Super El Niño: Why 3 Degrees of Pacific Warming Rattles Global Weather

The equatorial Pacific has been warming steadily since September. Chinese climate monitors say it is on track to become the strongest El Niño on record, likely peaking around November.

Sea-surface temperatures in the central and eastern equatorial Pacific have risen more than 3 degrees Celsius above normal. According to China's National Climate Center, this is the first time the index has crossed that level on record at the seasonal scale. Forecasters expect the warming to keep fluctuating upward.

That raises an obvious question: A two or three degree swing in the air feels unremarkable, so how can a slightly warmer stretch of ocean disrupt weather around the world? Two things are easy to overlook.

First, El Niño is not a small patch of warm water. It is a temperature anomaly spanning a vast stretch of the equatorial Pacific, and it usually lasts for months. Second, the impact depends not just on how much the water warms, but on where. The tropical ocean is a key source of heat and moisture for the atmosphere. When the water is warmer, more moisture rises into the air, feeding clouds and rain. That, in turn, changes where air rises and sinks. El Niño shifts those zones, rearranging winds, pressure and rainfall across enormous regions.

Normally, the equatorial Pacific is warm in the west and cool in the east. Trade winds blow from east to west, piling warm surface water up near Indonesia. Meanwhile, cold water from deeper down wells up in the east to keep it cooler. The warm west feeds rising air and heavy rain. The air then flows east at high altitude and sinks over the cooler east. This east-west loop is known as the Walker circulation.

During El Niño, those trade winds weaken. Warm water spreads toward the central and eastern Pacific, and the upwelling of cold water that usually cools the surface fades. The zones of thunderstorms and heavy rain shift east with it. The changes then reinforce one another: shifting pressure weakens the trade winds further, which pushes still more warm water east. Ocean and atmosphere feed on each other. That is why El Niño is not simply "warmer sea water" but a coupled process.