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

Abstract

The components of the moisture balance equation are calculated for the Middle East/North Africa regions based on NASA/GEOS-1 multiyear reanalysis data set. These include the Evaporation (E), Precipitation (P), moisture flux divergence (∇ · Q), and errors associated with the incremental analysis updates of the specific humidity, or IAU(q). The Annual mean ∇ · Q corresponds well to the results of Vitart et al. [1996], based on NCEP data. IAU(q) reveals a strong moisture source over the eastern Mediterranean and also confirms the paradoxical net moisture sink over the Arabian-Iraqi desert found by Alpert and Shay-El [1993]. Over the North African Sahara Desert the moisture flux was shown to converge through the northern and southern boundaries mainly at low levels (∼900 hPa) and to diverge through the eastern and western boundaries at higher levels (∼700 hPa). Starr and Peixoto [1958] have classified North Africa as a net moisture source. Area averaging of ∇ · 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 of the continent only then can it be classified as net source or divergence zone. Inspection of the intermonthly and diurnal variability, as well as the 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 water/land boundary, or due to various atmospheric diffusion processes such as the sea breeze cycle and cloud intrusion and evaporation. Copyright 1999 by the American Geophysical Union.