El Nĩo in a changing climate

El Nĩo 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 Nĩo events in a changing climate is therefore of broad scientific and socioeconomic interest. Recent studies show that the canonical El Nĩo has become less frequent and that a different kind of El Nĩo 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 Nĩo, termed the central Pacific El Nĩo (CP-El Nĩo; also termed the dateline El Nĩo, El Nĩo Modoki or warm pool El Nĩo), differs from the canonical eastern Pacific El Nĩo (EP-El Nĩo) in both the location of maximum SST anomalies and tropical-midlatitude teleconnections. Here we show changes in the ratio of CP-El Nĩo to EP-El Nĩo under projected global warming scenarios from the Coupled Model Intercomparison Project phase 3 multi-model data set. Using calculations based on historical El Nĩo indices, we find that projections of anthropogenic climate change are associated with an increased frequency of the CP-El Nĩo compared to the EP-El Nĩo. When restricted to the six climate models with the best representation of the twentieth-century ratio of CP-El Nĩo to EP-El Nĩo, the occurrence ratio of CP-El Nĩo/EP-El Nĩo 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.