Southern African Climate Devin Mastrodomenico, Benjamin Walton, Ziying Wang Iowa State University, Ames, Iowa

Abstract The climate of southern Africa has many contributing factors. There exists a cold ocean current to the west and a warm ocean current to the east causing great variability from west to east. The region extends from the southern tropics to the southern mid-latitudes giving it a range of dominant atmospheric flow. This causes many different climate types to arise anywhere from tropical in the north and along the east coast, arid conditions in the west and temperate in the central and southern regions. Maize grows well during the Austral summer do to adequate rainfall. This is important since the economy depends greatly on the agriculture of the region. The economy relies heavily on agriculture, even though it constitutes only about five percent of the region’s economy. Agriculture related business is roughly twenty-five percent of the economy in these countries.

1. Introduction used classification tool in use. Due to this fact, it is logical to start any discussion Southern Africa has many unique about climate at this point. Southern Africa features that dictate the climate. It extends has climates ranging from tropical to arid to from the tropical region in the north to the temperate (Peel et. al., 2007). Table 1 mid-latitude region in the south. It is shows the breakdown of the classification surrounded by ocean on the west, south and system used, and the temperature and east. The Agulhas warm current is off the precipitation guidelines accompanying it east coast and the Benguela cold current off (Peel et. al., 2007, Table 1). The following the west coast. All of this, coupled with a is shown in Figure 1. mountainous region in the southeast makes for a wide range of different average Along the east coast of Mozambique temperatures and rainfall. The following there is tropical savannah. This area does sections detail the climate classifications, not extend much off of the coast (Peel et. al., general circulations in the ocean and 2007, Figure 4). The warmer temperatures atmosphere, agriculture and economy of arise from the warm Agulhas Ocean current southern Africa. in the western Indian Ocean. This creates a warm sea breeze which allows for warmer 2. Köppen-Geiger Classification temperatures in lower latitudes (Jury et. al., 1993). There is also a small pocket of The Köppen-Geiger classification tropical savannah over extreme eastern system dates back to 1900. It was derived to Angola and western Zambia (Peel et. al., compare climates from different areas around the world. The Köppen 2007, Figure 4). classification system is still the most widely

