Significant Stratospheric Moistening Following Extreme El Niño Events

The moistening impact of El Niño on the tropical lower stratosphere has been extensively studied, yet a long-standing challenge is its potential nonlinearities regarding the strength of El Niño. Extreme El Niño’s hydration in 2015/2016 was unprecedented in the satellite era, providing a great opportunity to distinguish the differential response of water vapor to extreme and moderate El Niño. Using ERA5 and MERRA-2 reanalysis data from 1979–2019, we compare the composite tropical lower stratospheric water vapor anomalies throughout all extreme and moderate El Niño episodes since the satellite era. We validate the variations in the lower stratospheric water vapor during the two distinct El Niño episodes using a three-dimensional chemistry transport model simulating the same period. The model reproduces the observed pattern in lower stratospheric water vapor. Both demonstrate that robust moistening during extreme El Niño events occurs throughout the tropical lower stratosphere. However, moderate El Niño events seem to have a weak effect on lower stratospheric water vapor. In comparison to moderate El Niño, the strong convective activities induced by extreme El Niño release large amounts of latent heat, causing extensive and intense warming in the tropical upper troposphere and lower stratosphere, thus greatly increasing the water vapor content in the tropical lower stratosphere. Additionally, moderate El Niño events have strong seasonality in their hydration effect in the tropics, whereas the intense moistening effect of extreme El Niño events prevails in all seasons during their episodes.