P. D. Tyson

A 3000-year high-resolution stalagmitebased record of palaeoclimate for northeastern South Africa

High-resolution stable isotope variations and growth structure analyses of the last three millennia of a 6600-year stalagmite record at Cold Air Cave, Makapansgat Valley, South Africa, are presented. Growth layers, which are measurable over the last 250 years, are shown to be annual. The correlation between the width of growth layers and precipitation is strongly positive. Changes inδ18O andδ13C are positively correlated and inversely correlated to changes in the colour of the growth layers in the stalagmite. Variations in colour are directly correlated with mean annual temperature. Dark colouration is the product of increased temperature and mobilization of organic matter from the soil, and is associated with wetter summers and enhanced growth of C4 grasses. Darker colouring and enrichedδ18O andδ13C reflect a warmer, wetter environment, whereas lighter colouring and depleted isotopic values are indicative of cooler, drier conditions. The dominant episode in the 3000-year record is the cool, dry 500-year manifestation of the‘Little Ice Age’, from ad 1300 to about 1800, with the lowest temperatures at around ad 1700. The four centuries from ad 900 to 1300, experiencing above-average warming and high variability, may be the regional expression of the medieval warming. Other cool, dry spells prevailed from around ad 800 to 900 and from about ad 440 to 520. The most prolonged warm, wet period occurred from ad 40 to 400. Some extreme events are shown to correspond well with similar events determined from the Greenland GISP2 ice-core record and elsewhere. Distinct periodicities occur within the record at around 120, 200–300, 500–600 and at about 800 years BP.

The application of a present-day climatic model to the late quaternary in southern Africa

A present-day climatic model is presented in which extended wet spells of near-decadal duration and dry spells of similar length are explained on the basis of surface and upper tropospheric circulation variations. Wet spells are shown to be the result of increased tropical atmospheric disturbances and tropical-temperate interaction, and to be linked to variations in the Walker Circulation. Conversely, dry spells are shown to result from diminished tropical activity over southern Africa, equatorward movement of westerly storm tracks and temperate perturbations in the westerlies.The present-day analogue is compared to preliminary spatial reconstructions of the climate of southern Africa over the last twenty-five millennia and is shown to have wide applicability in the explanation of the late-Quaternary palaeoclimates of the subcontinent. In particular, it is argued that the Last Glacial Maximum was associated with northward-displaced circulation conditions similar to those of present-day dry spells over the summer rainfall region, whereas the extensive moist conditions that prevailed for several thousand years after 9000 BP were analogous to present-day wet spell conditions with little apparent displacement of major circulation features.

AN AIR TRANSPORT CLIMATOLOGY FOR SUBTROPICAL SOUTHERN AFRICA

An air transport climatology is derived for subtropical southern Africa (Africa south of 15°S) by classifying daily synoptic situations into predominant circulation types. The annual variation of these provides the basis for determining month-by-month transport. Percentage zonal transport in easterly and westerly directions, levels of transport, and times of transit are derived from forward trajectory analyses using European Centre for Medium- range Weather Forecasts (ECMWF) data for a 7-year period. It is shown that semi-permanent subtropical continental anticyclones, transient mid-latitude ridging anticyclones and mid-latitude westerly disturbances produce major transport into the south-western Indian Ocean in the Natal plume. Only quasi-stationary tropical easterly waves result in appreciable transport into the tropical South Atlantic Ocean in the Angolan plume. Total transport is a function of circulation type and frequency, as well as plume dimensions. Transport in continental highs follows an annual cycle reaching peak values in excess of 70 per cent in winter. That in easterly waves also exhibits an annual cycle, but one peaking in summer, when up to 55 per cent transport may occur in north-western regions. Transport in ridging highs and westerly perturbations is much less and occurs throughout the year, with a slight tendency to peak in spring. Recirculation of air is shown to be considerable when anticyclonic conditions prevail. Monthly, seasonal, and annual mass fluxes over and out of southern Africa are determined from transport fields, frequency of occurrence of circulation types and from measurements of aerosol concentrations. An annual mass flux of aerosols some 134 Mtons is generated over the subcontinent. About 60 Mtons year−1 are deposited, and approximately 29 Mtons year−1 are exported westward over the Atlantic Ocean and 45 Mtons year−1 eastward over the Indian Ocean. Twenty-six million tons of the 74 Mtons of aerosols exported annually to the adjacent oceans on each coast are a product of recirculation. Deposition within 10° latitude of the coast is nearly 10 times greater on the east than on the west coast.

