Climate of Southern Africa
Summary and Keywords
Southern Africa extends from the equator to about 34°S and is essentially a narrow, peninsular land mass bordered to its south, west, and east by oceans. Its termination in the mid-ocean subtropics has important consequences for regional climate, since it allows the strongest western boundary current in the world ocean (warm Agulhas Current) to be in close proximity to an intense eastern boundary upwelling current (cold Benguela Current). Unlike other western boundary currents, the Agulhas retroflects south of the land mass and flows back into the South Indian Ocean, thereby leading to a large area of anomalously warm water south of South Africa which may influence storm development over the southern part of the land mass. Two other unique regional ocean features imprint on the climate of southern Africa—the Angola-Benguela Frontal Zone (ABFZ) and the Seychelles-Chagos thermocline ridge (SCTR). The former is important for the development of Benguela Niños and flood events over southwestern Africa, while the SCTR influences Madden-Julian Oscillation and tropical cyclone activity in the western Indian Ocean. In addition to South Atlantic and South Indian Ocean influences, there are climatic implications of the neighboring Southern Ocean.
Along with Benguela Niños, the southern African climate is strongly impacted by ENSO and to lesser extent by the Southern Annular Mode (SAM) and sea-surface temperature (SST) dipole events in the Indian and South Atlantic Oceans. The regional land–sea distribution leads to a highly variable climate on a range of scales that is still not well understood due to its complexity and its sensitivity to a number of different drivers. Strong and variable gradients in surface characteristics exist not only in the neighboring oceans but also in several aspects of the land mass, and these all influence the regional climate and its interactions with climate modes of variability.
Much of the interior of southern Africa consists of a plateau 1 to 1.5 km high and a narrow coastal belt that is particularly mountainous in South Africa, leading to sharp topographic gradients. The topography is able to influence the track and development of many weather systems, leading to marked gradients in rainfall and vegetation across southern Africa.
The presence of the large island of Madagascar, itself a region of strong topographic and rainfall gradients, has consequences for the climate of the mainland by reducing the impact of the moist trade winds on the Mozambique coast and the likelihood of tropical cyclone landfall there. It is also likely that at least some of the relativity aridity of the Limpopo region in northern South Africa/southern Zimbabwe results from the location of Madagascar in the southwestern Indian Ocean.
While leading to challenges in understanding its climate variability and change, the complex geography of southern Africa offers a very useful test bed for improving the global models used in many institutions for climate prediction. Thus, research into the relative shortcomings of the models in the southern African region may lead not only to better understanding of southern African climate but also to enhanced capability to predict climate globally.
Although strictly southern Africa extends from the equator to about 34°S, in this article it is defined as Africa south of the Congo Basin and therefore also excludes the equatorial bimodal rainfall region of northern Tanzania and Kenya. A similar definition of southern Africa was taken by Lindesay (1998) in her discussion of the regional climate, while, in their review, van Heerden and Taljaard (1998) considered it as Africa south of the equator. In comparison to the other Southern Hemisphere continents, southern Africa is essentially a narrow peninsular land mass bordered to the west, south, and east by oceans and that terminates in the subtropics, unlike South America or southeastern Australia. The termination in the mid-ocean subtropics has important consequences for regional climate, since it allows the strongest western boundary current in the world ocean (warm Agulhas Current) to be in close proximity to an intense eastern boundary upwelling current (cold Benguela Current).
A further distinction between southern Africa and the other land masses is that the topography is mainly dominated by an interior plateau, at a height of about 1 to 1.5 km, which is separated from the coast by a series of mountain ranges. By contrast, Australia is relatively flat and at low altitude save a narrow range that runs up the east coast (Great Dividing Range), whereas South America is dominated by the high-altitude Andes near its west coast. Another difference is the presence of the large mountainous island of Madagascar located some 150 to 350 km off the east coast of tropical southern Africa. All these geographical features imprint themselves on the regional climates of southern Africa.
Unlike other western boundary currents in the world ocean, the Agulhas retroflects south of the southern African land mass and then flows back into the South Indian Ocean. About 65 to 75 Sverdrups of tropical water is transported along the east coast of South Africa by the Agulhas Current before it moves away from the coast near Port Elizabeth as the continental shelf there widens (Lutjeharms, 2006). The current then flows southwestward some 300 to 400 km south of the coast before it retroflects back into the South Indian Ocean as the Agulhas Return Current, thereby leading to a large area of anomalously warm water south of the subcontinent that may influence storm development over the southern part of the land mass. The contrast between the warm surface water (20–24°C) and the cool, relatively dry South Atlantic air masses sweeping across the retroflection near 38 to 45°S in the mid-latitude westerly winds leads to a large latent heat flux into the lower atmosphere that can reach over 1,000 Wm−2 on occasion (Rouault et al., 2003a). Many studies have provided evidence as to how this latent heat loss from the southern Agulhas can intensify storms tracking across southern South Africa and hence influence rainfall (Blamey & Reason, 2009; Reason, 2001a; Rouault et al., 2002; Singleton & Reason, 2006, 2007a; Walker & Mey, 1988). Thus, there is substantial evidence that the Agulhas Current plays an important role in the weather of at least the southern part of southern Africa. On interannual and decadal time scales, much research has shown relationships between sea-surface temperature (SST) variability in the Agulhas Current region and seasonal rainfall over large parts of South Africa (Mason & Jury, 1997; Reason & Mulenga, 1999; Walker, 1990), a topic further discussed in the section “Climate Variability.” On even greater scales, Beal et al. (2011) reviewed the implications of the Agulhas Current for the meridional overturning circulation and for global climate. In particular, the leakage of warm, salty Agulhas water into the South Atlantic via rings, eddies, filaments, and noneddy fluxes (Loveday et al., 2015) is important for global climate.
In addition to the Agulhas retroflection, there are two other unique regional ocean features that impact the climate of southern Africa: the Angola-Benguela Frontal Zone (ABFZ) and the Seychelles-Chagos thermocline ridge (SCTR; Figure 1).
The former serves as the boundary between the cold, nutrient-rich Benguela upwelling system off the Namibian coast and the warm tropical Angola Current to its north and is evident in Figure 1 as a sharp gradient in upper ocean chlorophyll that extends a few hundred kilometers in a west northwest direction from the west coast. Located near 17 to 18°S, the ABFZ is characterized by SST gradients of approximately 4°C/100 km (Veitch et al., 2006) and seems to result from a convergence between the southeastward-flowing South Equatorial Counter Current toward southern Angola and the northward-flowing upwelling coastal jet off the northern Namibian coast. In turn, the upper ocean flows are driven by the regional wind stress curl and hence are sensitive to the position and strength of the South Atlantic semi-permanent anticyclone (Colberg & Reason, 2006). Shifts in the location or the intensity of the ABFZ are important for coastal fisheries and typically occur during Benguela Niño events, leading to enhanced rainfall over southwestern Africa (Florenchie et al., 2003, 2004; Hansingo & Reason, 2009; Rouault et al., 2003b). South of the ABFZ, the Benguela upwelling system is evident in Figure 1 as the narrow band of high chlorophyll values all along the west coast. The upwelled cold SST, which is driven by the southerly winds associated with the semi-permanent South Atlantic anticyclone, is important for stabilizing the atmosphere along the coast south of the ABFZ to about 34°S (Cape of Good Hope). This process, together with subtropical subsidence in the atmosphere, leads to semi-arid or arid conditions along the west coast. A further climatic implication of the Benguela Current system is that it lies in close proximity to the southern Agulhas Current, thereby allowing strong latent heat flux exchange and the potential for storm development on the south coast of South Africa. It is worth noting that the geography of southern Africa means that the Benguela upwelling system is the only eastern boundary current system in the global ocean bounded at both its poleward and equatorial margins by warm water and is therefore distinct from its counterparts off the west coasts of North Africa (Canary Current), South America (Humboldt/Peru Current), and the United States (California Current).
