Influence of synoptic patterns on surface ozone variability over the eastern United States from 1980 to 2012

摘要

We investigate the effect of synoptic-scale weather patterns on observedmaximum daily 8-hour average (MDA8) surface ozone over the eastern UnitedStates during 1980–2012 in summer (June–August, JJA). Zonally averaged, therelative standard deviation (SD) of daily MDA8 JJA ozone shows a bimodalstructure, with peaks at 28–32 and 40–45° N,and we show that those regions are most influenced by thevariability in daily weather. We apply empirical orthogonal functions (EOFs)to understand the causes of this structure. The first three leading EOFpatterns explain 53 % of the total variance in detrended surface ozone,displaying (1) a widespread response of ozone in the eastern United Statesassociated with north–south movement of jet wind latitude, (2) a north–southpattern linked to the Bermuda High system when its west boundary is locatedalong the east coast, and (3) an east–west pattern characteristic of awestward extension of the Bermuda High and an enhanced Great Plains lowlevel jet (GPLLJ). The northern peak of ozone relative SD can be explainedby polar jet activity, while the southern peak appears related tovariability in the Bermuda High and GPLLJ. We define a new metric polar jetfrequency as the total number of days the jet traverses the Midwest andnortheast each summer. In the Midwest and northeast, we find that thecorrelation coefficient r between detrended mean JJA MDA8 ozone and the polarjet frequency ranges between ?0.76 and ?0.93 over 1980–2012 depending on thetime period selected, suggesting that polar jet frequency could provide asimple metric to predict ozone variability in future climate regimes. In thesoutheast, the influence of the Bermuda High on mean JJA MDA8 ozone dependson the location of its west edge. For those summers when the averageposition of the west edge is located west of ~ 85.4°W, a westward shift in the Bermuda High west edge increases ozone in thesoutheast by ~ 1 ppbv deg?1 in longitude. For allsummers, a northward shift in the Bermuda High west edge increases ozoneover the entire eastern United States by 1–2 ppbv deg?1 in latitude.None of the synoptic patterns identified in this study show a significanttrend from 1980 to 2012, confirming that the observed ozone decrease overthe eastern United States during this time period is mainly caused byemission controls. Our work underscores the impact of synoptic patterns onozone variability and suggests that a combination of changing local andsynoptic meteorology together with trends in background ozone will determinethe influence of climate change on US ozone air quality in future decades.The observed relationships of US surface ozone and synoptic circulationsin this study can also be used to validate models of atmospheric chemistry.