Rapid climate shifts in the southern African interior throughout the Mid to Late Holocene

A detailed climate proxy record based on δ18O, δ13O, and grey index of a well-dated stalagmite from Cold Air Cave in the Makapansgat Valley of north-eastern South Africa suggests that regional precipitation, temperatures and vegetation oscillated markedly and rapidly over the last ∼6500 years on centennial and multi-decadal scales. The mid-Holocene prior to 5200 years ago was humid and warm. A fundamental transition occurred 3200 years ago, leading to drier and cooler conditions that culminated at 1750 AD. Comparisons with ice core records suggest synchronous changes implicating rapid global teleconnections.

Statistical downscaling of GCM simulations to Streamflow

Abstract A multi-tiered forecast procedure is employed to simulate real-time operational seasonal forecasts of categorized (below-normal, near-normal and above-normal) streamflow at the inlets of twelve dams of the Vaal and upper Tugela river catchments in South Africa. Forecasts are made for the December to February (DJF) season over an 8-year independent period from 1987/1988 to 1994/1995. A physically based model of the atmosphere system, known as a general circulation model (GCM), is used to simulate atmospheric variability over southern Africa, the output of which is statistically downscaled to streamflow. The GCM used is the COLA T30, and is forced at the boundary with predicted monthly-mean global sea-surface temperatures. The monthly-mean sea-surface temperature fields are first predicted over lead-times of several months using a canonical correlation analysis (CCA) model. GCM simulations are then obtained for an area including most of southern Africa and adjacent oceans. The GCM simulations are downscaled to catchment level from coarse resolution gridded climate variables, using a perfect prognosis approach: bias-corrected GCM simulations are substituted into the perfect prognosis equations to provide the downscaled categorized streamflow forecasts. Although surface characteristics of each catchment that affect the variability of streamflow are not considered in the proposed downscaling system, successful forecasts of streamflow categories were obtained for some of the years forecast independently. The schemes operational utility is thus demonstrated, albeit over short lead-times.

CHANGING GRADIENTS OF CLIMATE CHANGE IN SOUTHERN AFRICA DURING THE PAST MILLENNIUM: IMPLICATIONS FOR POPULATION MOVEMENTS

Climates of equatorial East Africa and subtropical Southern Africa have varied inversely over long periods of time. The high-resolution δ 18O stalagmite record from Cold Air Cave in the Makapansgat valley in South Africa and a similar resolution lake-level record for Lake Naivasha in Kenya have been in anti-phase for much of the last thousand years. A similar relationship is evident in the twentieth century meteorological record. The changes in rainfall in the two regions on multi-decadal to centennial scales have influenced both settlement patterns and livelihoods of Iron Age agriculturalists. The resulting latitudinal gradient of change may have been a significant factor in promoting southward migration of Sotho-Tswana speaking people from equatorial East Africa during the first few centuries of the last millennium and earlier. This would have occurred at times when environments in the north were deteriorating and those to the south were ameliorating.

