Evidence of meridional motion in the summer lower stratosphere adjacent to monsoon regions

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Abstract
Twenty‐one years of rawinsonde data were used together with 8 years of uninitialized European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses to describe the climatological structure of large‐scale circulations adjacent to monsoon regions in northern and southern hemisphere summers. In the upper troposphere and lower stratosphere, equatorward and poleward motions occur on the eastern and western sides of monsoon regions, respectively. It is shown that significant meridional velocities (>1 ms−1) penetrate the lower stratosphere up to a maximum height of 50–30 mbar. Largest meridional velocities are observed in connection with the Asian monsoon in northern summer. Although evanescent in height, these motions are relatively important for horizontal transport of constituents in the summer lower stratosphere, when planetary waves are otherwise small. Asian and Mexican monsoons in this season are displaced sufficiently far from the equator, in close proximity to the tropopause break, to have a significant role in stratosphere‐troposphere (S/T) exchange. The companion paper by Chen (1995) provides evidence of irreversible S/T exchange in the “upper middle world” during northern summer.