Continuous measurements of total ozone (by Dobson spectrophotometers) across
the contiguous United States began in the early 1960s. Here, we analyze
temporal and spatial 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 addition 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 dynamical features in the
Dobson data. Methods from statistical extreme value theory (EVT) are used to
characterize days with high and low total ozone (termed EHOs and ELOs,
respectively) 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 Oscillation (NAO) and El Niño–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, particularly for the
positive phase of the NAO, at all five US monitoring sites. Results from the
STL decomposition support 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
Long-term monitoring of ozone is critical because it is instrumental in controlling the levels of ultraviolet radiation reaching the planet's surface and thus plays an important role in the existence of life on Earth (e.g., Tourpali et al., 2009; Bais et al., 2011; McKenzie et al., 2011). The 25th anniversary of the Montreal Protocol (signed in 1987) marked an important milestone in the phasing-out of man-made chemicals such as chlorofluorocarbons (CFCs), commonly referred to as ozone-depleting substances (ODSs). ODSs have very long lifetimes in the stratosphere (some as long as 100 years); they are lofted throughout the stratosphere from the tropical troposphere, transported into the middle and high latitudes, and recirculate, providing chlorine (and bromine) atoms for chemical ozone destruction (WMO, 2011; Rigby et al., 2013).
Analyses of interannual and long-term variability in total column ozone on regional (e.g., Mäder et al., 2007; Rieder et al., 2010a, b, 2011; Fitzka et al., 2014) and global (e.g., Frossard et al., 2013; Rieder et al., 2013) scales have been presented in a number of recent studies. There is now a broad consensus that long-term negative ozone trends are dominated by ODSs, while short-term trends and variability, particularly at midlatitudes, are also significantly influenced by synoptic-scale meteorological variability (e.g., Steinbrecht et al., 1998; Shepherd, 2008), decadal climate variability (e.g., Chandra et al., 1996; Hood, 1997) and dynamical modes such as the El Niño–Southern Oscillation (ENSO) (e.g., Brönnimann et al., 2004; Ziemke et al., 2010; Hood et al., 2010; Gabriel et al., 2011), the North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) (e.g., Appenzeller et al., 2000; Thompson and Wallace, 2000), and volcanic eruptions (e.g., Jaeger and Wege, 1990; Solomon, 1999; Robock, 2000; Mäder et al., 2007). For the European sector it has been reported that dynamical variability accounts for about a third of the observed ozone changes between the 1970s and 1990s (e.g., Mäder et al., 2007; Wohltmann et al., 2007). The particular importance of dynamical changes for column ozone at midlatitudes has also been highlighted in more recent work that attributes the slight increase in column ozone since the 1990s primarily to dynamics and to a lesser extent to the decrease in ODSs (after their peak around 1997) (e.g., Harris et al., 2008; Hegglin and Shepherd, 2009; WMO, 2007, 2011).
In addition, recent work analyzing the tails of the ozone distribution (i.e., the extremes) in relation to the bulk properties (i.e., the mean) has shown that analysis of the tails allows for a more systematic attribution of ozone changes to dynamical features than mean value analysis can achieve (e.g., Rieder et al., 2010a, b, 2011, 2013; Frossard et al., 2013). These studies also showed that even moderate NAO and ENSO events can have significant effects on the midlatitude ozone field.
Furthermore, while column ozone at northern midlatitudes reached its lowest values in the early 1990s following the 1991 eruption of Mt. Pinatubo, it has been noted that the effect of this eruption was partially masked by atmospheric dynamics in the Southern Hemisphere (e.g., Schnadt Poberaj et al., 2011; Rieder et al., 2013), again emphasizing the importance of atmospheric dynamics to ozone trends and variability.
In this paper we attempt to assess the information contained in the integrated total ozone column derived from the continental US network of Dobson measurements. We discuss interannual variability in column ozone, how it has changed over the last 50 years, what controls it, and how trends are statistically related to dynamical and chemical proxies.
