Southern Africa Precipitation Research Articles

Non–ENSO control on southern Africa precipitation variability

The objective of this study is to quantify the nature of the mechanisms of southern Africa rainfall variability, unrelated to the El Nino Southern Oscillation, by means of a simple empirical composite analysis, as a baseline for a more detailed study.

A Spatial Decision Support System for Radar‐meteorology Data in South Africa

AbstractThe South African Weather Service (SAWS) operates a radar station network, providing data continuity back to 1994, which is unique for southern Africa. The Geographical Resources Analysis Support System (GRASS) GIS was introduced to the SAWS “Meteorological Systems and Technology” (METSYS) radar research center in 1999 where it is still used for meteorologic research. The predominantly convective nature of precipitation in southern Africa creates a public demand for severe weather information systems for convective cells. Such a system was set up in GRASS GIS, using rule‐based expert systems to classify convective clouds. It isolates storm cells from stacks of reflectivity fields and derives information about their development stages. This data is both archived and also used to send out customised messages to target groups in specific areas of interest. Further, nationwide HTML‐maps can be created, serving as an interactive front‐end for a web‐based weather information system. This service can also be made accessible from remote locations by broad‐casting it as a datastream from satellite through the Worldspace digital radio system.

The Influence of Pacific SST Variability on the Precipitation over Southern Africa

Abstract This study is an analysis of AGCM model results to understand the dynamics of the response of precipitation over southern Africa (SA) to anomalies in the sea surface temperature (SST) over the Pacific Ocean. The pattern of interannual precipitation anomaly over SA and its temporal variations are quite similar in both the ensemble mean of the control (where AGCM is forced with observed SSTs in all ocean basins) and experimental runs (where AGCM is forced with seasonally varying climatological SST over the Pacific Ocean). However, the amplitude of the variability is found to be relatively reduced in the experimental runs. This is shown to be a result of the modulation of the Walker circulation by the variability of Pacific Ocean SST. The regional teleconnection pattern between the dominant mode of SA precipitation variability and SST anomalies over the eastern Indian Ocean is also influenced by the variations in Pacific SST. The nature of the teleconnection between SA precipitation and eastern Indian SST is apparent only when the Pacific SST variability is excluded. This is corroborated from observations as well.

A Southern Hemisphere Wave Response to ENSO with Implications for Southern Africa Precipitation

Ensemble GCM simulations with an imposed, idealized warming of the eastern Pacific Ocean reveal two wave anomalies in the Southern Hemisphere, one in the eastern and one in the western hemisphere. Both are statistically significant at the 99% confidence level. Application of a steady-state linear model and a Rossby wave source analysis is used to diagnose the causes of the waves. The western hemisphere wave is forced by the advection and stretching of planetary vorticity by the divergent flow from the Southern Hemisphere component of the central Pacific ‘‘twin anticyclones’’ that straddle the equator during warm events. The eastern hemisphere wave is a result of the northeastward shift of the South Indian convergence zone (SICZ) that, in turn, is forced from the upper troposphere by convergence to the north. An upper-level convergence maximum over the equatorial Indian Ocean induces divergence to the south, encouraging vertical motion and precipitation to the northeast of the SICZ’s normal position. The resulting anomalous upper-level convergence in the climatological position of the SICZ, as well as the anomalous vorticity flux convergence by the transients associated with an equatorward shift of the storm track behind the SICZ, force the eastern hemisphere Rossby wave. Since a shift of the SICZ is a fairly robust observed consequence of ENSO events, these results suggest the mechanism by which drought conditions develop over southern Africa at the height of many warm events. Seasonal prediction capabilities in this region can be improved by monitoring and understanding the details and consequences of the adjustment of the Walker circulation near the equator outside of the Pacific Ocean basin.

Isotopic evidence for the past climates and vegetation of southern Africa

The stable isotopes of hydrogen, oxygen and carbon can potentially provide evidence of past climates. The most detailed information has been obtained from variations in the oxygen-18 content of foraminifera from ocean floor cores, the analysis of which has produced a record of ocean temperature changes through the Quaternary and beyond. The use of isotope analysis of continental materials to reveal climatic change is more limited, but some advances have been made in recent years. One approach has been to utilize the variations in the isotopic composition of precipitation as recorded in ancient groundwater. Thus groundwater samples from a confined aquifer on the southern Cape coast show a marked rise in temperature since the Last Glacial maximum. The temperature changes during the Upper Pleistocene and Holocene are also reflected in the oxygen-18 content of stalagmites from the Cango caves in the same region. The widespread occurrence of C4 grasses in the warmer summer rainfall areas of southern Africa provides a novel possibility of observing temporal shifts of climatic boundaries. The distinctly high carbon-13 content of C4 plants is clearly reflected in the skeletons of grazers so that faunal material from suitably situated archaeological sites can be used to observe changes in the composition of the local grass-cover. The evidence thus far accumulated suggests only minor changes since the Upper Pleistocene. The combined evidence to date indicates that temperatures and also precipitation in southern Africa have changed since the Last Glacial maximum, about 18 000 years ago, but that shifts in the boundaries of the various veld-types were probably not very extensive.

On the components of time series; the removal of spatial dependence

A linear model is fitted to the precipitation series of each group of stations formed by principal component analysis on 157 precipitation records (Dyer 1975). Precipitation can thus be decomposed into independent components one each for regional temporal effect, regional spatial effect, and a residual or micro-effect. Each region has a temporal effect which is analysed for trend, enabling the conclusion to be made that Southern Africa's precipitation budget is stationary. On the other hand, trend on a microscale is present over randomly distributed parts of the country. Spectral analysis shows the oscillatory behaviour of the regional temporal effects, and provides information useful to the fitting of stochastic forecasting models to the data. The technique solves the problem of dependence between meteorological time series, and can be applied to any variable.

On the components of time series; the removal of spatial dependence

AbstractA linear model is fitted to the precipitation series of each group of stations formed by principal component analysis on 157 precipitation records (Dyer 1975). Precipitation can thus be decomposed into independent components one each for regional temporal effect, regional spatial effect, and a residual or micro‐effect. Each region has a temporal effect which is analysed for trend, enabling the conclusion to be made that Southern Africa's precipitation budget is stationary. On the other hand, trend on a microscale is present over randomly distributed parts of the country. Spectral analysis shows the oscillatory behaviour of the regional temporal effects, and provides information useful to the fitting of stochastic forecasting models to the data. The technique solves the problem of dependence between meteorological time series, and can be applied to any variable.