Meteorological Aspects of Dust Storms - Mineral Dust: A Key Player in the Earth System - page 123

visibility of less than 1,000 m near the area of convergence in the Sistan region,
where the borders of Iran, Afghanistan and Pakistan meet (Fig. 2 in Middleton
1986 ). Farther to the west, the northwesterly summer Shamal winds, which fre-
quently carry dust, blow across the Tigris and Euphrates river valleys of Iraq and
the Persian Gulf states into the heat low over the Arabian Peninsula (Fig. 6.1 ).
In a similar way, the weaker West African or Saharan heat low generates marked
circulations and convergence from late spring into summer (Fig. 6.1 ). Due to the
orographic barriers of the Atlas and Hoggar Mountains the three most important
inflow regions are the very strong and persistent Atlantic inflow over Western Sahara
and Mauritania (Knippertz 2008 ; Grams et al. 2010 ; Todd et al. 2013 ), the southerly
monsoon flow over the Sahel (Parker et al. 2005 ; Flamant et al. 2007 ;BouKaram
et al. 2008 ) and the northeasterly inflow from the eastern and central Mediterranean
Sea (Knippertz et al. 2009a ) (all marked in Fig. 6.1 ). All three airstreams pass
over dust source regions, leading to a marked maximum near the centre of the heat
low, where low-level convergence is strong (Engelstaedter and Washington 2007 ).
Similar but weaker inflows into continental heat lows are found for other monsoon
regions with less pronounced dust activity such as Australia with its persistent
southerly winds over the southern coastal region in austral summer, and North
America with persistent inflow from the Gulf of Mexico into the southern Great
Plains (not shown).

6.3 Synoptic-Scale Aspects

Synoptic-scale weather systems are the primary control on episodic, large, intense
dust events in many source regions. The most important prerequisite is the establish-
ment of a sufficiently large low-level pressure gradient to drive strong winds capable
of dust emission and often also long-range transport. On the synoptic scale, these
are frequently associated with extratropical cyclonic disturbances and particularly
the trailing cold fronts associated with them, but can also occur in connection with
strong winds around anticyclones or with tropical African easterly waves (AEWs;
Knippertz and Todd 2010 ). AEWs are disturbances that frequently form along the
midlevel easterly jet over the Sahel during summer.
The importance of individual dust storm types is a strong function of season
and position relative to the main stormtracks and subtropical anticyclones. The
former are located over the North Atlantic/Europe, the Mediterranean/Middle East,
Siberia, the North Pacific and a circumpolar band in the southern hemisphere
between about 40 and 60 ı S. Activity maxima typically occur in the local winter and
spring. Naturally, most dust source regions are found on the equatorward side of the
stormtracks, where precipitation is low. The subtropical anticyclones are centred
over the eastern parts of all major ocean basins in both hemispheres year-round,
typically around 30 ı latitude with moderate seasonal shifts. In addition, there are
shallow, cold continental highs over central Asia and Canada during winter.