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.
Highlights
Water vapor is the gas with most important greenhouse effect in the atmosphere, and the feedback associated with stratospheric water vapor in response to increasing anthropogenic greenhouse gas emissions is around half of that for global mean surface albedo or cloud feedbacks (Forster and Shine, 1999; Solomon et al, 2010; Dessler et al, 2013; Banerjee et al, 2019; Li and Newman, 2020)
This study focuses on the impact of El Niño–Southern Oscillation (ENSO) on the stratosphere on interannual timescales; in order to remove any impacts on longer timescales due to climate change and to remove any linear impacts from the quasi-biennial oscillation, which is known to affect water vapor (Reid and Gage, 1985; Zhou et al, 2001, 2004; Fujiwara et al, 2010; Liang et al, 2011; Kawatani et al, 2014; Brinkop et al, 2016), we first use multiple linear regression (MLR) to remove the linear variability associated with greenhouse gases and the QBO from all time series
The goal of this study is to understand how the comprehensive models that are used for the projection of future ozone and climate capture the connection between the dominant mode of interannual variability in the tropical troposphere, the El Niño–Southern Oscillation (ENSO), and entry stratospheric water vapor
Summary
Water vapor is the gas with most important greenhouse effect in the atmosphere, and the feedback associated with stratospheric water vapor in response to increasing anthropogenic greenhouse gas emissions is around half of that for global mean surface albedo or cloud feedbacks (Forster and Shine, 1999; Solomon et al, 2010; Dessler et al, 2013; Banerjee et al, 2019; Li and Newman, 2020). Lower-stratospheric water vapor concentrations are mainly determined by the tropical temperatures near the cold point, where dehydration takes place as air parcels transit into the stratosphere (Mote et al, 1996; Zhou et al, 2004, 2001; Fueglistaler and Haynes, 2005b; Fueglistaler et al, 2009; Randel and Park, 2019). Several different processes have been shown to influence these cold-point temperatures, and the goal of this work is to revisit the influence of one of these processes – the El Niño–Southern Oscillation (ENSO) – on entry water vapor in the lower stratosphere. In the tropical tropopause layer (TTL), water vapor increases in the region with warm anomalies and decreases in the region with cold anomalies by ∼ 25 % (Gettelman et al, 2001; Hatsushika and Yamazaki, 2003; Konopka et al, 2016)
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