Constraints on 2-way transport across the Arctic tropopause based on O-3, stratospheric tracer (SF6) ages, and water vapor isotope (D, T) tracers

Based upon airborne trace gas and isotope observations in the winter months 1991/1992 to 1994/1995, transport pathways across the mid-latitude and Arc tic tropopause are investigated. A powerful set of contrasting transport tr acers are examined, such as deuterated water vapor (HDO) which is shown to trace the passage of water vapor from the troposphere into the lowermost st ratosphere (LS), or the 'SF6 age' defined as the residence time of an air p arcel within the stratosphere since its entry at the tropopause. Cross-trop opause transport in both directions was found near mid-latitude cyclones (a t baroclinic flanks of troughs in the polar front), in which about 80% of t he stratosphere-to-troposphere flux proceeded along potential temperature ( theta) surfaces of 300 +/- 10 K. As these isentropes are the lowest which r each into the LS (in winter), a mixing zone just above the Arctic tropopaus e (at least 1.5 km thick) is formed. Here, upwelling tropospheric air is mi xed with downwelling LS air which is affected by air from higher altitudes, the surf-zone and the polar vortex. The observed elevated D/H isotope rati o of water vapor within the mixing zone can be explained by injection of su btropical water vapor that is transported to the tropopause by the warm con veyor belt associated with mid-latitude cyclones. Downward vertical transpo rt of Arctic LS air, which may be influenced by ouflowing chemically distur bed polar vortex air, into the Arctic troposphere was found to be small.