Subtropical Southern Africa Research Articles

Late Quaternary hydroclimate variability in Madagascar and its connection to atmospheric circulation patterns

Large uncertainties still exist about the long-term mechanisms influencing the hydroclimate variability of southeast Africa where proxy data and model simulations indicate rainfall dipoles between subtropical and tropical areas. The topography of Madagascar, located off the southeastern coast of Africa, modulates these dipoles while its climate is influenced by the position of the Intertropical Convergence Zone (ITCZ) and the Subtropical High as well as the sea surface temperature (SST) of SW Indian Ocean. The island can thus be considered a key location for the understanding of the tropical SE African climatic variability and the interplay between atmospheric patterns. However, the scarcity of continuous records from Madagascar has made the evolution of regional late Quaternary climate and its driving mechanisms difficult to assess. Here, we present a 26-kyr record of the deuterium/hydrogen isotope ratio (δD) of biomarkers (n-alkanes) from the central eastern part of the island at Antananarivo at around 1250 m a.s.l. Preliminary summary pollen data are also presented as a comparison. The δD profiles of aquatic plant and terrestrial plant-derived n-alkanes generally exhibit similar trends implying that they all record changes in the isotope composition of source water, namely meteoric water that recharges soil and lake waters. In this tropical region, the δD variability of precipitation is mainly influenced by the amount effect reflecting the intensity of precipitation associated with the monsoon. We observe: (i) stable and wet conditions during the Last Glacial Maximum, (ii) drier conditions from 18.5 to 15 ka (ka before present) during the Heinrich Stadial 1 (iii) high humidity after 15 ka culminating at the Younger Dryas (YD), (iv) drier conditions from 11.7 ka to 8.2 ka, (v) a return to humid climate until 2.8 ka, and (vi) an arid phase followed by increased wetness after 0.9 ka, although the record is likely influenced by human-induced vegetation changes the last 1.2 ka. This climate signal is similar to other records from the Mozambique Channel but opposite to records from the East African mainland and the subtropical southern Africa, especially between 20 and 25°S. Although there is a good correspondence of our record with insolation- driven migrations of ITCZ during the LGM and the early Holocene, the dipoles are largely consistent with the modern rainfall anomaly and are best explained by the interlinked effects of the SST changes and the variability of the Mozambique Channel Trough.

The role of drylines in thunderstorm initiation and record rainfall during an anomalously convective early summer season in South Africa

An average of 12 drylines are associated with thunderstorm development (convective drylines) during December in the western plateau of subtropical southern Africa. During December 2021, 20 convective drylines occurred. This was the highest number since the record began in 2010. December 2021 was also unusually wet over a large part of the summer rainfall region in South Africa. Monthly rainfall records were broken in the Northern Cape, Northwest and Free State provinces. Above-average rainfall had dire consequences for the maize industry over the central interior. The large-scale circulation during December 2021 was very conducive to dryline formation over the western plateau with a deeper than usual surface trough and strong low-level northeasterly moisture flux towards the climatologically dry western plateau region. Above normal wind speeds of mid-tropospheric northwesterlies and large-scale uplift contributed to a favorable environment for strong thunderstorms, as seen during a 6-day convective dryline event. Thunderstorms which were triggered along the dryline travelled east and contributed to the above normal rainfall observed during December 2021. Dryline and rainfall frequencies show large inter-annual variability. However, a positive Pearson correlation (r = 0.45; p < 0.05) between the number of dryline days and rainfall amounts were found between 1979 and 2021. This correlation increased (r = 0.53; p < 0.05) when rainfall amounts immediately east of the western plateau domain were also included, suggesting that dryline-related convection can have a significant influence on rainfall over the western plateau and further downstream.