Positioned to the northeast of Madagascar, the SCTR is an area of relatively shallow thermocline depth that influences Madden-Julian Oscillation and tropical cyclone activity in the western Indian Ocean (Annamalai et al., 2005; Malan et al., 2013; Mawren & Reason, 2017; Xie et al., 2002). The SCTR is evident in Figure 1 as a region of slightly higher chlorophyll values than the surrounding waters and is associated with upwelling driven by the cyclonic wind stress curl over the western tropical South Indian Ocean. Since the thermocline decreases to less than 70 m depth in the SCTR, Rossby wave signals that have been generated by ENSO or the Indian Ocean Dipole and that propagate across the basin from the east are then able to interact with the SST near the eastern African coast and hence produce rainfall (Hermes & Reason, 2008, 2009a; Yokoi et al., 2008). The depth of the SCTR influences the amount of heat in the upper ocean available to be taken up by tropical storms and seems to influence the number of severe tropical cyclone days in the southwest Indian Ocean as well as regional rainfall (Malan et al., 2013; Xie et al., 2002). Although its chlorophyll values are low by comparison to the Benguela system, the SCTR is a region of higher nutrients relative to the tropical South Indian Ocean waters to its north and south and hence exhibits greater marine productivity.
Various aspects of the tropical and subtropical oceans neighboring southern Africa therefore not only are important for the climate of the region but also are somewhat unusual compared to other regions of the world at similar latitudes. It can also be argued that the vast expanse of the Southern Ocean that lies between the Agulhas Return Current (near 42°S) and the Antarctic Current has substantial effects on southern African weather and climate. Globally, the Southern Ocean is of great importance for climate because of its ability to store large amounts of carbon dioxide exchanged from the atmosphere. For example, Sallée et al. (2012) estimated that over 40% of the anthropogenic carbon dioxide in the global ocean has entered it south of 40°S. This region is also one of relatively strong fronts in SST, upper ocean density, sea surface height, and chlorophyll (Belkin & Gordon, 1996; Billany et al., 2010). The marked greenish band across the Southern Ocean evident to the south of South Africa in Figure 1 reflects the Subtropical Convergence or Subtropical Frontal Zone, with the Sub-Antarctic Front apparent further south near the bottom of Figure 1. There is some evidence that variability in the Southern Ocean fronts may affect midlatitude westerly storms tracking toward the southern part of South Africa (Burls & Reason, 2006; Nakamura et al., 2004; Reason & Murray, 2001).
Regional Topography and Vegetation
Figure 2 shows the topography of southern Africa, along with a schematic of the main atmospheric circulation features during summer.
Most of the region south of the Congo Basin consists of an interior plateau that reaches its highest point (2.5–3 km) in Lesotho and neighboring KwaZulu Natal, South Africa. Between the plateau and the coast are a series of mountain ranges that are most prominent in South Africa. The coastal plain is relatively narrow in Angola, Namibia, and South Africa, but, on the east coast from northern KwaZulu Natal northward, it widens substantially so that most of Mozambique and southeastern Tanzania lie at altitudes of less than 500 m. East of Mozambique are several islands, of which by far the largest is Madagascar. Oriented roughly SSW–NNE, this island is dominated by mountain ranges that run the entire length of the east coast and reach over 1500 m in some places. As a consequence, much of western and southwestern Madagascar lies in a rain shadow from the moist southeasterly trade winds. In this regard, it is likely that Mozambique may receive less rainfall than it would if Madagascar were absent.
The interior plateau of southern Africa is dissected near its coastal margins by various rivers that are more prominent on the east coast than on either the south or the west coasts (e.g., the Thukela in KwaZulu Natal, South Africa, the Limpopo and the Zambezi in Mozambique, and the Rufiji in Tanzania). Rainfall strongly decreases from the eastern highlands to the west coast, so there are relatively few rivers discharging into the southeast Atlantic; the most prominent are the Orange/Gariep and the Kunene rivers. The eastern part of southern Africa is also characterized by two of the African Great Lakes—namely, Lakes Tanganyika and Malawi—as well as several other relatively large lakes, which have important consequences for the local climates of the countries that border them (western Tanzania, northern Zambia, southeastern Democratic Republic of Congo, and Malawi). The man-made Lakes Kariba (border of northern Zimbabwe/southern Zambia) and Cabora Bassa in northwestern Mozambique, which resulted from hydroelectric dams on the Zambezi River, also exert mesoscale influences on the rainfall and temperatures of the areas nearby.
The regional topography has important consequences for rainfall, soil conditions, vegetation, and fire risk. Highest rainfall occurs near the east coast, where the adjacent SSTs are warmest. The rainfall is further enhanced by uplift of moist marine air masses advecting toward the topography, such as at the eastern side of the Madagascan mountains, the Drakensberg in eastern South Africa, and the Chimanimani mountains along the Zimbabwean/Mozambican border. Thus, coastal forest is the indigenous vegetation along the coast from about East London northward to Kenya, while tropical rainforest is found along the east coast of Madagascar. Afro-montane forest is found in the higher altitudes of the mountains of tropical southern Africa and at relatively low altitudes on the windward slopes or in gorges of the mountains in eastern and southern South Africa.
Moving inland from the east coast, the vegetation becomes savanna, which is either mainly grassy savanna in KwaZulu Natal, southern Zimbabwe, and southern Mozambique, or woody savanna in northern Mozambique, Zambia, and Tanzania. Central South Africa contains a large area of grassland, whereas further north in the Limpopo province and eastern Botswana, grassy savanna dominates. Further west lies the Kalahari Desert, which covers much of western Botswana and Namibia and parts of northwestern South Africa. The vegetation in the Kalahari is sparse grass and scrubland with occasional acacia trees. Characterized by low-lying scrub, limited grass cover, and areas of open soil and rock, the Karoo semi-desert lies to the south of the Kalahari and extends over much of western South Africa. At the west coast, the Karoo merges into succulent semi-arid fynbos south of about 30°S, with the hyper-arid Namib Desert extending from this latitude to about 16°S in southwestern Angola. The southwest and south coastal region of South Africa (winter-rainfall-dominated, unlike elsewhere in southern Africa) manifests its own floral kingdom (fynbos) before merging into a small region of temperate rainforest along the Tsitsikamma coast (all-season rainfall enhanced by topography) between about George and just west of Port Elizabeth. A sense of the vegetation gradient across southern Africa from the lusher areas near the east coast to the arid west coast can also be seen in the satellite-derived chorophyll image in Figure 1, where dark green over the land mass reflects high leaf density (forest), lighter green reflects lower leaf density (e.g., savanna), and the yellow and brown regions are semi-desert or desert.
While rainfall seasonality and totals influence the type of vegetation that naturally grows in a particular place, in regions mainly dominated by convective rainfall, it has long been thought that changes in vegetation cover or soil moisture can influence rainfall frequency and intensity (Charney, 1975; Charney et al., 1977; Taylor & Lebel, 1998). However, as reviewed in Nicholson (2015), most of the evidence for these rainfall effects comes from theory or from models, due to the expense of mounting appropriate field experiments and the difficulty of designing the experiments such that the influences of the various important processes on precipitation can be separated from each other. For example, there is uncertainty in determining the evaporative fraction and its relationships with soil moisture and the overlying atmospheric boundary layer, and hence with the subsequent precipitation (Guillod et al., 2014). Also needing to be better understood is the coupling between latent heat release and the thermal instability of the boundary layer (Tawfik & Dirmeyer, 2014). The impacts of land-surface processes on triggering convection in the lower atmosphere relative to their role in modulating rainfall intensity is currently not well understood. Whether the surface processes act more to mainly redistribute the precipitation, as opposed to increasing it, is another aspect that is also not well understood (Nicholson, 2015).
