| Summary: | The stratospheric potential vorticity (PV) field in the current climate, its variations around the occurrence of a sudden stratospheric warming, and possible future changes are examined. The PV presents a compact way to describe the state of the atmosphere, and is linked to all other dynamical fields through the invertibility principle. Local changes in the PV have a nonlocal effect on the wind field, so that changes in the stratospheric PV can be related to circulation changes in the troposphere, making it possible to study the role of the stratosphere in climate. The seasonal cycle of the stratospheric polar cap PV anomaly (the PV anomaly is defined as that part of the PV that induces a wind field according to the PV inversion equation) is related to radiative effects. A positive polar PV anomaly forms in autumn and winter due to radiative cooling in the polar night, and vanishes in spring due to absorption of solar radiation by ozone. The formation of the vortex in autumn is similar for the Northern Hemisphere (NH) and the Southern Hemisphere (SH), but waves affect the NH stratosphere throughout winter, weakening the vortex compared to the less disturbed SH vortex. The stronger and colder SH vortex allows for (more) ozone depletion in spring, leading to a delayed break up of the SH vortex. In summer, the easterly stratospheric flow prohibits wave propagation to the stratosphere in both hemispheres, resulting in small interhemispheric differences. Wave forcing of the stratosphere from below can not only explain interhemispheric differences, but also interannual variability of the winter stratosphere. On average, about 50% of the interannual variability in the state of the stratosphere that is observed in the NH can be explained by the interannual variations in the 100 hPa heat flux, which is a measure of the wave forcing of the stratosphere. For the monthly mean climatology, the influence of the stratosphere on the tropospheric winds is small, but for individual winters the influence can be substantial, on ...
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