Continental-scale dust outbreak over Africa

by Jochen Kerkmann (EUMETSAT) and Thomas F. Lee (Naval Research Laboratory)

For much of the month, dust blew across Africa's Sahara desert and over the Atlantic Ocean. A monitoring station on the island of Barbados in the Caribbean recorded the highest dust concentrations over the island in three years.

The trade winds (also called trades) are the prevailing surface winds in subtropical areas. They blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. Over the continents trade winds may deviate from these directions due to local thermal lows like the summer heat low over Afghanistan and northern Pakistan. Therefore, in the Middle East the dominating summer trade wind is a northwesterly wind, called Shamal, while in northern Africa the prevailing wind is a northeasterly wind, called Harmattan.

When strong enough, these winds can lift large amounts of dust up into the air, and this effect can be quite persistent lasting several weeks (e.g. the so-called 40-day summer Shamal in the Middle East). Trade winds or monsoon winds are also responsible for transporting the huge dust clouds raised over northern Africa or the Middle East westward across the Atlantic Ocean into the Caribbean Sea, or southeastward across the Indian Ocean towards India.

The synoptic situation on 18 March 2010 (Figure 1) shows the typical pattern that causes these large dust outbreaks over northern Africa: a high pressure area over northern Africa (strengthened by cold air outbreaks from Europe) that presses against a low pressure area over the southern Sahel area (i.e. the ITCZ which during the month of March starts to move back northward), and strong Harmattan winds in between. This leads to continental-scale dust outbreaks, which can be seen on this image over Sudan, Chad, Niger, Mali and other countries. In most cases, numerical models (dust forecasting models) can handle such situations quite well.

See also same area for 19 March 2010, 12:00 UTC and 20 March 2010, 12:00 UTC

The huge dust outbreaks over northern Africa in March 2010 were also observed by MODIS on Terra and Aqua. The image composite shown below (Figure 2) compares the MSG (Meteosat-9) SEVIRI Dust RGB product (left) to the Aqua MODIS Dust RGB product (right). Note that for the MODIS RGB the MSG SEVIRI tuning has been applied. Over land, apart from the higher spatial resolution of the MODIS instrument, both satellite products show the enormous dust clouds in very similar colours, but over water the MSG SEVIRI Dust RGB product seems to do a better job in detecting dust clouds as shown off the west coast of Africa (see green oval). The reason for this is not fully understood, but could be due to water vapour absorption differences in the IR channels, in particular the IR8.7 channel.