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 biweekly 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 a feedback cycle acting between the monsoonal winds and their clouds, soil moisture, and surface temperature. ¿ 2001 American Geophysical Union