The Influence of Mid-Latitude Cyclones on European Background Surface Ozone

Abstract. The relationship between springtime mid-latitude cyclonesand background ozone (O 3 ) is explored using a combination of observational and reanalysis data sets. First, the relationship between surface O 3 observations at two rural monitoring sites on the west coast of Europe – MaceHead, Ireland, and Monte Velho, Portugal – and cyclonetrack frequency in the surrounding regions is examined. Second, detailed case study examination of four individual mid-latitude cyclonesand the influence of the associated frontal passage on surface O 3 is performed. Cyclonetracks have a greater influence on the O 3 measurements at the more northern coastal European station, MaceHead, located within the main North Atlantic (NA) storm track. In particular, when cyclonestrack north of 53° N, there is a significant relationship with high levels of surface O 3 (> 75th percentile). The further away a cycloneis from the NA storm track, the more likely it will be associated with both high and low ( 25th percentile) levels of O 3 at the observation site during the cyclone's life cycle. The results of the four case studies demonstrate (a) the importance of the passage of a cyclone's cold frontin relation to surface O 3 measurements, (b) the ability of mid-latitude cyclonesto bring down high levels of O 3 from the stratosphere, and (c) that accompanying surface high- pressure systemsand their associated transport pathways play an important role in the temporal variability of surface O 3 . The main source of high O 3 to these two sites in springtime is from the stratosphere, either from direct injection into the cycloneor associated with aged airstreams from decaying downstream cyclonesthat can become entrained and descend toward the surface within new cyclonesover the NA region.