Synoptic patterns associated with dusty and non-dusty seasons in the Sahara

The difference in the synoptic situation between years with a large amount of dust and years with a relatively small amount, in the Sahara, have been examined for the period 1979-1992. A comparison has been made separately for each season. For every month the dustiest and least dustiest years, determined by the deviation from the mean, were chosen and the average of the three months of each classical season for these years was examined. The examination was made for wind flow, geopotential height and temperature at the 700 hPa level, and appropriate maps of these variables have been prepared. The data used were the daily aerosol index (AI) from the TOMS satellite-borne instrument and daily NCEP/NCAR reanalysis data for the years 1979-1992. It was found that there is significant difference in the atmospheric variables between dusty and non-dusty years in the Sahara and the area to the north and to the west. In the spring, summer and autumn seasons there is a significant increase in cyclonic flow during dusty years relative to non-dusty years in western Europe and western North Africa. Accordingly, there is strong cooling and a decrease in geopotential height in this area. In the central and eastern Sahara and the central Mediterranean anticyclonic flow predominates, there is significant relative warming and an increase in the geopotential height. In the winter season, the distribution of cyclonic and anticyclonic activity, instead of east to west, becomes orientated north to south. Mean cyclonic activity, together with cooling and decreased height, is found in south-eastern Europe, and anticyclonic activity is found in the Sahara.