ENSO — the El Niño-Southern Oscillation — is the largest source of year-to-year climate variability on Earth. It shifts rainfall patterns across the tropics, modulates global temperatures, and drives economic consequences from fisheries to agriculture. Whether ENSO will intensify or weaken as the climate warms has been one of the most consequential open questions in climate science, and the answer from different models has been contradictory.
Tuckman and Yang (arXiv:2603.03458, March 2026) show that both answers are correct — at different times. ENSO first intensifies under greenhouse warming, then weakens. The trajectory is not monotonic. It rises and falls.
The rise comes from enhanced upper-ocean stratification. As the surface warms faster than the deep ocean, the temperature gradient between layers steepens. A steeper stratification amplifies the thermocline feedback — the mechanism by which subsurface temperature anomalies influence the surface. Stronger thermocline feedback means larger ENSO swings.
The fall comes from two competing mechanisms that eventually overtake the stratification effect. The Walker circulation — the east-west atmospheric pressure gradient across the Pacific — weakens under sustained warming, reducing the wind-driven coupling that drives ENSO oscillations. Simultaneously, stronger surface flux damping (warm surfaces radiate more efficiently) suppresses temperature anomalies. Both effects grow with cumulative warming and eventually dominate.
The result that interests me most is the rate dependence. Faster emissions produce stronger peak ENSO variability even when total emissions are identical. The same total CO2 released over 50 years produces a more extreme ENSO peak than the same total released over 200 years. The ocean's transient response — the disequilibrium between surface and deep warming — is what drives the intensification, and faster forcing sustains that disequilibrium longer. The rate of change changes the outcome, not just the amount.
A lag-linear model using only global mean sea surface temperature explains approximately 90% of simulated ENSO changes. The predictive power of a single variable suggests the dynamics, despite appearing complex, reduce to the relationship between current warming and its recent history. ENSO's future isn't determined by where the climate is. It's determined by how fast it got there.
Tuckman and Yang, "The Rise and Fall of ENSO in a Warming World," arXiv:2603.03458 (March 2026).