A modern climate model is used to test the hypothesis that changes observed during El Niños are, in part, forced by changes in the temperature of the North Equatorial Counter Current. This is a warm current that flows eastwards across the Pacific, a few degrees north of the Equator, close to the Inter-Tropical Convection Zone, a major region of deep atmospheric convection. The tests generate a significant El Niño type response in the ocean, giving confidence that the hypothesis is correct.

Research on strong El Niños has shown that they may be a result of a stronger-than-normal North Equatorial Counter Current, itself triggered by lower-than-normal sea levels that develop early in the year. A numerical model study of the 1981–1982 El Niño shows that the low sea levels are due to local winds in the west Pacific, and this is shown also to be true for the 1997–1998 and 2015–2016 El Niños. As a result, we now have a much better understanding of the mechanism causing strong El Niños.

Results from a high-resolution ocean model show that during the strong El Niños of 1983 and 1998, transport of warm water in the equatorial Pacific was dominated by the North Equatorial Counter Current and not by equatorial Kelvin waves. The results show why the NECC fails to do this in most years and how stronger than normal annual Rossby waves near the Equator can both trigger the El Niño in the western Pacific and help to ensure that the warm water arrives off South America around Christmas.