A Climatology of Dryline‐Related Convection on the Western Plateau of Subtropical Southern Africa

AbstractDrylines occur regularly during summer over the interior of sub‐tropical southern Africa. The highest frequency of drylines is over the western interior where it is also climatologically much drier than the eastern parts. An existing drylines data set is compared with lightning data between 2010 and 2021, to determine how often drylines were associated with convective storms. A minimum stroke threshold is applied to eliminate instances of weak convection. Seventy‐six percent of drylines during austral summer (October–March) were associated with significant convection with a standard deviation of 8.6. There is a peak frequency of 12 convective dryline days during December. Composite synoptic analyses were conducted for 591 convective and 186 non‐convective dryline days to determine the ideal formation environment for convective drylines. Composites were also created for the days leading up to convective and non‐convective drylines, showing the importance of moisture advection from the southwest Indian Ocean, through the Zambezi and Limpopo River Valleys. Early summers tend to be drier than late summers over the southern African interior and colder in the mid‐levels. These differences affect the formation environment of convective drylines. Composites show that dryline‐related convection during early summer tends to occur on days with a deep surface trough and a trough at 500 hPa further west. Convection during late summer is associated with increased surface moisture, higher instability, and a weaker trough at 500 hPa than during early summer.

Diversity and Ranking of ENSO Impacts along the Eastern Seaboard of Subtropical Southern Africa

El Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate variability over southern Africa during the summer half of the year. It is widely accepted that El Niño (La Niña) core summers (December–February) are typically warmer and drier (cooler and wetter) than average over the region. Although it is recognized that the ENSO impacts are nonlinear and not all events result in the expected impact, little or no work has been carried out to systematically explore the diversity and ranking of these impacts. Here, parameter-space bubble plots involving various rainfall and temperature metrics are used to study how such impacts vary over the eastern seaboard of subtropical southern Africa to determine the ENSO events with the strongest impacts, and to identify the most anomalous ENSO cases. Comparison of neutral summers experiencing the strongest droughts/floods with ENSO impacts is also performed. These metrics are designed to be applicable to the interests of farmers and other user groups. It is found that 1987/1988 (2017/2018) was the most unusual El Niño (La Niña) and neutral 1981/1982 had a severe drought, worse than occurs during most El Niños. These unusual cases are explained in terms of regional circulation and SST anomalies. Implications of the results for seasonal forecasting and for farmers are discussed.

Microscopic charcoals in ocean sediments off Africa track past fire intensity from the continent

Fires in Africa account for more than half of global fire-carbon emissions but the long-term evolution of fire activity and its link to global climate change remains elusive. Paleofire records provide descriptive information about fire changes through time, going beyond the range of satellite observations, although fire regime characteristics are challenging to reconstruct. To address this conceptual gap, we report here the abundance and morphometric data for a large set of microscopic charcoal samples (n = 128) recovered from surface ocean sediments offshore Africa. We show that in subtropical Southern Africa, large and intense fires prevailing in open savanna-grassland ecosystems produce a high abundance of small and elongated microcharcoal particles. In contrast, in the forest ecosystems of equatorial and tropical regions of western and central Africa, low-intensity fires dominate, producing low amounts of squared microcharcoal particles. Microcharcoal concentration and morphotype in marine sediment records off Africa are thus indicative of fire regime characteristics. Applied to down-core marine charcoal records, these findings reveal that at orbital time-scale intense and large, open grassland-savanna fires occurred during wet periods in the sub-tropical areas. A strong contribution of fire carbon emissions during periods of precession and summer insolation maxima in the geological record is thus expected.

A Climatology of Drylines in the Interior of Subtropical Southern Africa

Abstract A climatology of synoptic drylines on the subtropical southern African interior plateau (SAP) is developed using ERA5 reanalysis specific humidity and surface temperature gradients and an objective detection algorithm. Drylines are found to occur regularly during spring and summer (September–March), and almost daily during December of that period, but rarely in winter. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November with a mean of 10 drylines and then over the central (mean of 12) and western SAP (mean of 20) in December. During midsummer, drylines over the eastern SAP are negatively correlated with drylines in the west. Between 1980 and 2020, a significant correlation exists between ENSO and dryline days over the eastern (r = 0.44; p value = 0.004) and central (r = 0.41; p value = 0.008) SAP with fewer drylines (up to 10) occurring during years with increased surface moisture and more drylines (up to 45) occurring during years with decreased surface moisture. Drylines forming over the eastern parts of the SAP were more likely to move westward than drylines over the central and western parts. Onset times across the SAP show that drylines have a tendency to form during either the late morning to early afternoon (1100 and 1400 LST) or during the early evening hours (1700 and 2000 LST), suggesting that the surface heat trough (Kalahari heat low) and westward moisture transport mechanisms, such as the Limpopo low-level jet and ridging highs, are responsible for the formation of most drylines across the SAP. Significance Statement “Drylines” are used to describe boundaries separating regions of very dry air from those with much higher moisture content. The importance of these drylines is that they tend to act as a trigger for thunderstorms, which can produce severe weather. In this study, we build a long-term climatological description of drylines in subtropical southern Africa. We find that drylines are most frequent over eastern South Africa during the early summer, a time when storms with large hail and damaging winds are most likely to occur. Drylines are sensitive to moisture circulation patterns and respond differently during El Niño and La Niña years, with generally more drylines during El Niño over eastern South Africa and fewer during La Niña.

