Past and Future Climate Trends in the Tropical Pacific: Equatorial Cooling Versus an El Niño-like State

The response of the tropical Pacific to global warming remains an intensely debated topic with a high degree of uncertainty. Over the past three decades, there has been a pronounced cooling trend in the eastern equatorial Pacific and off the coast of South America, along with a strengthening of the Walker circulation—climate phenomena that climate models cannot replicate. Does this reflect the ocean dynamical thermostat, initially proposed by Clement, Seager, Cane, and Zebiak (1996) and extended in subsequent studies, or is it due to internal variability in the system, or perhaps both? On the other hand, by the end of the 21st century, the majority of climate models suggest a robust warming pattern in the eastern equatorial Pacific (EP) and a weaker Walker circulation. Likewise, proxy observations for the warm Pliocene—the most recent geological epoch with atmospheric CO2 concentrations close to or slightly higher than 400 ppm—show a significant reduction in the Pacific east-west SST gradient, indicating a permanent El Niño-like state (a term introduced by Molnar and Cane 2002). Most climate models cannot reproduce the full extent of this SST gradient reduction for CO2 levels similar to today’s, nor can they replicate the pronounced changes in the hydrological cycle that led to much wetter conditions over western North America during the Pliocene. This contrasts with climate projections for the future of this region, which predict much drier conditions. I will discuss these problems in the context of contemporary climate change and efforts to reduce uncertainty in future climate projections.