Wind velocities at 925 hPa

ITCZ goes with the minimum pressure in surface, zone where the trade winds converge at the ground level. This convergence of low layers favours the could cover development. The intensity of the monsoon flux is linked to the winds, these are stronger when the pressure difference is wider between high pressures of the St Helens anticyclone and the low pressures of Sahara. The measurements are done at 925 hPa, in order to eliminate the roughness and relief effects. The wind velocity is given in m.s-1. The following maps show the seasonal variations of the winds on West and Central Africa. The arrows figure the wind direction (combination of meridian and zonal components). The stronger the winds are, the more visible the arrow is.

The trade winds converge to the ITCZ, whatever the considered season is. But, the vertical system is also identified over the Sahara, the saharo-mediterranean anticyclone centered over Libya, where the harmattan comes from. In August, the monsoon system sets up: an intensification of the winds bordering the Azores and St Helens anticyclones. A movement northward of the ITCZ is also observed as well as a deviation of the trade winds under the effect of the Coriolis force. If the harmattan is too powerful, it blocks the movement northward of the trade winds, and the dynamic of the monsoon is diminished. Around September-October, the system moves southward, the wind intensity diminishes pregressively.