A case study of trace gas transports near the tropopause

Abstract The CRISTA experiment provided global measurements of atmospheric trace gases in the stratosphere and upper troposphere during its second mission in August 1997. A new data product has recently been developed, yielding water vapor mixing ratios around the tropopause. The high data density of CRISTA makes an efficient assimilation of the water vapor field (by means of the NCAR ROSE CTM) possible. From these data turbulent trace gas fluxes v ′ χ ′ are derived. The global distribution of the H 2 O fluxes is found to be highly structured at a given pressure level. Zonal means of fluxes indicate transport barriers at the equator and near the tropopause. This is compared to effective diffusivities of P. Haynes and E. Shuckburgh [J. Geophys. Res. 105 (2000) 22795] and general agreement is obtained. There is one important discrepancy, though: the effective diffusivities show a transequatorial “bridge” at about 350 K potential temperature (about 12 km), a “bridge” which is also seen in the CRISTA water vapor data, but at a considerably higher altitude. This finding is confirmed by ozone data from CRISTA and from UARS.