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.

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