Continuous measurements of total column ozone (TOC) over the continental United States began in the early 1960s. Individual measurements were also made earlier at some sites, but more sporadically and mainly in conjunction with the International Geophysical Year 1957.
The backbone of the World Meteorological Organizations (WMO) ozone monitoring network is the Dobson ozone spectrophotometer, an instrument developed in the 1920s specifically for high-accuracy measurements of total column ozone (e.g., Dobson, 1957, 1968). The concept of the Dobson measurement is the differential absorption of ozone at selected wavelengths in the solar ultraviolet spectrum. Two pairs, where one spectral range absorbs light more strongly than the other, are combined to minimize the effect of aerosol interference on the measurements. Measurements made using the direct solar beam are used to determine the TOC based on Lambert–Beer law, while measurements made with the scattered light from the zenith are converted to a TOC value based on the statistics of quasi-simultaneous measurements of both direct sun and zenith.
Operational instrument calibration is maintained by monthly tests with reference and discharge lamps, plus regular intercomparison with two standard instruments: D083 (world primary standard) or D065 (world secondary standard). The calibration of the primary standard is maintained by Langley Plot Campaigns at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory Mauna Loa Observatory (Hawaii).
In the contiguous USA, column ozone has been measured routinely at five observational sites (Bismarck, ND; Boulder, CO; Caribou, ME; Wallops Island, VA; Nashville, TN) since the 1960s. In this study we analyze the total ozone records from these five sites spanning from the 1960s through 2012. A detailed overview on geographical location and information on record length, data completeness and time series properties of the individual station records is provided in Fig. 1.
Geographical overview and site-specific information for the USA long-term Dobson total ozone monitoring sites.
There is now a broad consensus that long-term trends in total ozone are driven primarily by changes in the atmospheric concentration of ODSs (WMO, 2011). Nevertheless, active research in the field has shown that besides ODSs, several other processes have significant influence on total ozone changes and variability. The 11-year solar cycle, the quasi-biennial oscillation (QBO) and volcanic eruptions are among the most prominent explanatory variables often used to describe the influence of atmospheric variability on column ozone (WMO, 2011). At midlatitudes other dynamical features also show a significant influence on column ozone on seasonal and interannual timescales. In particular, synoptic-scale meteorological variability, described by, for example, the NAO (e.g., Appenzeller et al., 2000; Orsolini and Doblas-Reyes, 2003; Rieder et al., 2010a, 2011; Frossard et al., 2013) and climate modes such as ENSO (e.g., Rieder et al., 2010a, 2013; Brönnimann et al., 2004), has been shown to significantly influence ozone variability and trends.
In the present study we focus particularly on the influence of atmospheric
dynamics on variability and trends in column ozone over the USA. To this aim
we use a set of indices describing ENSO and NAO modes on a seasonal basis in
the statistical analysis. For ENSO we are using the seasonal NINO3.4 index
provided by NOAA's Climate Prediction Center – available at
Recent work has introduced concepts of statistical extreme value theory (EVT) into the field of total ozone research (Rieder et al., 2010a, b, 2011, 2013; Frossard et al., 2013; Fitzka et al., 2014). Here we build on these methodologies to analyze events of extremely low and high ozone (termed ELOs and EHOs, respectively) in the US long-term total ozone records.
The generalized Pareto distribution (GPD) is a commonly used distribution in
the framework of extreme value theory (e.g., Davison and Smith, 1990; Ribatet
et al., 2009) because it arises as the natural distribution for the
exceedance of a random variable (here total ozone) over a threshold. Below we
briefly describe the modeling procedure for values over a threshold. Note
that the modeling for values below a threshold is precisely the same; the
only thing to be done is to negate the values and apply exactly the same
procedure as for values above a high threshold as
min(
The GPD, which is the limiting distribution of exceedances over a threshold,
is defined as
Threshold values
Thresholds for extreme highs (EHOs, dark-grey dashed lines) and
lows (ELOs, light-grey dashed lines) of total ozone and climatological
monthly means of total ozone (black crosses) at
Here the well-known seasonal cycle with ozone minima in fall and maxima in spring, as well as the latitudinal dependence of total ozone mean values and thresholds (i.e., higher TOC at northern sites (Bismarck and Caribou) due to transport of ozone-rich air from high latitudes), is visible.
