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

Abstract. 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 (TKB) and Hateruma (HAT), Japan, showed interannual

variations in phase with the observed variations in the global ocean heat

content (OHC). We calculated secularly increasing trends of 0.75 ± 0.30

and 0.89 ± 0.60 per meg per year from the Ar∕N2 ratio observed at

TKB and HAT, respectively, although these trend values are influenced by

large interannual variations. In order to examine the possibility of the

secular trend in the surface Ar∕N2 ratio being modified significantly

by the gravitational separation in the stratosphere, two-dimensional model

simulations were carried out by arbitrarily modifying the mass stream

function in the model to simulate either a weakening or an enhancement of

the Brewer–Dobson circulation (BDC). The secular trend of the Ar∕N2

ratio at TKB, corrected for gravitational separation under the assumption of

weakening (enhancement) of BDC simulated by the 2-D model, was 0.60 ± 0.30 (0.88 ± 0.30) per meg per year. By using a conversion factor of

3.5 × 10−23 per meg per joule by assuming a one-box ocean with a temperature

of 3.5 ∘C, average OHC increase rates of 17.1 ± 8.6 ZJ yr−1 and 25.1 ± 8.6 ZJ yr−1 for the period 2012–2019 were

estimated from the corrected secular trends of the Ar∕N2 ratio for the

weakened- and enhanced-BDC conditions, respectively. Both OHC increase

rates from the uncorrected- and weakened-BDC secular trends of the Ar∕N2

ratio are consistent with 12.2 ± 1.2 ZJ yr−1 reported by ocean

temperature measurements, while that from the enhanced-BDC is outside of the

range of the uncertainties. Although the effect of the actual atmospheric

circulation on the Ar∕N2 ratio is still unclear and longer-term

observations are needed to reduce uncertainty of the secular trend of the

surface Ar∕N2 ratio, 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.