Abstract/Summary

Large-scale overturning mass transport in the stratosphere is commonly explained through the action of potential vorticity (PV) rearrangement in the flank of the stratospheric jet. Large-scale Rossby waves, with wave activity source primarily in the troposphere, stir and mix PV and an overturning circulation arises to compensate the zonal torque imposed by the breaking waves. In this view, any radiative heating is relaxational and the circulation is mechanically driven. Here we present a fully thermodynamic analysis of these phenomena, based on ERA-Interim data. Stream functions in a thermodynamic, log-pressure-temperature space are computed. The sign of a circulation cell in these coordinates directly shows whether it is mechanically driven, converting kinetic energy to potential and thermal energy, or thermally driven, with the opposite conversion. The circulation in the lower stratosphere is found to be thermodynamically indirect, i.e. mechanically driven. In the middle and upper stratosphere thermodynamically indirect and direct circulations coexist, with a prominent semiannual cycle. A part of the overturning in this region is thermally driven, whilst a more variable indirect circulation is mechanically-driven by waves. The wave driving does not modulate the strength of the thermally direct part of the circulation. This suggests that the basic overturning circulation in the stratosphere is largely thermally driven, while tropospheric waves add a distinct indirect component to the overturning. This indirect overturning is associated with poleward transport of anomalously warm air parcels.