Another significant factor for some vegetation types in southern Africa is fire. The tendency for fire to occur is important for the propagation of many fynbos species as well as for the balance between grass and tree cover in many savanna areas (van Wilgen, 2009) or for preventing shrub encroachment in these regions if the fire frequency is high enough (Roques et al., 2001). Grazing, either by agricultural species or by wildlife in areas like the Kruger National Park, is also of great importance for the type of cover that dominates. For example, large populations of elephant can quite rapidly reduce the proportion of tree cover in game reserves or other wild areas and help promote a more grassy savanna than would otherwise occur.
Mean Spatial Patterns of Temperature
Southern Africa’s temperature and rainfall patterns are moderated to a large extent by the surrounding oceans and also the topography. Due to the altitude of the interior plateau (at least 1 km above sea level), temperatures over most of inland southern Africa are several degrees cooler than they would be otherwise. Thus, unlike in Australia, average January maximum temperatures do not exceed 30°C except near the Mozambican coast, low- lying areas, such as the Zambezi and Limpopo River valleys, or the areas of highest insolation in the Kalahari and Karoo semi-desert (Figure 3a).
Elsewhere, average maximums are 25 to 30°C, except in the highest-altitude mountains of Lesotho and Drakensberg, and along the Namibian coast, where southerly wind-driven upwelling leads to SST of 10°C or less, and cold air advection and regular low-level stratus or fog cools the narrow coastal strip. These patterns are more or less replicated in the patterns of average minimum temperature (Figure 3b), which are about 26°C in coastal Madagascar/Mozambique/Tanzania, almost as warm in the Zambezi and Limpopo River valleys, and then decrease westward as expected in the areas of high insolation and relatively low cloud cover.
Over most of tropical southern Africa, winter maximum temperatures (Figure 3c) are not much cooler than in summer, since winter involves very little cloud cover. Only over southern South Africa and Lesotho and the perennial upwelling areas off the northern Namibian coast do July maximums fail to reach 20°C on average. Average winter minimum temperatures (Figure 3b) are less than 5°C in the high altitude, almost cloudless sky regions of interior South Africa, southern Botswana, and southeastern Namibia, and below freezing in the highest mountain regions of Lesotho/KwaZulu Natal/Free State. The combination of relatively cloud-free skies in interior South Africa/Botswana during winter and its high altitude mean that the diurnal temperature range here is 3 to 5°C more than at comparable latitudes in Australia or South America at the same time of year (Figure 4b).
In summer, interior southern Africa also displays a greater diurnal range than the other continents, but the difference is less pronounced due to increased cloud cover in summer (Figure 4a).
The influence of the neighboring oceans is reflected in average annual air temperatures at coastal stations. For example, Luderitz (26.6°S) on the west coast in the central Benguela upwelling system has a mean temperature about 7°C cooler than Maputo (25.9°S) on the east coast near the northern Agulhas Current. Another measure of the warming influence of the Agulhas Current is reflected in the fact that the South African east coast station Durban (30.0°S) has a mean annual temperature almost a degree warmer than the Australian east coast station of Brisbane (27.5°S), despite lying about 250 km further from the equator.
Mean Spatial Patterns of Rainfall
With the exception of southwestern South Africa and the south coast, southern Africa receives most of its rainfall during the summer, from October to March (ONDJFM). Hence, it is convenient to begin discussion of rainfall with the early summer (OND; Figure 5a) which is the time of the rainy season onset.
The latter can be defined in different ways, but if one uses a simple definition based on the amount of pentad (5-day) rainfall needed for germination of the staple crop, maize (Hachigonta et al., 2008; Tadross et al., 2005), then onset over southern Africa usually occurs sometime in October. This definition has the advantage of being easy to calculate but neglects consideration of the associated rain-bearing weather systems, which can lead to counterintuitive results, such as southern Mozambique having an earlier onset date than further north due to the contribution of cut-off lows, for example.
In simple terms, as the Inter-tropical Convergence Zone (ITCZ) starts to move south of the equator in October and the winter mean subsidence over the land mass starts to weaken, one might expect early summer rainfall over tropical, and to lesser extent, subtropical, southern Africa to begin. Figure 5a shows a general pattern of increasing rainfall as one moves east from the arid west coast, but which becomes more complex over the eastern third of the land mass due to the effects of the topography. For example, the influence of the Drakensberg/Maluti/Lebombo mountains can be seen in Lesotho, eastern South Africa, and Swaziland, as can the influence of the Chimanimani mountains in Zimbabwe and west-central Mozambique, while the low-lying Limpopo and Zambezi River valleys are relatively dry. Northern Mozambique is somewhat drier than further inland or the area to its south, which may be due to relative divergence near the coast of the monsoonal westerly flow toward northern Madagascar. Strong topographic effects are also apparent on Madagascar, with the east and north much wetter than the southwest due to the mountain chain that runs the length of the island and the prevailing southeasterly trade winds incident on the mountains.
Tropical southern Africa and the southwest Indian Ocean islands (Madagascar, Mauritius, and Reunion) are much wetter in JFM (Figure 5b) than in OND as the ITCZ reaches its southernmost position across central Madagascar and Mozambique in February. Further south, the increase in mean rainfall in JFM relative to OND is less pronounced, although the low rainfall belt in the west substantially decreases in width, such that the Kalahari and eastern half of the Karoo receive 30 to 90 mm/month on average. In the far southwest, in the Mediterranean-type climate region near Cape Town, JFM is noticeably drier than OND, due to the southward displacement of mid-latitude frontal systems in summer. The all-season rainfall region of the south coast receives about the same amount of rainfall in the late summer as in the previous season, mainly due to ridging anticyclones and topographic uplift of moist marine air by the coastal mountains (Engelbrecht et al., 2015; Weldon & Reason, 2014).
With the exception of southwestern South Africa and eastern Tanzania, almost all of southern Africa is substantially drier in the austral autumn (Figure 5c) than in summer. In the east, the ITCZ moves north across the equator in April, leading to the so-called long rains in Tanzania and Kenya, while the northward shift of the subtropical anticyclones and frontal systems leads to increased rainfall over the far southwest and relative subsidence/dry conditions over most of the rest of the subcontinent.
The same general pattern is evident in winter (Figure 5d), which is even drier than AMJ over almost the entire region except in the far southwest, where local topographic enhancement of the rainfall slightly inland from the coast is evident. Subsidence over southern Africa from the outflow from the northern hemisphere monsoons leads to dry conditions, even in near-equatorial East Africa and the southern Congo Basin. As in AMJ, there is still considerable rainfall experienced along the south coast of South Africa in winter.
As for temperature, there are also oceanic influences on regional rainfall. Excepting its equatorial and far southern margins, all of the west coast of southern Africa with its cold upwelled SST is arid or semi-arid. Here the coast is like western South America at the same latitudes; however, both continents differ from Western Australia, which does not have a hyper-arid coastal desert comparable to the Namib or Atacama deserts. The obvious difference off Western Australia is lack of cold upwelled coastal SST due to the presence of the warm poleward-flowing Leeuwin Current, which overwhelms the effect of the upwelling favorable southerly winds. Thus, the coastal atmosphere of Western Australia tends to be less stable and subsiding than that of Namibia or northern Chile/Peru. On the east coast, where each continent experiences a warm poleward-flowing western boundary current, annual mean rainfalls at, for example, 30°S are similar for each continent. However, further south (34°S), the South African station at Port Elizabeth has an average annual rainfall of 624 mm, whereas Sydney and Buenos Aires, at virtually the same latitude, receive 1,213 mm and 1,306 mm, respectively, with far more rainy days per year. One clear distinction among the three stations is the cooler coastal SST at Port Elizabeth, since the Agulhas Current moves away from the South African coast at a point about 100 km north of the city. The large increase in average rainfall along the South African east coast north of Port Elizabeth toward Durban (1,003 mm) was attributed by Jury et al. (1993) to the closeness of the Agulhas Current to the coast north of about 32 to 33°S. Idealized atmospheric model experiments (Reason, 2001a), in which the presence of the Agulhas Current was smoothed out from the SST forcing, have confirmed the influence of the current on regional rainfall.
