Synoptic circulation and its influence on spring and summer surface ozone concentrations in southern Sweden

The influence of synoptic circulation patterns on surface ozone concentrations at three monitoring sites in southern Sweden was investigated for the spring (April–May) and summer (June–August) periods 1990–2005. Synoptic circulation patterns were classified into six groups based on the Lamb Weather Types (LWTs). The analyses show that the anticyclonic weather pattern (A) and the directional flows from southeast/east (SEE) and southwest/south (SWS) were most frequently associated with high ozone levels. It was estimated that 85.5%, 73.3% and 83.5% of the ozone episode days at Rorvik/Rao, Norra Kvill and Vavihill, respectively, were observed under these three circulation patterns. There were apparent spatial differences in the ozone concentrations during nighttime under condition A that were related to the high altitude position of Norra Kvill and Vavihill. The wind component indices u and v, and the total vorticity index ζ for each circulation pattern reflect the intensity of synoptic circulation and they all have an impact on the variation of surface ozone concentrations. The total vorticity index seems to be the key variable in terms of synoptic influence on surface ozone. A statistic model for the relations between synoptic circulation and ozone concentrations was established based on the frequencies and intensities of the six LWTs. It is able to explain 85% and 71% of the total variances in the observed mean ozone concentrations in spring and summer, respectively, over the period 1998–2005. The results demonstrated the strong impacts of synoptic circulations on surface ozone concentrations in southern Sweden.