Constraints on 2-Way Transport across the Arctic Tropopause Based on O3, Stratospheric Tracer (SF6) Ages, and Water Vapor Isotope (D, T) Tracers

Abstract

Based upon airborne trace gas and isotope observations in the winter months 1991/1992 to1994/1995, transport pathways across the mid-latitude and Arctic tropopause areinvestigated. A powerful set of contrasting transport tracers are examined, such asdeuterated water vapor (HDO) which is shown to trace the passage of water vapor from thetroposphere into the lowermost stratosphere (LS), or the `SF6 age' defined as theresidence time of an air parcel within the stratosphere since its entry at thetropopause. Cross-tropopause transport in both directions was found near mid-latitudecyclones (at baroclinic flanks of troughs in the polar front), in which about 80% of thestratosphere-to-troposphere flux proceeded along potential temperature (θ)surfaces of 300 ± 10 K. As these isentropes are the lowest which reach into the LS(in winter), a mixing zone just above the Arctic tropopause (at least 1.5 km thick) isformed. Here, upwelling tropospheric air is mixed with downwelling LS air which isaffected by air from higher altitudes, the surf-zone and the polar vortex. The observedelevated D/H isotope ratio of water vapor within the mixing zone can be explained byinjection of subtropical water vapor that is transported to the tropopause by the warmconveyor belt associated with mid-latitude cyclones. Downward vertical transport ofArctic LS air, which may be influenced by ouflowing chemically disturbed polar vortexair, into the Arctic troposphere was found to be small.