Following Rieder et al. (2010b), total ozone observations at the individual
US sites are categorized into three groups (see Eqs. 2–4):
Seasonal trend decomposition of time series based on LOESS (LOcally wEighted Scatterplot Smoothing) (e.g., Cleveland et al., 1990) decomposes a data record (here ozone) into seasonality, trend, and residual components. When applied to the US column ozone data it returns a well-known picture: (i) a strong seasonal cycle with maxima in spring and minima in winter/fall, in accordance with the understanding of the influence of the Brewer–Dobson circulation on column ozone (transport of ozone-rich air towards northern midlatitudes during boreal winter); (ii) a negative trend component dominated by the influence of ODSs on column ozone; and (iii) a highly variable residual component representing the effects of local-scale meteorology on column ozone. An example of an STL decomposition for the site in Boulder, CO, is shown in Fig. S1 in the Supplement of this article. For a more detailed description of the STL procedure we refer the interested reader to the paper of Cleveland et al. (1990), which describes the method.
STL-trend components represent smoothed TOC residuals after the seasonal cycle is removed and are thus not reliable measures for TOC trend analysis. Statistical trend analysis in this manuscript is solely based on linear regression analysis (see Sect. 4.4). Here we utilize STL because the resulting trend component provides a more detailed picture of the seasonal and interannual variability in the overall TOC time series compared to, for example, a simple linear trend component, and thus is suitable for the secondary assessment of “fingerprints” of NAO and ENSO events as long-term time series variability is preserved.
The main goal of this paper is to analyze the influence of dynamical features such as NAO and ENSO on column ozone over the USA. To this aim, EVT modeling and STL decomposition are applied to the long-term total ozone time series to derive fingerprints of the dynamical covariates.
In Sect. 3.1 we described the classification of total ozone observations into days with extremely low, extremely high, and non-extreme ozone. Here we focus on the frequency distribution of the extremes and analyze the influence of ENSO and NAO events on column ozone at the five US ozone monitoring sites. As the general features are very similar among the individual sites, we mainly show the results for Boulder and Caribou, which give the envelope of column ozone observations over the USA, in the main body of the paper. For convenient reference, illustrations for other sites are available in the Supplement for this article.
“Fingerprints” of the NAO and ENSO as detected for Boulder in the
seasonal frequency time series of EHOs (right axis, top to bottom) and ELOs
(left axis, bottom to top) for
As Fig. 3 but for Caribou.
In Figs. 3 and 4 we plot the observed frequency of ELOs, NEOs, and EHOs as time series for Boulder and Caribou (the results for the remaining sites are shown in Figs. S2–S4). Next, we turn to fingerprints of atmospheric dynamics in the frequency distribution of EHOs and ELOs at these sites.
First, we turn to the North Atlantic Oscillation, the leading mode in the Atlantic sector, which influences the direction and intensity of the tropospheric jet stream (e.g., Orsolini and Limpasuvan, 2001) and represents the main driver of the interannual variability in storm tracks during the cold season (e.g., Lau, 1988). A positive NAO phase leads to lower ozone over Europe and the USA and higher ozone over the Labrador Sea and Greenland, and vice versa for a negative NAO phase (see Rieder et al., 2010b, and references therein, as well as Frossard et al., 2013, for a spatial representation of NAO influence on column ozone at northern midlatitudes).
The North Atlantic Oscillation in its negative phase (NAO index <
Correlation of the NAO index and the average number (no.) of EHOs and ELOs and mean column ozone (TOC) on a seasonal basis at the five US long-term total ozone monitoring sites.
