Southern African Summer Rainfall Research Articles

Rainfall variability over Malawi during the late 19th century

ABSTRACTEstablishing long‐term records of rainfall variability is essential for understanding changes in the magnitude and frequency of extreme events. The need is particularly pressing for much of Africa, where the instrumental meteorological record rarely stretches back beyond the early 20th century. This study extends the rainfall record for present‐day Malawi back to the mid‐19th century through the analysis of historical documentary materials from British and African archives. Textual evidence within documents is used to construct a semi‐quantitative chronology of rainfall variability spanning the period 1858–1900. Widespread and severe droughts are identified during the austral summer rainy seasons of 1861–1863, 1877–1879, 1885–1888, and 1892–1894, and two unusually wet periods in 1889–1892 and 1894–1898. Instrumental rainfall data from Cape Maclear, Bandawe, and Kaningina spanning the period 1876–1880 – the earliest so far discovered for Malawi – are compared with nearby long‐term records from the Global Historical Climatology Network database. These analyses confirm the classifications for the equivalent years in the semi‐quantitative chronology. The results of this study are considered alongside other annually resolved rainfall reconstructions for the southern African summer rainfall zone to assess the spatial extent of late 19th century drought and wetter episodes and to explore the distribution of teleconnections arising from historical El Niño events. The very strong 1877 El Niño event was associated with drought from northern Malawi to the Eastern Cape. In contrast, drier conditions during the strong 1885 El Niño extended from Malawi as far south as the southern Kalahari, with Lesotho and KwaZulu‐Natal experiencing relatively wet conditions. The very strong 1891 El Niño was associated with very wet conditions from Malawi to KwaZulu‐Natal. In contrast, wetter conditions only extended as far south as Zimbabwe following the moderate 1896 El Niño. The study concludes with suggestions on how to extend historical climate information for the region.

Seasonal rainfall variability in southeast Africa during the nineteenth century reconstructed from documentary sources

Analyses of historical patterns of rainfall variability are essential for understanding long-term changes in precipitation timing and distribution. Focussing on former Natal and Zululand (now KwaZulu-Natal, South Africa), this study presents the first combined annual and seasonal reconstruction of rainfall variability over southeast Africa for the 19th century. Analyses of documentary sources, including newspapers and colonial and missionary materials, indicate that the region was affected by severe or multi-year drought on eight occasions between 1836 and 1900 (the rainy seasons of 1836–38, 1861–63, 1865–66, 1868–70, 1876–79, 1883–85, 1886–90 and 1895–1900). Six severe or multi-year wet periods are also identified (1847–49, 1854–57, 1863–65, 1879–81, 1890–91 and 1892–94). The timing of these events agrees well with independent reconstructions of 19th century rainfall for other parts of the southern African summer rainfall zone (SRZ), suggesting subcontinental scale variability. Our results indicate that the relationship between El Nino and rainfall in southeast Africa was relatively stable, at least for the latter half of the 19th century. El Nino conditions appear to have had a more consistent modulating effect upon rainfall during the 19th century than La Nina. The rainfall chronology from this study is combined with other annually-resolved palaeoclimate records from mainland southern Africa and surrounding oceans as part of a multi-proxy rainfall reconstruction for the SRZ. This reconstruction confirms (i) the long-term importance of ENSO and Indian Ocean SSTs for modulating regional rainfall; and (ii) that summer precipitation has been declining progressively over the last 200 years.

Impacts of convection schemes on simulating tropical-temperate troughs over southern Africa

This study examines southern African summer rainfall and tropical temperate troughs (TTTs) simulated with three versions of an atmospheric general circulation model differing only in the convection scheme. All three versions provide realistic simulations of key aspects of the summer (November–February) rainfall, such as the spatial distribution of total rainfall and the percentage of rainfall associated with TTTs. However, one version has a large bias in the onset of the rainy season. Results from self-organizing map (SOM) analysis on simulated daily precipitation data reveals that this is because the occurrence of TTTs is underestimated in November. This model bias is not related to westerly wind shear that provides favorable conditions for the development of TTTs. Rather, it is related to excessive upper level convergence and associated subsidence over southern Africa. Furthermore, the model versions are shown to be successful in capturing the observed drier (wetter) conditions over the southern African region during El Nino (La Nina) years. The SOM analysis reveals that nodes associated with TTTs in the southern (northern) part of the domain are observed less (more) often during El Nino years, while nodes associated with TTTs occur more frequently during La Nina years. Also, nodes associated with dry conditions over southern Africa are more (less) frequently observed during El Nino (La Nina) years. The models tend to perform better for La Nina events, because they are more successful in representing the observed frequency of different synoptic patterns.