How well does MPAS-atmosphere simulate the characteristics of the Botswana High?

The Botswana High is a prominent mid-tropospheric system that modulates rainfall over subtropical southern Africa, but the capability of a global climate model (GCM) to reproduce it remains unknown. This study examines the capability of a GCM with quasi-uniform resolution (Model Prediction Across Scales, hereafter MPAS) in simulating the characteristics of the Botswana High. The MPAS is applied to simulate the global climate at 240 km quasi-uniform resolution over the globe for the period 1980–2010. The model results are validated against gridded observation dataset (Climate Research Unit, CRU), satellite dataset (Global Precipitation Climatology Project, GPCP), and reanalysis datasets (Climate Forecast System Reanalysis, CFSR; the National Oceanic and Atmospheric Administration, NOAA; and the European Centre for Medium-Range Weather Forecasts version 5, ERA5). In general, MPAS replicates all the essential features in the climatology of temperature, rainfall, 500 hPa geopotential height and vertical motion over southern Africa, reproduces the spatial and temporal variation of the Botswana High, and captures the influence of the Botswana High on droughts and deep convections over the sub-continent. In addition, the model reproduces well the anomalies in vertical motion over subtropical southern Africa during +ve and −ve phases of the Botswana High. However, the model struggles to reproduce the precipitation pattern associated with the positive and negative modes of the Botswana High. The results of this study have an application in understanding the characteristics of the Botswana High and in improving MPAS for seasonal forecasting over southern Africa.

Role of ocean mesoscale structures in shaping the Angola-Low pressure system and the southern Africa rainfall

Southern African climate is under the influence of both tropical and subtropical systems which result in a complex region where important interactions co-exist over a large spectrum of spatiotemporal scales. The Angola Low (AL), situated on boundary between tropical and subtropical southern Africa, has been diagnosed as a key driver of moisture distribution in the region on daily to seasonal time scales. It has been demonstrated that the AL pressure system is sensitive to the dynamics of the neighbouring oceans, but to date no study has considered the model resolution of air–sea interactions required to simulate this sensitivity. Using sensitivity experiments with a regional atmospheric model, which differ only in the mesoscale sea surface temperature (SST) forcing characteristics (either the full spectrum of SST variability or only its large-scale components are included), we first quantify the importance of SST gradients on the AL strength and variability. The results suggest that the mesoscale SST variability of the Angola–Benguela Frontal Zone (ABFZ) plays a key role in AL activity, particularly during the late summer. Synoptic-scale tropical lows, which form the AL, are automatically detected, and the results suggest more extreme events occur when the model is forced by mesoscale SSTs (everywhere and in the ABFZ area only). The rainfall resulting from those events suggests that tropical-low episodes are associated with nearly 15% of the total rain in Angola and Namibia. The link between AL dynamics and wet spells is also discussed, with the former showing a different spatial pattern as well as frequency when the ocean is fully resolved.

Potential Geographic Distributions of Two Parthenogenetic Weevils (Coleoptera: Curculionidae) Associated with Citrus in Argentina and Brazil

Parapantomorus fluctuosus (Boheman) and Pantomorus cinerosus (Boheman) (both Coleoptera: Curculionidae) are parthenogenetic broad-nosed weevils native to South America, and they damage citrus in Argentina and Brazil. The objectives of this contribution are to apply ecological niche modelling to compare the suitable areas of occurrence of these species with those of other parthenogenetic Naupactini distributed in similar areas, and to analyze their potential range in relation to the distribution of citrus groves, mainly in Argentina and Brazil. Pantomorus cinerosus and P. fluctuosus were predicted to be distributed in tropical and subtropical South America, Mexico, Central America, Cuba, southeastern USA (Florida), southern Africa, Madagascar, eastern Australia, and southern China. However, neither has colonized other countries outside the native range. The potential distribution of P. fluctuosus is broader than that of P. cinerosus, and the predicted areas of both species are partially coincident with main production areas of citrus in Argentina, Brazil, and throughout the world. Although ecological niche modelling is an effective tool for predicting the potential spread and colonization of weevils in distant cultivated areas, weevil establishment is most likely the result of multiple factors.

