Summer Monsoon Water Vapour Workshop, Boulder, USA, 11 October 2001

Andrew Gettelman, NCAR, Boulder, USA (andrew@ucar.edu)
Timothy J. Dunkerton, NWRA, Bellevue, USA (tim@nwra.com)

A mini-workshop took place at the NCAR to review what is known about the structure of the Northern Hemisphere (NH) summer monsoons over Asia and North America, and to discuss the role of these circulation systems in the seasonal and long-term evolution of water vapour in the upper troposphere and lower stratosphere (UT/LS). The workshop discussion included a comprehensive overview of existing and planned observations that provide information on monsoon circulations and transport in the UT/LS. These observations consist of ground-based, airborne, and satellite measurements of water vapour, other trace species, aerosol particles, and meteorological parameters.

A working group was formed as a result of the workshop, consisting of experts, mostly from the USA, in monsoon circulations and transport, water vapour, and stratosphere/troposphere exchange. Additional participation by the international community is encouraged, particularly by scientists from south or south-east Asia directly affected by the summer monsoon, or those aware of local observations that would be of value to us.

The NH summer monsoons are uniquely interesting in the UT/LS for three reasons. (1) Much of the deep moist convection occurs over land, implying that cloud microphysical properties and the dynamics of convection itself differ significantly from tropical maritime convection. Evidence suggests that the role of convection in the water budget of the lower stratosphere differs between NH summer and winter, causing hydration of the layer in the summer, and dehydration in the winter. (2) Temperature soundings in the monsoon regions of NH summer have a tropical character, with a steep lapse rate and high tropopause near 100 hPa. These profiles and cloud observations imply that convective penetration of the UT/LS occurs equatorward of the jet stream. (3) Monsoon systems in NH summer are displaced relatively far from the equator, generate circulations asymmetric about the equator, with inter-hemispheric transport, and penetrate the lower stratosphere with malies measurable up to the 30 hPa level. Upward penetration is more striking in the Asian summer monsoon than in the North American monsoon. Throughout the summer the jet stream is rather weak and constitutes an ineffective barrier to horizontal transport.

Satellite observations demonstrate a moistening of the lowermost stratosphere within or near the NH summer monsoon regions. There is an apparent connection between monsoon-induced water vapour anomalies and subsequent horizontal transport into the NH extratropics (on the west side of monsoon anticyclones) and tropics (on the east side of monsoon anticyclones). The tropical transport path is interesting insofar as the tropical lower stratosphere is a gateway to the entire middle atmosphere via the Brewer-Dobson circulation.

While certain details of these water-vapour transport paths may be accessible from existing observations, a special experiment to acquire in situ measurements in the UT/LS during NH summer has also been discussed. Such an experiment would complement recent and proposed studies of the cold trap during NH winter. Some insights might also be obtained on the so-called 'tape recorder' in stratospheric water vapour, and on whether summer monsoons have played a role in the observed trend of water vapour in the middle atmosphere, a trend unexplained by tropopause temperature.