ABSTRACT:The projected climate-change signal in simulations by the Conformal-Cubic Atmospheric Model (CCAM) over southern Africa is presented, with particular emphasis on the projected changes in circulation over the region. Current (1975Current ( -2005 and future (2070-2100; A2 scenario) climate simulations are used for this purpose.In the austral winter of the future climate, frontal rain bands are displaced to the south as a result of the subtropical highpressure belt intensifying to the south of the subcontinent. In spring and autumn, mid-and upper-level highs are simulated to become more prominent over the eastern and central parts of southern Africa. The enhanced subsidence associated with these systems results in generally lower rainfall totals over much of the south-eastern subcontinent. To the north of these highs, enhanced westward moisture advection contributes to increased rainfall totals over northern Mozambique, whilst along the western periphery of the anomalously strong highs, enhanced southward moisture advection results in higher rainfall totals over Namibia, Botswana and the central and western interior of South Africa. In mid-summer, the Indian Ocean High (IOH) is simulated to intensify most over the south-western Indian Ocean (IO). This seemingly results in the more frequent occurrence of the cloud bands that constitute the South Indian Convergence Zone (SICZ) over the southeastern subcontinent -resulting in generally wetter conditions over this region.

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Mid-tropospheric closed-lows (cold-core cut-off lows and warm-core tropical lows) are important rain producing weather systems for the southern Africa region. Over South Africa, most wide-spread flood events are caused by these systems. It is therefore important to explore the potential impact of anthropogenic forcing on the occurrence of closed-lows and extreme rainfall events over the region. Coupled global circulation models (CGCMs) can not be directly applied for this purpose because of their relatively low spatial resolution -some form of downscaling is required to adequately resolve these systems and the rainfall they cause. In this study, a variable-resolution atmospheric global circulation model is applied as a regional climate model to simulate closed-low characteristics over southern Africa under current and future forcings. The model is forced with greenhouse gas concentrations according to the A2 SRES scenario and with sea surface temperatures (SSTs) and sea-ice as specified by the CSIRO Mk3 CGCM. The model projects a general decrease in closed-low frequencies over the region, which occurs in association with a general strengthening of the subsiding branch of the Hadley cell. However, the climate-change signal shows variation in time and space and certain sub-regions are projected to experience an increase in closed-low frequencies during certain seasons. A general increase in extreme rainfall events is projected over southern Africa despite the projected decrease in closed-low frequencies. It is deduced that this increase in extreme rainfall events is driven by intense convective rainfall events occurring within more frequently forming tropical-temperate cloud bands. Over Mozambique, extreme rainfall events are projected to increase in association with more frequently occurring closed-lows.

ABSTRACT:The study provides perspective on the contribution of landfalling tropical systems (cyclones, depressions, storms and lows) from the southwest Indian Ocean (SWIO) towards rainfall over the eastern interior of southern Africa, over the period . Although these systems contribute to <10% of the annual rainfall occurring over the region, their relative contribution to local and widespread heavy rainfall events is shown to be highly significant. About 50% of widespread heavy rainfall events over northeastern South Africa are caused by landfalling tropical systems. Fourier analysis performed on the time series of rainfall occurring over northeastern South Africa in association with these systems reveals the existence of a quasi-18-year cycle. The cycle coincides with the well-known quasi-18-year Dyer-Tyson cycle in rainfall over the summer rainfall region of South Africa. These results suggest that atmospheric and surface conditions leading to wet phases of the Dyer-Tyson cycle also favour the landfall and subsequent westward movement of tropical systems from the SWIO over southern Africa -and their eventual contribution to rainfall over northeastern South Africa and southern Zimbabwe.

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The accurate prediction of rainfall events, in terms of their timing, location and rainfall depth, is important to a wide range of social and economic applications. At many operational weather prediction centres, as is also the case at the South African Weather Service, forecasters use deterministic model outputs as guidance to produce subjective probabilistic rainfall forecasts. The aim of this research was to determine the skill of a new objective multi-model, multi-institute probabilistic ensemble forecast system for South Africa. Such forecasts are obtained by combining the rainfall forecasts of 2 operational high-resolution regional atmospheric models in South Africa. The first model is the Unified Model (UM), which is operational at the South African Weather Service. The UM contributes 3 ensemble members, each with a different physics scheme, data assimilation techniques and horizontal resolution. The second model is the Conformal-Cubic Atmospheric Model (CCAM) which is operational at the Council for Scientific and Industrial Research, which in turn contributed 2 members to the ensemble system based on different horizontal resolutions. A single-model ensemble forecast, with each of the ensemble members having equal weights, was constructed for the UM and CCAM models, respectively. These UM and CCAM single-model ensemble predictions are then combined into a multi-model ensemble prediction, using simple un-weighted averaging. The probabilistic forecasts produced by the single-model system as well as the multi-model system have been tested against observed rainfall data over 3 austral summer 6-month periods from 2006/07 to 2008/09, using the Brier skill score, relative operating characteristics, and the reliability diagram. The forecast system was found to be more skilful than the persistence forecast. Moreover, the system outscores the forecast skill of the individual models.