How can the same El Niño cause drought in one part of the world and floods in another?

The Pacific Ocean is warming unusually quickly, and this year’s El Niño has already surpassed previous records. Forecasters expect it to strengthen even further before approaching its usual peak around December.

El Niño, the warm phase of a natural fluctuation in the Earth’s climate system, can reshape weather thousands of miles away, bringing drought to some regions while increasing the risk of heavy rain and flooding in others. To understand why, you need to follow what happens to the Pacific’s warm water – and the air above it.

Usually, winds over the tropical Pacific blow from east to west, pushing warm surface water towards the western Pacific, where it piles up around Indonesia and nearby regions. These are the trade winds. As warm water moves westward, colder water from deeper in the ocean rises to replace it in the east — a process called upwelling. This is why the western Pacific is usually much warmer than the eastern Pacific.

Warmer water means more evaporation, much like heating water on a stove makes more of it turn into vapour. The warm western Pacific therefore feeds more moisture into the air above it. The ocean also warms that air, making it less dense than the cooler air around it, so it rises.

As the air rises, there is less air above it pressing down, so the pressure around it decreases. The rising air expands in this lower-pressure environment and cools. The water vapour then condenses into clouds and rain. When this happens, the energy carried upwards by the water vapour is released as heat into the atmosphere.

High above the Pacific, the rising air spreads out. Some of it moves eastward before sinking again over the cooler eastern Pacific. Near the surface, air flows back westward as the trade winds, completing the large circulation loop known as the Walker circulation.

This is one reason places around Indonesia and the western Pacific receive so much rainfall, while the eastern tropical Pacific and much of coastal Peru are normally far drier. In fact, some parts of Indonesia can receive more rain in a single month than Lima receives in an entire year.

The same ocean–atmosphere pattern also helps keep itself going. The warm western Pacific encourages air to rise, while cooler conditions in the east favour sinking air. This helps maintain the east-to-west trade winds. Those winds then keep pushing warm water westward and bringing colder water to the surface in the east, preserving the temperature contrast that helped drive the winds in the first place. In this way, the ocean and atmosphere reinforce each other – a process scientists call the Bjerknes feedback.

How El Niño changes the tropical Pacific:

So what changes during El Niño?

Some years, the tropical Pacific stops following its usual script. The east-to-west trade winds weaken, so less warm surface water is pushed towards the western Pacific. More warm water remains in the central and eastern Pacific, while the western Pacific can become relatively cooler. This is El Niño.

Once the warm water shifts eastward, the atmosphere above it begins to shift too. Think of moving the stove: the region of strongest evaporation, rising moist air, cloud formation, and rainfall also moves farther east into the central and eastern Pacific. This is why El Niño can bring much wetter conditions to parts of the eastern Pacific and nearby regions, including coastal Peru.

But that shift has consequences elsewhere. With more air rising over the central and eastern Pacific, air higher in the atmosphere spreads outward, and some of it sinks over the western Pacific. Sinking air makes cloud formation more difficult, so places such as Indonesia and the surrounding region near the western Pacific can become much drier than usual – conditions that can also increase the risk of forest and peatland fires in Indonesia.

At the same time, the temperature difference between the western and eastern Pacific becomes smaller. That weakens the pressure contrast that helps drive the trade winds, so the winds can weaken even further. The whole Walker circulation therefore becomes weaker and shifts eastward.

How El Niño reaches around the world

But El Niño’s influence does not stop at the Pacific. It can affect weather thousands of kilometres away through what scientists call teleconnections.

Remember the heat released when water vapour condenses into clouds? As the main zone of rising moist air shifts eastward during El Niño, so does this atmospheric heating. That can warm the middle and upper tropical atmosphere far beyond the Pacific.

In places such as the Sahel – the southern fringe of the Sahara – this makes it harder for warm, moist air near the surface to keep rising, suppressing rain-producing clouds and favouring drier conditions. Fewer clouds can also allow more sunlight to reach the surface, further warming and drying the land. Scientists refer to this as the tropospheric temperature mechanism.

That is one pathway. Another involves huge ripples in the atmosphere, triggered when the main zone of rising air and rainfall shifts eastward. These ripples can travel far from the Pacific, changing winds and storm paths along the way. Even Britain can feel the effects: El Niño can increase the chance of wetter and stormier weather in autumn and early winter, and also raise the risk of colder spells later in winter. But these links are less certain than many tropical impacts, because UK weather is also shaped by several other atmospheric influences.

A third route involves El Niño altering conditions in other oceans. These changes can interact with the Indian Ocean dipole, for instance, a see-saw in temperatures between the western and eastern Indian Ocean that strongly influences rainfall over East Africa and parts of India.

But El Niño alone does not determine the weather on any given day. For example, Thailand generally tends towards drier conditions during El Niño, yet parts of it, including Bangkok, experienced heavy rainfall and flooding this September. India also has shown a similar contrast: the monsoon has been unusually dry overall this year, while some regions have still experienced intense rainfall and flooding. El Niño shifts the odds, but local weather systems can still produce extremes in the opposite direction.

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This article is republished from The Conversation under a Creative Commons license. Read the original article.

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