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. As for the near‐surface response to SSWs, the models, however, are in good agreement as to this signal over the North Atlantic: There is no indication of a change under 4xCO2 forcing. Over the Pacific, however, the change is more uncertain, with some indication that there will be a larger mean response. 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.

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

“…4. PJOs are identified by applying a slight variation of criteria established by Hitchcock et al (2013), as the original required finer vertical resolution than available. The new metric has been validated in reanalysis to ensure that similar results are obtained in this case as to those obtained by applying the original one (not shown).…”

“…In this subsection we focus on specific events (PJOs) that are closely related to SSWs and the stratosphere-troposphere coupling (Hitchcock et al, 2013). As indicated in section 1, their strong and persistent tropospheric response explains the interest in investigating possible changes in the occurrence of these events for increasing CO 2 concentrations.…”

“…In most of these theories, the role of the circulation anomalies of the lower stratosphere was found to be extremely important to define the impact on the troposphere. Indeed, recently, Hitchcock et al (2013) defined a subset of SSWs, called Polar-night Jet Oscillation events (PJOs), which are characterized by a very persistent warm polar lower stratosphere and whose signal in the troposphere is particularly strong and persistent too.…”

The role of Barents–Kara sea ice loss in projected polar vortex changes

“…Variability in vortex strength is not only linked to stratospheric ozone concentrations, but, due to downward coupling to the troposphere, it also strongly affects mid-latitude weather. In particular, extreme states of the SPV are known to influence the phase and persistence of the North Atlantic Oscillation (NAO) and associated storm tracks and weather regimes (Baldwin and Dunkerton, 2001;Kidston et al, 2015).…”

Numerical impacts on tracer transport: A proposed intercomparison test of Atmospheric General Circulation Models

“…In this paper, we propose an update of the HS94 benchmark, designed to take into account two advances in climate research over the past 25 years. First, there has been a growing awareness that the tropospheric circulation is tightly coupled with that of the stratosphere, such that climate and weather prediction is improved by accurately representing the stratosphere (e.g., Gerber et al, 2012;Butler et al, 2018;Ayarzagüena et al, 2020). We update the HS94 recipe to allow one to explore the impact of numerics and resolution on the coupled stratosphere-troposphere circulation based on the work of Polvani and Kushner (2002).…”