Future southern African summer rainfall variability related to a southwest Indian Ocean dipole in HadCM3

Southern Africa is subject to strong interannual summer rainfall extremes associated with modes of climate variability. Interaction between these modes and the climate change signal make improving understanding of their future operation and links with southern African rainfall a priority. This paper examines future southern African summer rainfall variability related to a meridional dipole mode of southwest Indian Ocean SST variability (the SWIOD) in coupled climate model HadCM3. The SWIOD is central to southern African rainfall in a 1000‐year control simulation, and remains so during the twenty‐first century under two different CO2 forcing scenarios: SRES A2 and B1. Future rainfall variability connected with this mode is found to be sensitive to the emissions pathway, particularly in the second half of the century. Flooding episodes with anomalies near 4 mm day−1 in already wet regions and which are unprecedented in a 1000 year control run are simulated under A2 SRES forcing.

Influence of the Madden–Julian Oscillation on Southern African Summer Rainfall

Abstract Composite maps of outgoing longwave radiation (OLR) anomalies over the Madden–Julian oscillation (MJO) cycle show marked intraseasonal fluctuations over southern Africa (south of 15°S). Large-scale convective clusters are seen to propagate eastward and then northward over the continent, mainly between 10° and 20°S. The corresponding response of the rainfall field presents the alternation, over the cycle, of dry and humid phases, which are both significant. Moisture flux anomalies indicate an intraseasonal modulation of the midtropospheric easterly flow over the Congo basin at 700 hPa; these fluctuations are coupled to meridional flux anomalies that extend from the tropical to the subtropical austral latitudes, and favor occurrences of wet or dry conditions over the domain. Though statistically significant, the influence of the MJO on southern Africa is however not homogeneous spatially, and only the tropical areas exhibit sharp periodicities in the 30–60-day period range. The OLR dipole observed in previous studies at the interannual and synoptic time scales between the hinterland parts of southern Africa and the southwestern Indian Ocean in the north of Madagascar is investigated next, as it also shows strong fluctuations at the intraseasonal time scale. The study points out that the dipole is partly influenced by the MJO, though the strongest periodicities are found for slightly longer periods (35–80 days) than those typically associated with the oscillation. The forcing of the MJO on the OLR dipole, though significant, remains thus partial.

Predicting southern African summer rainfall using a combination of MOS and perfect prognosis

A statistical‐dynamical approach to probabilistic precipitation forecasts of southern African summer rainfall is described and validated. An ensemble of seasonal precipitation and circulation fields is obtained from the ECHAM4.5 atmospheric general circulation model (AGCM). Model output statistics (MOS) then spatially recalibrate the AGCM fields relative to observations. Although the MOS equations are built using the simulation data, in which observed SSTs force the AGCM, the same set of equations can be applied to the predicted data, in which predicted SSTs force the AGCM. The use of prediction data in a set of equations developed for simulations, assumes that the AGCM forecast skill approximates its simulation skill and that the systematic biases of the AGCM do not change in a prediction setting; this assumption is analogous to a perfect prognosis (PP) approach. Probabilistic forecast skill is assessed using this MOS‐PP‐recalibration scheme for 3 equi‐probable categories using a 3‐year‐out cross‐validation approach. High skill scores are found over the north‐eastern interior of the region, with marginal skill over the remainder of the austral summer rainfall regions. When skill is assessed for only the wettest and driest of the years, high skill appears over most of the region.

