CLEANER AIR, STRONGER PACIFIC WINDS

The tropical Pacific sets much of the climate’s year-to-year rhythm. Its trade winds and the Walker circulation — a giant loop of air that rises over the warm western Pacific and sinks over the east — decide where tropical rain falls. Out of this ocean–atmosphere machine grows El Niño, which shifts rain and temperature on every inhabited continent.

Anything that changes the Pacific’s average state can therefore change climate risks far away. One such change is coming: as countries tackle air quality, emissions of aerosols — tiny polluting particles that send part of the sunlight back to space and so cool the surface below — and of the gases that form low-level ozone are expected to fall steeply, most of all in Asia. What that does to the Pacific’s circulation has been contested: some studies suggested that removing aerosols would weaken it, others found no change on average. None looked at El Niño.

Twelve models, two futures

Ioana Colfescu, of the National Centre for Atmospheric Science at the University of St Andrews, and colleagues from St Andrews, Edinburgh and Bologna used experiments designed for this very question. In both, greenhouse gases keep rising along the same path. In one, air pollution stays high; in the other, strict air-quality controls cut it. The difference isolates the effect of cleaner air.

They analysed 12 Earth system models, counting each model once in every statistical test.

Stronger trades after 2040

By 2036–2054, cleaner air strengthens the equatorial Pacific trade winds:

• the push of the wind on the western Pacific grows by about 4%, in 11 of the 12 models;
• the pressure difference that drives the Walker circulation steepens, again in 11 of 12;
• the tilt of the ocean surface and of the thermocline — the boundary between warm surface water and cold deep water, traced here by the depth at which the water is at 20 °C — steepens in every model that provides ocean data: stronger trades pile more warm water up in the west.

Before 2035, none of this stands out from natural variability; the signal only clears the noise in the 2040s. It is also seasonal, strongest from May to December.

The scale matters. Without the cuts, the same models project the western-Pacific trades to weaken by mid-century. Cleaner air cancels about half to two-thirds of that weakening — the uncertainty does not even rule out a complete offset.

The Indian Ocean connection

When three of the models are run with sea temperatures held fixed, the effect vanishes, even though Asia still receives more sunshine at the surface through a sky cleared of aerosols. So the ocean must be part of the chain.

The clue the authors point to: under the cuts, the Indian Ocean warms 0.045 °C more than the tropical Pacific, mostly from August to December — exactly when the trades strengthen most. Earlier work showed that an Indian Ocean warmer than the Pacific drives easterly winds — winds blowing from the east, like the trades — over the western Pacific. Experiments that remove aerosols one region at a time agree: cutting aerosols over Africa and the Middle East, which warms the Indian Ocean relative to the Pacific, strengthens the trades; cutting them over East Asia, which cools it slightly, does not.

The authors stay careful. Within each model, this link is hard to separate from El Niño itself, and they write that they have not shown the Indian Ocean warming causes the response.

El Niño holds still, its surroundings do not

El Niño’s strength — the typical size of its sea-surface temperature swings in the central equatorial Pacific — changes by −0.014 °C, statistically nothing: any real change is smaller than about a tenth of its amplitude. The same test does detect the larger change driven by greenhouse gases, so it is not blind. An AI emulator of El Niño, trained without ever seeing the clean-air simulations, predicts them well, and predicts no change either. Why? In these models, El Niño is a damped oscillation — one that would die out on its own — kept going by weather noise, and a 4% change in the trades is too small to alter it.

The way El Niño acts on distant regions shows no detectable change either. Yet its effects still change, because the climate it acts on does. Under the cuts, the background warms by 0.12 to 0.29 °C over India, the Maritime Continent, East Africa and the southern United States. Over north-eastern Brazil, the chance that an El Niño season lands in the hottest third of temperatures of the future without cuts rises from 0.75 to 0.96. Rainfall shifts in India and southern Africa are only suggestive.

The authors’ practical message: forecasts and impact studies that keep the background climate fixed would misjudge the heat that comes with El Niño in these regions. Their caveat: only 8 or 9 models provided ocean data for the clean-air scenario, and a change in El Niño smaller than a tenth of its amplitude would slip through their test.