THE DIURNAL AND SEASONAL CYCLES OF WIND-BORNE DUST OVER AFRICA NORTH OF THE EQUATOR

This article presents a study of the diurnal and seasonal cycles of du st over North Africa, using surface visibility as an indicator of dust . The diurnal cycle shows a reduction of visibility during tile daytim e hours in the areas where dust is generated, a consequence of the eli mination of the nocturnal inversion. The annual cycle reveals that, at latitudes from 5 degrees to 16 degrees N, there is a latitudinal incr ease in the duration of the presence of aerosols over the course of th e year. The presence of aerosols dimishes in the latitudes from 20 deg rees to 35 degrees N, indicating that the aerosol content of the Sahar an air is lower than that over the semiarid sub-Saharan zones, such as the Sahel. A comparison of three periods, 1957-61, 1970-74, and 1983- 87, shows a continually increasing presence of dust, particularly in t he western Sahel. The interannual variability of the dust and its annu al cycles in these three periods throughout North Africa bear a strong relationship to rainfall fluctuations in the Sahel. Overall, the resu lts indicate that over the last few decades the Sahel region has repla ced the central Sahara as the source of atmospheric aerosols over most of North Africa.