Influence of the El Niño-Southern Oscillation on entry stratospheric water vapor in coupled chemistry-ocean CCMI and CMIP6 models

The connection between the dominant mode of interannual variability in the tropical troposphere, the El Nino-Southern Oscillation (ENSO), and the entry of stratospheric water vapor is analyzed in a set of model simulations archived for the Chemistry-Climate Model Initiative (CCMI) project and for Phase 6 of the Coupled Model Intercomparison Project. While the models agree on the temperature response to ENSO in the tropical troposphere and lower stratosphere, and all models and observations also agree on the zonal structure of the temperature response in the tropical tropopause layer, the only aspect of the entry water vapor response with consensus in both models and observations is that La Nina leads to moistening in winter relative to neutral ENSO.

For El Nino and for other seasons, there are significant differences among the models. For example, some models find that the enhanced water vapor for La Nina in the winter of the event reverses in spring and summer, some models find that this moistening persists, and some show a nonlinear response, with both El Nino and La Nina leading to enhanced water vapor in both winter, spring, and summer. A moistening in the spring following El Nino events, the signal focused on in much previous work, is simulated by only half of the models.