Gravitational separation in the stratosphere – a new indicator of atmospheric circulation

Abstract. As a basic understanding of the dynamics of the atmospheric circulation, it has been believed that gravitational separation of atmospheric components is observable only in the atmosphere above the turbopause. However, we found, from our high-precision measurements of not only the isotopic ratios of N₂, O₂ and Ar but also the mole fraction of Ar, that gravitational separation occurs even in the stratosphere below the turbopause; their observed vertical profiles are in good agreement with those expected theoretically from molecular mass differences. The O₂/N₂ ratio observed in the middle stratosphere, corrected for gravitational separation, showed the same mean air age as estimated from the CO₂ mole fraction. Simulations with a 2-dimensional model of the middle atmosphere indicated that a relationship between gravitational separation and the age of air in the stratosphere would be significantly affected if the Brewer–Dobson circulation was enhanced due to global warming. Therefore, gravitational separation is usable as a new indicator of changes in the atmospheric circulation in the stratosphere.