Mean Winds in the Tropical Stratosphere and Mesosphere during January 1993, March 1994, and August 1994

Matthew H. Hitchman, Erhan Kudeki, David C. Fritts, Joleen M. Kugi, Clint Fawcett, Gregory A. Postel, Chia-Yi Yao, David Ortland, Dennis Riggin, and V. Lynn Harvey

Radar observations of winds and momentum fluxes in the stratosphere and mesosphere at the Jicamarca Radio Observatory in Peru (JRO; 12 degrees South, 77 degrees West) were taken during three 10 day campaigns in January 1993, March 1994, and August 1994. In order to interpret features in the campaign mean JRO wind profiles, we examined global circulation patterns as depicted by long time series of radiosonde profiles, analyses from the European Center for Medium Range Weather Forecasting (ECMWF), and winds from the High Resolutin Doppler Imager (HRDI) aboard the Upper Atmosphere Research Satellite. In the tropical stratosphere, large scale analyses show that a geographically varying annual cycle significantly affects winds over JRO, as does the quasi-biennial oscillation (QBO). The spatial structure of the annual cycle and the QBO is shown for the three campaigns, emphasizing the upward influence of the subtropical tropospheric monsoon anticyclones. These anticyclones tilt poleward and merge zonally, underlying the zonal summer easterlies, which also merge zonally and tilt poleward with altitude. The annual cycle at Singapore includes a substantial easterly acceleration during March-August, which causes an apparent stalling of descending QBO westerlies or a more rapid descent of QBO easterlies. In the mesosphere, JRO and HRDI winds agree reasonably well, with zonal winds over JRO varying on a semiannual basis and meridional winds exhibiting structures expected from the diurnal tide. For vertical motion, separate north-south and east-west beam pair estimates agree, yet campaign-averaged vertical motions are large: ~1-5 cm/s in the stratosphere and ~10-50 cm/s in the mesosphere. In both the stratosphere and mesosphere, vertical winds are anticorrelated with horizontal wind. Possible explanations for the large vertical motions include aspect sensitivity and the diurnal tide. Uncertainties in the meaning of radar vertical motions create a challenge for interpreting momentum fluxes.

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