ABSTRACT: The contribution of African wave disturbances (AWD) to the interannual variability of precipitation in the Sahel is considered. Synoptic circumstances associated with the occurrence of precipitation at Niamey (Niger) during July-August 1987 and 1988 are investigated using European Center for Medium Range Weather Forecasts (ECMWF) gridded analyses and Niamey station observations of several meteorological variables. AWD are detected as oscillations in time series of 700 mb meridional winds and as inverted U-shaped cyclonic circulations on latitude-longitude analyses of the 700 mb wind field over West Africa. Spectral analyses of the time series indicate several prominent AWD periods with peaks ranging from 3 to 6 d, with somewhat different results for Niamey radiosonde data and the co-located ECMWF winds. Three passages of AWD at Niamey are examined in detail and the circumstances associated with ample precipitation during the first two are contrasted with the factors that led to a deficiency of rainfall during the third AWD. The study provides evidence that rainier seasons in the Sahel do not necessarily experience a greater number of AWD or even more intense AWD than drier summers. Occasional heavy precipitation can result from uplift driven by upper tropospheric divergence unrelated to AWD. On the other hand, AWD can provide an important trigger mechanism for convective rainfall during an otherwise dry spell.

KEY WORDS: African waves . Sahel . ECMWF analyses

Published in CR Vol. 7, No. 1 (1996) on August 22
ISSN: 0936-577X. Copyright © Inter-Research, Oldendorf/Luhe, 1996