Influence of ENSO 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, El Niño Southern
Oscillation (ENSO), and entry of stratospheric water vapor, is analyzed in a set of the 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 also agree
 on the zonal structure of the response in the tropical tropopause layer, the only aspect of the entry water vapor with consensus
is that La Niña leads to moistening in winter relative to neutral ENSO. For El Niño and for other seasons there are significant
differences among the models. For example, some models find that the enhanced water vapor for La Niña in the winter of the
event reverses in spring and summer, other models find that this moistening persists, while some show a nonlinear response
with both El Niño and La Niña leading to enhanced water vapor in both winter, spring, and summer. A moistening in the spring
 following El Niño events, perhaps the strongest signal in observations, is simulated by only half of the models. Focusing on
Central Pacific ENSO versus East Pacific ENSO, or temperatures in the mid-troposphere as compared to temperatures near the
surface, does not narrow the inter-model discrepancies. Despite this diversity in response, the temperature response near the
cold point can explain the response of water vapor when each model is considered separately. While the observational record is
too short to fully constrain the response to ENSO, it is clear that most models suffer from biases in the magnitude of interannual
variability of entry water vapor. This bias could be due to biased cold point temperatures in some models, but others appear to
be missing forcing processes that contribute to observed variability near the cold point