In the Pacific Ocean, there is a warm current that flows eastward against all the others. It is called the North Equatorial Countercurrent, or NECC. In normal years, this band of warm water, a few hundred kilometers wide, quietly carries heat from Indonesia toward the Americas. But before the strongest El Niños of 1982-83, 1997-98, and 2015-16, unusually warm water was noticed in it. The question arose: is this just a sign of an impending event, or can the current itself trigger it?

To test this, scientists used the CESM climate model. They ran 52 paired simulations: in each forced experiment, sea surface temperatures in a narrow band along the NECC (roughly 4000 by 400 kilometers, about the area of India) were artificially raised by 1°C starting August 1. The control run remained unchanged. Then, during September, they compared how the atmosphere responded to this small heating.

It turned out that the warm water intensifies deep convection over the region: the air begins to rise faster. At an altitude of about 2 kilometers (the 800 hPa level), the vertical velocity increased from nearly zero to -0.06 Pa/s when the surface temperature reached 30°C. At an altitude of 9 kilometers (the 300 hPa level), the same thing happened. Convection shifted southward, and precipitation decreased over Indonesia and Southeast Asia. In the sea-level pressure field, a pattern resembling the Southern Oscillation emerged: lower pressure in the eastern Pacific and higher pressure in the Indian Ocean. This weakened the easterly trade winds along the equator and reduced the upwelling of cold water off the coast of South America.

These changes resemble what happens during the development of El Niño. But here they were caused not by temperature differences between the eastern and western Pacific, as in the classic Bjerknes mechanism, but by local heating along the current. Analysis of meridional cross-sections showed that the Hadley cell shifted: rising air intensified near 8°N, while subsidence between 15°N and 30°N weakened. Comparison with ERA5 reanalysis data from 1990-2020 confirmed a linear relationship between convection and sea surface temperature in the NECC region.

Before this work, it was unclear whether NECC warming was a cause or a consequence of El Niño. Now it is clear that a local temperature increase of 1°C can trigger a cascade: enhanced convection, a shift in the Hadley cell, and weakened trade winds. This is an independent mechanism, separate from the classic one. The practical benefit is that threshold relationships between convection and temperature can be used to improve El Niño forecast models, especially for early warning of strong events.

The warm current turned out to be not just a witness but a trigger. A band of water that in normal years merely transports heat can, with a slight warming, alter weather across the entire planet.