Diagnosing the stratosphere-to-troposphere flux of Ozone

We use high-resolution simulations of Ozone from a chemistry-transport model (CTM) to design and test new diagnostics of the Ozone Stratosphere-troposphere exchnage (STE). The CTM uses meteorological fields developed at U. Oslo from the ECMWF Integrated Forecast System at a resolution of about 200 km (horizontal) by 0.6 km (vertical). These simulations reproduce the tropopause structures, folds, and intrusions observed during the TRACE-P campaign. Our goal is to design a diagnostic O$_3$ flux that does not simply record the net flux of Ozone across a climatological tropopause or a pressure level, but one that counts stratospheric O$_3$ molecules when they have effectively mixed into the troposphere and participate in tropospheric chemistry. For example, a stratospheric fold located below the tropopause but containing Ozone at hundreds of ppb with very low water vapor and hydrocarbons has chemistry like the lower stratosphere rather than the troposphere. Thus we design a budget diagnostic that follows stratospheric O$_3$ molecules as they enter the troposphere, recording a net flux only when they mix into the troposphere (i.e., dilute to below 100 ppb). We are able to derive latitude-by-longitude maps of this STE Ozone flux, and thus show where stratospheric is most likely to influence the tropospheric abundance.

With these met fields, the global Ozone fluxes are calculated to be about 600 Tg/yr with an equal contribution from each hemisphere, and larger spring-time contributions for both hemispheres. For the northern hemisphere, regions of high Ozone fluxes are closely related to the cyclogenesis/baroclinic activity and therefore follow the jet streams. However, in late spring/early summer, the maximum Ozone flux is located in the subtropical region, particularly in the region of the Tibetan Plateau. This flux is caused by the entrainment of stratospheric air associated with the rapid rise of the tropopause over the Tibetan Plateau. In contrast, the Ozone fluxes in the southern hemisphere have less temporal and zonal variations and occur mainly in the subtropical region (25S-35S) regardless of season. The zero wind lines at 200 mb are shown to be a good marker for the equatorward margin of Ozone STE and overlap with local STE maxima in the subtropical western Pacific during the summer. This is the signature of Ozone STE due to stationary Rossby wave breaking over this region. In some winter months, the Ozone STE extends deep into the eastern Pacific tropics between the zero wind lines due to the intrusion of waves through the westerly duct.