The Impact of ENSO on Surface Ozone Concentrations Over California

Casual examination of the number of days per year in which southern California experiences unhealthy (>84ppb) levels of surface ozone shows that there is significant inter-annual variability in yearly mean surface ozone concentration. Furthermore, the years of known major El Nino/Southern Oscillation events (e.g. 1983 and 1997) seem to have less days with high ozone concentrations. In this study, we examine the processes by which surface ozone over California is impacted by inter-annual climate variability using four dimensional statistical analysis techniques, with specific attention paid to the effect of the El Nino/Southern Oscillation (ENSO). The data sets we examine include the California Air Resources Board's observed surface ozone time series, the National Center for Environmental Prediction's global reanalysis data sets, and the University of Washington's Joint Institute for the Study of the Atmosphere and Ocean's Cold Tongue Index time series, with all data sets spanning the period 1972-2006. We employ several statistical methods in this study, including single and multiple linear regression, empirical orthogonal function analysis, and maximum covariance analysis. Initial results indicate that a southern shift in the mean track of mid-latitude cyclones entering the west coast - a phenomenon linked to ENSO events - may be responsible for altering the stagnation time period of air masses over California. We also explore inter-regional differences in surface ozone sensitivity to ENSO and other global climate processes amongst various air basins within California.