Abstract
Abstract. We show that the frequency of summertime mid-latitude cyclones tracking across eastern North America at 40°–50° N (the southern climatological storm track) is a strong predictor of stagnation and ozone pollution days in the eastern US. The NCEP/NCAR Reanalysis, going back to 1948, shows a significant long-term decline in the number of summertime mid-latitude cyclones in that track starting in 1980 (−0.15 a−1). The more recent but shorter NCEP/DOE Reanalysis (1979–2006) shows similar interannual variability in cyclone frequency but no significant long-term trend. Analysis of NOAA daily weather maps for 1980–2006 supports the trend detected in the NCEP/NCAR Reanalysis 1. A GISS general circulation model (GCM) simulation including historical forcing by greenhouse gases reproduces this decreasing cyclone trend starting in 1980. Such a long-term decrease in mid-latitude cyclone frequency over the 1980–2006 period may have offset by half the ozone air quality gains in the northeastern US from reductions in anthropogenic emissions. We find that if mid-latitude cyclone frequency had not declined, the northeastern US would have been largely compliant with the ozone air quality standard by 2001. Mid-latitude cyclone frequency is expected to decrease further over the coming decades in response to greenhouse warming and this will necessitate deeper emission reductions to achieve a given air quality goal.
Highlights
Regional pollution episodes in the eastern US develop under summertime stagnant conditions with clear skies, a situation associated with weak high-pressure systems (Logan, 1989; Vukovich, 1995; Hegarty et al, 2007)
The red line shows the number of ozone pollution days predicted from the number of mid-latitude cyclones in the National Oceanic and Atmospheric Administration (NOAA) weather maps and in the NCEP/NCAR Reanalysis 1 if anthropogenic emissions had not changed over the period
We showed that the frequency of mid-latitudes cyclones tracking across eastern North America in the 40◦–50◦ N latitudinal band is a strong predictor variable of the frequency of summertime pollution episodes in the eastern US
Summary
Vukovich, 1995; Hegarty et al, 2007). The pollution episode ends when the warm, stagnant air mass is pushed to the Atlantic by the cold front of a passing mid-latitude baroclinic cyclone and replaced with cooler, cleaner air (Merrill and Moody, 1996; Cooper et al, 2001; Dickerson et al, 1995; Li et al, 2005). Mid-latitude cyclone frequency is an attractive meteorological predictor for air quality on several accounts It encapsulates to some extent the information in the local meteorological predictors (temperature, solar radiation, wind speed), while providing direct information on boundary layer ventilation. It represents a non-local single metric to serve as explanatory variable for air quality on a regional scale. Since mid-latitude cyclones are an important aspect of the general circulation of the atmosphere, cyclone frequency can be expected to be robustly simulated by GCMs and provide a useful and general metric for probing the effect of climate change on air quality
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