The influence of the North Atlantic Oscillation and El Niño-Southern Oscillation on mean and extreme values of column ozone over the United States

Continuous measurements of total ozone (by Dob- son spectrophotometers) across the contiguous United States began in the early 1960s. Here, we analyze temporal and spa- tial variability and trends in total ozone from the five US sites with long-term records. While similar long-term ozone changes are detected at all five sites, we find differences in the patterns of ozone variability on shorter timescales. In ad- dition to standard evaluation techniques, STL-decomposition methods (Seasonal Trend decomposition of time series based on LOESS (LOcally wEighted Scatterplot Smoothing)) are used to address temporal variability and "fingerprints" of dy- namical features in the Dobson data. Methods from statisti- cal extreme value theory (EVT) are used to characterize days with high and low total ozone (termed EHOs and ELOs, re- spectively) at each station and to analyze temporal changes in the frequency of ozone extremes and their relationship to dynamical features such as the North Atlantic Oscilla- tion (NAO) and El Nino-Southern Oscillation. A comparison of the fingerprints detected in the frequency distribution of the extremes with those for standard metrics (i.e., the mean) shows that more fingerprints are found for the extremes, par- ticularly for the positive phase of the NAO, at all five US monitoring sites. Results from the STL decomposition sup- port the findings of the EVT analysis. Finally, we analyze the relative influence of low- and high-ozone events on seasonal mean column ozone at each station. The results show that the influence of ELOs and EHOs on seasonal mean column ozone can be as much as 5 %, about as large as the overall long-term decadal ozone trends.