Secular change in atmospheric Ar/N2 and its implications for ocean heat uptake and Brewer-Dobson circulation

Systematic measurements of the atmospheric Ar/N2 ratio have been made at ground-based stations in Japan and Antarctica since 2012. Clear seasonal cycles of the Ar/N2 ratio with summertime maxima were found at middle to high latitude stations, with seasonal amplitudes increasing with increasing latitude. Eight years of the observed Ar/N2 ratio at Tsukuba and Hateruma, Japan showed not only secular increasing trends, but also interannual variations in phase with the observed variations in the global ocean heat content (OHC). The observed secular trend of the Ar/N2 ratio was 0.75±0.30 per meg yr-1. Sensitivity test by using a 2-dimensional model with the Brewer-Dobson circulation (BDC) scenarios indicated the possibility of the secular trend in the surface Ar/N2 ratio being modified significantly by the gravitational separation in the stratosphere. The secular trend of the Ar/N2 ratio, corrected for gravitational separation under the assumption of weakening of BDC simulated by the 2D model, was 0.60±0.30 per meg yr-1. By using a conversion factor of 3.5x10-23 per meg J-1 by assuming a 1-box ocean with a temperature of 3.5 °C, then an average OHC increase rate of 17.1±8.6 ZJ yr-1 for the period 2012–2019 was estimated from the corrected secular trend of the Ar/N2 ratio. This value is consistent with 12.2±1.2 ZJ yr-1 reported by ocean temperature measurements. The effect of the actual atmospheric circulation on the Ar/N2 ratio is still unclear, however the analytical results obtained in the present study imply that the surface Ar/N2 ratio is an important tracer for detecting spatiotemporally-integrated changes in OHC and BDC.