Tuesday, October 11, 2011

Rare rainfall in the deserts of North Africa A look at recent shower activity in the usually dry Sahara

Fighters heading towards Sirte under clear blue skies [GALLO/GETTY]
The battle for control of Libya has become a familiar sight on our television screens over the last few months. A less familiar sight has been that of the fighting taking place during heavy rain. This has fallen over the Libyan coast as a trough of low pressure has moved through the Gulf of Sirte.
Other North African states have also seen some heavy rainfall. Algeria, at the start of the month, saw flooding around the town of el-Bayadh, which killed eight people, including three children swept away in a river flood.
Rainfall in the northern parts of Africa usually increases at this time of year as high pressure, situated over the Mediterranean, begins to withdraw. This allows frontal systems to move in from the northwest. In fact, climatologists identify 20th October as the date when there is a significant pressure drop and much of North Africa can expect to enter the rainy season.
‘Rainy’ is a relative term, of course. There is a marked rainfall gradient from west to east across the North African coast. October rainfall in Benghazi is less than half that of Tripoli, 1000 kilometers to the west.
Over the last week there has even been some penetration of troughs into southern parts of both Algeria and Tunisia. Unfortunately weather observations, despite the best efforts of the World Meteorological Organization, are rather limited in this region, but computer forecasts certainly indicated the possibility of desert showers late last week.
Where rain did fall, then it is likely that dormant plant life would have been rejuvenated. The World Wildlife Fund estimates that on such occasions 50 percent of sand dunes and 20 percent of gravel plains may spring into life.
This is quite an unusual event. Most troughs and frontal systems tend to peter out before they reach the Sahara.
The other source of rain in this region is from the monsoon trough which meanders up from the south during the late summer. But it rarely reaches the southern part of the Sahara and it soon retreats southwards.
Throughout the Sahara rainfall does not exceed 25 millimetres (mm) per annum and in the eastern part of the desert it is less than 5mm. It is also hugely inconsistent and no rain may fall in some areas for many years.