El Niño and La Niña: ENSO and the Indian Ocean Dipole Effect

At a Glance

• El Niño Warming of the central-eastern Pacific
• La Niña Cooling of the same region
• Frequency Every 2 to 7 years
• ENSO Ocean warming plus pressure seesaw
• IOD West-east temperature difference in the Indian Ocean
• Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic begins the part of the macro on large-scale climate cycles, after the study of local and regional weather. It leads to the topic on how these cycles affect the Indian monsoon and to the classification of climates. Previous topic: Polar Vortex and Its Relationship with Ozone Depletion. Next topic: Influence of El Niño/La Niña on the Indian Monsoon.

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: Hadley Cell, Ferrel Cell, Polar Cell, since it is an east-west cell along the equator.

What Are the Normal Conditions in the Pacific?

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 Major Ocean Currents of the World and Their Effects, 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.

What Is El Niño and How Does It Develop?

In short: in an El Niño year the trade winds weaken, warm water flows east, and the heat and rain of the Pacific shift.

El Niño, Spanish for the Christ child, was originally the name for a warm current that appears every few years off the coast of Peru near Christmas. Today it refers to the warm phase of ENSO, the El Niño-Southern Oscillation, the combined ocean and atmospheric phenomenon. In an El Niño, the trade winds weaken or even reverse for some time. The warm pool in the west Pacific then moves east, the thermocline deepens in the east and rises in the west, and the upwelling of cold water off Peru is reduced or suppressed. The surface of the eastern and central Pacific becomes several degrees warmer than normal, the pressure falls over Tahiti and rises over Darwin, and the Southern Oscillation Index turns negative. The rising branch of the Walker Circulation shifts from Indonesia to the central Pacific. The warm ocean in turn weakens the winds even more, which is the Bjerknes feedback that makes the event grow.

It is monitored using the Niño 3.4 region of the equatorial Pacific, and the Oceanic Niño Index, ONI, which is the three-month average temperature anomaly in that area. El Niño conditions are declared when the anomaly stays at or above +0.5 degrees Celsius, together with the atmospheric response. Events are usually called weak, moderate, strong or very strong, depending on the size of the anomaly.

What Are the Effects of El Niño?

The warm waters in the east also make the Pacific release more heat to the atmosphere, so that years with strong El Niño often become records for global warmth. The years 2015 and 2016, and 2023 and 2024, were records, and 2024 was the warmest year on record. Food markets are affected through droughts in rice, wheat, palm oil and sugar producers: in July 2023, India restricted the export of non-basmati white rice, in part because of the concern about the El Niño and the monsoon.

What Is La Niña?

In short: La Niña is an exaggeration of normal conditions: stronger trade winds and a colder eastern Pacific.

La Niña, Spanish for the little girl, is the cool phase of the El Niño-Southern Oscillation, and is sometimes called El Viejo or the anti-El Niño. The trade winds blow stronger than normal, pushing more warm water into the western Pacific, and upwelling of cold water off South America is enhanced. The sea surface in the central and eastern equatorial Pacific is below normal, and the Niño 3.4 anomaly stays at or below minus 0.5 degrees Celsius. The pressure is higher than normal at Tahiti and lower at Darwin, so the Southern Oscillation Index is strongly positive. When the Pacific is close to normal, with neither El Niño nor La Niña, the state is called ENSO-neutral. La Niña usually occurs after an El Niño, though not always, and may last for one or two years: the triple-dip La Niña of 2020 to 2023 lasted for three northern winters.

What Is the Indian Ocean Dipole?

The Indian Ocean Dipole, IOD, is an irregular see-saw in sea surface temperature between the western Indian Ocean, near Africa, and the eastern Indian Ocean, near Sumatra. It was described by Saji and colleagues in 1999, and is measured by the Dipole Mode Index, which is the temperature difference between the two regions.

The dipole can work with or against ENSO. In 1997, a positive IOD offset the effect of the strong El Niño, so that the Indian monsoon was normal. In 2019, a very strong positive IOD helped the monsoon to end at about 110 per cent of the average, although the Pacific was close to neutral, and it also contributed to the severe bushfires of Australia that year. As an analytical point, the Indian monsoon is thus the product of multiple oceans, and good forecasting needs the Pacific, the Indian Ocean and the land together, as in Monsoon Mechanism: Traditional and Contemporary Theories (Air Mass Theory, Jet Stream Theory).

Why Is This Topic Important?

• Recurring theme El Niño is asked every year
• India Monsoon and food prices
• Current affairs Strong event in 2026

What Came Next?

The next topic explains the influence of El Niño and La Niña on the Indian monsoon. Next topic: Influence of El Niño/La Niña on the Indian Monsoon.

Key People and Terms

• ENSO El Niño-Southern Oscillation
• ONI Oceanic Niño Index
• Niño 3.4 Pacific region used to monitor ENSO
• Bjerknes feedback Self-reinforcing ocean-atmosphere loop
• Indian Ocean Dipole East-west temperature difference in the Indian Ocean
• Teleconnection Distant weather link

Exam Corner

Mains Angle

Questions ask for effects and the Indian link. These points add depth.

• Cause Weak winds and warm east
• India Weaker monsoon, but offset by IOD
• Economy Food inflation and policy response