First palynological results of accessorial elements from the Langhian Schaßbach clay pit, Lavanttal Basin (Austria) – LM and SEM investigations of Cornales and Ericales

Preliminary results of the palynoflora from the Schaßbach clay pit comprise accessorial floral elements of three Cornales and five Ericales pollen taxa that have modern living equivalents from different continents. The eight taxa are relatively rare in the palynological assemblages and are affiliated with the genera Nyssa, Mastixia, Erica, Vitellariopsis, Rehderodendron and Polyspora. Nyssa sp. resembles closely the deciduous Nyssa sylvatica of temperate America, the two Erica spp. resemble the south European tree heathers Erica arborea and Erica lusitanica, and Vitellariopsis sp. looks like evergreen Vitellariopsis marginata from subtropical southern Africa. The living equivalents of Mastixia spp., Rehderodendron sp. and Polyspora sp. are evergreen taxa from subtropical and tropical Asia.

Controls on the Diversity in Climate Model Projections of Early Summer Drying over Southern Africa

Abstract Ninety-five percent of climate models contributing to phase 5 of the Coupled Model Intercomparison Project (CMIP5) project early summer [October–December (OND)] rainfall declines over subtropical southern Africa by the end of the century, under all emissions forcing pathways. The intermodel consensus underlies the Intergovernmental Panel on Climate Change (IPCC) assessment that rainfall declines are “likely” and implies that significant climate change adaptation is needed. However, model consensus is not necessarily a good indicator of confidence, especially given that there is an order of magnitude difference in the scale of rainfall decline among models in OND (from &amp;lt;10 mm season−1 to ~100 mm season−1), and that the CMIP5 ensemble systematically overestimates present-day OND precipitation over subtropical southern Africa (in some models by a factor of 2). In this paper we investigate the uncertainty in the OND drying signal by evaluating the climate mechanisms that underlie the diversity in model rainfall projections. Models projecting the highest-magnitude drying simulate the largest increases in tropospheric stability over subtropical southern Africa associated with anomalous upper-level subsidence, reduced evaporation, and amplified surface temperature change. Intermodel differences in rainfall projections are in turn related to the large-scale adjustment of the tropical atmosphere to emissions forcing: models with the strongest relative warming of the northern tropical sea surface temperatures compared to the tropical mean warming simulate the largest rainfall declines. The models with extreme rainfall declines also tend to simulate large present-day biases in rainfall and in atmospheric stability, leading the authors to suggest that projections of high-magnitude drying require further critical attention.

Stronger Local Overturning in Convective‐Permitting Regional Climate Model Improves Simulation of the Subtropical Annual Cycle

AbstractGlobal climate models fail to represent the annual cycle of tropical‐extratropical cloud bands and produce too much summer rainfall over subtropical southern Africa. This study demonstrates that running a regional convective‐permitting climate simulation alleviates these biases, counteracting biases that are present in the parent model. The improvement emerges from stronger vertical mass flux in the tropics, which forces a stronger local Hadley overturning into the summer hemisphere. This enhanced overturning increases upper‐level subsidence in the subtropics and amplifies the forcing of the local subtropical jet. Together, these improvements halve the wet subtropical rainfall bias and are associated with a 50% increase to an 80% match between the simulated and observed annual cycle of crucial tropical‐extratropical cloud band rainfall systems. The results advocate for the increased use of convective‐permitting climate models with domains that include regional tropical convection hot spots, in order to fully benefit from the explicit representation of deep convection.

Low-frequency variability in the Botswana High and southern African regional climate

Decadal variability in the Botswana High and its relationships with rainfall, temperature and circulation over the southern African region are examined. This High exists in the mid-troposphere over subtropical southern Africa from early spring through to early autumn. Although its seasonal and interannual variability have been previously studied, lower frequency variability in the Botswana High has not. An EOF analysis of 500 hPa geopotential height from the NCEP Twentieth Century reanalyses reveals two modes both of which display substantial decadal to multidecadal variability. These modes, which explain almost 90% of the variance, reflect shifts in the summer core position of the High over central Namibia / western Botswana and in its strength. This decadal variability in the Botswana High is associated with sizeable changes in regional rainfall and surface temperature as well as changes in the onshore flow of moist marine air and uplift over many parts of southern Africa. Such changes are large enough to have important impacts on regional society and may also obscure climate change signals over the region.