Figure 6 shows the coefficient of variation over the region for each season.
As expected, it is largest in the regions of lowest mean seasonal rainfall and varies substantially throughout the year. There is little seasonal change in the coefficient in the hyper-arid Namib Desert, where it hardly ever rains, or in areas that receive similar monthly rainfall amounts throughout the year, such as the south coast of South Africa and the east coast of Madagascar (Engelbrecht et al., 2015; Randriamahefasoa & Reason, 2017).
Although the rainfall total and coefficient of variation plots give some idea of mean conditions and their general variability, they are not all that helpful from a practical point of view. Given that a large proportion of the region depends to a substantial extent on subsistence agriculture, a more useful measure is the frequency of dry spell occurrence during the rainy season (Usman & Reason, 2004). This type of analysis shows the existence of a drought corridor across subtropical southern Africa near 20 to 23°S that is much larger in area than the semi-arid Limpopo Valley and that, outside the Namib or Kalahari desert regions, is most intense in southwestern Zimbabwe. Over the drought corridor region, more than half of the summers during the 1979–2002 period analyzed by Usman and Reason (2004) experienced more than half of the total number of pentads in a given summer as dry. Note that one pentad equals 5 days, so that a given 3-month summer period contains 18 pentads. As is discussed other sections, much of the variability in rainfall can be attributed to regional circulation and SST anomalies associated with ENSO and other climate modes.
Regional Circulation Patterns
In a broad sense, the mean temperature and rainfall patterns described in the previous two sections largely follow from the seasonal shifts in the semi-permanent anticyclones over the South Atlantic and South Indian Oceans (hereafter SAA and SIA, respectively) and from the ITCZ. Over eastern Africa, the ITCZ migrates from about 20°N in the late boreal summer to around 15 to 18°S in the late austral summer, reaching its southernmost position over central Madagascar/Mozambique (Figure 1). The southward shift in austral spring tends to be more rapid than the northward shift back into the Northern Hemisphere in autumn (Nicholson & Grist, 2003; Suzuki, 2011). Somewhere over southern Zambia in central southern Africa, the ITCZ forms a meridional arm in summer that stretches northward through the Congo Basin before turning west and exiting Africa at a latitude slightly north of the Gulf of Guinea coast of West Africa (Figure 1). In boreal summer, the ITCZ is more or less zonally oriented across the continent near 20°N, and thus its meridional shift over the western half of Africa between summer and winter is very small in comparison to that over the eastern half. The situation over western southern Africa is complicated in summer by the presence of a secondary tropical convergence zone over Angola that extends from the heat low (the Angola low) that forms in spring over southern Angola/northern Namibia to the meridional arm of the ITCZ (Figure 1 and Figure 7d).
Another heat low is present further south over the southern Kalahari Desert. In the late summer, the Angola Low is more like a tropical low and is associated with moist instability (Munday and Washington, 2017). A surface low is also present over the Mozambique Channel, likely due to the adjustment of the easterly low-level winds to the Madagascan orography. In summer, the SAA and SIA are located about 5 to 6° poleward of their mean winter position (they are centered farthest south in February).
On the eastern side of the subcontinent, there is a convergence zone of enhanced summer rainfall that extends from Mozambique southeast over the southwest Indian Ocean. This South Indian Convergence Zone (SICZ; Cook, 2000; Streten, 1973) is much weaker than its counterparts in the other southern basins, the South Atlantic Convergence Zone and the South Pacific Convergence Zone. Essentially, the SICZ is the seasonal aggregate of the tropical-extratropical cloud bands or tropical temperate troughs (TTT) that bring much of the summer rainfall (Fauchereau et al., 2009).
At mid- to upper levels in the troposphere, an important feature that influences summer rainfall is the Botswana Upper High. This tropical upper-level anticyclone, like the Bolivian High or the one in northwestern Australia (Bilybara High), forms as part of the circulation response to the heat released by high precipitation areas, such as the Amazon and Congo Basins, to their northeast (Lenters & Cook, 1997). Compared to the Bolivian and Bilybara Highs, the Botswana High forms earlier in the spring and tends to be stronger in magnitude (Reason, 2016). Its significance for summer rainfall has been highlighted for Zimbabwe by Matarira (1990) and Unganai and Mason (2002) and more generally throughout southern Africa by Reason (2016) and Driver and Reason (2017).
A zonal wind transect along 25°E through the land mass (Figure 8a) shows that, in summer, low-level easterlies exist over the land from about 15°S (the tropical convergence zone over southeastern Angola/southwestern Zambia) to just south of the south coast, with the mid-latitude westerlies apparent poleward of about 36°S.
At this time of year, the subtropical jet is located somewhere near 40°S, with a strong westerly aloft reaching about as far north as the Tropic of Capricorn. Further north, the tropical easterly jet is centered near 8°S, while in the lower troposphere, there is a low-level westerly component to the flow blowing in toward the tropical convergence zone. On the east coast of southern Africa, westerly flow exists between about 12°S and 18°S as the northeast monsoon recurves north of the Mozambique Channel to a northwesterly monsoonal flow toward northern Madagascar.
In winter (Figure 8b), almost all of southern Africa is under the influence of anticyclonic subsidence; this not only is due to the equatorward migration of the SAA and SIA, but also results from the large westward movement of the latter, which is centered near 85°E in summer but near 60°E in winter. A relative col in pressure exists just to the south of Cape Town in winter, which, together with the northwestward migration of the SAA (in winter centered near 28°S 4°W), allows mid-latitude frontal systems to frequently impact southwestern Africa, the only region in the subcontinent that receives mainly winter rainfall. At this time of year, the subtropical jet moves substantially northward, with strong westerly flow aloft extending to around 15°S, whereas, near the surface, there is mean westerly flow everywhere south of about 28°S (Figure 8b). By contrast to the SIA, the mean monthly central position of the SAA appears to show a semi-annual variation; this may help explain the small semi-annual variation in mean monthly rainfall observed at south coast stations, such as George and Port Elizabeth.
On the seasonal scale, linking regional circulation patterns to those of rainfall requires consideration of the large-scale moisture fluxes. In addition to moisture recycling over the Congo Basin, there are three major oceanic sources of moisture for precipitation over southern Africa: the most important is the tropical western Indian Ocean, the subtropical southwest Indian Ocean is also a substantial contributor of moisture for rain over subtropical southern Africa, whereas the South Atlantic has been traditionally regarded as a secondary source for summer rainfall (D’Abreton & Lindesay, 1993; Reason et al., 2006).
Figure 9 (derived from NCEP re-analyses, Kalnay et al., 1996), which shows the 850-hPa summer moisture flux and its convergence over the southern African region, further implies that the South Atlantic is less important than the South Indian Ocean for rainfall over the land mass.
