Tropopause sharpness and atmospheric general circulation
The general circulation of the atmosphere is primarily composed of planetary-scale features such as the overturning meridional circulations in the tropics and extratropics (the so-called Hadley and Ferrel cells) and meandering jet streams that at places curl up into eddies (mid-latitude highs and lows). These features are not just interesting for an overall depiction of the atmospheric circulation but in recent years have also received increased attention due to their varied responses to climate change. For example, climate model projections show a robust poleward shift of the mid-latitude jets, as well as a widening of the Hadley cells. Dynamical coupling between the jets and overturning circulation features occurs via Rossby waves that propagate vertically and meridionally away from their sources and amplify within the core of the tropopause-level jet, which acts as a waveguide. The strength of this waveguide is in part controlled by tropopause sharpness. In this talk, I will provide an overview of the role that tropopause sharpness may play in modulating aspects of the general circulation across the troposphere-stratosphere system. In particular, I will highlight robust relations between interannual-to-multidecadal variations in mid-latitude tropopause sharpness and features of the general circulation, such as jet latitude and strength of the Hadley cell. These relationships are found to hold in reanalyses and climate models, and for the variability across climate models. Furthermore, experiments with a mechanistic model show that a sharper tropopause promotes an intensified general circulation and an equatorward shifted jet.