On the Temporal and Spatial Structure of Troposphere-to-Stratosphere Transport in the Lowermost Stratosphere over the Asian Monsoon Region during Boreal Summer

This study produced a novel characterization of the troposphere-to-stratosphere transport (TST) over the Asian monsoon region during boreal summer, using a comprehensive analysis of 60-day backward trajectories initialized in the stratosphere. The trajectory datasets were derived from the high-resolution Lagrangian particle dispersion model (FLEXPART) simulation driven by the wind fields acquired from the National Center for Environmental Prediction (NCEP). The results indicate that the distribution of residence time (t _TST) of tropopause-crossing trajectories in the lowermost stratosphere represents a horizontal signature of the Asian summer monsoon. Vertically, the distribution of t _TST can be roughly separated into two layers: a consistent lower layer with t _TST <5 days forming a narrow band, corresponding to a layer ～3 km thick following the location of the tropopause, and an upper layer at a larger distance from the local tropopause. The maximum residence time was ～20 days, especially within the Asian high anticyclone consistent with its confinement effects. In general, the overall geographical distribution of dehydration points was not coincident with the location of tropopause crossing. TST trajectories, which were initialized in the stratosphere, underwent their Lagrangian cold points mostly in the tropics and subtropics 1–4 days after the TST event; they were characterized by a wide range of temperature differences, with a mean value of 3–12 K. The vertical extent of the influence of tropospheric intrusion on the Asian monsoon region in the stratosphere exhibited a peak at ～16.5–18.5 km, and the uppermost height was ～21 km.