Fast Transport Pathways Connecting the Northern Hemisphere Surface to the Lower Stratosphere during Boreal Summer

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. While studies agree that the summer monsoon system is an efficient pathway connecting the troposphere and the stratosphere, the exact mechanisms and transport pathways by which this coupling occurs are not well understood. Here we examine the transport pathways that connect the Northern Hemisphere surface to the UTLS during northern summer using rigorously defined Boundary Impulse Response (BIR) idealized tracers that partition air in the UTLS according to where and when it last contacted the planetary boundary layer. Specifically, a large ensemble of 90 BIR tracers, released weekly during July and August of nine summer months, are integrated in one simulation of the Whole Atmosphere Community Climate Model (WACCM) version 5 subject to historical sea surface temperatures and sea ice concentrations. We show that the fastest transport pathway into the UTLS occurs over the southern slope of the Tibetan Plateau, northern India, and the Arabian Sea and that it is associated with a modal timescale of about 5-10 days during July versus 15-20 days during August. By comparison, interannual variations in modal transport timescales are shown to be relatively smaller than the July versus August differences. Finally, a detailed budget analysis reveals that while convective processes are responsible for lifting boundary layer air masses to 200-300 hPa, the resolved dynamics, specifically the vertical eddy flux, dominate transport to 100-150 hPa. Variability in the resolved dynamics are also shown to dominate the spread among the ensemble.