Seasonal Ventilation of the Stratosphere: Robust Diagnostics from One-Way Flux Distributions

We present the first analysis of the seasonally varying ventilation of the stratosphere using one-way flux distributions. Robust transport diagnostics are computed using the state-of-the-art comprehensive GEOSCCM general circulation model subject to fixed present-day climate forcings. From the one-way flux distributions we determine the mass of the stratosphere in any given residence-time interval and stratospheric mean residence times. The seasonalities of the transport masses and mean residence times are quantified with respect to (1) when air enters the stratosphere at the tropical tropopause, (2) when air is in transit in the stratosphere, and (3) when air exits back to the troposphere. We find that the cross-tropopause flux of air back into the northern hemisphere (NH) high-latitude troposphere, with residence times less than three months, is 34% larger for air entering the stratosphere in summer than in winter. For months during late summer to early fall, air with residence times less than 6 months is about six times more likely to be flushed back into the troposphere than for months during winter and early spring. Integrated over all residence times, 51% and 39% of the annually-averaged mass in transit in the stratosphere that entered in the tropics will leave at midlatitudes in the NH and southern hemisphere after residing on average 5 to 6 years in the stratosphere. Most of this mass will eventually exit the stratosphere downstream of the Pacific and Atlantic storm tracks.