CMIP5 Projections of Two Types of El Niño and Their Related Tropical Precipitation

Abstract:

Future projections of the frequencies and amplitudes of the Eastern-Pacific (EP) and Central-Pacific (CP) types of El Niño-Southern Oscillation (ENSO), as well as their associated tropical precipitation variability, have been studied based on historical runs from the Coupled Model Intercomparison Project phase 5 (CMIP5) and model simulations for the 21st century corresponding to the Representative Concentration Pathways (RCP) 8.5. EP- and CP-El Niño events, as well as their related tropical circulation anomalies, were identified based on empirical orthogonal function (EOF) analysis of the tropical Pacific sea surface temperature (SST). Results from the historical runs indicate that while the majority of CMIP5 models are able to capture the EP-ENSO SST anomalies pattern, only a handful of models can reproduce that associated with CP-ENSO. Based on their performance in capturing the tropical Pacific SST variability, the best models for simulating the 2 types of ENSO were selected, and their projections for the 21st century climate state were further examined. There is evidence that SST anomalies associated with EP-ENSO will become weaker in the future climate; on the other hand, there seems to be no significant change in the CP-ENSO-related SST. There also exists no consensus on how the relative frequency of CP to EP El Niño may change in the future. However, the rainfall response to EP-El Niño (CP-El Niño) was found to be enhanced over the central-to-eastern (central-to-western) equatorial Pacific, consistent with the fact that the mean hydrological cycle will become stronger under a warmer background climate. Walker circulation anomalies associated with the two types of El Niño are also modified. The relationship between these circulation changes related to ENSO and those in the basic state due to a warmer climate will be discussed.