In both winter and spring the NAO index correlates negatively with the frequency of EHOs and positively with the frequency of ELOs, indicating an increase (decrease) in the frequency of high-ozone events during a negative (positive) NAO phase and vice versa for low-ozone events, manifested also in the seasonal means.
It has been noted that the NAO has tended towards a more positive phase in
recent decades (e.g., Hurrell, 1995; Thompson and Wallace, 2000), concomitant
with a strengthening of the northern polar vortex. Nevertheless, a strongly
negative NAO phase (NAO index <
The NAO fingerprints identified in the US column ozone records are in broad agreement with those for European sites and satellite data. Appenzeller et al. (2000) were among the first to report on the influence of the NAO on column ozone over Europe, based on their analysis of the world's longest total ozone record: Arosa, Switzerland. Rieder et al. (2010a) extended these investigations toward low and high ozone values and Rieder et al. (2011) documented the influence of the NAO in its positive (reduced column ozone, reduced frequency of high-ozone events) and negative (increased column ozone, increased frequency of high-ozone events) phases for five European ground-based sites in 1970–2010. These authors report a similar number of detected fingerprints and occasional misses at individual sites due to local effects. Frossard et al. (2013) extended investigations to larger spatial scales by analyzing the NIWA assimilated total ozone data set in 1979–2007. These authors report that the fingerprint of the NAO is of similar spatial extent for both mean values and ozone extremes but that the magnitude of influence on total ozone is larger for extremes than mean values. These results are in broad agreement with those presented here for the US long-term ozone records, documenting the significant influence of the NAO on column ozone variability throughout northern midlatitudes.
Next we turn to the El Niño–Southern Oscillation. Warm ENSO events are triggered by a high contrast between tropical and extratropical Pacific sea-surface temperatures, which are known to affect midlatitudes (in particular the North Pacific) via changes in the Hadley cell and Rossby wave generation (e.g., Trenberth, 1998; Alexander et al., 2002). During warm ENSO events, the meridional circulation in the stratosphere leads to enhanced ozone transport from the tropics to middle and high latitudes and a warmer lower stratosphere, both of which tend to increase midlatitude ozone (Rieder et al., 2013, and references therein).
The warm ENSO phase (El Niño, NINO3.4 index > 0.7) is, as expected, associated with higher ozone over the USA during winter/spring, visible in the frequency distribution of the extremes. During the study period, moderate to strongly positive ENSO events were recorded 11 times during winter and 4 times during spring. Most wintertime events (except those in 1983, 1992, and 1995) and springtime events (except 1983 and 1992) can be identified in the frequency distribution of ozone extremes. The absence of ENSO fingerprints in the remaining 3 years is consistent with their occurrence immediately after the two major volcanic eruptions of the last century (El Chichón in 1982 and Mt. Pinatubo in 1991), when the effects of the volcanic eruptions (enhanced ozone depletion on sulfate aerosols) would have masked the dynamical signal. As was the case for the NAO, the ENSO results for the US sites are in good agreement with findings for European sites and satellite data (e.g., Rieder et al., 2010a, 2013), illustrating the importance of ENSO in modulating column ozone at northern midlatitudes. The correlation analysis between ENSO and column ozone (or the frequency of EHOs and ELOs) is less conclusive then for the NAO, probably because of the rather small number of strong ENSO events.
STL-trend component anomaly (in DU) in 1963–2012 for Boulder (top) and Caribou (bottom) with underlying marks (colored vertical bars) for fingerprints of positive and negative NAO modes (left panels) and warm ENSO phases (right panels) on a seasonal basis. NAO positive (negative) phase is indicated for winter in red (blue) and for spring in orange (light blue). The warm ENSO phase is indicated for winter in green and spring in light green.