Note that the 850- hPa level is just above the height of the interior plateau that exists over much of southern Africa. Over this land mass, the cyclonic circulation and strong convergence over tropical southwestern Africa mark the Angola Low and its connection to the meridional arm of the ITCZ through the eastern Congo Basin, schematically shown in Figure 1 and Figure 2. Further east, in the southern Mozambique Channel, there is another cyclonic area of low-level moisture convergence which may also act as a source region for cloud bands. Moisture flux from the subtropical southwest Indian Ocean feeds into this feature and over subtropical southeastern Africa. This cyclonic feature is associated with the adjustment of the easterly trade winds to the Madagascan topography. Relative divergence and anticyclonic flow occur over most of South Africa and southern Botswana/southern Zimbabwe, some of it is due to the dynamical adjustment of the mean flow to the high-lying areas over eastern South Africa. Northeast of Madagascar and stretching across the tropical South Indian Ocean is a region of low-level convergence associated with the ITCZ. Another area of moisture convergence, fed by the northeast monsoonal flow from the tropical western Indian Ocean and a weaker westerly flow from the tropical southeast Atlantic/Congo Basin, is apparent over equatorial East Africa, just to the south of Lake Victoria.
On this seasonal scale, it is apparent from Figure 9 that most of the low-level moisture flux over southern Africa emanates from the western Indian Ocean, with the tropical southeast Atlantic Ocean playing rather a minor role, since the anticyclonic circulation of the South Atlantic High essentially exports moisture away from the land mass. However, on synoptic and intraseasonal scales, as well as on seasonal scales during certain years, moisture from the South Atlantic plays an important role (Cook et al., 2004; Hart et al., 2010; Vigaud et al., 2009). A warm pool (SST 26–29°C) develops in the tropical southeast Atlantic Ocean, off the central and northern coast of Angola and north of the ABFZ, during the austral summer and lasts until at least April. This tropical southeast Atlantic–sourced moisture flux also feeds into the Angola Low, which acts as the source region for the main synoptic summer-rainfall-producing system over subtropical southern Africa, the tropical temperate trough (TTT) or tropical extratropical cloud band (Harrison, 1984; Hart et al., 2010). The importance of tropical Atlantic moisture for rainfall over the Congo Basin has also been highlighted by Vigaud et al. (2007) and for Tanzania by Kijazi and Reason (2012), while Vigaud et al. (2007) presented evidence that South Atlantic moisture flux anomalies associated with SST dipole events in the mid-latitudes (Fauchereau et al., 2003; Hermes & Reason, 2005) could also contribute to summer rainfall over subtropical southern Africa. Although the emphasis here is on summer, the main rainy season for most of southern Africa, it should also be noted that the South Atlantic is the main moisture source for the small winter rainfall region in western South Africa (Reason & Jagadheesha, 2005a).
Major Weather Systems
Both tropical and mid-latitude weather systems and their interactions can produce significant rainfall over much of southern Africa. The classic example of the latter is the TTT that results from the interaction between a tropical disturbance situated over low-latitude southern Africa and a westerly system passing south of the land mass (Harrison, 1984; Todd et al., 2004; Figure 1). Although baroclinic westerly systems are necessary for TTT formation, in summer only about 40% of such systems actually lead to formation of a TTT (Macron et al., 2014), since latent energy in the tropics is also required. Moisture convergence and subsequent advection from the tropics to the mid-latitudes in the TTT are facilitated by strong easterly (westerly) flow from the tropical South Indian (mid-latitude South Atlantic) Ocean. TTTs bring a large portion of the summer rainfall south of about 15°S (Hart et al., 2010, 2013; Manhique et al., 2011; Ratna et al., 2012; Todd & Washington, 1998; Tozuka et al., 2014; Washington & Todd, 1999). Furthermore, TTTs developing over southern Africa or the Mozambique Channel may act as precursors for intraseasonal wet spells over Madagascar (Macron et al., 2016). The TTT results in organized deep convection and can include, or lead to, the development of mesoscale convective complexes (MCC) and heavy rainfall/flooding over some regions. For example, an intense TTT event resulting in severe flooding over the lower Limpopo Basin in Mozambique and causing over 100 deaths occurred in January 2013 (Manhique et al., 2015). MCC over southern Africa have been studied by Laing and Fritsch (1993) and more recently by Blamey and Reason (2013), who showed that they can contribute up to about 20% of the summer rainfall over northeastern South Africa and southern Mozambique. By comparison, Hart et al. (2013) found that TTTs, which are more common during November to January than in October or in the late summer months, can contribute 30% to 60% of the mean summer rainfall over South Africa.
While TTTs can occur anywhere over southern Africa south of about 15°S latitude, including Madagascar, other summer-rainfall-producing weather systems tend to be located in specific regions that favour their occurrence. Thus, the warm SST of the southwest Indian Ocean and generally easterly midlevel flow allow tropical cyclones to be a significant contributor to rainfall over eastern Madagascar and the western Indian Ocean islands of Mauritius and Reunion. As the SST and upper ocean heat content in the tropical western Indian Ocean has increased over the last few decades so has the number of very intense tropical cyclones risen in this region (Mawren & Reason, 2017). Although only about 5% of southwest Indian Ocean cyclones make landfall on the eastern mainland of southern Africa, they can make a very large contribution to central and southern Mozambican rainfall in some seasons (e.g., 2000, 2007), with devastating flooding and often significant loss of life (Reason & Keibel, 2004; Vitart et al., 2003). On the other hand, the upwelled cold SST off Namibia precludes tropical storm development south of the ABFZ.
Widespread heavy rainfall north of about 25 to 30°S can occur due to the passage of easterly waves or lows across southern Africa in summer without TTT development (Dyson & van Heerden, 2001). Easterly waves over southern Africa tend to be semi-stationary rather than propagating. In some cases, tropical storms over southern Africa may be the remnants of tropical cyclones that have made landfall in Mozambique. Reason and Keibel (2004) examined ex-Tropical Cyclone Eline, which, after landfalling in central Mozambique in late February 2000, tracked across to northern Namibia as a tropical storm, leading to heavy rainfall over not only the area near the escarpment in eastern Zimbabwe and northeastern South Africa but also parts of the Kalahari Desert much further to the west.
Further south, most flood events that have occurred are due to cut-off lows, which are most common in the transition seasons (Singleton & Reason, 2007a; Taljaard, 1985), although some cases with severe flooding have occurred in summer. Cut-off lows are cold-cored westerly systems forming in the mid- and upper troposphere on the equatorward side of the subtropical jet. Ndarana and Waugh (2010) showed that almost 90% of cut-off low events are associated with Rossby wave breaking, usually upstream, on or before the day of cut-off low formation. When cut-off lows move over potentially unstable air at lower levels, near-surface cyclogenesis can occur, leading to heavy rainfall (Fuenzalida et al., 2005). These systems can occasionally bring heavy rainfall in a single event that is many times greater than the seasonal average rainfall when they occur in arid areas, such as Laingsburg in the Little Karoo in January 1981 (Estie, 1981) and Luderitz in the Namib Desert in April 2006 (Muller et al., 2008). Sometimes a near-surface meso-low develops during the evolution of the cut-off system, leading to a low-level wind jet. Occurrence of such a low-level moist jet (marked as an arrow over the south coast in Figure 1), crossing the Agulhas Current and impacting on the coastal mountains, has been implicated in some cases of flooding in southern South Africa (Singleton & Reason, 2006, 2007a). Similar low-level jets crossing the current, in this case off the Kwa-Zulu Natal coast, have been identified in MCC events (Blamey & Reason, 2009).
The passage of cold fronts over southern South Africa, or south of the land mass, is important not only for TTT development in the summer half of the year but also for postfrontal ridging anticyclones along the south and east coasts. These systems can bring substantial rainfall, particularly where the onshore moist flow meets coastal mountains. The onshore flow and associated low-level cloud due to anticyclonic ridging east of South Africa sometimes bring cool, drizzly weather as far north as Zimbabwe, where it is colloquially known as guti weather. In the winter half of the year, the westerly storm track is further north and the fronts tend to directly impact the west and southwest coasts of South Africa, thereby bringing most of the region’s annual rainfall. Occasional cut-off lows can also contribute substantially to the winter rainfall in some years (Favre et al., 2012; Molekwa et al., 2014) and to spring rainfall over the south coast of South Africa (Engelbrecht et al., 2015; Weldon & Reason, 2014). When there is a large meridional component to the upper-level westerly flow, postfrontal ridging can result in cold air outbreaks over South Africa, with the low-level air originating from deep in the Southern Ocean. Such winter outbreaks may lead to substantial snowfall over the higher mountains of southern and eastern South Africa/Lesotho.
