The seasonal vertical distribution of the Saharan Air Layer and its modulation by the wind

The Saharan Air Layer (SAL) influences large scale environment from West Africa to eastern tropical America, by carrying large amounts of dust aerosols. However, the vertical distribution of the SAL is not well established due to a lack of systematic measurements away from the continents. This can be overcome by using the observations of the space lidar CALIOP on board CALIPSO. By taking advantage of CALIOP capability to distinguish dust aerosols from other types of aerosols through depolarization, the seasonal vertical distribution of the SAL is analysed at 1 degree horizontal resolution over a period of 5 yr (June 2006-May 2011). This study shows that SAL can be identified all year round displaying a clear seasonal cycle. It occurs higher in altitude and more northern in latitude during summer than during winter, but with similar latitude extent near Africa for the four seasons. The south border of the SAL is determined by the Intertropical Convergence Zone (ITCZ), which either prohibits dust layers to penetrate it or reduces significantly the number of dust layers seen in or south of it, as over the eastern tropical Atlantic. Spatially, near Africa, it is found between 5° S-15° N in winter going at 5-30° N in summer. Towards America (50° W), SAL is observed between 5° S-10° N in winter and 10-25° N in summer. During spring and fall, SAL is found between the position of winter and summer not only spatially, but also vertically. In winter, SAL occurs in the altitude range 0-3 km off West Africa, decreasing to 0-2 km close to South America. During summer, SAL is found to be thicker and higher near Africa at 1-5 km, reducing to 0-2 km in the Gulf of Mexico, farther west than during the other seasons. SAL is confined to one layer, of which the mean altitude is decreasing with westward transport by 13 m deg−1 during winter and 28 m deg−1, after 30° W, during summer. Its mean geometrical thickness is decreasing by 25 m deg−1 in winter and 9 m deg−1 in summer. Spring and fall present similar characteristics for both mean altitude and geometrical thickness. Wind plays a major role not only for the transport of dust within the SAL, but also by sculpting it. During winter, the trade winds transport SAL towards South America, while in spring and summer they scavenge dust aerosols below it by bringing maritime air masses from North Atlantic up to about 50° W. The North Atlantic westerlies, with their southern border occurring between 15° N and 30° N (depending on the season, the longitude and the altitude), prevent the SAL to develop further northward. In addition, their southward shift with altitude gives SAL its characteristic oval shape in the northern part. The effective dry deposition velocity of dust particles is estimated to be 0.07-0.08 cm s−1 in winter, 0.13-0.15 cm s−1 in spring and fall, and 0.2 cm s−1 in summer. Finally, the African Easterly Jet (AEJ) is observed to collocate with the maximum dust load of the SAL and this might promote the differential advection for SAL parts, especially during summer.