darwin, high at Tahiti.
The thermocline is deeper in the west.
Peru has upwelling and rich fisheries.
SOI is positive in normal conditions.

Do Not Confuse With

Walker cell and Hadley cell East-west versus north-south
Thermocline and upwelling Temperature layer versus rising water
SOI and ENSO A pressure index versus the whole phenomenon

Memory Hook

Trades blow west, the warm pool piles up, Peru’s cold water rises, Darwin low and Tahiti high.

Mains Angle

Questions need the baseline before El Niño. These points add depth.

Coupling Ocean and atmosphere together
Effect Fisheries and rainfall
India Monsoon teleconnection

A Question You May Face

An original practice question, not a past paper question.

“Describe the normal conditions in the tropical Pacific and explain the role of the Walker Circulation.”

How to Answer

Normal Warm west, cool east.
Circulation Rising west, sinking east.
Index Southern Oscillation.
Impact Fisheries and the monsoon.

GS Relevance

Prelims: normal Pacific conditions, Southern Oscillation, Walker cell, upwelling, thermocline. GS Paper 1: climatology; base for El Niño and the monsoon.

Frequently Asked Questions

What is the Walker Circulation?

The Walker Circulation is an east-west loop of air over the equatorial Pacific. Air rises over the warm western Pacific near Indonesia, flows east aloft, sinks over the cooler eastern Pacific off South America and returns west along the surface as the easterly trade winds.

What are the normal conditions in the Pacific Ocean?

Under normal conditions, the trade winds push warm surface water to the western Pacific, creating a warm pool and heavy rain there. Off Peru, cold water rises by upwelling, the thermocline is shallow, and the air pressure is high, making the coast dry.

What is the Southern Oscillation?

The Southern Oscillation is a see-saw in air pressure between the eastern and western tropical Pacific, discovered by Gilbert Walker. When the pressure is high over Tahiti, it is low over Darwin, and vice versa. It is measured by the Southern Oscillation Index.

What is the thermocline?

The thermocline is the layer of ocean water in which the temperature falls sharply with depth, lying between warm surface water and cold deep water. In the Pacific it is deep in the west, about 150 metres or more, and shallow in the east, close to the surface.

Why is the coast of Peru rich in fish?

Easterly trade winds and the offshore winds along the Peruvian coast move surface water away, so cold, nutrient-rich water rises from the depth by upwelling. The nutrients support plankton and large anchoveta fish stocks, making Peru one of the leading fishing nations.

PYQ Practice — Statement Analysis

1 In normal conditions, the air pressure is high over Tahiti and low over Darwin.

True

This gives a positive SOI.

2 The thermocline is shallower in the western Pacific than in the eastern.

False

It is deeper in the west.

3 Gilbert Walker described the Southern Oscillation.