Title: Tracer-Independent Approaches to Stratosphere-Troposphere Exchange and Tropospheric Air Mass Composition
Description: Two transport processes are examined. The first addresses the interaction between the stratosphere and the troposphere. We perform the first analyses of stratosphere-troposphere exchange using one-way flux distributions; diagnostics are illustrated in both idealized and comprehensive contexts. By partitioning the one-way flux across the thermal tropopause according to stratospheric residence time τ and the regions where air enters and exits the stratosphere, the one-way flux is quantified robustly without being rendered ill-defined by the short-τ eddy-diffusive singularity. Diagnostics are first computed using an idealized circulation model that has topography only in the Northern Hemisphere (NH) and is run under perpetual NH winter conditions; suitable integrations are used to determine the stratospheric mean residence time and the mass fraction of the stratosphere in any given residence-time interval. For the idealized model we find that air exiting the stratosphere in the winter hemisphere has significantly longer mean residence times than air exiting in the summer hemisphere because the winter hemisphere has a deeper circulation and stronger eddy diffusion.

The complicated response of mean residence times to increased topography underlines the fact that flux distributions capture the integrated advective-diffusive tropopause-to-tropopause transport, and not merely advection by the residual-mean circulation. Extending one-way flux distributions to non-stationary flow we quantify the seasonal ventilation of the stratosphere using the state-of-the-art GEOSCCM general circulation model subject to fixed present-day climate forcings. From the one-way flux distributions, we determine the mass of the stratosphere that is in transit from the tropical tropopause back to the troposphere, partitioned according to stratospheric residence time and exit location. We find that poleward of 45N, the cross-tropopause flux of air that has resided in the stratosphere three months or less is 34 ± 10 % larger for air that enters the stratosphere in July compared to air that enters in January. During late summer and early fall the stratosphere contains about six times more air of tropical origin that is destined to exit poleward of 45S/N in both hemispheres, after an entry-to-exit residence time of six months or less, than is the case during other times of year.

We find that 51 ± 1 % and 39 ± 2 % of the stratospheric air mass of tropical origin, annually averaged and integrated over all residence times, exits poleward of 10N/S in the NH and SH, respectively, with most of the mass exiting downstream of the Pacific and Atlantic storm tracks. The mean residence time of this air is found to be ~ 5.1 years in the NH and ~ 5.7 years in the SH. The second transport process addresses new diagnostics of tropospheric transport. We introduce rigorously defined air masses as a diagnostic of tropospheric transport in the context of an idealized model. The fractional contribution from each air mass partitions air at any given point according to either where it was last in the planetary boundary layer (PBL), or where it was last in contact with the tropopause. The utility of these air-mass fractions in isolating the climate change signature on transport alone is demonstrated for the climate of a dynamical-core circulation model and its response to a specified heating.

For an idealized warming that produces dynamical responses that are typical of end-of-century comprehensive model projections, changes in air-mass fractions are order 10% and reveal the model's climate change in tropospheric transport: poleward shifted jets and surface intensified eddy kinetic energy lead to more efficient stirring of air out of the midlatitude boundary layer, suggesting that in the future there may be increased transport of industrial pollutants to the Arctic upper troposphere. Correspondingly, air is less efficiently mixed away from the subtropical boundary layer. The air-mass fraction that had last stratosphere contact at midlatitudes increases all the way to the surface, in part due to increased isentropic eddy transport across the tropopause. A weakened Hadley circulation leads to decreased interhemispheric transport in the model's future climate.