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We show that the frequency of summertime mid-latitude cyclones trackingacross eastern North America at 40°–50° N (the southernclimatological storm track) is a strong predictor of stagnation and ozonepollution days in the eastern US. The NCEP/NCAR Reanalysis, goingback to 1948, shows a significant long-term decline in the number ofsummertime mid-latitude cyclones in that track starting in 1980 (−0.15 a<sup>−1</sup>).The more recent but shorter NCEP/DOE Reanalysis (1979–2006) showssimilar interannual variability in cyclone frequency but no significantlong-term trend. Analysis of NOAA daily weather maps for 1980–2006 supportsthe trend detected in the NCEP/NCAR Reanalysis 1. A GISS general circulationmodel (GCM) simulation including historical forcing by greenhouse gasesreproduces this decreasing cyclone trend starting in 1980. Such a long-termdecrease in mid-latitude cyclone frequency over the 1980–2006 period mayhave offset by half the ozone air quality gains in the northeastern USfrom reductions in anthropogenic emissions. We find that if mid-latitudecyclone frequency had not declined, the northeastern US would have beenlargely compliant with the ozone air quality standard by 2001. Mid-latitudecyclone frequency is expected to decrease further over the coming decades inresponse to greenhouse warming and this will necessitate deeper emissionreductions to achieve a given air quality goal

Sensitivity of US air quality to mid-latitude cyclone frequency and implications of 1980–2006 climate change