Mean age from observations in the lowermost stratosphere: an improved method and interhemispheric differences

Abstract. The age of stratospheric air is a concept commonly used to evaluate transport timescales in atmospheric models. The mean age can be derived from observations of a single long-lived trace gas species with a known tropospheric trend. Commonly, deriving mean age is based on the assumption that all air enters the stratosphere through the tropical (TR) tropopause. However, in the lowermost stratosphere (LMS) close to the extra-tropical (exTR) tropopause, cross-tropopause transport needs to be taken into account. We introduce the new exTR–TR method, which considers exTR input into the stratosphere in addition to TR input. We apply the exTR–TR method to in situ SF6 measurements from three aircraft campaigns (PGS, WISE and SouthTRAC) and compare results to those from the conventional TR-only method. Using the TR-only method, negative mean age values are derived in the LMS close to the tropopause during the WISE campaign in Northern Hemispheric (NH) fall 2017. Using the new exTR–TR method instead, the number and extent of negative mean age values is reduced. With our new exTR–TR method, we are thus able to derive more realistic values of typical transport times in the LMS from in situ SF6 measurements. Absolute differences between both methods range from 0.3 to 0.4 years among the three campaigns. Interhemispheric differences in mean age are found when comparing seasonally overlapping campaign phases from the PGS and the SouthTRAC campaigns. On average, within the lowest 65 K potential temperature above the tropopause, the NH LMS is 0.5±0.3 years older around March 2016 than the Southern Hemispheric (SH) LMS around September 2019. The derived differences between results from the exTR–TR method and the TR-only method, as well as interhemispheric differences, are higher than the sensitivities of the exTR–TR method to parameter uncertainties, which are estimated to be below 0.22 years for all three campaigns.

[1]  E. Atlas,et al.  Age spectra and other transport diagnostics in the North American monsoon UTLS from SEAC4RS in situ trace gas measurements , 2022, Atmospheric Chemistry and Physics.

[2]  F. Haenel,et al.  The impact of sulfur hexafluoride (SF6) sinks on age of air climatologies and trends , 2022, Atmospheric Chemistry and Physics.

[3]  P. Hoor,et al.  Comparison of inorganic chlorine in the Antarctic and Arctic lowermost stratosphere by separate late winter aircraft measurements , 2021, Atmospheric Chemistry and Physics.

[4]  E. Atlas,et al.  Age Spectra and Other Transport Diagnostics in the North American Monsoon UTLS from SEAC4RS In Situ Trace Gas Measurements , 2021 .

[5]  J. Hormaechea,et al.  SOUTHTRAC-GW: An Airborne Field Campaign to Explore Gravity Wave Dynamics at the World’s Strongest Hotspot , 2021, Bulletin of the American Meteorological Society.

[6]  A. Engel,et al.  Sensitivity of age of air trends to the derivation method for non-linear increasing inert SF6 , 2020 .

[7]  P. Jöckel,et al.  Bromine from short-lived source gases in the extratropical northern hemispheric upper troposphere and lower stratosphere (UTLS) , 2020 .

[8]  P. Hoor,et al.  A convolution of observational and model data to estimate age of air spectra in the northern hemispheric lower stratosphere , 2020, Atmospheric Chemistry and Physics.

[9]  R. Weiss,et al.  The increasing atmospheric burden of the greenhouse gas sulfur hexafluoride (SF6) , 2020, Atmospheric Chemistry and Physics.

[10]  M. Pitts,et al.  Polstracc: Airborne Experiment for Studying the Polar Stratosphere in a Changing Climate with the High Altitude and Long Range Research Aircraft (HALO) , 2019 .

[11]  A. Engel,et al.  Deriving stratospheric age of air spectra using an idealized set of chemically active trace gases , 2019, Atmospheric Chemistry and Physics.

[12]  R. Weiss,et al.  History of chemically and radiatively important atmospheric gases from the Advanced Global Atmospheric Gases Experiment (AGAGE) , 2018, Earth System Science Data.

[13]  H. Oelhaf,et al.  Mixing and ageing in the polar lower stratosphere in winter 2015–2016 , 2017 .

[14]  C. Crevoisier,et al.  Mean age of stratospheric air derived from AirCore observations , 2017 .

[15]  D. Marsh,et al.  Quantification of the SF6 lifetime based on mesospheric loss measured in the stratospheric polar vortex , 2017 .

[16]  M. Riese,et al.  Hemispheric asymmetries and seasonality of mean age of air in the lower stratosphere: Deep versus shallow branch of the Brewer‐Dobson circulation , 2015 .

[17]  H. Pumphrey,et al.  Tropical troposphere to stratosphere transport of carbon monoxide and long-lived trace species in the Chemical Lagrangian Model of the Stratosphere (CLaMS) , 2014 .

[18]  N. Butchart.  The Brewer‐Dobson circulation , 2014 .

[19]  C. Sweeney,et al.  Tropospheric SF6: Age of air from the Northern Hemisphere midlatitude surface , 2013 .

[20]  L. Polvani,et al.  Air‐mass origin as a diagnostic of tropospheric transport , 2013 .

[21]  R. Weiss,et al.  Re-evaluation of the lifetimes of the major CFCs and CH 3 CCl 3 using atmospheric trends , 2012 .

[22]  Andrew Gettelman,et al.  THE EXTRATROPICAL UPPER TROPOSPHERE AND LOWER STRATOSPHERE , 2011 .

[23]  R. Weiss,et al.  History of atmospheric SF 6 from 1973 to 2008 , 2010 .

[24]  T. Birner,et al.  Residual circulation trajectories and transit times into the extratropical lowermost stratosphere , 2009 .

[25]  P. Hoor,et al.  Quantifying transport into the lowermost stratosphere using simultaneous in-situ measurements of SF 6 and CO 2 , 2008 .

[26]  R. Weiss,et al.  Medusa: a sample preconcentration and GC/MS detector system for in situ measurements of atmospheric trace halocarbons, hydrocarbons, and sulfur compounds. , 2008, Analytical chemistry.

[27]  M. Hegglin,et al.  Highly resolved observations of trace gases in the lowermost stratosphere and upper troposphere from the Spurt project: an overview , 2005 .

[28]  D. Waugh,et al.  AGE OF STRATOSPHERIC AIR: THEORY, OBSERVATIONS, AND MODELS , 2002 .

[29]  Dylan B. A. Jones,et al.  Empirical age spectra for the midlatitude lower stratosphere from in situ observations of CO2: Quantitative evidence for a subtropical “barrier” to horizontal transport , 2001 .

[30]  S. Wofsy,et al.  Empirical age spectra for the lower tropical stratosphere from in situ observations of CO2: Implications for stratospheric transport , 1999 .

[31]  Michael R. Gunson,et al.  Evaluation of source gas lifetimes from stratospheric observations , 1997 .

[32]  Timothy M. Hall,et al.  Age as a diagnostic of stratospheric transport , 1994 .

[33]  R. Prinn,et al.  The Atmospheric Lifetime Experiment: 4. Results for CF2Cl2 based on three years data , 1983 .

[34]  R. Prinn,et al.  A methodology for determining the atmospheric lifetime of fluorocarbons , 1978 .

[35]  R. A. Plumb.  Stratospheric Transport , 2002 .

[36]  H. Kida.  General Circulation of Air Parcels and Transport Characteristics Derived from a hemispheric GCM: Part 1. A Determination of Advective Mass Flow in the Lower Stratosphere@@@第一部下部成層圏の質量移流の決定 , 1983 .