Circulation changes and teleconnections between glacial advances on the west coast of New Zealand and extended spells of drought years in South Africa

ABSTRACTTwentieth century changes in the terminal position of the Franz Josef Glacier on the west coast of New Zealand are comparedwith an area-averaged mean annual rainfall series for the summer rainfall region of South Africa. Distinctive teleconnectionsare evident in an out-of-phase relationship between the two series, each of which exhibits an oscillation of 18–20 years.Periods of glacial advance are shown to coincide with extended dry spells in South Africa, when drought years are prevalent.Reconstructed pressure anomaly fields are presented for periods of advance and recession of the glacier and for the inter-decadal wet and dry spells in South Africa. In both cases major regional atmospheric circulation adjustments take place in aquasi-regular fashion over time to produce an enhancement in westerly airflow during the periods of drought in Africa andglacial advance on the west coast of New Zealand. Extended wet periods and glacial recession are likewise shown to beassociated with an enhancement of easterly components of the circulation. Changes in precipitation, temperature and moisturetransport are examined and associations with the Southern Oscillation Index are investigated.Franz Josef glacial advances are shown to occur 4–5 years after the onset of enhanced south-westerly airflow on to the westcoast of New Zealand. Advances occur on average 4 years after the onset of extended dry spells in South Africa. Similaratmospheric circulation anomalies in the respective sectors of the Southern Hemisphere and adjustments in the locations of thepositions of the ridges in standing wave three are responsible for this correspondence. # 1997 Royal Meteorological Society.Int. J. Climatol., Vol. 17: 1499–1512 (1997)(No. of Figures: 5. No. of Tables: 0. No. of References: 89)

TOWARDS AN INVERSION CLIMATOLOGY OF SOUTHERN AFRICA: PART II, NON-SURFACE INVERSIONS IN THE LOWER ATMOSPHERE

ABSTRACT Radiosonde data for the years 1969–1972 have been processed to determine non-surface inversion conditions over Southern Africa. Results are presented to show, first, the spatial and point characteristics of mean non-surface inversions by season and by month, and secondly, the nature of inversion occurrence within six layers of the lower atmosphere.

The modulation of sea surface temperature and rainfall associations over southern Africa with solar activity and the quasi‐biennial oscillation

Correlations between solar activity and southern African rainfall, stratified according to the phase of the Quasi-Biennial Oscillation (QBO), reveal significant zero and 3-month lag associations in both the summer and the winter rainfall regions. The solar-rainfall associations are significant for in-season rainfall only when the QBO is in its easterly phase, while for out-of-season rainfall, significant associations are evident when the QBO is in its westerly phase. When the QBO is in its easterly phase, an increase (decrease) in early season solar activity (October-December in the summer rainfall region, April-June in the winter region) produces a synchronous increase (decrease) in rainfall over both the summer and the winter rainfall regions. The same increase (decrease) in solar activity produces a significant decrease (increase) in rainfall over the regions in the second half of the same rainfall season, 3 months later. When the QBO is in its westerly phase, out-of-season changes occur with both the winter precipitation of the summer rainfall region and the summer precipitation of the winter rainfall region, showing a tendency to become somewhat higher (lower) following an increase (decrease) in solar activity. During the early and late summer seasons, when the QBO is in its easterly phase, anomalous rainfall receipts over the summer rainfall region are the result of anomalies in the tropical atmospheric circulation of the region, which can in part be understood by reference to the sea surface temperature departures of the southwest Indian Ocean. In contrast, the rainfall anomalies observed in the southwest Cape Province during its peak rainfall season in winter can be related to anomalies in the temperate atmospheric circulation and sea surface temperature anomalies in the South Atlantic Ocean. Out-of-season associations between solar activity and southern African rainfall are evident for the summer rainfall region when the QBO is in its westerly phase and can be explained by reference to sea surface temperature anomalies in both the South Atlantic and the southwest Indian oceans. The phase of the QBO appears to modulate the association between solar activity and rainfall and also between solar activity and sea surface temperatures.

Climatic change in southern Africa: Past and present conditions and possible future scenarios

A brief review of climatic changes over the last 800 000 years in southern Africa is presented. The greatest emphasis is placed on those changes occurring during the Holocene and the period of meteorological record. Twentieth-century rainfall variations and their spatial manifestations are examined, and scenarios of possible future conditions are presented.