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Namibia, Botswana, and parts of Malawi and western Mozambique, central Angola, South Africa, Zimbabwe and Zimbabwe and eastern South Africa. Along Mozambique have an arid climate. This arid the coast of South Africa there is a region of area is partly caused by a rain shadow temperate with no dry season. The southeast formed by the mountains on the east coast. coast has a hot summer and the southern As the warm moist air coming off of the coast has a warm summer (Peel et. al., 1993, Indian Ocean hits the mountains it rises, and Figure 4). the water vapor condenses and precipitates out. As the air descends on the other side of It has been claimed that temperatures the mountains it warms and dries causing in southern Africa have been increasing due the rain shadow. The arid is divided into a to climate change. During the period of steppe with hot temperatures and a hot 1940-1989, average daily maximum desert. The southwest coast of Angola, temperatures in South Africa were shown to northern Namibia, most of Botswana, decrease slightly while minimum extreme northern South Africa, extreme temperatures increased. The increase in southern Zimbabwe, and southern minimum temperatures was greater than the Mozambique is a hot desert. The decrease in maximum temperatures. The northwestern coast of Angola and areas east weather stations where these measurements of the desert in northern Namibia, most of were taken from have seen a change in the Botswana, extreme northern South Africa, landscape around them. Most of the stations and extreme southern Zimbabwe have a hot are in urban areas and development in these steppe climate. Western South Africa is a areas has taken off in the last three decades. cold steppe (Peel et. al., 2007, Figure 4). It appears that the urbanization of the region There is a small pocket of hot steppe with an has caused the apparent increase in area of hot desert inside it in southern temperature, since the nineteen non-urban Mozambique. This may be caused by the stations in South Africa have shown no Agulhas Current. When the continental significant warming over this period shelf is short, the current is closer to the (Hughes and Balling, 1996). It should be shore. This causes mesoscale subsidence made clear that this data is 15 years old, and over the land. When the shelf is longer the doesn’t necessarily agree with what is going current is farther from shore and the on at present. subsidence is pulled offshore allowing for 3. Circulations convection on land. The current goes between Africa and Madagascar here and is Longwaves in the atmosphere closer to the shore causing less rain to fall propagate in a clockwise direction in the (Jury et. al., 1993). Southern Hemisphere. Within these longwaves are shortwaves called ridges and The north and east, not including the troughs. These are responsible for the coast, are generally a temperate climate with weather and thus, the climate of southern a dry winter and a hot summer. This covers Africa. The dominant features of circulation most of Angola, southern Democratic in the region are the semi-permanent Republic of Congo, Zambia, Malawi, subtropical anticyclones over the Atlantic western Mozambique, Zimbabwe and Ocean to the west and the Indian Ocean to eastern South Africa. There are small the east, and a low circumpolar vortex pockets of temperate with a dry winter and between them over southern Africa (Kelbe warm summer over south Angola, south 1988). The anticyclones are caused by high Democratic Republic of Congo, eastern pg. 2 pressure over the oceans and the low Indian Ocean and Atlantic Ocean (Reason, circumpolar vortex is caused by low 1999). pressure over the landmass. Precipitation in the region is sensitive to sea surface 4. Agriculture temperature (SST) anomalies, especially More than half of the population in during Austral summer (Cook et. al., 2006). southern Africa lives in rural areas. The The main rainfall season for the region is main crop is maize, and it accounts for about Austral summer (Reason, 1999). seventy percent of the total human caloric There is also a Walker-style intake (Martin et. al., 2000). Maize is also circulation that affects rainfall variability used in other agricultural products. The over the region. Under normal growing season is in from December to circumstances, this circulation is such that February during the rainy season which is there is convergence near the surface over critical to the food supply of southern South America, divergence over the South Africa. In southern Africa people do not Atlantic Ocean, and finally, convergence have enough water to irrigate crops; almost over southeastern Africa. This convergence all are fed by rain. As the climate changes over Africa creates a low pressure in the in southern Africa there is a negative effect area, bringing precipitation (Williams, on the agriculture. Rainfall has decreased as 2008). Cold anomalies in SST in the South much as forty percent in some parts and Atlantic Ocean cause this Walker circulation temperature has increased up to six degrees to shift westward by increasing the Celsius, causing prolonged drought temperature gradient there. This increases conditions (Palitza 2009). The limited rainfall over southwestern Africa and also rainfall and increased temperature in some increases the number in extreme events regions in southern Africa contributes to (Williams, 2008). Figure 2 shows the sharp reductions in crop yields. structure of the Walker circulation under El Niño has an effect on rain normal conditions (a) and during cold SST amounts. An El Niño brings westerly wind anomalies (b). to the eastern highlands and increases The Indian Ocean also plays a role in evaporation. This causes warm and dry the position of the Walker circulation. The conditions during the austral summer. Agulhas Current retroflection is just Major El Niño events occurred in 1964, southeast of southern Africa. This is where 1973, 1983, 1992, and 1998 (Jury 2002). the current moves away from the continental During that time rainfall over southern shelf and turns back on itself. The warmer Africa was about sixty percent of normal, water of the current, combined with the temperatures increased by five degrees Celsius and evaporative losses were more convergence from the Walker circulation, -1 work to form a tropical-temperate trough than 10 mm day (Jury 2002). Total land (TTT). The TTT is an elongated low used for crop production shrinks by thirty pressure area usually over southeastern percent during an El Niño. Africa where storms form (Reason, 1999) 5. Economy and (Williams, 2008). Warmer SST near the Agulhas retroflection cause increased The climate of southern Africa has a precipitation in southern Africa due to large influence on the GDP of the area. increased convergence of moisture from the Agriculture is the main factor for the economy of the region. Dry-land crops pg. 3 contribute around one billion U.S. dollars 6. Conclusion every year, but conditions during an El Niño cause that to decline by thirty percent (Jury The climate of southern Africa is 2002). Agriculture itself accounts for only diverse. One can almost take a complete about five percent of economic activity; world tour of climates just by stopping in however its indirect influence is estimated to this small sector of the globe. The be about twenty-five percent, due to combination of a cold ocean current on the manufacturing and service support needed west coast, and a warm current on the east for commercial farming operations. It is coast, make for a spread of both also about thirty-six percent of the regions precipitation amounts and temperatures. exports. The GDP in dry periods is about This contributes to the importance of the eighty-three percent of what it is in wet agriculture since any variations in these periods. Limitations of water cause quotas currents effects the atmospheric circulations in domestic and industrial areas leading to and precipitation over the region. Since production losses. During the drought in agriculture plays a vital role in the 1992 about seventy-percent of the rural economies of southern Africa, any change in population requested relief. Sixty million the climate will have a drastic change on the U.S. dollars were lost in foreign exchange in livelihood of the people who inhabit the Zimbabwe alone (Jury, 2001). area.

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7. Appendix

Table 1. Breakdown of the Köppen-Geiger classification system

Figure 1. Map of Köppen-Geiger classifications for southern Africa

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Figure 2. Walker-Style circulation over southern Africa during normal conditions (a), and during cold South Atlantic SST anomalies

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