Internal Variability Insufficient to Explain Recent Equatorial Pacific Trends

The temperature contrast between the western and eastern equatorial Pacific Ocean (hereafter zonal temperature gradient) plays a central role in driving the Walker Circulation, an atmospheric circulation pattern that affects climate across the globe. Over the past forty years, observations show that this zonal temperature gradient has strengthened. However, fewer than 1% of simulations from the latest generation of climate models reproduce this observed trend.

Two possible explanations have been proposed for this discrepancy. First, the strengthening could be a response to human-driven climate change that models fail to represent accurately. Second, it could reflect natural, long-term fluctuations of the climate system that models underestimate. Here we examine the second possibility.

We show that climate models underestimate the magnitude of low-frequency natural variability in the tropical Pacific, partly because they rarely simulate extreme El Niño events. El Niño is a recurring climate phenomenon in which the zonal temperature gradient weakens. During extreme events, this gradient can nearly vanish, which substantially increases temperature variability on decadal timescales. When we statistically account for the rarity of such extreme events in models, agreement with observations improves only modestly: even after correction, only about 5% of simulations reproduce the observed strengthening.

These results indicate that it is very unlikely that the recent strengthening of the Pacific temperature contrast arises from natural variability alone. This finding instead points to potential deficiencies in how climate models represent the tropical Pacific’s response to global warming.