TY - JOUR
T1 - Long-Term Variation in the Mixing Fraction of Tropospheric and Stratospheric Air Masses in the Upper Tropical Tropopause Layer
AU - Inai, Yoichi
N1 - Funding Information:
This work was supported by Grants-in- Aid for Scientific Research (15 K17760 and 26220101) from the Japanese Society for the Promotion of Science and the Arctic Challenge for Sustainability (ArCS) Project by the Ministry of Education, Culture, Sports, Science and Technology, Japan. The author thanks Fumio Hasebe and Satoshi Sugawara for helpful discus sions. Engel’s AoA data were down loaded from supporting information of Engel et al. (2009; URL: https://images. nature.com/full/nature-assets/ngeo/ journal/v2/n1/extref/ngeo388-s1.pdf) for 1975−2005 and are referred to in the text as Engel et al. (2017) for 2015 and 2016. Appreciation is given to ECMWF for providing the ERA-Interim data and to the NASA Jet Propulsion Laboratory for providing the Aura/MLS data. All figures were produced with the GFD-DENNOU Library. The author is also grateful to the Editor William Randel and sincerely appreciates three reviewers who provided constructive comments that greatly improved the manuscript.
Publisher Copyright:
©2018. American Geophysical Union. All Rights Reserved.
PY - 2018/5/27
Y1 - 2018/5/27
N2 - Observational records of the stratospheric mean age of air (AoA), the average transit time of an air parcel since entering the stratosphere, must reflect changes in the in-mixing from the extratropical lower stratosphere to the tropical tropopause layer (TTL). Focusing on the mixing fraction of tropospheric and stratospheric air masses in the upper TTL, the impact of mixing processes that occur prior to air masses entering the stratosphere separately from those that occur within the stratosphere is investigated. Using trajectory analysis, the mixing fraction during 1980–2016 is evaluated, and AoA and water vapor mixing ratios in the upper TTL are reconstructed. The interannual variation in the reconstructed water vapor shows good agreement with observations in the tropics. Furthermore, the reconstructed AoA exhibits long-term variation with a positive trend between 1980 and 1999. To compare the reconstructed AoA with the observed AoA in the midlatitude stratosphere, the transit time spectra are estimated for the 1989–2016 period, and the AoA in the midlatitude stratosphere is estimated by integration of the spectrum together with the AoA in the upper TTL. As a result, although the explicitly calculated transit time shows a clear underestimation compared with previous studies, it is suggested that the “aging” effects are 3.1–4.2 years for transit time in the stratosphere, 0.2–0.7 years for in-mixing prior to entering the stratosphere, 0.1 years for transit time from the troposphere to upper TTL, and 0.0–1.6 years for subgrid-scale mixing in the stratosphere.
AB - Observational records of the stratospheric mean age of air (AoA), the average transit time of an air parcel since entering the stratosphere, must reflect changes in the in-mixing from the extratropical lower stratosphere to the tropical tropopause layer (TTL). Focusing on the mixing fraction of tropospheric and stratospheric air masses in the upper TTL, the impact of mixing processes that occur prior to air masses entering the stratosphere separately from those that occur within the stratosphere is investigated. Using trajectory analysis, the mixing fraction during 1980–2016 is evaluated, and AoA and water vapor mixing ratios in the upper TTL are reconstructed. The interannual variation in the reconstructed water vapor shows good agreement with observations in the tropics. Furthermore, the reconstructed AoA exhibits long-term variation with a positive trend between 1980 and 1999. To compare the reconstructed AoA with the observed AoA in the midlatitude stratosphere, the transit time spectra are estimated for the 1989–2016 period, and the AoA in the midlatitude stratosphere is estimated by integration of the spectrum together with the AoA in the upper TTL. As a result, although the explicitly calculated transit time shows a clear underestimation compared with previous studies, it is suggested that the “aging” effects are 3.1–4.2 years for transit time in the stratosphere, 0.2–0.7 years for in-mixing prior to entering the stratosphere, 0.1 years for transit time from the troposphere to upper TTL, and 0.0–1.6 years for subgrid-scale mixing in the stratosphere.
KW - Brewer-Dobson circulation
KW - age of air
KW - stratosphere
KW - stratospheric water vapor
KW - trajectory analysis
KW - tropical tropopause layer
UR - http://www.scopus.com/inward/record.url?scp=85047630989&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85047630989&partnerID=8YFLogxK
U2 - 10.1029/2018JD028300
DO - 10.1029/2018JD028300
M3 - Article
AN - SCOPUS:85047630989
VL - 123
SP - 4890
EP - 4909
JO - Journal of Geophysical Research: Atmospheres
JF - Journal of Geophysical Research: Atmospheres
SN - 2169-897X
IS - 10
ER -