AQAST research highlights

Influence of synoptic patterns on surface ozone variability
over the eastern United States from 1980 to 2012
by L. Shen, L. J. Mickley, and A. P. K. Tai

Atmos. Chem. Phys., 15, 10925-10938, 2015.

Shen 2015

Caption: Slopes of mean summertime MDA8 ozone in the eastern United States vs. the longitude of the western edge of the Bermuda High over the 1980-2012 time period. The slopes were constructed using residuals of the seven-year moving averages of MDA8 ozone in each grid box. Positive values indicate increasing ozone with westward shift of the Bermuda High west edge. The left panel shows results for those summers when the west edge was located in the West Regime, west of 85.4°W. The right panel shows results for the East Regime, with the west edge east of 85.4°W. The boundary between the two regimes is denoted by the blue arrow in the left panel.

We investigate the effect of synoptic-scale weather patterns on observed maximum daily 8-hour average (MDA8) surface ozone over the eastern United States during 1980-2012 in summer (June-August, JJA). Zonally averaged, the relative standard deviation (SD) of daily MDA8 JJA ozone shows a bimodal structure, with peaks at 28°-32°N and 40°-45°N, and we show that those regions are most influenced by the variability in daily weather. The northern peak of ozone relative SD can be explained by polar jet activity, while the southern peak appears related to variability in the Bermuda High and Great Plains Low Level Jet. We define a new metric, polar jet frequency, as the total number of days the jet traverses the Midwest and Northeast each summer. In the Midwest and Northeast, we find that the correlation coefficient r between detrended mean JJA MDA8 ozone and the polar jet frequency ranges between -0.76 and -0.93 over 1980-2012 depending on the time period selected, suggesting that polar jet frequency could provide a simple metric to predict ozone variability in future climate regimes. In the Southeast, the influence of the Bermuda High on mean JJA MDA8 ozone depends on the location of its west edge. For those summers when the average position of the west edge is located west of ~85.4°W, a westward shift in the Bermuda High west edge increases ozone in the Southeast by ~1 ppbv deg-1 in longitude. For all summers, a northward shift in the Bermuda High west edge increases ozone over the entire eastern United States by 1-2 ppbv deg-1 in latitude. None of the synoptic patterns identified in this study show a significant trend from 1980 to 2012, confirming that the observed ozone decrease over the eastern United States during this time period is mainly caused by emission controls. Our work underscores the impact of synoptic patterns on ozone variability and suggests that a combination of changing local and synoptic meteorology together with trends in background ozone will determine the influence of climate change on U.S. ozone air quality in future decades.

Dec 2015