PV-θ view of the zonal mean state of the atmosphere

The relation between zonal mean potential vorticity (PV) in potential temperature (θ) coordinates and the zonal mean zonal wind in January and in July is studied. PV-anomalies are defined with respect to a reference state that is at rest with respect to the rotating earth. Two important PV-anomalies are identified. One PV-anomaly, the ‘Ex-UTLS PV-anomaly’, coincides approximately with the extratropical tropopause (310–360 K). It is a permanent feature of the zonal mean state. The other PV-anomaly is located higher in the stratosphere. It exhibits a strong seasonal cycle, i.e. in winter, it is strongly positive, while in summer, it is weakly negative. In the Northern Hemisphere winter, the Ex-UTLS PV-anomaly and the stratospheric PV-anomaly are separated by a ‘surf-zone’, which is characterised by a negative PV-anomaly pole-wards of a positive PV-anomaly. Piecewise PV-inversion reveals that (1) the Ex-UTLS PV-anomaly induces the westerly winds in the troposphere and the lower stratosphere, including the subtropical jet, that (2) the positive stratospheric PV-anomaly induces the stratospheric polar night jet and that (3) the negative polar cap stratospheric PV-anomaly in summer reduces the westerly wind speeds in the troposphere and induces easterly winds in the stratosphere. The Ex-UTLS PV-anomaly is manifest mainly as an isentropic density- (or mass-) anomaly. Piecewise PV-inversion of these anomalies in isolation should account for this by an appropriate adjustment of the lower boundary condition.