[A-AS11] Atmospheric Chemistry

[AAS11-P03] 10-years observation of atmospheric Ar/N₂ ratio and its comparison with some indexes associated with climate change

*Shigeyuki Ishidoya¹, Yasunori Tohjima², Satoshi Sugawara³, Daisuke Goto⁴, Kazuhiro Tsuboi⁵, Shohei Murayama¹, Kentaro Ishijima⁵, Yosuke Niwa², Nobuyuki Aoki¹ (1.National Institute of Advanced Industrial Science and Technology (AIST), 2.National Institute for Environmental Studies, 3.Miyagi University of Education, 4.National Institute of Polar Research, 5.Meteorological Research Institute)

Keywords:Atmospheric Ar/N2 ratio, gravitational separation in the atmosphere, ocean heat content, global surface temperature , Pacific Decadal Oscillation

Variations in the tropospheric and the stratospheric Ar/N₂ ratio are driven mainly by air-sea Ar and N₂ flux due to changes in solubility and gravitational separation of Ar and N₂, respectively (e.g. Keeling et al., 2004; Ishidoya et al., 2013). Recently, Ishidoya et al. (2021) reported temporal variations of gravitational separation in the middle atmosphere could also modify the long-term trend of the tropospheric Ar/N₂ ratio. Therefore, Ar/N₂ ratio has been applied not only to evaluating seasonal air-sea heat flux (Morgan et al., 2021) and atmospheric circulation in the stratosphere (e.g. Sugawara et al, 2018), but also to estimate changes in the ocean heat content (OHC) by simultaneous analyses of the surface and the stratospheric long-term trends (Ishidoya et al., 2021). We have continued observations of the Ar/N₂ ratio by using a mass spectrometer at Tsukuba (36°N, 140°E) and Hateruma Island (24°N, 124°E) for 10 years since 2012, and started observations at Cape Ochiishi (43°N, 146°E), Ryori (39°N, 142°E), Takayama (36°N, 137°E), and Minamitorishima (24°N, 154°E), Japan and Syowa station (69°S, 40°E), Antarctica since 2013 – 2020. In this study, we compared the long-term Ar/N₂ ratio data with some indexes associated with climate change; global OHC, global surface temperature, ENSO and Pacific Decadal Oscillation (PDO). The observed Ar/N₂ ratio showed secular increases before 2018 and subsequent decreases, although the global OHC showed secular increase throughout the period (updated from Levitus et al., 2012, https://www.ncei.noaa.gov/access/global-ocean-heat-content/). It was also found that the Ar/N₂ ratio at Tsukuba and Hateruma showed interannual variations in phase with the global OHC, and the minima of the rate of change of Ar/N₂ratio and the global OHC appeared at the beginning of 2016 and 2020 when maxima in the surface temperature anomaly appeared (JMA, https://www.data.jma.go.jp/cpdinfo/temp/nov_wld.html). Secular increases and subsequent decreases of Ar/N₂ ratio also seems to be corresponding to the variations in PDO (updated from Mantua, 1999, https://www.ncdc.noaa.gov/teleconnections/pdo/), of which links to variability in Earth’s energy imbalance have been suggested by past studies (Loeb et al., 2021). It seems that we have detected long-term variations in Ar/N₂ ratio corresponding to climate change, however, further studies are indispensable for quantitative interpretation.

Acknowledgements
We thank staff of Global Environmental Forum (GEF), the Science Program of the Japan Antarctic Research Expedition (JARE) and Japan Meteorological Agency (JMA) for their works to collect the air samples at Hateruma and Ochiishi stations (GEF), Syowa station (JARE), Ryori and Minamitorishima (JMA), respectively. This study was partly supported by the JSPS KAKENHI (grant nos. 15H02814, 19H01975 and 19K03963) and the Global Environment Research Coordination System from the Ministry of the Environment, Japan (grant nos. METI1454 and METI1953).

References
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., Secular change in atmospheric Ar/N₂ and its implications for ocean heat uptake and Brewer-Dobson circulation, Atmos. Chem. Phys., 21, 1357-1373, 2021.
Keeling, R. F., et al., Measurement of changes in atmospheric Ar/N₂ ratio using a rapid-switching, single-capillary mass spectrometer system, Tellus B, 56, 322–338, 2004.
Levitus, S., et al., World ocean heat content and thermosteric sea level change (0–2000 m), 1955–2010, Geophys. Res. Lett., 39, L10603, 2012.
Loeb, N. G., et al., Satellite and Ocean Data Reveal Marked Increase in Earth’s Heating Rate, Geophys. Res. Lett., 48, e2021GL093047, 2021.
Mantua, N. J., The Pacific Decadal Oscillation, A brief overview for non-specialists, Encyclopedia of Environmental Change, 1999.
Morgan, E. J., et al., An atmospheric constraint on the seasonal air-sea exchange of oxygen and heat in the extratropics, J. Geophys. Res., Oceans, 126, e2021JC017510, 2021.
Sugawara, S., et al., Age and gravitational separation of the stratospheric air over Indonesia, Atmos. Chem. Phys., 18, 1819-1833, 2018.