[A-AS09] Stratosphere-troposphere Processes And their Role in Climate

[AAS09-05] Long-term changes in gravitational separation of the stratospheric air over Japan

*Shigeyuki Ishidoya¹, Satoshi Sugawara², Shinji Morimoto³, Sakae Toyoda⁴, Hideyuki Honda³, Shuji Aoki³, Takakiyo Nakazawa³, Daisuke Goto⁵, Yoichi Inai⁶, Kentaro Ishijima⁷, Fumio Hasebe⁸ (1.National Institute of Advanced Industrial Science and Technology (AIST), 2.Miyagi University of Education, 3.Tohoku University, 4.Tokyo Institute of Technology, 5.National Institute of Polar Research, 6.Japan Meteorological Agency, 7.Meteorological Research Institute, 8.Hokkaido University)

Keywords:gravitational separation of major atmospheric components, lower-to-middle stratosphere over Japan, age of air, Brewer-Dobson circulation

Stratospheric air samples over Japan have been collected at altitudes between 10 and 35 km using a cryogenic sampler since 1985 (e.g. Aoki et al., 2003). We have analyzed Ar/N₂ ratio and stable isotopic ratios of N₂, O₂ and Ar for the air samples, and found gravitational separation of the stratospheric air (Ishidoya et al., 2008, 2013, 2018; Sugawara et al., 2018). Numerical simulations of gravitational separation by using 2-D and 3-D models have also been conducted (Sugawara et al., 2018; Belikov et al., 2019; Birner et al., 2020), and we showed that a secular change in the stratospheric gravitational separation will induce a slight secular change in Ar/N₂ ratio at the surface (Ishidoya et al., 2021). Recently, we analyzed archived air samples collected in the lower-to-middle stratosphere over Japan since 1989 to clarify long-term variations in gravitational separation. It was found that the vertical gradients of gravitational separation vary roughly in opposite phase with the middle stratospheric mean age of air (AoA) (CO₂ and SF₆ ages). Moreover, if we follow the 2-D model simulation by Ishidoya et al. (2013), then secular enhancement of the Brewer-Dobson circulation is suggested from the relationships between the observed long-term changes in gravitational separation and those in AoA. However, there are many issues left unsolved for gravitational separation such as fluctuations found in each observed vertical profile which have not been reproduced by the 2-D and 3-D model. Therefore, more studies are needed both for observations and simulations to understand detail mechanisms of the spatiotemporal distributions in gravitational separation.

Acknowledgements
We deeply thank the Scientific Ballooning (DAIKIKYU) Research and Operation Group of the Institute of Space and Astronautical Science, JAXA. This study was partly supported by the JSPS KAKENHI (grant nos. 22H05006, 19H01975 and 19K03963).

References
Aoki, S., et al., Carbon dioxide variations in the stratosphere over Japan, Scandinavia and Antarctica. Tellus, 55B, 178–186, 2003.
Birner, B., et al., Gravitational separation of Ar/N₂ and age of air in the lowermost stratosphere in airborne observations and a chemical transport model, Atmos. Chem. Phys., 20, 12391– 12408, 2020.
Belikov, D., et al., Three-dimensional simulation of stratospheric gravitational separation using the NIES global atmospheric tracer transport model, Atmos. Chem. Phys., 19, 5349–5361, 2019.
Ishidoya, S., et al., Gravitational separation of major atmospheric components of nitrogen and oxygen in the stratosphere, Geophys. Res. Lett., 35, L03811, 2008.
Ishidoya, S., et al., Gravitational separation in the stratosphere – a new indicator of atmospheric circulation, Atmos. Chem. Phys., 13, 8787–8796, 2013.
Ishidoya, S., et al., Gravitational separation of the stratospheric air over Syowa, Antarctica and its connection with meteorological fields, Atmos. Sci. Lett., e857. 2018.
Ishidoya, S., et al., Secular change in atmospheric Ar/N₂ and its implications for ocean heat uptake and Brewer-Dobson circulation, Atmos. Chem. Phys., 21, 1357–1373, 2021.
Sugawara, S., et al., Age and gravitational separation of the stratospheric air over Indonesia, Atmos. Chem. Phys., 18, 1819–1833, 2018.