Systematic Climate Model Rainfall Biases over Southern Africa: Links to Moisture Circulation and Topography

AbstractAn important challenge for climate science is to understand the regional circulation and rainfall response to global warming. Unfortunately, the climate models used to project future changes struggle to represent present-day rainfall and circulation, especially at a regional scale. This is the case in southern Africa, where models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) overestimate summer rainfall by as much as 300% compared to observations and tend to underestimate rainfall in Madagascar and the southwest Indian Ocean. In this paper, we explore the climate processes associated with the rainfall bias, with the aim of assessing the reliability of the CMIP5 ensemble and highlighting important areas for model development. We find that the high precipitation rates in models that are wet over southern Africa are associated with an anomalous northeasterly moisture transport (~10–30 g kg−1 s−1) that penetrates across the high topography of Tanzania and Malawi and into subtropical southern Africa. This transport occurs in preference to a southeasterly recurvature toward Madagascar that is seen in drier models and reanalysis data. We demonstrate that topographically related model biases in low-level flow are important for explaining the intermodel spread in rainfall; wetter models have a reduced tendency to block the oncoming northeasterly flow compared to dry models. The differences in low-level flow among models are related to upstream wind speed and model representation of topography, both of which should be foci for model development.

The Angola Low: relationship with southern African rainfall and ENSO

The main states of the Angola Low (AL) are identified using clustering analysis applied to daily anomalous patterns of 700-hPa wind vorticity over Angola and adjacent countries from November to March for the 1980/81–2014/15 period. At the daily timescale, we examine the extent to which the main states of the AL modulate daily rainfall over southern Africa. At the interannual timescale, we assess both the relationship between the occurrence of these AL states and El Nino southern oscillation (ENSO) and the role of the AL in explaining ENSO’s failure in driving southern African rainfall at times. Three reanalyses are considered to account for uncertainties induced by the scarcity of data available for assimilation over southern Africa. Three preferential states of the Angola Low are identified: AL state close to its seasonal climatology with slight zonal displacements, anomalously weak AL state and anomalously strong AL state with meridional displacements. These different states all significantly modulate daily southern African rainfall. Near-climatological AL state promotes wet rainfall anomalies over eastern subtropical southern Africa and dry rainfall anomalies over its western part. A slight westward shift in the near-climatological position of the AL leads to reversed zonal gradient in rainfall. The remaining regimes significantly modulate the meridional gradient in southern African rainfall. Anomalously weak and anomalously northward AL states promote wet rainfall anomalies over tropical southern Africa and dry rainfall anomalies over subtropical southern Africa. The reverse prevails for anomalously southward AL. At the interannual timescale, ENSO significantly modulates the seasonal occurrence of most AL states in the three reanalyses. Anomalously weak and southward AL states are more strongly correlated with regional rainfall than ENSO in all reanalyses, suggesting that accounting for AL variability may improve seasonal forecasts. Case study analysis of the major 1982/83 and 1997/98 El Nino events suggests that the weak rainfall anomalies and strong seasonal AL in 1997/98 may result from counteracting effects between ENSO and Indian Ocean coupled modes of variability.

