Seasonal characteristics of trace gas transport into the extratropical upper troposphere and lower stratosphere

Yoichi Inai,Hidekazu Matsueda,Kazuhiro Tsuboi,Toshinobu Machida,Yousuke Sawa,Shinji Morimoto,Keiichi Katsumata,Ryo Fujita,Shuji Aoki,Takakiyo Nakazawa

doi:10.5194/acp-19-7073-2019

Abstract

Abstract. To investigate the seasonal characteristics of trace gas distributions in the extratropical upper troposphere and lower stratosphere (ExUTLS) as well as stratosphere–troposphere exchange processes, origin fractions of air masses originating in the stratosphere, tropical troposphere, midlatitude lower troposphere (LT), and high-latitude LT in the ExUTLS are estimated using 10-year backward trajectories calculated with European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim data as the meteorological input. Time series of trace gases obtained from ground-based and airborne observations are incorporated into the trajectories, thus reconstructing spatiotemporal distributions of trace gases in the ExUTLS. The reconstructed tracer distributions are analyzed with the origin fractions and the stratospheric age of air (AoA) estimated using the backward trajectories. The reconstructed distributions of SF6 and CO2 in the ExUTLS are linearly correlated with those of AoA because of their chemically passive behavior and quasi-stable increasing trends in the troposphere. Distributions of CH4, N2O, and CO are controlled primarily by chemical decay along the transport path from the source region via the stratosphere and subsequent mixing of such stratospheric air masses with tropospheric air masses in the ExUTLS.

Highlights

The extratropical upper troposphere and lower stratosphere (ExUTLS; e.g., Gettelman et al, 2011) account for about 40 % of the total stratospheric air mass (Appenzeller et al, 1996) and about 20 % of stratospheric aerosols (Andersson et al, 2015)
Distributions of origin fractions in a ∅eq–θ cross section are shown in Fig. 9 for January together with the climatology of monthly average potential vorticity for the period 2012–2016 obtained from ERA-Interim
Air masses in regions lower than 310 K generally originate in the midlatitude lower troposphere (LT) with mixing fractions up to ∼ 70 %, with few air masses originating in the high-latitude LT

Summary

Introduction

The extratropical upper troposphere and lower stratosphere (ExUTLS; e.g., Gettelman et al, 2011) account for about 40 % of the total stratospheric air mass (Appenzeller et al, 1996) and about 20 % of stratospheric aerosols (Andersson et al, 2015). Trace gases and aerosols in the ExUTLS play an important role in atmospheric radiative processes. These species are transported to the ExUTLS from the deep stratosphere via stratospheric circulation (Brewer–Dobson circulation, BDC; Brewer, 1949; Dobson, 1956) and from the lower troposphere or the tropical troposphere via local convection, frontal cyclones, Rossby wave breaking at/along the subtropical jet, monsoon activity, and other systems (e.g., Holton et al, 1995; Wernli and Bourqui, 2002; Manney et al, 2011; Pan et al, 2016; Vogel et al, 2016; Boothe and Homeyer, 2017; Ploeger et al, 2017). Appenzeller et al (1996) estimated the mass flux across the 380 K isentrope due to global-scale meridional circulation and found that the downwelling mass flux from the stratosphere varies from 8 × 109 kg s−1 in summer to 15 × 109 kg s−1 in winter, whereas the Asian summer monsoon and local convection, which supply tropospheric air to the ExUTLS, are active only during the summer and early autumn

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