Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak

Abstract In March 2004, a large‐scale, strong and persistent outbreak of Saharan dust on to the adjacent tropical and subtropical Atlantic Ocean was observed in satellite imagery. Surface observations show that the event was accompanied by unusual weather conditions across large parts of North Africa including low temperatures and strong winds over the Sahara, extreme precipitation in Libya and a heat wave on the Guinea coast. The dust outbreak was associated with an unseasonally southerly position of the Intertropical Convergence Zone and a delayed northward progression of the African monsoon. The dust front was initially related to a density current caused by strong evaporational cooling along a precipitating cloud‐band that penetrated into the northern Sahara ahead of an upper‐level trough. At later stages, massive upper‐level convergence, sinking, low‐level divergence and an explosive anticyclogenesis over northwest Africa caused strong northerly flow and a quick spreading of the dust front to the south and west. The strong pressure gradients over North Africa were further enhanced by the formation of a cyclone ahead of the upper‐trough. A dynamical analysis based on potential‐vorticity (PV) theory provides new insights into the generation of North African harmattan surges. Using a PV inversion, it is shown that the explosive anticyclogenesis was caused largely by unbalanced dynamics related to the density current, divergent outflow from the precipitation over Libya and inertial instability in the entrance region of the strong subtropical jet and tropical plume along the eastern side of the upper trough. These processes enhanced the effects of the balanced advection of negative vorticity and cold air by the large‐scale wave. Moreover, we show the importance of an explosive cyclogenesis event over the central North Atlantic for the amplification of the downstream wave associated with the penetration of the cold front into North Africa. Copyright © 2006 Royal Meteorological Society.