Heavy rainfall events in many subtropical and mid-latitude regions have been associated with atmospheric rivers, or narrow plumes of enhanced winds that transport large amounts of tropical-sourced moisture into the higher latitudes over large distances (Gimeno et al., 2016; Zhu & Newell, 1994, 1998). Although not specifically studied over subtropical southern Africa in detail as yet, similar narrow bands of tropical moisture have been identified in TTT (Hart et al., 2010), cut-off low (Singleton & Reason, 2007a), and MCC (Blamey & Reason, 2009) events. In the former case, the bands originate from the tropical southwest Atlantic, whereas in the latter, the Mozambique Channel/tropical southwest Indian Ocean appears to have been the main source. A preliminary analysis of atmospheric rivers over southern South Africa suggests that about 70% of heavy rainfall events in winter over South Africa may be linked to the existence of an atmospheric river affecting the west coast of South Africa (Blamey et al., forthcoming).
Prefrontal flow can sometimes lead to dust storms over central South Africa. The interior tends to be very dry in the winter half of the year, and the relative lack of vegetation cover and dry, crumbly topsoil in this season mean that strong northwesterly winds ahead of cold fronts tracking across southern South Africa are able to advect dust over a large distance. Resane et al. (2004) documented a case on August 11, 2003, where dust, likely sourced from the Makgadikgadi salt pans in central Botswana, reached Johannesburg. These salt pans, together with those at Etosha in northern Namibia, are two of the major sources of dust in the Southern Hemisphere (Bhattachan et al., 2015). In another case, in October 2014, a dust cloud advected 800 km across the Northwest, Gauteng, and Free State Provinces, with wind gusts reaching 65 km/hr at Bloemfontein just prior to the arrival of the dust front. Summers with below-average rainfall are also prone to dust storms. The risk increases over areas where fields that are usually thickly covered with maize or other crops contain stunted plants and large areas of bare soil. Such was the case in the very dry summer of 2015–2016, when a severe dust storm occurred on January 13, 2016, near Bloemfontein and Hoopstad in strong northwesterly winds. Although dust storms are not very common in South Africa, they occur quite frequently in the drier, sparsely vegetated areas of Namibia and Botswana.
Random air mass thunderstorms bring a substantial portion of the summer rainfall over much of southern Africa. Deep convection develops at certain times when the combination of diurnal heating and low-level moisture convergence associated with either mesoscale or synoptic forcing facilitates sustained cumulus cloud growth. Variations in local topography with associated mountain/valley winds often facilitate low-level moisture convergence and the uplift of unstable air, leading to thunderstorm development. The resulting thunderstorms can either be single-cell or multi-celled and are sometimes organized into squall lines (Admirat et al., 1985; Mader et al., 1986; Rouault et al., 2002). Over southern Africa, Collier et al. (2006) showed that maxima in lightning flash rates occur over northwestern Madagascar and central South Africa as well as in the Congo Basin. Many thunderstorms also produce damaging hail episodes, particularly over the highveld of South Africa. Smith et al. (1998) showed that over the summer half of the year, this region experiences on average 21.9 and 2.8 cases of non-severe and severe hail episodes, respectively. An episode is classed as severe if the maximum hailstone size is at least 31 mm.
In some cases, tornadoes also occur over eastern South Africa (de Coning & Adam, 2000), which can lead to severe damage and sometimes loss of life. A well-publicized case occurred in December 1998 in Mthatha when then President Nelson Mandela was visiting the city (Rouault et al., 2002). MSG satellite data are now used operationally to identify and track rapidly developing intense thunderstorms over southern Africa where lightning or radar data are not available (de Coning et al., 2015). Over the highveld of South Africa, early summer thunderstorms tend to develop in a conditionally unstable atmosphere, with large values of convective available potential energy (CAPE) and strong upper-level winds, as the large-scale circulation is more extratropical in character (Dyson et al., 2015). A climatology of potential severe convective environments, based on CAPE and deep-layer vertical shear from CFSR data for 1979–2010, showed that the early summer had the most favourable conditions for severe storm development over the interior of South Africa (Blamey et al., 2017). However, later in summer, the regional circulation becomes more tropical in nature, so there is more low-level moisture available and hence storms develop in a convectively unstable atmosphere.
The complex terrain of southern Africa leads to a variety of other local/mesoscale circulation systems. For example, mountain winds in winter and emissions from local transport and (heating/cooking) fires often combine to produce significant air-quality problems over many cities, such as Pretoria, as well as regions like the KwaZulu Natal midlands and the Cape Flats. Nocturnal low-level jets have been shown to be important over Botswana and Namibia for night-time rainfall events (Monaghan et al., 2010). A combination of mountain/valley and lake/land breeze systems often generates convective cloud and sometimes thunderstorms near the Great Lakes of southern Africa (Lakes Malawi and Tanganyika) as well as large dams (Kariba on the Zimbabwe / Zambia border, Cabora Bassa in Mozambique, and Gariep in central South Africa).
At the coast, land–sea breeze circulations are commonly observed in the absence of strong synoptic flow. However, because of the topography, they do not usually penetrate very far inland in South Africa or Namibia. On the Namib Desert coast, the sea breeze makes a vital contribution to local ecosystems by advecting marine fog a few tens of kilometers inland. On the east coast as well as the southwest Indian Ocean islands, sea breezes are important for triggering afternoon or evening thunderstorms. As the easterly trade winds encounter variations in the topography of the islands, they can generate relative wind shadows or local jets; an example of the latter is the tip wind jet evident at the northern tip of Madagascar (Collins et al., 2012). The strong southerly flow that blows throughout the year along the west coast north of about 30°S and on most summer days between 30 and 34°S forms a distinct jet at times (Nicholson, 2010; Winant et al., 1988). Adjustment of this flow to pronounced mountainous headlands, such as the capes near and north of Cape Town, lead to relative wakes and bands of stronger flow that impact the upwelling and the local marine ecosystems (Burls & Reason, 2008; Jury, 1985). In some cases, the bands of stronger southerly flow between Table Mountain and the mountains on the other side of False Bay, or to lesser extent between the mountainous headland at Cape Columbine and the mountains further inland, are reminiscent of gap winds. These are defined as jet-like mesoscale flows through a gap in the coastal mountains or between two islands or land masses (e.g., Straits of Gibraltar). Examination of QuikSCAT wind data in the Mozambique Channel suggests that gap-wind flow may sometimes occur through the Comoros Islands (Collins et al., 2012).
Another coastal mesoscale system of importance for Namibia and South Africa is the coastal low, a type of coastally trapped disturbance (CTD; Reason & Steyn, 1990). A CTD may occur in this region, as well as other subtropical mountainous coastal areas, such as California and Chile, when synoptic forcing pulses the marine subsidence inversion and the inversion lies at a height beneath the crests of the coastal mountains. Typically, the low is generated somewhere on the Namibian coast and then propagates over a few days as a type of internal Kelvin wave southward and then eastward along the coast as far as at least Durban (Gill, 1977; Reason & Jury, 1990). Coastal lows bring about a change in weather from relatively hot, dry conditions to cool marine stratus, sometimes with fog or drizzle. They also pulse the upwelling in the coastal ocean in Namibia/western South Africa on time scales of hours to a day or so. This pulsing reduces the upwelling of cold, nutrient-rich water, warms the upper ocean, and increases the risk of harmful algal bloom development.
