El Nino in a Changing Climate

Abstract

El Ni? events, characterized by anomalous warming in the eastern equatorial Pacific Ocean, have global climatic teleconnections and are the most dominant feature of cyclic climate variability on subdecadal timescales. Understanding changes in the frequency or characteristics of El Ni? events in a changing climate is therefore of broad scientific and socioeconomic interest. Recent studies1, 2, 3, 4, 5 show that the canonical El Ni? has become less frequent and that a different kind of El Ni? has become more common during the late twentieth century, in which warm sea surface temperatures (SSTs) in the central Pacific are flanked on the east and west by cooler SSTs. This type of El Ni?, termed the central Pacific El Ni? (CP-El Ni?; also termed the dateline El Ni?2, El Ni? Modoki3 or warm pool El Ni?5), differs from the canonical eastern Pacific El Ni? (EP-El Ni?) in both the location of maximum SST anomalies and tropical？idlatitude teleconnections. Here we show changes in the ratio of CP-El Ni? to EP-El Ni? under projected global warming scenarios from the Coupled Model Intercomparison Project phase 3 multi-model data set6. Using calculations based on historical El Ni? indices, we find that projections of anthropogenic climate change are associated with an increased frequency of the CP-El Ni? compared to the EP-El Ni?. When restricted to the six climate models with the best representation of the twentieth-century ratio of CP-El Ni? to EP-El Ni?, the occurrence ratio of CP-El Ni?/EP-El Ni? is projected to increase as much as five times under global warming. The change is related to a flattening of the thermocline in the equatorial Pacific.