Elevated stratopause events in the current and a future climate: A chemistry-climate model study

Abstract

The characteristics and driving mechanisms of Elevated Stratopause Events (ESEs) are examined in simulations of the ECHAM/MESSy Atmospheric Chemistry (EMAC) chemistry-climate model under present and projected climate conditions. ESEs develop after sudden stratospheric warmings (SSWs) in boreal winter. While the stratopause descends during SSWs, it is reformed at higher altitudes after the SSWs, leading to ESEs in years with a particularly high new stratopause. EMAC reproduces well the frequency and main characteristics of observed ESEs. ESEs occur in 24% of the winters, mostly after major SSWs. They develop in stable polar vortices due to a persistent tropospheric wave forcing leading to a prolonged zonal wind reversal in the lower stratosphere. By wave filtering, this enables a faster re-establishment of the mesospheric westerly jet, polar downwelling and a higher stratopause. We find the presence of a westward-propagating wavenumber-1 planetary wave in the mesosphere following the onset, consistent with in-situ generation by large-scale instability. By the end of the 21st century, the number of ESEs is projected to increase, mainly due to a sinking of the original stratopause after strong tropospheric wave forcing and planetary wave dissipation at lower levels. Future ESEs develop preferably in more intense and cold polar vortices, and tend to be shorter. While in the current climate, planetary wavenumber-2 contributes to the forcing of ESEs, future wave forcing is dominated by wavenumber-1 activity as a result of climate change. Hence, a persistent wave forcing seems to be more relevant for the development of an ESE than the wavenumber decomposition of the forcing.

The Climatology of Elevated Stratopause Events in the UA‐ICON Model and the Contribution of Gravity Waves

“…From the composite plots presented in Figures 5 and 6, we found some features that are listed below. These points broadly agree with previous studies with EMAC and WACCM (Chandran, Collins, et al., 2013; Limpasuvan et al., 2016; Scheffler et al., 2022), however, also some differences can be found. It is worthwhile to mention that generally the differences between ES and SSW‐only events are not statistically significant.…”

“…The enhancement in the altitude of the reformed stratopause is consistent with the stronger residual circulation at about 70–100 km around and after day 0 in both types of events, although the enhancement in the residual circulation around and after day 0 is stronger in ESEs compared to SSW‐only events, as shown in Figure 7g. While there is a clear jump in the stratopause height following ES events, the reformed stratopause only weakly shifts upward during SSW‐only events which is consistent with the previous studies (Chandran, Collins, et al., 2013; Scheffler et al., 2022). In general, following the SSWs, the stratosphere is colder in ES composites compared to the SSW‐only around and after day 0 (Figure 7a).…”

“…This might be due to the relatively small number of events considered in the present study and also in the study of Scheffler et al. (2022) and given a large wind variability during and around SSW onset, such differences between models are expected. The easterlies extend to the mesosphere (approximately up to about 80 km) in the composites of ESEs but the easterlies are limited to below 60 km in the case of SSW‐only events.…”

“…With the improvements in the spatial and temporal resolutions of middle and upper atmospheric satellite data, detailed analyses of ESEs have become feasible in recent years (France, Harvey, Alexander, et al, 2012;Manney et al, 2008;Naren Athreyas et al, 2022;Orsolini et al, 2010;Shi et al, 2022). Modeling studies with WACCM (France & Harvey, 2013;Limpasuvan et al, 2016;Orsolini et al, 2017), the Canadian Middle Atmosphere Model (CMAM; McLandress et al, 2013), a high-resolution middle atmosphere GCM developed for the KANTO project (Tomikawa et al, 2012), the ECHAM/MESSy Atmospheric Chemistry (EMAC; Scheffler et al, 2022) and with the Japanese Atmospheric General circulation model for Upper Atmosphere Research (JAGUAR, Okui et al (2021)) provided insights into the sequence of events associated with the occurrence of a typical ES event. For example, Limpasuvan et al (2016) using WACCM showed that while PWs are important at the onset of ESEs, GWs are crucial in explaining the residual circulation before and after ESEs.…”

Zonal Asymmetry of the Stratopause in the 2019/2020 Arctic Winter

“…Descent-ascent-descent of the polar stratopause usually occur during the onset and recovery phases of the SSW events, when the zonal wind at 60°N reverses to easterly and the polar vortex breaks down [4,21,22,32]. Special attention was paid to the elevated stratopause events characterized by a rapid reformation of the polar stratopause near 75-80 km and its subsequent gradual descent over a period of several weeks [3,7,23,32,51]. Because of the absence of the zonal wind reversal, this event is not typical for winter 2019/2020 (Figure 3).…”