Seasonal transport patterns of intense Saharan dust events at the Mediterranean island of Lampedusa

Saharan dust (SD) episodes occurring at the Mediterranean island of Lampedusa (35.52° N, 12.63° E) from May 1999 to December 2005 have been shown to occur in the 26% of the cloud-free days [Meloni, D., di Sarra, A., Biavati, G., DeLuisi, J.J., Monteleone, F., Pace, G., Piacentino, S., Sferlazzo, D.M., 2007. Seasonal behavior of Saharan dust events at the Mediterranean island of Lampedusa in the period 1999–2005, Atmos. Environ. 41, 3041–3056]. In this paper we focus on intense SD events detected until September 2006, characterized by large values of the Saharan Dust Event Index (SDEI), the sum of the daily average aerosol optical depth at 500 nm, τ, over the duration of the dust episode. The SDEI index provides an indication about the intensity of SD events, due either to a long duration and/or to high dust optical depth. A total of 24 episodes characterized by large values of SDEI are examined. The NCEP-based maps of geopotential height and temperature at 700 mbar are used to identify the main circulation patterns driving SD to the Central Mediterranean and Lampedusa. Dust transport episodes in summer last for several days, and the corresponding SDEI values are the highest of the year. These episodes are mainly governed by two circulation patterns: the trough extending near the Atlantic coast of Europe and the high pressure system present in North Africa, generally above 25° N. This configuration causes strong south-westerly flows from the Sahara towards Southern Italy. The time evolution of τ for these long SD events shows that the largest values (> 0.3) are usually observed when the two patterns are present simultaneously, while low (< 0.3) τ values are measured when the Atlantic trough influence is weak. Moreover, the most probable loading region typically shows a warm kernel in North-Western Sahara, with a tongue extending north-eastward towards Sicily. In spring, moderate to high τ are measured, and SD episodes last as long as 13 days. Two synoptic configurations can be distinguished. Days with large τ are associated with the 700 mbar geopotential height features typical of summer. Days with moderate τ are found to be characterized by the Saharan high, but with the close low replaced by the Atlantic trough descending West of Africa or above the Iberian peninsula. In winter the typical meteorological conditions are determined by the presence of westerly cyclones. Depressions centered over Portugal, Central/Northern Europe, and Western Mediterranean ha