Summary of detected and missed fingerprints at all five US stations
for
Average fraction of days (in %) identified as EHO and ELO
during
At all sites we find a more consistent presence of fingerprints of NAO and
ENSO in extreme values of column ozone (upper panels in Figs. 3 and 4) than
in its mean values (lower panels of Figs. 3 and 4). While the extremes show
a pronounced response (increasing or decreasing frequency) to the prevailing
ENSO and NAO phases, the mean values often do not show large differences
compared to neighboring years without ENSO or NAO events. This is
particularly evident for NAO
Here we contrast the findings of the EVT-based analysis with results from the STL-decomposition approach. In Fig. 5 we show the anomaly of the trend components of the STL decomposition for the two selected US sites – Boulder and Caribou – as above in the EVT analysis (the results for the remaining sites are shown in Fig. S5). While the overall trend curves show a steady decline that is most pronounced in the 1980s and 1990s, as expected from the strong negative influence of ODSs on column ozone (e.g., WMO, 2011), there is also a large degree of interannual variability in these curves. This variability is not related to seasonality in the ozone field, since the seasonal component has been removed from the data prior to the trend computations within the STL procedure.
As for the EVT analysis we now identify fingerprints of ENSO and NAO events in the STL trend component. The colored vertical bars in Fig. 5a and c mark positive and negative winter- and springtime NAO events. The analysis of the STL trend component shows that positive NAO events are associated with a decreasing tendency in the trend curve, and thus with lower column ozone, while negative NAO events are associated with an increasing tendency in the STL trend component, and thus with higher column ozone. Figure 5b and d show the corresponding results for warm ENSO events, which tend to enhance column ozone. The good agreement between the results in Fig. 5 with those from the EVT analysis (Figs. 3 and 4) provides further evidence of the significant influence of strong NAO and ENSO events on column ozone variability over the continental USA.
Despite the overall similarity in trends and patterns of variability, it is important to note that fingerprints of individual NAO and ENSO events are not always found at all five stations analyzed. Figure 6 provides a summary of all major ENSO and NAO events over the 1963–2012 time period and their detection (or absence) in the individual station records. Solid squares in Fig. 6 mark fingerprints detected, while open squares mark “absent” fingerprints at individual sites. The majority of ENSO and NAO events are detected at all five US total ozone monitoring sites, but some individual events are not discernible at individual (or multiple) sites such as the negative NAO event of spring 1996. The absence of individual fingerprints is not too surprising given the large spatial distance between individual sites and their regional location (see Fig. 1). The occasional masking of large-scale ozone variability by localized synoptic-scale meteorology (e.g., the influence of the subtropical jets and localized tropopause variations) is associated with the regional patterns of advection and convergence or divergence that are related to changes in tropospheric and stratospheric pressure systems as has been previously reported for regions other than the USA (e.g., Koch et al., 2005; Mäder et al., 2007; Wohltmann et al., 2007).
Direct correlations of daily TOC between sites are rather inconclusive due to the difficulty in accounting for local meteorological effects at a station or temporal lags between stations due to transport. Unfortunately, vertical investigations are limited by the absence of vertically resolved ozone profiles at most of the stations (except for Boulder, CO). In addition, seasonal comparisons between years with fingerprints and without are restricted to a small sample size (i.e., a few missing fingerprints on a site basis). Nevertheless, a comparison of cumulative distribution functions (CDFs) on a site basis between neighboring years with and without fingerprints reveals the absence of high- or low-ozone events associated with the NAO or ENSO (see Fig. S6 in the Supplement). Thus, instead of individual effects, we quantify the overall contribution of extremes to seasonal mean column ozone by calculating the influence of ELOs and EHOs at each site.
