West African monsoon crosses the Sahara desert

By Philip Mulholland

They were part of an active monsoon that extended across the Sahel, producing rain in the Sudan as far north as Khartoum, where a vigorous dust storm or haboob (see reference) was reported on 2 May.

As well as the Khartoum example, dust storm activity has also been notable this summer in West Africa. On 2 August the typical north-east wind that comes out of the Sahara initiated an organised dry-line front with enhanced afternoon thunderstorm activity. This front tracked south-west across Mali (see Powerpoint presentation (slide 1)) demonstrating that advection of dry Saharan air into the ITCZ was occurring at this time.

Three days later the weather pattern has changed. On 5 August at 15:00 UTC a mega storm cell associated with a tropical wave is tracking west from Niger into Burkina Faso with an extensive dust veil to the north that is itself being entrained into the desert wind coming out of the central Sahara (slide 2).

Over the next 48 hours this mega storm cell tracks west into southern Mali (7 August at 15:00 UTC, slide 3) before adopting a northern track towards Mauritania. During the subsequent 18 hours the cell centre stepped north into Mauritania (8 August 09:00 UTC, slide 4) before reaching the Anti-Atlas in Morocco at 15:00 hours UTC on 8 August (slide 5).

Weather reports for southern Morocco on 8 August show thunderstorms occurring at five locations namely; Agadir, Demnat, Ouarzazate, Taroudannt and Tiznit. The rainfall at each of these localities can be explained as the orographic effect of the Atlas hinterland, triggering convective activity in the moist air mass associated with the arrival of the west African storm cell in Morocco (slide 5).

Twenty-four hours later this organised storm, centred on the High Atlas, covers Morocco (slide 6). During the next day (10 August) the storm system moved rapidly east through Algeria (were rainfall was reported from Batna) and reached Tunisia (slide 7) before leaving Africa, passing east into the Mediterranean Sea.

A single weather event does not in itself define a trend; however, on 19 August a new weather pulse arrived in North Africa bringing rain from the south. Weather reports from Algeria show rainfall in Ain Beida, Batna, Biskra, Guelma, Setif and Tebessa on the southern slopes of the Atlas Mountains while reports of rain in Tunisia include Al Kaf, Bajah, Duz and Gafsa.

On 22 August another smaller rainfall event occurred in Algeria with reports of rain from Batna on the southern flanks of the Tell Atlas.

This route of moist air passing north from west Africa across the Sahara continued through 23 August where rain was reported from El Golea on the edge of the Great Western Erg of the central Sahara, Algeria. The next morning a rainfall event occurred in Corsica after a storm system, passed north from Algeria and rejuvenated over the warm waters of the Mediterranean Sea (slide 8).

On 25 August the moist tropical air continued to invade southern Europe, but this time Spain was the focus, as the axis of the monsoon plume from west Africa was dragged to the west across the continent. The incursion into Spain marked the end of this major northward movement of moist tropical air across the Sahara during August 2007 (slide 9).

Rainfall events of the type and duration seen during August 2007 are not represented in the modern summer rainfall records for North Africa. Climate records of North Africa ubiquitously record a summer rainfall minima for this region, associated with the Mediterranean climate that brings rain in the winter.

To find an analogue for this year's summer rainfall in the southern Maghreb, we have to study the historical records (see for example The Ancient Lakes of the Sahara), but even here the timing of these past rainfall events is not always known for certain, but they are usually considered by scientists as being due to summer monsoonal rains.

Additional content

Full powerpoint presentation (source: P. Mulholland)
Met-9 WV6.2 image of 9 August 2007, 15:00 UTC
Met-9 Dust RGB composite of 9 August 2007, 15:00 UTC