The true size of Africa
Figure 1: Graphic of the true size of Africa
Rainfall variability
Rainfall in Africa displays high levels of variability across a range of spatial and temporal scales. Annual rainfall is estimated to be 20360 km3. The central region of Africa, which covers approximately 20% of the total land area, receives 37% of all precipitation. Conversely, the northern region which has a similar area receives less than 3% of the total precipitation (UNECA, 2011).
Figure 2: Average annual rainfall and Variability in annual rainfall
Note rainfall variability is represented by the coefficient of variability (CV), calculated as the standard deviation devised by the mean annual rainfall. It is expressed as a percentage and indicates how much rainfall varies from average annual rainfall.
Figure 2 shows how rainfall in Africa varies within and between regions, dependent on latitude, topography, seasons, continentality, and global climate circulations (which I will go on to talk about). Map 1 shows that rainfall tends to decrease away from the equator, and is scarce in the Sahara, Eastern Somalia, and South Africa. Rainfall is most abundant in parts of the highlands of Eastern Africa, large areas of the Congo Basin, central Africa and parts of coastal West Africa. Map 2 shows that most of Northern Africa experiences highly variable rainfall. Variation is the measure of how much rainfall varies from the annual average- This regions variation coefficient is > 45%. Comparable to the Congo Basin which is much less variable, with a variation of ~10%. Much of the continent has a variability coefficient of ~15-35%.
The inter-tropical Convergence Zone (ITCZ)
The ITCZ is the convergence of 2 'Hadley Cells' where warmer air rises, cools, sheds precipitation, moves poleward and descends delivering little precipitation (See video 1 for more information).
Video 1: The inter-tropical convergence Zone (ITCZ)
The ITCZ is arguably the most important control over the spatial and temporal variability in rainfall. The many different features of atmospheric circulation (which control the ITCZ) drives the seasonal development of the tropical rainfall belt over Africa. The ITCZ moves North and South throughout the year in response to solar radiation: the northern hemisphere have their rainy season in July when solar radiation is at its peak (summer) and the Southern Hemisphere have their rainy season in January (Taylor, 2004). The movement of the ITCZ across Africa means that the northernmost and southernmost latitudes only have one annual wet season, whereas the regions between these latitudes have 2 (figure 3).
Figure 3: Modern rainfall variability over Africa
(a) January and (b) August. Colours indicate days per month with measurable rainfall. The climate over large parts of Africa is characterised by a strong seasonality with summer monsoonal rainfall and the approximate position of the ITCZ (red band) migrating between the north and south of the continent over the course of the year
As mentioned before, it is not only global climate circulation that drives African climate, and therefore rainfall variations. It is several processes that are interrelated in complex ways and is still not yet fully understood yet (Conway, 2009).
Change in Rainfall variability
In the C20th there has been observed warming in Africa, especially since the late 1960s. This has occurred at a rate of about 0.5 ℃ per decade, however this increased in the last 3 decades of the century. This trend is not consistent over the whole continent. In coastal and major inland lakes of East Africa decreasing trends in temperature were observed.
For most of Africa there has been either a decline or no long-term trend in annual rainfall, an increase in inter-annual variability has however been observed. From the early 1970s there has been reports of 'drying' in the Sahel region. However, parts of East Africa showed a mean linear increase in annual rainfall of ~10-20% from 1901 to 1995, which is part of a wetting trend seen across equatorial Africa (UNECA, 2011). Both trends have been linked with El Nino- Southern Oscillation (ENSO).
The most significant change recorded has been a long- term reduction of rainfall in the semi-arid regions of West Africa and the Sahel.
The importance of Rainfall Variation
Throughout Africa, this variability brings significant implications for society.
Where most agriculture is rain-fed, crop growth is limited by water availability- rainfall variability during a growing season generally translates into variability in crop production.
There are many examples of the challenges posed on Africa due to the large variability in water supplies including: Lake Chad fisheries; balancing supply and demand on the Nile for Egypt; Irrigation management in the Greater Ruaha River in Tanzania. The main driver of much of the variability in river flows is rainfall (Conway, 2009). A 10% drop in precipitation reduces river discharge by 17- 50% (De Wit, 2006).
Climate Change
As the impacts of anthropogenic climate change begin to manifest and we see a shift in flows and variability across Africa (figure 4), we need to have a better understanding of the drivers of rainfall variability and rain-fall runoff interactions.
Figure 4: Projected rainfall changes in Africa
Climate change has the potential to impose additional pressures on water availability and demand in Africa. Next weeks post will focus on how climate change will impact water resources in Africa.
Do you think other climate phenomena such as the Indian Ocean Dipole will have equal, more or less impacts on rainfall over East and other regions of Africa, compared to ENSO?
ReplyDeletei was planning in the next couple of weeks to focus on ENSO in East Africa (as thats the region I'm focussing on), and i had not even considered the Indian Ocean Dipole, so thank you for bringing that to my attention.
ReplyDeleteI have done a bit of reading around this, one particular one i came across was (Yamagata, 2005). In the study based on East Africa, their models showed that the IOD had a much greater influence on short rains compared to ENSO.
However i do not think its really about what one has more or less impacts on rainfall, but more about their relationship as the IOD nteracts with ENSO. ~ 30% of positive IOD events co-occur with El Nino events (Rao, 2002). The fact that when positive IOD is combined with ENSO, it strengthens the EL Nino influence, is of importance. Especially with a ever warming climate, the Droughts and Flooding associated with ENSO will be exacerbated.
Rhona and Hong, this is a very good exchange and I agree with Rhona's reply.
ReplyDelete