Uncertainty in the response of sudden stratospheric warmings and stratosphere-troposphere coupling to quadrupled CO2 concentrations in CMIP6 models

Abstract

Major sudden stratospheric warmings (SSWs), vortex formation and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for stratosphere-troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO2 forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO2 (4xCO2) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above.In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO2 forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically significant change. The models, however, are in good agreement as to the impact of SSWs over the North Atlantic: there is no indication of a change under 4xCO2 forcing. Over the Pacific, however, the change is more uncertain. Finally, the models show robust changes to the seasonal cycle in the stratosphere. Specifically, we find a longer duration of the stratospheric polar vortex, and thus a longer season of stratosphere-troposphere coupling.

GISS Model E2.2: A Climate Model Optimized for the Middle Atmosphere—2. Validation of Large‐Scale Transport and Evaluation of Climate Response

“…As described in R20, E2.2 consists of 102 vertical levels spanning the surface up to 0.002 hPa, as compared to the lower vertical resolution of ModelE (E2.1), which consists of 40 levels extending up to 0.1 hPa. Orographic and non‐orographic gravity wave drag (GWD) is parameterized following Lindzen (1984) and Rind et al (1988), producing in E2.2 a QBO that compares well with observations as well as improved stratospheric polar vortex variability (Ayarzagüena et al, 2020; R20). We refer the reader to R20 for an in‐depth discussion of the model.…”

“…Among the different model versions discussed in R20 here we focus on the “Altered‐Physics” (‐AP) Version E2.2‐AP because this is the configuration that was submitted to CMIP6 and presented in Ayarzagüena et al (2020). While this version does differ from the “standard” model Version E2.2 in certain respects (i.e., convective mass flux profiles, high cloud cover, planetary albedo, and shortwave absorbed at the surface) the climatologies of both model versions agree overall quite well, especially with respect to their stratospheric transport properties, as discussed in sections 3 and 4.…”

“…Finally, there is some indication of larger interannual ozone variability in E2.2‐AP over the Northern Hemisphere, relative to E2.1. How (if) this is linked to the more realistic polar vortex variability in that model (R20, Ayarzagüena et al, 2020) will be examined in future studies.…”

“…The dynamical responses in the E2.2(‐AP) CO 2 experiments were discussed only briefly in R20, specifically with regard to changes in global mean surface temperature and the response of the North Atlantic meridional overturning circulation. The only study examining the stratospheric response so far (Ayarzagüena et al, 2020) only did so with respect to stratospheric polar vortex variability (i.e., sudden warming events). Therefore, though not exhaustive in our analysis, here we take the opportunity to present basic measures of the stratospheric and tropospheric dynamical responses that were not examined in the previous studies but are important for interpreting the transport response, which is the focus of this study.…”

Stratospheric drivers of extreme events at the Earth’s surface

“…However, in the NH, the amplified surface warming in the Arctic reduces the surface equator-to-pole temperature gradient at the surface and shifts the storm tracks equatorward, leading to a "tug of war" between the upper and lower tropospheric temperature gradient changes [123][124][125] . These counteracting influences contribute to highly uncertain trends in the stratospheric polar vortex, with no consensus among climate models on whether the vortex will strengthen or weaken 126 . The uncertain future of the NH polar stratosphere is linked to uncertainties in the projection of the tropospheric jet streams and associated climate extremes 127,128 .…”