On the relationship between the strength of the Brewer-Dobson circulation and the age of stratospheric air

[1] The strength of the Brewer-Dobson circulation is computed for multi-decadal simulations of a coupled chemistry-climate model covering the period 1960 to 2100. The circulation strength, as computed from the tropical mass upwelling, generally increases throughout the simulations. The model also includes an age of air tracer which generally decreases during the simulations. The two different transport concepts of mass upwelling and reciprocal of the age of air are investigated empirically from the model simulations. The results indicate that the variables are linearly related in the model but with a change of gradient some time near 2005. Possible reasons for the change of gradient are discussed. Citation: Austin, J., and F. Li (2006), On the relationship between the strength of the Brewer-Dobson circulation and the age of stratospheric air, Geophys. Res. Lett., 33, L17807, doi:10.1029/2006GL026867. [2] The so-called Brewer-Dobson circulation [Holton et al., 1995] is a global circulation which in the annual average consists of motion which is upward in the tropics and downwards in the middle and high latitudes. The tropical mass upwelling in the very low stratosphere is a measure of the strength of the Brewer-Dobson circulation [Butchart and Scaife, 2001] and in multi-decadal simulations of a climate model was shown to increase mainly due to an enhancement of the planetary wave driving in the lower stratosphere. The year to year fluctuations in the circulation were found to be large, but over long time scales an overall increase in tropical upwelling was found to occur. This is supported by recent results from a wide range of simulations of independent models [Butchart et al., 2006]. [3] Model simulations suggest that the rate of increase of the mass upwelling varies from decade to decade. For an ensemble of simulations for the recent past, Austin et al. [2006] found that the tropical upwelling was approximately constant for the period 1960 to 1975, but then increased rapidly for the next 20 years. It is possible that the change in circulation trend from 1975 may have been related to the loss of ozone from that time. The coupling of the system is thus suggested by the likelihood that changes in circulation will change ozone transport and that the ozone itself will also affect the circulation. [4] An alternative measure of circulation strength is the age of stratospheric air [e.g., Hall and Plumb, 1994; Waugh and Hall, 2002], which is essentially the mean time since the air parcel previously resided in the troposphere. It might be expected that mean age of air and the strength of the Brewer-Dobson circulation are closely connected, but despite the established trend in simulations of the latter, there has so far been no clear indication in the published literature that age of air might change systematically either in the atmosphere or in models. The relationship with atmospheric transport is also not yet clearly established. Although several works have recognized the relationship between heat fluxes and ozone transport [Salby and Callaghan, 2002; Weber et al., 2003] these have referred primarily to interannual and not to secular variations. [5] In this work, we use multi-decadal simulations of the GFDL coupled chemistry climate model (AMTRAC — Atmospheric Model with TRansport And Chemistry) to relate the strength of the Brewer-Dobson circulation with the stratospheric age of air.