The hot, dry conditions ahead of the coastal low are caused by adiabatic warming of winds originating from the interior around the trailing edge of the anticyclone as the latter ridges east to the south of South Africa. These winds, known locally as berg winds, transport dust and other aerosols from the interior out over the coastal plain and are accompanied by a lowering of the marine subsidence inversion, leading to poor air quality as well as anomalously high temperatures at the coast. Dust blown off the Namib and Kalahari Deserts over the coastal ocean by berg winds acts a source of nutrients for the Benguela upwelling system (Summerhayes et al., 1995). The winds are also able to enhance filaments at the upwelling front (Lutjeharms et al., 1991). In autumn, as the SIA migrates further westward from the South Indian Ocean toward the southern African land mass, the tendency for berg winds to blow, particularly on the south and east coasts of South Africa, increases. Since the inland air mass is still warm, this situation can lead to stations like Port Elizabeth experiencing their warmest day of the year in April or May. In most cases, the synoptic cycle in the South African region is such that a run of several days of abnormally high temperatures does not occur as the berg wind is replaced by the next cold front in the sequence. The lack of land mass beyond the subtropics and the strong maritime influence in South Africa are likely factors that help explain the tendency for extended periods of unusually hot or cold conditions not to prevail, unlike in southern Australia, for example. An exception occurred during the April 2014 heat wave over the southwestern Cape, with almost a week of abnormally warm temperatures that on occasion reached about 40°C in some parts of Cape Town, or around 17°C above the long-term average maximum for April.
Due to its geographical position and regional land–sea distribution, southern Africa is a region of pronounced climate variability on all scales, ranging from the intraseasonal to centennial and longer. The climate variability of the region is complex and sensitive to a number of different drivers originating from both low and high latitudes that are still not well understood. Nevertheless, there are well-recognized large-scale patterns of African rainfall variability, such as opposite signals between the equatorial and subtropical regions, or between equatorial East Africa and most of southern Africa, that have been observed at least over the last two centuries (Nicholson, 2014).
Over southern Africa, variability is strongly expressed in rainfall and to lesser extent temperature, with much literature on the former, often motivated by its impacts on agriculture and water resources. On the intraseasonal scale, Levey and Jury (1996) identified strong 30- to 60-day periodicities in southern African rainfall. Madden Julian Oscillation (MJO) activity (Zhang, 2005) is pronounced over eastern tropical Africa/western Indian Ocean and there is strong evidence of rainfall and wind oscillations on this scale over Tanzania (Kijazi & Reason, 2005; Mapande & Reason, 2005). Pohl et al. (2007) demonstrated how large-scale convective clusters resulting from MJO activity propagate southeastward from Angola toward South Africa and then northeastward toward Tanzania, while Pohl et al. (2009) showed that El Niño events are associated with suppressed intraseasonal convective activity over much of southern Africa. Hart et al. (2013) provided evidence that, over South Africa, TTT occurrence and associated rainfall are influenced by MJO activity. There may also be a connection between MJO activity and South Indian Ocean tropical cyclone tracks and frequencies (Ho et al., 2006). Intraseasonal oscillations in coastal winds over the Benguela Current system have been found by Risien et al. (2004), while links between the wind variability and intraseasonal convection over southern Angola/northern Namibia were found by Hermes and Reason (2009b). Teleconnections on intraseasonal scales also exist between anomalous convection over South America and regional circulation and rainfall over South Africa at all times of the year (Grimm & Reason, 2015).
On the interannual scale, ENSO is the dominant mode in the global climate system and is known to strongly impact on southern African circulation, rainfall, biomass production, and regional SST (Anyamba et al., 2002; Cook, 2001; Lindesay, 1988; Nicholson & Kim, 1997; Reason et al., 2000). El Niño (La Niña) events tend to lead to below- (above-) average rainfall in the summer, or peak and mature phases of ENSO, over southern Africa. Further north, equatorial East Africa tends to experience the opposite signal; i.e., above- (below-) average rains in the OND (short rain season) during El Niño (La Niña) events. There is a nonlinear relationship between the magnitude of the rainfall impacts over southern Africa and the strength of an ENSO event. For example, the 1997/1998 El Niño event was one of the strongest in the last century but it did not lead to the expected severe drought over subtropical southern Africa. Reason and Jagadheesha (2005b) and Lyon and Mason (2007) showed evidence that the lack of a bad drought was due to the fact that the Angola Low (the source of the TTT) was not anomalously weak, unlike during other El Niño events. Landman and Mason (1999) argued that a change in relationship between ENSO and western Indian Ocean SST since the late 1970s led to changes in rainfall impacts over parts of South Africa and Namibia. Richard et al. (2000, 2001) also suggested that the character of the ENSO impact over southern Africa was different in the last few decades of the 20th century than earlier in the century.
In tropical southern Africa, the ENSO signal is transmitted through tropical atmospheric waves that either impact the regional convection directly or lead to changes in SST in the tropical Indian and Atlantic Oceans that then modulate the low-level winds and moisture convergence over the neighboring land and hence rainfall. Interannual tropical SST anomalies in these regions also occur via the Indian Ocean Dipole (Saji et al., 1999) and the Atlantic Niño/Zonal mode (Zebiak, 1993). However, these modes mainly impact on equatorial African rainfall so they are not discussed here. ENSO also influences the mid-latitude South Atlantic and South Indian Oceans (Colberg et al., 2004; Reason et al., 2000).
Another tropical mode of importance for southern Africa is the Benguela Niño, which expresses large SST anomalies off the coast of southern Angola/northern Namibia, thereby disrupting the ABFZ and regional fisheries (Florenchie et al., 2003, 2004; Gammelsrod et al., 1998; Shannon et al., 1986). A connection between SST variability and southwestern African rainfall has long been established (Hirst & Hastenrath, 1983; Nicholson & Entekhabi, 1986; Rouault et al., 2003b), but it appears that the relationship is strongly nonlinear and also sensitive to whether there are concomitant SST anomalies in the southwest Indian Ocean (Hansingo & Reason, 2009; Reason & Smart, 2015).
Influences of southwest Indian Ocean SST on southern African rainfall have been found on intraseasonal through to decadal time scales (Mason & Jury, 1997; Reason & Mulenga, 1999; Washington & Preston, 2006). Boulard et al. (2013) highlighted the importance of SST variability in the South Indian Ocean for the regional impacts of ENSO over southern Africa. A particular pattern, known as the South Indian Ocean subtropical dipole (SIOD), was identified by Behera and Yamagata (2001) and confirmed in AGCM experiments by Reason (2001b, 2002) as being important for late summer rainfall over much of subtropical southern Africa. A similar dipole in South Indian Ocean SST was shown by Washington and Preston (2006) to be responsible for the very wet 1974 and 1976 summers over southern Africa. Fauchereau et al. (2003) and Hermes and Reason (2005) found relationships between the SIOD and similar patterns in the South Atlantic and South Pacific. Both ENSO and the Southern Annular Mode (SAM) may be implicated in these hemispheric dipole patterns. For example, the extratropical atmospheric Rossby waves generated during ENSO propagate as the Pacific South America (PSA) pattern (Mo & Paegle, 2001) toward subtropical southern Africa and directly impact on SST and wind variability in the South Atlantic and southwest Indian Oceans (Colberg et al., 2004).
South of about 30°S, the dominant mode of Southern Hemisphere climate variability on a range of scales is the SAM. This mode leads to hemispheric-scale changes in mid-latitude winds and SST and has been shown to impact on both the winter (Reason & Rouault, 2005) and the summer (Gillett et al., 2006) rainfall regions of South Africa. Further north, there may be interactions between the SAM and the ENSO signals over southern Africa. Wet winters in western South Africa often occur during negative phases of the SAM when there are high-pressure anomalies over Antarctica and low-pressure anomalies in the mid-latitudes leading to a northward expansion of the westerly storm track (Reason & Rouault, 2005).