Invasion of the guttural toads

The Western Cape region of South Africa is a modern Garden of Eden packed with indigenous species spread across two globally recognised biodiversity hotspots. So, when the snore-like call of invasive guttural toads (Sclerophrys gutturalis) threatening to displace native populations was first heard in Cape Town in 2000, alarm bells began ringing. Giovanni Vimercati from Stellenbosch University, South Africa, says, ‘The ability of this species to invade Cape Town is surprising’, as guttural toads are usually better suited to the more humid and warmer environments of subtropical southern Africa than the cooler, dry Mediterranean climate of the Western Cape. Yet, despite the mismatch, the population flourished. ‘As the invading population expanded, individuals would need to cope with the new climate by rapidly adapting to their drier cooler surroundings’, says Vimercati; and they have done so remarkably quickly. Concerned by the toads’ success, Vimercati and his PhD supervisors, John Measey and Sarah Davies, decided to investigate how the invaders’ ability to withstand dehydration had altered during the two decades following the occupation.Explaining that it is relatively easy to find the toads by torchlight as they congregate after dark around ponds during the breading season, Vimercati says that the main problem was the reluctance of homeowners in Cape Town and Durban to allow the scientists into their gardens. ‘They and their dogs were not always enthusiastic’, chuckles Vimercati. As soon as the toads were captured, the trio measured how dehydrated the animals were by popping them into a shallow tub of water and weighing them until they absorbed no more. Not surprisingly, the Cape Town toads were more dehydrated (∼90% hydrated) than the toads from Durban (∼95% hydrated), and when the team blew a gentle breeze over the fully hydrated animals in the lab, the two groups lost water at roughly the same rates, so the invaders had not improved their waterproofing. However, the team noticed that the Cape Town toads hunkered down more, tucking their legs in beneath their body and pulling their head down close to the ground in a bid to conserve water. The invaders were also better at reabsorbing water that had been lost; they were better equipped to cope with water losses. But how did the dehydration affect their performance?‘I spent hours running round in small circles after reluctant toads’, laughs Vimercati, as he describes chasing the dehydrated animals (90% and 80% hydration) around a small racetrack in the dead of night to find out how well the Cape Town invaders and their Durban cousins coped when they were low on water. Impressively, the invaders were more robust than their indigenous cousins, with five of the 90% hydrated animals covering distances of over 500 m, in contrast to the Durban toads, which only managed an average distance of 250 m. And when the team checked how well the animals coped with colder conditions by gently chilling them until they could no longer right themselves after toppling over, the toads from cooler Cape Town were able to return to a squatting position at temperatures down to 7.1°C, while the Durban animals were only able to recover at temperatures above 8.2°C.‘Although they've only been there for 20 years, invading toads in Cape Town are better suited than their native cousins to survive and reproduce in the new surroundings’, says Vimercati and he warns ‘These kinds of rapid adaptations may allow guttural toads to invade climates quite unlike their native conditions, making it hard to predict their potential success in new areas’.

Mature Larval Dispersal and Adult Emergence of the Economically Significant Pest, Scirtothrips aurantii Faure (Thysanoptera: Thripidae), in Commercial Citrus.

Scirtothrips aurantii Faure (Thysanoptera: Thripidae) is a major pest of citrus fruit in subtropical southern Africa. Population monitoring is an important aspect of S. aurantii control, but additional information is required on its phenology. Dispersal of mature larvae onto the soil surface from the tree canopy, and emergence of adults, were assessed using dispersal/emergence (D/E) traps in an untreated citrus orchard in South Africa. Overall, 90.7% of adult Thysanoptera emerging from soil / leaf litter beneath the citrus trees were S. aurantii, of which 35.7% were males, and 64.3% were females. Female S. aurantii having survived winter as adults oviposited on the early spring flush. This resulted in the first population peak of larvae dropping to the ground to pupate and adults emerging in spring, September to early October, as the first vegetative flush of the citrus-growing season hardened, and fruit was set as blossoming ended. Initial infestation of young fruit occurred after a build-up of larval numbers on the soft citrus flush late July to early August. The second generation larval and adult peak occurred late November to early December, whether there was new flush or not, as young fruit could support the S. aurantii population. A third peak occurred mid-December to late January, depending on year. Thus, there were three generations of S. aurantii during the period of citrus fruit susceptibility to thrips damage (September–January). After the autumn flush in April, another peak of larvae and adults occurred before the population declined to a minimum from May to July.

Biomarker records of palaeoenvironmental variations in subtropical Southern Africa since the late Pleistocene: Evidences from a coastal peatland

