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Ozone and Potential Vorticity

As the mean PV distribution on isentropic surfaces shows high PV values in the polar region and low values in the (sub)tropics, PV also holds information about the origin of air masses arriving at the measurement site in Payerne.
Inside the wintertime polar vortex PV values are particularly high.Thus very high PV values along the trajectories indicate transport of air masses from inside the vortex.
In the following PV deviations are calculated as the difference between PV averaged along the 10-day trajectories and 22-year PV averages in percent.
The time series in Fig.4 show the high year to year variability of PV deviations between 340K and 440K, that correlate well with the ozone deviations. This confirms the trajectory analysis shown in Fig. 3.

Fig. 4: Ozone and PV deviations for January - March at 340K - 440K

As already stated before, there is no trend in ozone and no trend in the deviations from the fitted line on these levels.

In opposition to that a negative trend in the observed ozone values can be seen at altitudes of 70hPa - 35hPa that correspond to potential temperature levels between 460K and 560K. Ozone and PV deviations are negatively correlated, but only in the 1980s (see fitted line in Fig.5).



Fig. 5:  Ozone Deviation and Origin of Air Masses for January - March at 460K - 560K

A similar correlation can also be found in spring and summer. However in the winters of the 1990s ozone values are systematically lower than expected from this correlation. This is in contrast to spring, summer and autumn where this correlation can be seen until the year 2000. This can be viewed as evidence for chemical ozone depletion during northern hemisphere winter.

Further studies will be performed to separate the influence of ozone depletion in the polar vortex to mid-latitude ozone profiles from in-situ processing of air at mid-latitudes.