Tropopause Based on O3, Stratospheric Tracer (SF6) Ages, and Water Vapor Isotope (D, T) Tracers

• Main Author: Andreas Zahn
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1999
• Subjects: Arctic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.523.8251; http://www-imk.fzk.de/asf/chemie/papers/JAC_Zahn_2001.pdf

Abstract. Based upon airborne trace gas and isotope observations in the winter months 1991/1992 to 1994/1995, transport pathways across the mid-latitude and Arctic tropopause are investigated. A powerful set of contrasting transport tracers are examined, such as deuterated water vapor (HDO) which is shown to trace the passage of water vapor from the troposphere into the low-ermost stratosphere (LS), or the ‘SF6 age ’ defined as the residence time of an air parcel within the stratosphere since its entry at the tropopause. Cross-tropopause transport in both directions was found near mid-latitude cyclones (at baroclinic flanks of troughs in the polar front), in which about 80 % of the stratosphere-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) is formed. Here, upwelling tropospheric air is mixed 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 ratio of water vapor within the mixing zone can be explained by injection of subtropical water vapor that is transported to the tropopause by the warm conveyor belt associated with mid-latitude cyclones. Downward vertical transport of Arctic LS air, which may be influenced by ouflowing chemically disturbed polar vortex air, into the Arctic troposphere was found to be small. Key words: extra-tropical stratospheric-tropospheric exchange, isotope composition of tropospheric and stratospheric water vapor. 1.