Impact of the Guinea Coast upwelling on atmospheric dynamics, precipitation and pollutant transport over Southern West Africa

Gaëlle de Coëtlogon (1) , Adrien Deroubaix (2, 3) , Cyrille Flamant (1) , Laurent Menut (3) , Marco Gaetani (4)

1 TROPO - LATMOS
2 IUP - Institut für Umweltphysik [Bremen]
3 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
4 IUSS - Istituto Universitario di Studi Superiori

In West Africa, the zonal band of precipitation is generally located around the southern coast in June before migrating northward towards the Sahel in late June / early July. This gives way to a relative dry season for coastal regions from Ivory Coast to Benin called "little dry season" which lasts until September-October. Previous studies have noted that the coastal rainfall cessation in early July seems to coincide with the emergence of an upwelling along the Guinea Coast: the aim of this study is to investigate the mechanisms by which this upwelling impacts the precipitation, using a set of numerical simulations performed with the regional atmospheric model Weather Research and Forecasting (WRF v 3.7.1). Sensitivity experiments highlight the response of the atmospheric circulation to an intensification, or conversely a reduction, of the strength of the coastal upwelling: they clearly show that the coastal upwelling emergence is responsible for the cessation of coastal precipitation by weakening the northward humidity transport, thus decreasing the coastal convergence of the humidity transport and inhibiting the deep atmospheric convection. In addition, the diurnal cycle of the low-level circulation plays a critical role: the land breeze controls the seaward convergence of diurnal anomaly of humidity transport, explaining the late night / early morning peak observed in coastal precipitation. The emergence of the coastal upwelling strongly attenuates this peak because of a reduced land-sea temperature gradient in the night and a weaker land breeze. The impact on the inland transport of anthropogenic pollution is also shown with numerical simulations of aerosols using the CHIMERE chemistry-transport model: warmer (colder) SST increase (decrease) the inland transport of pollutants, especially during the night, suggesting an influence of the upwelling intensity on the coastal low-level jet. The mechanisms described have important consequences for inland humidity transport and the predictability of the West African Monsoon precipitation in summer.

Mots clés: Guinea Coast; coastal precipitation; ocean influence; seasonal cycle; diurnal cycle; coastal upwelling; West African Monsoon

Domaines: Physique Atmosphérique et Océanique [physics.ao-ph]; Sciences de l'environnement; Météorologie

Gaëlle de Coëtlogon, Adrien Deroubaix, Cyrille Flamant, Laurent Menut, Marco Gaetani. Impact of the Guinea Coast upwelling on atmospheric dynamics, precipitation and pollutant transport over Southern West Africa. Atmospheric Chemistry and Physics Discussions, inPress, ⟨10.5194/egusphere-2023-681⟩. ⟨insu-04083138v1⟩