Several studies have linked the occurrence of multiple tropopauses to Rossby wave breaking events along the subtropical jet (Homeyer and Bowman, 2013, and references therein) and to associated tropospheric intrusions (e.g., Pan et al., 2009). Climatological maxima in multiple tropopause occurrence have been linked to observed changes in vertical profiles of satellite-observed trace gases that are consistent with air from the tropical tropopause layer being drawn into the region between the two tropopauses; specifically, climatological ozone mixing ratios in midlatitude multiple tropopause regions are substantially lower than those in regions with a single tropopause (Schwartz et al., 2015). Schwartz et al. (2015) estimated that in Northern Hemisphere winter midlatitudes, when multiple tropopauses are most common, climatological ozone values can be as much as 20 % lower than they would be without multiple tropopauses.
These results are consistent with the observed association of lower column ozone with multiple tropopauses (e.g., Castanheira et al., 2012; Mateos et al., 2014). Mateos et al. (2014) also noted more common occurrence of such tropospheric intrusion events during NAO positive phases, suggesting a role for dynamical modes such as NAO and ENSO in modulating multiple tropopause occurrence and thus their corresponding effects on ozone.
In addition, there is a maximum in multiple tropopause occurrence frequency over the USA in winter and spring, extending poleward from the region where upper tropospheric jets are most common (Manney et al., 2014). Boulder, Nashville, and Wallops Island are near the latitude of maximum multiple tropopause occurrence just poleward of the subtropical upper tropospheric jet, while Bismarck and Caribou are at the northern edge of the region of enhanced multiple tropopause activity (Manney et al., 2014) and are thus less frequently affected by processes in multiple tropopause regions.
The absence of individual fingerprints on a site basis and their underlying cause is of general interest but beyond the spatial and climatological scope of the presented study. Nevertheless, further analysis (including vertical information from sounding profiles) is suggested for future site-specific analysis addressing effects of local dynamics on column ozone variability.
In this section we turn the focus to column ozone trends at the five US Dobson sites.
Seasonal linear trends (in % per decade) for observed and
extremes removed winter (DJF) and spring (MAM) column ozone time series in
1970–1995 and 1996–2010 at the five US ozone monitoring sites. Standard
errors are given in parentheses;
To analyze the influence of extremes (both low and high) on ozone trends we
contrast linear trends for the entire observational time series (i.e., all
observational data included) with trends for time series with extremes
removed. We focus on two main time periods: 1970–1995,
with almost linearly increasing ODSs, which includes the peak in ozone depletion (following the
Mt. Pinatubo eruption), and 1996–2010, which extends from
the maximum in ODSs (
Comparing the entire observational records with those with extremes removed, we find that trends are only about half as strong in the latter case. This is particularly interesting as no statistically significant trend (at a 95 % level) is found for the magnitude of EHOs or ELOs over 1970–1995. The individual time series show the well-known pattern of large interannual variability but no robust increase (or decrease) in the average magnitude of the extremes themselves. Thus the influence of extremes on seasonal mean column ozone (see below) can be understood as a function of their occurrence frequency, driven by chemical ozone depletion and dynamics.
Turning now to the more recent past, i.e., 1996–2010 (Table 2), we find
positive trends at most sites, an anticipated result since stratospheric
chemistry in this period is impacted by slowly but steadily declining ODSs.
The key interest in the trends for 1996–2010 is thus not the sign of the
trends but their significance. Observational and modeling studies suggest
that chemical ozone depletion ceased to increase around the turn of the
century (e.g., WMO, 2011), but whether significant ozone recovery has
started is still undetermined. Positive trends at the majority of sites
indicate that ozone has stopped declining over the USA, particularly during
winter, suggesting that chemical depletion may have ceased (Table 2).
