Reassessment of the moisture source over the Sahara Desert based on NASA reanalysis

The components of the moisture balance equation are calculated for the Midd le East/North Africa regions based on NASA/GEOS-1 multiyear reanalysis data set. These include the Evaporation (E), Precipitation (P), moisture flux d ivergence (del.Q), and errors associated with the incremental analysis upda tes of the specific humidity, or IAU(q). The Annual mean del.Q corresponds well to the results of Vitart et al. [1996], based on NCEP data. IAU(q) rev eals a strong moisture source over the eastern Mediterranean and also confi rms the paradoxical net moisture sink over the Arabian-Iraqi desert found b y Alpert and Shay-El [1993]. Over the North African Sahara Desert the moist ure flux was shown to converge through the northern and southern boundaries mainly at low levels (similar to 900 hPa) and to diverge through the easte rn and western boundaries at higher levels (similar to 700 hPa). Starr. and Peixoto [1958] have classified North Africa as a net moisture source. Area averaging of del.Q over a box with varying dimensions reveals that it can be classified as a net sink if the box is small enough and located over the center of the desert. If the box is big enough to include the boundaries o f the continent only then can it be classified as net source or divergence zone. Inspection of the intermonthly and diurnal variability, as well as th e model biases, weakens also the net source argument. It is suggested that the earlier finding of a net source might be due to the smoothing of the wa ter/land boundary, or due to various atmospheric diffusion processes such a s the sea breeze cycle and cloud intrusion and evaporation.