| Summary: | The zonal mean state of the atmosphere in the Northern Hemisphere in winter is determined by the temperature at the Earth’s surface and by two potential vorticity (PV) anomalies (defined as that part of the PV field that induces a wind field) centred over the North Pole: one in the upper troposphere/lower stratosphere (UTLS), extending to the Subtropics, and the other over the polar cap in the lower to middle stratosphere. Isentropic PV inversion demonstrates that the UTLS PV anomaly induces themain part of the zonalmean wind in the troposphere, including the subtropical jet stream, while the stratospheric PV anomaly induces the polar night stratospheric jet. The stratospheric PV anomaly has a greater amplitude and extends further downwards if the Arctic Oscillation (AO) index is positive. Also, the UTLS PV anomaly has a slightly larger amplitude if the AO index is positive, but the meridional PV gradient in the Subtropics that is associated with this anomaly is greatest when the AO index is negative, resulting in a stronger subtropical jet when the AO index is negative. PV inversion translates the UTLS PV anomaly into a wind anomaly and a static stability anomaly. The resulting differences in the vertical wind shear and in the Brunt–V¨ais¨al¨a frequency between the two AO phases show a larger baroclinicity in the extratropics when the AO index is positive. This explains why more extratropical cyclones are observed when the AO index is positive
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