Recalibration of general circulation model output to austral summer rainfall over southern Africa

AbstractEmpirical techniques are developed to adjust dynamic model forecasts on the seasonal time scale for southern African summer rainfall. The techniques, perfect prognosis and model output statistics (MOS), are utilized to ‘recalibrate’ the CSIRO 9 general circulation model (GCM) large‐scale fields statistically to three equi‐probable rainfall categories for December to February. The recalibration is applied to a GCM experiment where simultaneously observed sea‐surface temperature fields serve as the lower boundary forcing. An optimal canonical correlation analysis model is designed for MOS and perfect prognosis and the 700 hPa geopotential height field is selected as the single predictor field in the two sets of statistical equations that are subsequently used to produce recalibrated rainfall simulations over a 10 year independent test period. MOS produced the higher forecast skill for southern Africa over the independent test period. Copyright © 2003 Royal Meteorological Society

Exploratory Long-Range Models to Estimate Summer Climate Variability over Southern Africa

Abstract Teleconnection predictors are explored using multivariate regression models in an effort to estimate southern African summer rainfall and climate impacts one season in advance. The preliminary statistical formulations include many variables influenced by the El Nino–Southern Oscillation (ENSO) such as tropical sea surface temperatures (SST) in the Indian and Atlantic Oceans. Atmospheric circulation responses to ENSO include the alternation of tropical zonal winds over Africa and changes in convective activity within oceanic monsoon troughs. Numerous hemispheric-scale datasets are employed to extract predictors and include global indexes (Southern Oscillation index and quasi-biennial oscillation), SST principal component scores for the global oceans, indexes of tropical convection (outgoing longwave radiation), air pressure, and surface and upper winds over the Indian and Atlantic Oceans. Climatic targets include subseasonal, area-averaged rainfall over South Africa and the Zambezi river basin, an...

On the Dry Valleys of the Kalahari: Documentary Evidence of Environmental Change in Central Southern Africa

Dry 'fossil' valleys (mekgacha) are an important landscape feature in the semi-arid to arid Kalahari Desert of central southern Africa. The majority of valleys do not contain flow, except in their headwater regions, but may contain pools of water on a seasonal basis. As a result, they are frequently mentioned in historical accounts of the region recorded by European travellers, and from these reports it is possible to assemble a picture of hydrological change. This paper describes the evidence for such change, for the period c. 1750 until c. 1910. Discussion mainly concerns the most frequently observed valleys, the Molopo and Kuruman, although Middle Kalahari systems are also considered. The picture which emerges is one of little change in Middle Kalahari systems. A decline in flow in the Kuruman since c. 1750 is tentatively identified, pre-dating the period of permanent human settlement in this region. The river appears to have established its present regime after 1800. A previously unrecognized flood event in the Kuruman in 1820 is also identified. These hydrological changes are placed within the context of historical climate evidence from the southern Cape of South Africa and show close correlation. This suggests that documentary evidence may be used to extend the climate record established for the southern African summer rainfall zone into the Kalahari.

REGIONAL TELECONNECTION PATTERNS ASSOCIATED WITH SUMMER RAINFALL OVER SOUTH AFRICA, NAMIBIA AND ZIMBABWE

Climatic determinants of southern African summer rainfall are analysed using statistical techniques. Summer rainfall time series are formulated for South Africa, Namibia and Zimbabwe areas and correlated with global indices and with field variables: sea-surface temperature (SST), outgoing longwave radiation (OLR), and tropospheric winds at 200 and 700 hPa levels. Linear regression correlations are performed using monthly standardized departures at various lags before and during the summer season. The SSTs in the central equatorial Indian Ocean (CEI) are identified as significant predictors/modulators of southern African rainfall. The SSTs at the CEI are best correlated with South Africa rainfall at r≶ − 0ċ6 at lags −2 and 0 months and are associated with the El Nino. The SSTs of the CEI modulate the overlying monsoon trough, as indicated by the OLR correlation maps. A centre of convective action alternates between southern Africa and the south-west Indian Ocean from year-to-year. A useful circulation index that emerges in the statistical analysis is spring-time zonal upper wind anomalies over the equatorial central Atlantic. This index is correlated with South Africa rainfall at r≶ −0ċ 8 at lags −4 and −2 months. Westerly (easterly) 200 hPa anomalies in spring are followed by a summer of below (above) normal rainfall. Other patterns that have a bearing on summer rainfall include a circulation gyre identified in 700 hPa wind correlations off the coast of south-east Africa. This circulation feature controls the flux of moisture between southern Africa and the northern Mozambique Channel. The correlation patterns offer statistical guidance in long-range forecasts and insights to climatic processes that govern the interannual variability of summer rainfall over southern Africa.