Southern Africa's unique global position has given rise to a dynamic climate influenced by large sea surface temperature gradients and seasonal fluctuations in the Inter Tropical Convergence Zone. Due to the semi-arid climate of the region, terrestrial palaeorecords are rare and our understanding of the long-term sensitivity of Southern African terrestrial ecosystems to climatic drivers is ambiguous. A 810cm continuous peat core was extracted from the Mfabeni peatland with a 14C basal age of c. 47thousandyears calibrated before present (kcalyr BP), positioning it as one of the oldest known sub-tropical coastal peatlands in Southern Africa. This peat core provides an opportunity to investigate palaeoenvironmental changes in subtropical Southern Africa since the late Pleistocene. Biomarker (n-alkane, n-alkanoic acid and n-alkanol) analysis, in conjunction with previously published bulk geochemical data, was employed to reconstruct organic matter (OM) sources, rates of OM remineralisation and peatland hydrology. Our results showed that the principal OM source into the peatland was emergent and terrestrial plants with exception of shallow lake conditions when submerged macrophytes dominated (c. 44.5–42.6, 29.7, 26.1–23.1, 16.7–7.1 and 2.2kcalyr BP). n-Alkane proxies suggest that local plant assemblages were predominantly influenced by peatland hydrology. By incorporating temperature sensitive n-alkanoic acid and n-alkanol proxies, it was possible to disentangle the local temperature and precipitation changes. We report large variations in precipitation intensities, but subdued temperature fluctuations during the late Pleistocene. The Holocene period was characterised by overall elevated temperatures and precipitation compared to the preceding glacial period, interspersed with a millennial scale cooling event. A close link between the Mfabeni archive and adjacent Indian Ocean marine core records was observed, suggesting the regional ocean surface temperatures to be the dominant climate driver in this region since the late Pleistocene.

Projections of rapidly rising surface temperatures over Africa under low mitigation

An analysis of observed trends in African annual-average near-surface temperatures over the last five decades reveals drastic increases, particularly over parts of the subtropics and central tropical Africa. Over these regions, temperatures have been rising at more than twice the global rate of temperature increase. An ensemble of high-resolution downscalings, obtained using a single regional climate model forced with the sea-surface temperatures and sea-ice fields of an ensemble of global circulation model (GCM) simulations, is shown to realistically represent the relatively strong temperature increases observed in subtropical southern and northern Africa. The amplitudes of warming are generally underestimated, however. Further warming is projected to occur during the 21st century, with plausible increases of 4–6 °C over the subtropics and 3–5 °C over the tropics by the end of the century relative to present-day climate under the A2 (a low mitigation) scenario of the Special Report on Emission Scenarios. High impact climate events such as heat-wave days and high fire-danger days are consistently projected to increase drastically in their frequency of occurrence. General decreases in soil-moisture availability are projected, even for regions where increases in rainfall are plausible, due to enhanced levels of evaporation. The regional dowscalings presented here, and recent GCM projections obtained for Africa, indicate that African annual-averaged temperatures may plausibly rise at about 1.5 times the global rate of temperature increase in the subtropics, and at a somewhat lower rate in the tropics. These projected increases although drastic, may be conservative given the model underestimations of observed temperature trends. The relatively strong rate of warming over Africa, in combination with the associated increases in extreme temperature events, may be key factors to consider when interpreting the suitability of global mitigation targets in terms of African climate change and climate change adaptation in Africa.

Decadal Rainfall Dipole Oscillation over Southern Africa Modulated by Variation of Austral Summer Land–Sea Contrast along the East Coast of Africa

Abstract A rainfall dipole mode characterized by negative correlation between subtropical southern Africa and equatorial eastern Africa is identified in instrumental observation data in the recent 100 years. The dipole mode shows a pronounced oscillation signal at a time scale of about 18 years. This study investigates the underlying dynamical mechanisms responsible for this dipole pattern. It is found that the southern African rainfall dipole index is highly correlated to the land–sea contrast along the east coast of Africa. When the land–sea thermal contrast strengthens, the easterly flow toward the continent becomes stronger. The stronger easterly flow, via its response to east coast topography and surface heating, leads to a low pressure circulation anomaly over land south of the maximum easterly flow anomalies and thus causes more rainfall in the south. On a decadal time scale, an ENSO-like SST pattern acts to modulate this land–sea contrast and the consequent rainfall dipole. During a “wet in the south and dry in the north” dipole, there are warm SSTs over the central Indian Ocean and cold SSTs over the western Indian Ocean. The cold SSTs over the western Indian Ocean further enhance the land–sea contrast during austral summer. Moreover, these cold western Indian Ocean SSTs also play an important role in regulating land temperature, thereby suppressing clouds and warming the land via increased shortwave radiation over the less-cloudy land. This cloud–SST coupling acts to further strengthen the land–sea contrast.