This work continues a line of research that began with the detection of anomalously warm water in the North Equatorial Countercurrent (NECC) before the strong El Niño events of 1982-83, 1997-98, and 2015-16. Previous studies based on satellite data and a single model run showed that warm water is transported from the Western Pacific Warm Pool by an intensified NECC, and then influences deep atmospheric convection over the Intertropical Convergence Zone (ITCZ). This study stands somewhat apart from neighboring research: those deal with El Niño prediction, ocean circulation, or thermal processes on icy moons, whereas here the focus is on a specific mechanism of ocean-atmosphere coupling in the tropical Pacific.
Version 2.1.3 of the CESM climate model was used. After a long control run, 52 paired experiments were performed: in each forced run, the sea surface temperature in a rectangular region (180°E–240°E, 4°N–7.5°N) was increased by 1°C relative to the control run, starting from August 1. Then, during September, key fields were averaged, and the difference between the forced and control runs was used to estimate the mean response and the signal-to-noise ratio. Additionally, comparisons were made with ERA5 reanalysis data for 1990–2020.
The forcing led to a significant intensification of deep convection over the affected area: at 800 hPa, the vertical velocity (Omega) dropped from near zero at 28°C to -0.06 Pa/s at 30°C, and at 300 hPa from near zero at 29°C to -0.06 Pa/s at 30°C. Convection shifted southward, while decreasing over Indonesia and Southeast Asia. The sea-level pressure field developed a pattern resembling the Southern Oscillation: low pressure in the eastern Pacific and high pressure in the Indian Ocean. This weakened the easterly trade winds along the equator and reduced upwelling. Analysis of meridional cross-sections revealed changes in the Hadley cell: enhanced rising motion near 8°N and weakened subsidence between 15°N and 30°N. Comparison with ERA5 confirmed a linear relationship between convection and sea surface temperature in the NECC region.
The work confirms that NECC-ITCZ interaction can be an independent mechanism for El Niño development, separate from the classic Bjerknes mechanism based on the east-west temperature gradient. Unlike neighboring studies that view El Niño as a response to changes in the cold tongue or as part of the general circulation, here it is shown that local warming along a narrow current band can trigger a large-scale atmospheric response. This shifts the focus from the cold tongue to the warm countercurrent as a potential trigger.
Further research could test whether the sensitivity of convection to NECC temperature is reproduced in other climate models, and how interactions with tropical instability waves affect heat transport. It may also be worthwhile to study the role of the NECC in central-Pacific El Niño events, which have become more frequent in recent decades.
The results are important for forecasting El Niño and its global impacts, including droughts in India and Australia, as well as for understanding changes in the Hadley cell under climate shifts.
The next step could be to incorporate NECC temperature anomalies into El Niño forecast models and test the predictive power of such an approach on historical data.
The work addresses a fundamental problem in climatology: what processes trigger El Niño and why some events become strong while others do not. Traditional theories based on the Bjerknes mechanism do not always explain the diversity of events, and the proposed mechanism via NECC-ITCZ may fill this gap.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
Using 52 paired CESM runs, the study shows that artificially warming the ocean surface along the NECC path by 1°C strengthens deep convection, alters the pressure field, and weakens the trade winds, consistent with ERA5 reanalysis data and supporting the hypothesis that the NECC-ITCZ interaction plays a role in the development of strong El Niño events.
Where its strength is
The strength of this work lies in its controlled model experiment, which isolates the contribution of a specific mechanism (local warming along the NECC) independently of the classical Bjerknes feedback, and in its consistency with observations.
Where it could go next
Worth trying
Why this may matter
If the NECC-ITCZ interaction is indeed an independent trigger of El Niño, this could change approaches to El Niño forecasting and our understanding of tropical climate variability.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.53, against a usual 0.64. It looks like few have drilled nearby.
The forcing region in the model is roughly 4000×400 km, comparable to the area of India. Raising the temperature by just 1°C in this strip of ocean caused changes in atmospheric pressure that were even noticeable near the Drake Passage.