Stratosphere-Troposphere Coupling and Global Warming

The atmospheric circulation response to the anthropogenic increase of CO₂ is known to generally involve a number of pathways of change, often leading to contrasting responses, the so called “tug of wars”. Multiple pathways of change in atmospheric dynamics are due to the development of direct and indirect dynamical responses to global warming, the latter being the thermodynamical response to the anthropogenic increase of CO₂. The most known direct dynamical response to global warming is the strengthening and poleward shift of the tropospheric eddy-driven jets, a direct consequence of the tropical upper troposphere warming. During Northern Hemisphere winter, where and when the two-way dynamical stratosphere-troposphere coupling is active, stratosphere-troposphere coupling has been shown to provide for indirect dynamical responses, which are relevant to regional circulation changes, over the North Atlantic and Eurasia, for instance affecting the European precipitation changes. However, it has been found that the dynamical response of the stratospheric vortex is highly uncertain. Changes from ensemble of models (such as the CMIP intercomparisons) show a spread in the stratospheric responses, although the stratospheric response can be significant by model. This spread in the stratospheric responses in turn leads to uncertainty in the impact on surface climate of the stratospheric change. For instance, a stratospheric vortex weakening with global warming will counteract to some extend in some regions, the strengthening and poleward shift of the tropospheric eddy-driven jets. However, a polar vortex strengthening will feedback positively on the strengthening and poleward shift of the tropospheric eddy-driven jets. Here we review our knowledge and include new selected results, based on CMIP6, on the role of dynamical stratosphere-troposphere coupling in providing for pathway of atmospheric circulation changes in response to global warming.