Table 1.
The empirically identified thresholds for the atmospheric circulation proxy.*
Fig. 1.
Monthly sum of observed and predicted days with high NO2 air pollution based on the proxy applied to ERA-Interim for the extended winter seasons 1979–2013. The horizontal lines show the seasonal means of the monthly sums.
Table 2.
Skill scoresSkill scores WE14Detection rate0.820.80False alarm rate0.550.62Correct null prediction rate0.870.84Correlation days per month0.890.85Correlation days per season0.930.93Fig. 2.
Relationship between the total duration of observed (left panels) and predicted (right panels) pollution events vs. the absolute maximum hourly mean NO2 concentration at DP (top panels). Relationship between the prior duration of pollution events vs. the daily maximum hourly mean NO2 concentration at DP (bottom panels). The red lines are the best estimates for linear regressions of the scatter plots; the green lines show the 95% confidence intervals for the regression lines based on the Matlab regression_line_ci function. R2 is the adjusted coefficient of determination. All R2 are significant at the 99% level.
Fig. 3.
Monthly sum of predicted days with high NO2 air pollution based on the proxy applied to MRI-AGCM for the extended winter seasons 1979–2003 and 2075–2099. The horizontal lines show the seasonal means of the monthly sums.
Fig. 4.
Probability distribution functions of the monthly and seasonal sums of predicted days with the potential for high air pollution in ERA-Interim, MRI-AGCM and NorESM. The indices h and p refer to the historic (1979–2003/1950–2000) and predicted (2075–2099/2050–2100) time-ranges for MRI-AGCM/NorESM, respectively. The x-axes are normalized by the maximum number of days of all records (given in parenthesis in the legend). For the distribution on a monthly basis, also the mean number of predicted days with the potential for high air pollution is shown after the slash. The y-axes are normalized by the record length in years. The NorESM curves extend beyond the limit of the y-axis.
Fig. 5.
First (left panels) and second (right panels) modes of co-variability between daily Za and the maximum hourly mean NO2 concentrations at DP and RHT during wintertime. The top panels show the heterogeneous correlation maps (Wallace et al., 1992) between Za and the expansion coefficients of the pollution measurements. The middle and bottom panels show the normalized expansion coefficients for Za and the pollution measurements. Both fields were smoothed with a 7-day running mean filter prior to the SVD analysis. The explained correlation fractions and coefficients of determination between the principal components are given in the titles of the contour-plots. All correlations are significant above the 99% level. Blue circles and crosses in the time-series of the principal components (PCs) denote days with daily maximum NO2 concentrations of more than 150 μg m−3 at DP and RHT, respectively. Regard the different y-axis limits for the bottom right panel.
Fig. 6.
Comparison between the observed high pollution events and the blocking indices over longitude. Blockings north and south are the blockings based on the central latitudes 56.25°N and 48.75°N, respectively. Significant correlations above the 99% level are marked with dots.
Fig. 7.
Response Operator Curves (ROCs) for high pollution events. The two lines marked with DP and RHT show the ROC of the proxy when modifying the threshold for the NO2 concentrations at the two measurement stations. The + marks the location of the minimum distance from the optimum point with false alarm rate 0 and detection rate 1 for the ROC curve of the proxy. The o and * mark NO2 concentration thresholds of 150 μg m−3 (high pollution) and 200 μg m−3 (very high pollution), respectively. The line marked with NAO shows the ROC for the first PC of Cn. The different false alarm and prediction rates are achieved by setting varying thresholds for the minimum in the expansion coefficient with frozen threshold for the NO2 concentration at DP of 150 μg m−3.
Fig. A1.
The dominating mode of variability of Za over the North Atlantic-European region. The top panel shows the first eigenvector of Cn (the first EOF). The bottom panel shows the time series of the expansion coefficient of the eigenvector (the first PC).