Changes in El Niño characteristics and air–sea feedback mechanisms under progressive global warming

In this study, we investigate the potential changes of El Niño characteristics, including intensity, frequency and CP/EP El Niño ratio, under progressive global warming based on the 140-year CMIP6 model simulation outputs with the 1pctCO2 experiment. Major air-sea feedback mechanisms attributing to the changes are also examined. The CMIP6 ensemble means project a slight enhancement of El Niño intensity by about 2% and a modest increase of El Niño frequency by about 4% from the first to the second 70-year periods. It is found that these small changes result from the opposite response to global warming between CP and EP El Niño, i.e., the intensity of EP El Niño is projected to weaken by nearly 4.6% while the intensity of CP El Niño is projected to increase by about 4.5%. Since CP El Niño occurs more frequently than EP El Niño in CMIP6 simulations, this leads to a slight enhancement of the total El Niño intensity if these two types of El Niño were not separated. A similar situation occurs in projecting the future change of El Niño frequency, i.e., the frequency of EP El Niño is projected to decrease by about 1.4% while the frequency of CP El Niño is projected to increase by about 2%, thereby leading to a modest increase of the total El Niño frequency. By comparing the variance explained by key air-sea feedback mechanism between the two 70-year periods, we also note that the increased CP/EP ratio can be explained by the enhanced role played by the SF (seasonal footprinting) mechanism in a warmer atmosphere. Our study also points out that, as long as a climate model can correctly produce the intensity (variance) of major air-sea feedback mechanisms, the relationship between changes in El Niño characteristics and changes in feedback mechanisms can be physically robust.