Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak

Abstract

SUMMARYIn 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 uppertrough. 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.

Discernible rhythm in the spatio/temporal distributions of transatlantic dust

“…These massive emissions occur only a few times per year. The unusual weather conditions during these events and part of their climatic impacts were described by Knippertz and Fink (2006) Following up on the periodicity gleaned from the time series of τ d , as shown in Fig. 4a and b, and plotting the daily averaged τ d vs. the day in the year, major differences be- tween the SRP and the NRP are revealed (Fig.…”

How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa?

“…Maheras et al, 2001;Schepanski and Knippertz, 2011). Several studies suggest that cyclones can cause dust storms (Bou Karam et al, 2010;Hannachi et al, 2011;Schepanski et al, 2009;Schepanski and Knippertz, 2011), although a case study by Knippertz and Fink (2006) for the exceptionally strong and continental-scale dust storm in March 2004 gives evidence that a cyclone only produces one part of the associated dust emission. The remaining dust mobilization is linked to strong northeasterly Harmattan winds.…”

“…These troughs advect cool air masses at their western side towards the Sahara and transport Saharan air northwards at their eastern side (e.g. Maheras et al, 2001;Knippertz and Fink, 2006). The interaction with orography can lead to cyclogenesis at the lee side of mountain ranges.…”

Dynamical mechanisms controlling the vertical redistribution of dust and the thermodynamic structure of the West Saharan atmospheric boundary layer during summer

“…In addition to sometimes leading to deep convection and cold-pool outflows (Section 3.2.2; Knippertz and Fink, 2006), cold air fronts associated with midlatitude troughs can induce low-level intrusions of clean and cold air into the northern Sahara. Likewise, in the southern Sahara the cooler moist monsoon air is often observed to undercut the SABL during summer.…”

“…Such events are often associated with mesoscale convective systems (MCSs) over the Sahel in the summer (Hamilton and Archbold, 1945). They may also be triggered by convection over mountainous regions, such as the Atlas (Knippertz and Fink, 2006;Knippertz et al, 2007). Cold pool outflows may be moister or drier than their surroundings (Miller et al, 2008).…”