Nevertheless, since the trend estimates over the 15-year period of 1996–2010
are not significant at the 95 % level (see
Discriminating the effects of the individual dynamical proxies on column ozone is difficult because (i) fingerprints for multiple proxies are found in several years (e.g., a strongly positive NAO and a warm ENSO phase) and (ii) the occurrence frequency of the individual fingerprints is highly variable. Also correlations between sites are rather noisy on daily timescales (local effects) and seasonal comparisons between years with fingerprints and without are restricted to a small sample size (i.e., too few missing fingerprints on a site basis). Thus, instead of individual effects, we quantify the overall contribution of extremes to seasonal mean column ozone by calculating the influence of ELOs and EHOs at each site:
Influence (in %) of events of extremely low (ELOs, light histogram) and
high (EHOs, dark histogram) ozone and net influence of extremes
(white curve) on winter (DJF) mean ozone at
Pattern correlation of the net influence of extremes on winter (DJF) and spring (MAM) mean column ozone among the five US ozone monitoring sites. For station code see Fig. 1.
Next we analyze the pattern correlation of the net contribution of the
extremes (i.e.,
As Fig. 8 but for spring (MAM).
In this study we analyze data from the five long-term Dobson stations across the contiguous USA to investigate the influence of the North Atlantic Oscillation (NAO) and the El Niño–Southern Oscillation (ENSO) on total ozone variability and trends since the 1960s. In addition to standard evaluation techniques we utilize a STL-decomposition method (Seasonal Trend decomposition procedure based on LOESS) and statistical extreme value theory (EVT) to address the temporal variability and trends in the Dobson data in relation to synoptic-scale meteorological and climate variability.
The results show that fingerprints of the dynamical features are better
captured in the tails (i.e., the extremes) than in the bulk (i.e., the mean)
of the observational records, a result in broad agreement with earlier work
for European monitoring sites (Rieder et al., 2010a, 2011)
and satellite data (e.g., Frossard et al., 2013; Rieder et al., 2013).
Fingerprints of individual ENSO and NAO events are coherently captured at
the majority of the sites, indicating the large-scale influence of these
features on column ozone. The observed increase in the frequency of ELOs and
decrease in the frequency of EHOs from the 1970s on is in agreement with the
notion of increasing ODSs. Further, ELOs are indicative of the extension of
the subtropical jet to the north of the station, which brings in tropical
air masses with low ozone content, while EHOs are indicative of an
equatorward excursion of the polar jet and advection of O
In agreement with earlier work we find significant negative trends in column
ozone over the USA in 1970–1995 (the period with almost linearly increasing
ODSs). Although column ozone values over the USA ceased to decrease around
the turn of the century, the observational records for 1996–2010 generally
show positive, but insignificant, trends and thus do not yet show a clear
signature of the onset of ozone recovery. Trends derived excluding extremes
from the records are much smaller than those derived from the full records,
consistent with previous results for other regions and data sets. The
contribution of low- and high-ozone events to winter and spring mean column
ozone is bounded by about
Pattern correlations of the contribution of low- and high-ozone events to seasonal mean column ozone are highest for neighboring sites (i.e., Bismarck–Boulder and Nashville–Wallops Island), though not homogenous among sites (e.g., seasonally dependent and time varying among individual sites). Trends for individual sub-periods (i.e., 1970–1995 and 1996–2010 (Table 2); 1970–2000 and 1990–2010; see Supplement) are mostly of the same sign at all sites but differ in magnitude and significance among seasons and time periods analyzed.
The results presented here highlight the importance of a continued spatially distributed long-term ozone monitoring program to address future ozone changes and to detect and confirm the onset and progress of ozone recovery in the context of the Montreal Protocol.
The authors wish to express their appreciation to the NOAA Weather Service personnel, whose efforts in making the Dobson ozone measurements over more than half a century allowed us to study some of the longest atmospheric constituent time series in existence. The authors thank the NOAA Climate Prediction Center and NCAR/UCAR climate data center for providing ENSO and NAO indices used in this study via their respective data portals. The authors are grateful to the two anonymous referees for helpful comments during the discussion phase of this paper.
The total ozone data were obtained from the World Ozone and Ultraviolet
Radiation Data Centre (WOUDC) operated by Environment Canada, Toronto,
Ontario, Canada, under the auspices of the World Meteorological Organization.
Data files can be found on the WOUDC ftp server,