Air Mass Origin in the Upper Troposphere/Lower Stratosphere: Signatures of Asian Boundary Layer Air

The distributions of trace gases in the tropical upper troposphere/lower stratosphere (UTLS) influence the structure of the ozone layer, with implications for both atmospheric chemistry and the radiative balance of the climate system. A complete understanding of trace species' budgets in the tropical UTLS in turn rests on disentangling the contributions made by transport from chemistry and emissions. A natural way to quantify transport is in terms of rigorously defined air masses that partition air in the UTLS according to where it last contacted the planetary boundary layer (PBL). Here we present seasonally-varying climatologies of air mass origin in the tropical lower stratosphere, computed from a time-slice integration of the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM) subject to present-day climate forcings. Annually averaged air mass fractions reveal that nearly half of the air between 20S-20N and between 70-100 mb bypasses the tropical boundary layer, with 30% of air originating in the northern hemisphere (NH), almost half of which last contacts the PBL over midlatitudes. Seasonal variations in air mass origin are largest during NH late summer/early fall, when nearly 20% of the air in the tropical lower stratosphere originates over Asia. Complementary analysis of the distribution of transit times since air in the lower stratosphere last contacted Asia reveals that transport from the Asian boundary layer during summer occurs on timescales as short as 20 days, consistent with rapid vertical transport out of the lower troposphere associated with the monsoon. Furthermore, transport pathways inferred from the transit time distribution reveal that air originating over Asia bypasses the tropical upper troposphere, instead crossing the tropopause near 30N. Our results build on growing observational and modeling evidence that the monsoon circulation provides an efficient pathway for pollution to enter the stratosphere via the extratropical tropopause.