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

Abstract

Abstract. The connection between the dominant mode of interannual variability in the tropical troposphere, the El Niño–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 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, some models find that this moistening persists, and 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, the signal focused on in much previous work, is simulated by only half of the models. Focusing on Central Pacific ENSO vs. East Pacific ENSO, or temperatures in the mid-troposphere compared with 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 the 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.

The roles of the Quasi-Biennial Oscillation and El Nino for entry stratospheric water vapour in observations and coupled chemistry-ocean CCMI and CMIP6 models

“…We only include CMIP6 models with interactive stratospheric chemistry as such a coupled chemistry-climate configuration has been shown to lead to more robust interannual variability of temperatures in the lower stratosphere as compared to models with fixed ozone (Yook et al, 2020). Note that -v02.6-198401-201912-latpress-2.5deg-L31 1994-2019 3D combinedanomh2oq swoosh-v02.6-198401-201912-lonlatpress-20deg-5deg-L31 2005-2019 most of the models nevertheless simulate a too-warm cold point and too-little interannual variability of entry water (Garfinkel et al, 2021).…”

“…All indices are deseasonalized by removing the long term monthly means. We do not consider seasonality in this work in order to maximize the degrees of freedom, though we certainly acknowledge that the regression coefficients for, say, ENSO change sign between midwinter and late spring (Garfinkel et al, 2018(Garfinkel et al, , 2021. For all of these predictor time series, we divide by the standard deviation before constructing a MLR or ML model.…”

“…We now consider the ability of CMIP6 and CCMI models to represent this connection, and for simplicity we focus on a simple regression of the QBO with entry water. (The ability of these models to represent the connection between ENSO and entry water vapor was considered in Garfinkel et al (2021) in detail. )…”

Contrasting Effects of Indian Ocean Basin and Dipole Modes on the Stratosphere

“…But the point to make here is, using a set of model simulations, Garfinkel et al. (2021) finds that only half of the models they considered show the moistening in the spring following El Niño events, indicating a complex and uncertain relationship between the ENSO and SWV. Anomalous propagation of Rossby waves induced by ENSO can reach the mid‐ to high‐latitude stratosphere, whereupon the waves exert influences on the polar vortex (e.g., Free & Seidel, 2009; Garfinkel & Hartmann, 2007; Li et al., 2016; Sassi et al., 2004; Taguchi & Hartmann, 2006; Yang et al., 2015).…”

The roles of the Quasi-Biennial Oscillation and El Niño for entry stratospheric water vapor in observations and coupled chemistry–ocean CCMI and CMIP6 models

“…While the lower stratosphere clearly was moister following the two largest EN events in the satellite era (in 1997/1998 and in 2015/2016) (Fueglistaler and Haynes, 2005a;Avery et al, 2017), moistening also was evident following two of the strongest La Niña events (in 1998/1999 and 1999/2000). The impact of more moderate events is less clear, and any net effect is not statistically significant considering the shortness of the data record (Garfinkel et al, 2018(Garfinkel et al, , 2021. Both La Niña and El Niño can lead to a moistening if the cold point moves zonally within the tropics (to the central Pacific for El Niño and to the far western Pacific for La Niña), and even though the lower stratospheric temperature response is opposite for El Niño and La Niña, the cold point appears to have warmed for both strong El Niño and strong La Niña events, explaining the moistening in 1997/1998, 1998/1999, 1999/2000and 2015(Garfinkel et al, 2018.…”