The influence of the North Atlantic Oscillation and El Ni?o–Southern Oscillation on mean and extreme values of column ozone over the United States

摘要

Continuous measurements of total ozone (by Dobson spectrophotometers) acrossthe contiguous United States began in the early 1960s. Here, we analyzetemporal and spatial variability and trends in total ozone from the five USsites with long-term records. While similar long-term ozone changes aredetected at all five sites, we find differences in the patterns of ozonevariability on shorter timescales. In addition to standard evaluationtechniques, STL-decomposition methods (Seasonal Trend decomposition of timeseries based on LOESS (LOcally wEighted Scatterplot Smoothing)) are used toaddress temporal variability and "fingerprints" of dynamical features in theDobson data. Methods from statistical extreme value theory (EVT) are used tocharacterize days with high and low total ozone (termed EHOs and ELOs,respectively) at each station and to analyze temporal changes in thefrequency of ozone extremes and their relationship to dynamical featuressuch as the North Atlantic Oscillation (NAO) and El Ni?o–Southern Oscillation.A comparison of the fingerprints detected in the frequency distribution ofthe extremes with those for standard metrics (i.e., the mean) shows thatmore fingerprints are found for the extremes, particularly for thepositive phase of the NAO, at all five US monitoring sites. Results from theSTL decomposition support the findings of the EVT analysis. Finally, weanalyze the relative influence of low- and high-ozone events on seasonal meancolumn ozone at each station. The results show that the influence of ELOsand EHOs on seasonal mean column ozone can be as much as ±5 %,about as large as the overall long-term decadal ozone trends.