Contrasting Effects of Central Pacific and Eastern Pacific El Niño on stratospheric water vapor

Targeted experiments with a comprehensive chemistry-climate model are used to demonstrate that seasonality and the location of the peak warming of sea surface temperatures dictate the response of stratospheric water vapor to El Niño. In boreal spring, El Niño events in which sea surface temperature anomalies peak in the eastern Pacific lead to a warming at the tropopause above the warm pool region, and subsequently to more stratospheric water vapor (consistent with previous work). However, in fall and in early winter, and also during El Niño events in which the sea surface temperature anomaly is found mainly in the central Pacific, the response is qualitatively different: temperature changes in the warm pool region and specifically over the cold point region are nonuniform, and less water vapor enters the stratosphere. The difference in water vapor in the lower stratosphere between the two variants of El Niño approaches 0.3 ppmv, while the difference between the winter and spring responses exceeds 0.5 ppmv.