Semyon A. Grodsky, and James A. Carton

Department of Meteorology Computer and Space Sciences Bldg. University of Maryland
College Park, MD 20742

Phone: 301-405-5330,
Fax: 301-314-9482
E-mail: senya@ocean2.umd.edu

    Rainfall in subSaharan West Africa is concentrated in a rainy season beginning in
summer whose arrival is of critical importance for local economies. Here we use new surface
wind observations made available by the QuickSCAT satellite and rainfall estimates from the
Tropical Rainfall Measuring Mission to provide the first documentation of a bi-weekly
oscillation in rainfall and wind that precedes the onset of the rainy season. This
oscillation in rainfall occurs in conjunction with monsoonal wind patterns and is
particularly noticeable in the zonal wind field. It is also associated with a cooling of
surface temperature and a reduction in zonal surface pressure gradient. Together the phasing
of these variables implies feedback cycle acting between the monsoonal winds and their
clouds, soil moisture and surface temperature. We discuss  the influence of these bi-weekly oscillations in winds on  surface fluxes diagnosed by the NCEP/NCAR reanalysis.