Variability of winter and summer surface ozone in Mexico City on the intraseasonal timescale

摘要

Surface ozone concentrations in Mexico City frequently exceed the Mexicanstandard and have proven difficult to forecast due to changes inmeteorological conditions at its tropical location. The Madden–JulianOscillation (MJO) is largely responsible for intraseasonal variability in thetropics. Circulation patterns in the lower and upper troposphere andprecipitation are associated with the oscillation as it progresses eastwardaround the planet. It is typically described by phases (labeled 1 through 8),which correspond to the broad longitudinal location of the active componentof the oscillation with enhanced precipitation. In this study we evaluate theintraseasonal variability of winter and summer surface ozone concentrationsin Mexico City, which was investigated over the period 1986–2014 todetermine if there is a modulation by the MJO that would aid in the forecastof high-pollution episodes.Over 1 000 000 hourly observations of surface ozone from five stationsaround the metropolitan area were standardized and then binned by activephase of the MJO, with phase determined using the real-time multivariate MJOindex. Highest winter ozone concentrations were found in Mexico City on dayswhen the MJO was active and in phase 2 (over the Indian Ocean), and highestsummer ozone concentrations were found on days when the MJO was active andin phase 6 (over the western Pacific Ocean). Lowest winter ozoneconcentrations were found during active MJO phase 8 (over the easternPacific Ocean), and lowest summer ozone concentrations were found duringactive MJO phase 1 (over the Atlantic Ocean). Anomalies of reanalysis-basedcloud cover and UV-B radiation supported the observed variability in surfaceozone in both summer and winter: MJO phases with highest ozone concentrationhad largest positive UV-B radiation anomalies and lowest cloud-coverfraction, while phases with lowest ozone concentration had largest negativeUV-B radiation anomalies and highest cloud-cover fraction. Furthermore,geopotential height anomalies at 250 hPa favoring reduced cloudiness, andthus elevated surface ozone, were found in both seasons during MJO phaseswith above-normal ozone concentrations. Similar height anomalies at 250 hPafavoring enhanced cloudiness, and thus reduced surface ozone, were found inboth seasons during MJO phases with below-normal ozone concentrations. Theseanomalies confirm a physical pathway for MJO modulation of surface ozone viamodulation of the upper troposphere.