The Impact of the Stratosphere on Tropospheric Climate

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 monthly as well as daily timescales. The stratospheric changes that accompanied the major sudden stratospheric warming in January 2009, for example, resulted in an easterly wind forcing of about 5 m/s on the tropospheric winds. Furthermore, the influence of the stratosphere on the tropospheric winds was felt until months after the warming event, indicating that inclusion of stratospheric processes in models might improve extended range weather forecasts. Due to CO2 increases, the SH stratospheric PV increases in winter, related to enhanced stratospheric emission of longwave radiation to space. In the NH, CO2 increases are associated with a decrease in the stratospheric polar PV in winter, related to an increased stratospheric wave forcing. The stratospheric changes that result from CO2 increases in a climate model also influence the tropospheric winds. Although the magnitude of the tropospheric response related to stratospheric changes is small, it is of the same order of magnitude as the total tropospheric response, indicating that changes in the stratosphere can certainly modify the tropospheric response to climate change.