EL-NINO Circulations

What is El Niño?

El Niño is the name for a warming of the ocean surface waters in the central and eastern Pacific, off the shores of Ecuador and Peru. When these waters warm, the regular upwelling of cool, nutrient rich deep ocean water is greatly reduced.

The name comes from Spanish and means "the little boy". Fishermen in Peru first noticed the warm water arriving around Christmas and New Year, so they linked it to the Christ child. A typical El Niño is a seasonal event that lasts from a few weeks to a few months, although a very strong event can last substantially longer than expected.

Normal conditions in the South Pacific

In a typical year, a surface low pressure system forms between northern Australia and Indonesia, while a high pressure system forms off the coast of Peru.

As a result, the Pacific Ocean trade winds blow strongly from east to west. These easterly trade winds carry warm surface water westward, generating convective storms and thunderstorms across Indonesia and coastal Australia. Along the coast of Peru, cold, nutrient rich water rises to the surface to replace the warm water that is being dragged west. This upwelling supports one of the richest fishing grounds in the world.

El Niño conditions

During an El Niño year, air pressure drops across much of the central Pacific and along the South American coast. In the western Pacific, a weak high pressure system replaces the usual pattern of low pressure. Because of this change in the pressure pattern, the trade winds are weakened.

The Walker Circulation

The Walker Circulation is the loop of air that runs along the equator between these areas of low and high pressure. Air rises over the areas of low pressure and sinks over the areas of high pressure. When the trade winds weaken, this Walker cell is rendered ineffective, and in some cases it can even reverse.

With the winds no longer holding the warm water in the west, the equatorial counter current (the current that runs parallel to the doldrums) allows warm ocean water to gather off the shores of Peru and Ecuador. This warm water pushes the thermocline in the eastern Pacific downwards, which prevents the upwelling of cold deep ocean water along the coast of Peru. As a result, El Niño weather patterns bring drought to the western Pacific, rain to the equatorial coasts of South America, and convective storms and hurricanes to the central Pacific.

What are the effects of El Niño?

• Marine life along the coasts of Peru and Ecuador suffers because of the warmer waters. Because there is no upwelling, the amount of fish caught off the South American coast is far lower than in a regular year.
• Australia, Indonesia, India and southern Africa can experience severe droughts.
• California, Ecuador and the Gulf of Mexico have all experienced significant rainfall.
• In India, monsoon rainfall is often reduced when the monsoon shifts eastward from its normal position, as it tends to do during El Niño years.

Forecasters watch the Southern Oscillation Index, or SOI, which is the difference in air pressure between Tahiti and Darwin. The seesaw pattern of pressure between the eastern and western Pacific is called the Southern Oscillation, and whether it is positive or negative helps decide whether India gets excess or insufficient rainfall. Together, the ocean warming and this pressure seesaw form the wider pattern known as the El Niño Southern Oscillation, or ENSO.

What is La Niña?

La Niña is the opposite phase of El Niño, and the other half of the El Niño Southern Oscillation. Its name means "the little girl" in Spanish. During La Niña, the sea surface temperature of the western and central Pacific falls below normal, a situation that is almost the mirror image of El Niño.

The high pressure over the southern Pacific strengthens, which brings back powerful trade winds. As these winds blow from east to west, they push the surface water westward and cause rapid upwelling of cold seas off the coast of Peru.

Effects of La Niña

• The west coast of South America becomes drier than normal, and drought can affect the coastal regions of Peru and Chile.
• Rainfall rises over the western equatorial Pacific, including Indonesia, Malaysia and northern Australia.
• During the northern hemisphere winter, South East Africa and northern Brazil tend to be warmer than usual.
• In India, the summer monsoon rainfall is usually greater than normal, especially over the north west. This is not permanent, because both the Indian Ocean Dipole and La Niña affect how strong the Indian monsoon becomes.

Why it still matters

El Niño is not just a topic in a textbook. It is happening right now. In its bulletin dated 13 August 2026, the United States National Oceanic and Atmospheric Administration, or NOAA, placed the Pacific under an "El Niño Advisory" and said the event was strengthening, with a greater than 90 percent chance of becoming a very strong event through the winter of 2026 to 2027. The July value of the Niño 3.4 index, one of the main measures of ocean warming, had already reached plus 1.4 degrees Celsius, and NOAA put the odds of a historic event for October to December 2026 at about 69 percent.

This is exactly why students, farmers and policymakers in India follow these ocean bulletins so closely. As the notes above explain, a strong El Niño tends to weaken the Indian monsoon. When a strong El Niño builds, Indian agencies flag the districts most at risk. Environmental reporting has identified between 150 and 200 vulnerable districts, many of them in rain fed farming belts across states like Maharashtra, Karnataka, Rajasthan and Madhya Pradesh, where roughly nine in ten hectares of pulses and oilseeds depend on the rains. The India Meteorological Department measures each season against a long period average of about 869 millimetres of rain, and in worrying El Niño years it has warned of below normal rainfall, as low as around 90 percent of that average.

So a warm patch of ocean thousands of kilometres away in the Pacific can help decide whether a farmer in India has enough water for the season, and whether food prices stay steady. You can explore how the ocean, the atmosphere and the monsoon connect in our Learnacy Hub, revise related climate topics in the geography notes, and browse more study guides across the resources library.

Sources

1. NOAA Climate Prediction Center, ENSO Diagnostic Discussion, El Niño Advisory issued 13 August 2026.
2. Down To Earth, El Niño impact on the Indian monsoon and the districts most at risk.

Key takeaways

• El Niño is a warming of ocean surface waters in the central and eastern Pacific that reduces upwelling of cool, nutrient-rich deep water, disrupting normal weather patterns.
• In normal conditions, trade winds blow east to west across the Pacific, carrying warm water westward and causing cold, nutrient-rich water to upwell along the Peruvian coast.
• During El Niño, the Walker Circulation weakens or reverses as trade winds weaken, allowing warm water to accumulate off South America and blocking upwelling.
• El Niño brings drought to Australia, Indonesia, India, and southern Africa, while causing heavy rainfall in California, Ecuador, and the Gulf of Mexico, and weakens the Indian monsoon.
• La Niña is the opposite phase of El Niño, with cooler Pacific waters, stronger trade winds, increased upwelling, and typically stronger monsoons in India.

Test yourself

What causes the warming of ocean waters during El Niño?

The trade winds weaken, which allows the equatorial counter current to carry warm ocean water from the western Pacific to the shores of Peru and Ecuador.

What is the Walker Circulation?

The Walker Circulation is the loop of air that runs along the equator between areas of low pressure over Australia/Indonesia and high pressure off Peru, where air rises over low pressure areas and sinks over high pressure areas.

How does El Niño affect the Indian monsoon?

A strong El Niño tends to weaken the Indian monsoon, often reducing rainfall to below normal levels, with the India Meteorological Department warning of rainfall as low as 90 percent of the long period average.