Keywords:Atmospheric Ar/N2 ratio, air-sea heat flux, seasonal cycle, atmospheric transport model
Variations of the atmospheric Ar/N2 ratio at the ground surface are driven principally by air-sea Ar and N2 fluxes due to changes in solubility in seawater (e.g. Keeling et al., 2004). Recently, we expanded our model study for the gravitational separation to Ar/N2 ratio, and found that temporal variations of gravitational separation in the middle atmosphere could also modify the long-term trend of the surface Ar/N2 ratio (Ishidoya et al., in preparation). Therefore, the surface Ar/N2 ratio is a unique tracer of the spatiotemporally-integrated air-sea heat flux and the circulation in the middle atmosphere. We have continued systematic observations of the Ar/N2 ratio by using a mass spectrometer at Cape Ochiishi (43°N, 146°E), Tsukuba (36°N, 140°E), Takayama (36°N, 137°E), Hateruma Island (24°N, 124°E) and Minamitorishima (24°N, 154°E), Japan and Syowa station (69°S, 40°E), Antarctica since 2012. Clear seasonal Ar/N2 ratio cycles with summertime maxima have been observed at the middle to high latitudinal stations, and the peak-to-peak amplitudes of the average seasonal cycles at Ochiishi, Tsukuba, Hateruma and Syowa were found to be 21, 11, 5 and 32 per meg, respectively. To evaluate the seasonal air-sea heat flux based on seasonal cycles of Ar/N2 ratio in the atmosphere, we carried out simulations of the Ar/N2 ratio using an atmospheric transport model (GSAM-TM) that incorporated the Ar (N2) flux derived from an equation of the relationship between the air-sea Ar (N2) flux and the air-sea heat flux (Keeling et al., 1993; Weiss, 1970). We use the air-sea heat flux components, which mainly drive spatiotemporal variations of the air-sea Ar (N2) fluxes, and sea surface temperature (SST) from the ERA5 (Hersbach et al., 2019). Thus simulated seasonal cycles of Ar/N2 ratio agreed well with those observed. On the other hand, the amplitudes of the seasonal cycles of Ar/N2 ratio simulated by using the TransCom seasonal air-sea N2 flux (Garcia and Keeling, 2001), widely used in the simulation of the atmospheric O2/N2 ratio and based on the past ECMWF seasonal air-sea heat flux, underestimate the observed seasonal cycles significantly. These facts suggest that the air-sea heat fluxes and SST from the ERA5 is reasonable to reproduce the atmospheric Ar/N2 variations on the seasonal time scale.
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) and Minamitorishima (JMA), respectively. This study was partly supported by the JSPS KAKENHI Grant Number 15H02814 and 18K01129, and the Global Environment Research Coordination System from the Ministry of the Environment.
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