Climatology of coastal wind regimes in Benin

Abstract

Using surface and lower-tropospheric wind observations at Cotonou (Benin), a comprehensive ten-year (2006–2015) climatology of coastal wind regimes at the West African Guinea Coast is presented for the first time. Four wind regimes are objectively identified: the monsoon and Harmattan winds, the nocturnal low-level jet (NLLJ) and the land-sea breeze (LSB) system. Amongst other things, these regimes are important for the occurrence of coastal squall line systems and for the redistribution of pollutants emitted from coastal cities. The seasonal cycle is split into the long (December–February) and short (July–September) dry seasons, the long (May–June) and short rainy (October–November) seasons, and the beginning of the long rainy season (March–April). In terms of the LSB, the lack of a diurnal reversal in wind direction obscures sea breeze activity at the height of the southwesterly monsoon flow in boreal summer and during a period of stronger winds in March-April related to large land-sea thermal contrasts. During these periods, the sea breeze manifests itself in a relative increase of the monsoon background flow during daytime, while early morning land breezes are infrequent. The latter expectedly, peak during the long dry season in December with a secondary peak during the long rainy season in May–June. The large–scale monsoon and Harmattan flows, mutually exclusive by definition, occur respectively, 88.2–98.4 % and 1.6–11.8 % per year with Harmattan days restricted to the long dry season and deepest monsoon flows of about 2.5 km confined to the long rainy season. The NLLJ is most prominent during the little dry season with, however, a similar and hitherto less documented peak in occurrence frequency during the beginning of the long rainy season. At this time of the year, the African Easterly Jet passes northward over Cotonou, causing the largest 3000–600 m vertical wind shear.