At lower frequencies, Nicholson (2000) showed evidence of decadal variability in southern African rainfall as well as the levels of Lake Malawi, while Tyson et al. (1975, 2002) found a strong signal near 20-year periodicity in southern African rainfall south of about 15 to 20°S. This decadal signal can be seen in many regions of southern and East Africa over about the last 200 years (Nicholson et al., 2012). There is also a suggestion of a slower-frequency pattern, since most of the 1800 to 1860 period was dry relative to the 1800 to 2000 mean. In terms of the near 20-year signal over parts of South Africa and neighboring countries, earlier work suggested that this rainfall signal could be related to the 18.6-year lunar nodal cycle and associated changes in regional atmospheric circulation and SSTs (Mason & Jury, 1997). More recently, evidence has been shown that it may be the Pacific Decadal Oscillation (Malherbe et al., 2016) or variability on these time scales in ENSO (Reason & Rouault, 2002) or the SAM (Malherbe et al., 2014) that could help explain this roughly bi-decadal rainfall signal. Based on coupled GCM output, Morioka et al. (2015) suggested that eastward advection of SST anomalies in the South Atlantic by the Antarctic Circumpolar Current into the southwest Indian Ocean could also play a role in bi-decadal climate variability over southern Africa. Decadal variations in ENSO were also highlighted by Gaughan and Waylen (2012), who examined rainfall changes since 1950 over the Okavango-Kwando-Zambezi catchment basins of southeastern Angola, western Zambia, and northern Botswana. They attributed the relative dryness of the 1975–1999 period compared to 1950–1974 to more El Niño events in the later period. Another quasi-decadal signal in Southern Hemisphere rainfall, including southern Africa, is related to changes in the midlevel tropical anticyclones, such as the Botswana High, that exist over the Southern Hemisphere land masses in the summer half of the year (Reason, 2016).
The fifth assessment report of the IPCC (2013; hereafter AR5) showed that a warming of about 1°C occurred over southern Africa between 1901 and 2012. Over South Africa, Kruger and Shongwe (2004) found a significant warming trend at most stations between 1960 and 2003, with the trends stronger at central stations than near the coast. Seasonal trends were greatest in autumn and weakest in spring. MacKellar et al. (2014) also found that maximum temperatures have increased significantly for all seasons throughout South Africa. Most areas also showed an increase in minimum temperatures, except for the central interior, where a significant decrease was found. For the arid Namaqualand region of western South Africa, Davis et al. (2016) analyzed CRU TS3.1 data for the last century and five weather stations to find that minimum (maximum) temperatures have increased by 1.4°C (1.1°C) over this period, with a significant increase (decrease) in the number of anomalously warm (cool) days. No significant changes in rainfall amounts, frequency, or intensities were found for this region however. Consistent with Davis et al. (2016), Kruger and Sekele (2013) showed that the western half of South Africa as well as its northeast had greater increase in hot extremes and decrease in cold extremes than elsewhere in the country.
For precipitation between 1951 and 2010, the CRU, GHCN, and GPCC data sets show a weak drying trend in eastern South Africa, Zimbabwe, and Botswana, with a weak wetting trend in western and central South Africa/southern Namibia, but the data sets disagree on the magnitude and the exact distribution. Using GPCP satellite data (post-1979), Jury (2013) noted a weak drying tendency over central southern Africa and a wetting tendency over eastern Madagascar and Tanzania. Over South Africa for 1910–2004, Kruger (2007) found some significant increases (decreases) in annual precipitation over central South Africa and the North West Province (eastern South Africa and the South Coast region) using South African Weather Service station data. Significant increases in heavy rainfall days and daily and pentad precipitation totals were found over the southern Free State/northern Eastern Cape region. Based on data from the South African Weather Service and the Computing Centre for Water Resources, MacKellar et al. (2014) found statistically significant decreases in rainfall and the number of rain days over central and northeastern South Africa in autumn and more rain days in the southern Drakensberg region in spring and summer.
AR5 concluded that it is very likely that all of Africa will continue to warm over the 21st century, and the warming over subtropical southern Africa is expected to be greater than that over equatorial Africa as the subtropical anticyclones are expected to shift poleward and intensify. Using three CMIP3 models, Lyon (2009) suggested that the probability of heat wave occurrence is projected to increase by more than 3.5 times in the next century relative to 1981–2000. In recent decades, the SAM has tended to be more in its positive phase (Marshall, 2003; AR5) leading to more anticyclonic conditions south of about 30 to 35°S. Vizy and Cook (2016) have attributed part of the summer SST warming tendency during 1982–2013 along the Angolan / Namibian coast to a poleward shift in the SAA and strengthening of the heat lows over the interior.
It was also concluded by AR5 that the tropics are expected to get wetter and the subtropics drier, that ENSO will remain the dominant mode of natural climate variability in the 21st century (high confidence), and that it is very likely that the IOD will remain very active during this period. A poleward shift and intensification of the SIA might, however, have the effect of increasing the moisture flux from the southwest Indian Ocean and the frequency and strength of TTTs (Engelbrecht et al., 2009), thereby leading to wetter conditions over subtropical southeastern Africa. Manatsa et al. (2013) also highlighted the role of changes in the SIA, as well as the Angola Low, in early summer warming trends across Angola, southern Zambia, and western Zimbabwe. An earlier study, based on IPCC Assessment Report 3 climate models (Shongwe et al., 2009), suggested that a northeastward shift of the mean position of the TTT cloud bands could be behind the projections of drier conditions in western southern Africa and increased rainfall in Zambia, Malawi, and northern Mozambique. Although biases still exist, regional climate models assessed through CORDEX are able to represent precipitation climates over southern Africa sufficiently accurately for there to be reasonable confidence in future climate projections over the region (Kalognomou et al., 2013). Given the projections of annual rainfall decreases over southern Africa by up to 20% by the 2080s and the resulting likelihood of reduced water availability and crop yields, removing trade barriers and improving the transfers of power and stored waters within the region are therefore a high priority (Conway et al., 2015).
This article provides an overview of understanding (as of 2016) of the weather and climate patterns of Africa south of the Congo Basin (termed southern Africa). With the exception of southwestern South Africa and the south coast, this region is one of mainly summer rainfall and excludes the bimodal rainfall areas of northern Tanzania and Kenya. Southern Africa is characterized by complex topography and is essentially a narrow, peninsular land mass surrounded to its west, east, and south by highly dynamic, variable, and large ocean areas. This geography imprints itself on the weather and climate patterns of the region, leading to sharp gradients in rainfall characteristics, soil moisture, and vegetation across southern Africa, which together with the similarly strong gradients in the SST, circulation, and marine productivity in the neighboring oceans, pose challenges for the application of models, whether they be for seasonal prediction or climate change projection. A further challenge to better understanding the regional climates, their variability, and change is that southern Africa receives climate signals from a variety of sources on a range of time scales. Substantial evidence exists for the importance of the tropical Indo-Pacific (ENSO), the tropical South Atlantic (Benguela Niño), the South Indian Ocean (SIOD), and the Southern Ocean (SAM) as well as remote land-based processes, such as the South American monsoon, for variability in temperature and rainfall characteristics across southern Africa. Although the existence of such complexity and multiple climatic influences can slow progress in understanding, it has the advantage of providing a very useful test-bed for improving climate models as well as continuing to make the study of southern African climate interesting and intellectually rewarding.
NOAA and the KNMI are thanked for providing the freely accessible online data and plotting tools used to produce some of the figures. The author is also grateful to the 2016 BSc (Honours) class in the Department of Oceanography, University of Cape Town, for suggestions and for proof-reading the manuscript.
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