THE CLIMATE OF THE KALAHARI TRANSECT

SUMMARY

The climate of the Kalahari Transect is defined by the nature of the general circulation of the atmosphere over Africa south of the equator and the regional circulation variations that determine local weather. Individual synoptic systems and the transport of moisture into the region by converging and diverging airstreams in both summer and winter govern conditions favouring or inhibiting the major rainfall-producing process of convective precipitation. The climate is illustrated by the spatial variation of moisture, rainfall, temperature, evaporation and other fields. It is shown that the climate of the transect varies from the subtropical in the south to the tropical in the north with characteristic gradients of most parameters between the two regions in both summer and winter.

The Kalahari Transect is a multi-disciplinary regional project having as its aim the advancement of the understanding of the effects of human and climatically induced global change over a large area of southern Africa. It has a near-unique advantage of being a study where the underlying regional geology, comprising Kalahari sand, is uniform over a considerable range of latitude. This offers the prospect of reducing the uncertainties associated with regional manifestations of global change. Climate and climatic change remain major determinants of all facets of the Kalahari Transect study. In this paper, the nature of the regional climate of the transect and how it may change in the future are discussed.

The Kalahari Transect has a latitudinal extent from 14°S to 28°S and a longitudinal equivalent from 21°E to 28°E. The delimited area covers much of central southern Africa, ranging from shrubland and grassy desert in the south to evergreen dense woodland in the north. The whole region is dominated by the southern hemisphere descending limb of the Hadley cell of the general atmospheric circulation, the southern part of the transect more so than the northern part. This means that anticyclonic circulation patterns are the dominant circulation type for most of the year. The attendant atmospheric conditions are characterised by subsiding, stable air masses, hazy lower tropospheric conditions and generally cloud-free skies.

South of the Tropic of Capricorn disturbances in the mid-latitude westerlies assume an increasingly important role in determining rainfall, whereas to the north seasonal migrations of the Inter-Tropical Convergence Zone (ITCZ) become the major determinant of the annual rainfall cycle. Over the whole region cumulus convective activity, enhanced by atmospheric disturbances associated with negative pressure departures and suppressed by those with positive departures, is the major determinant of rainfall, which everywhere exhibits a strong seasonal cycle with a summer maximum.

In general, it is difficult to acquire a dense coverage of continuous, long-period, modern meteorological records for Africa south of the Sahara. The same is true of the region covered by the Kalahari transect. Good general descriptions of the climate of the region are provided in the atlases of Jackson (1961) and Thompson (1965). The reviews of Torrance (1972), Schulze (1972, 1984). Hastenrath (1984) and Tyson (1986) provide additional and more up-to-date material. These sources and others have been used to compile a climatology of the Kalahari transect region. After a brief summary of the regional atmospheric circulation, the main parameters to be considered are rainfall, cloud cover, energy budget components, temperature, atmospheric moisture, evaporation and potential evapotranspiration. The summary of the transect climate concludes with estimates of possible future regional conditions that may follow from global greenhouse warming.