The relation between rainfall and the Normalized Di erence Vegetation Index (NDVI) in Africa south of 15ß S ( 1983± 1988) is studied. For 115 1ß by 1ß grid-points, the spatial distribution of annual NDVI and rainfall means is highly comparable. Both parameters have overall decreasing values from Mozambique to South-Western Africa. The strongest correlations occur when NDVI monthly values are compared with the bimonthly preceding rainfall amounts, attesting a time response of one to two months. At these time and space scales, NDVI does not appear to be sensitive to the seasonal and interannual rainfall variations in the Namib desert, South Namibia and western Cape Province. Along the Indian Ocean coast, it is weakly sensitive to the seasonal cycle only. It becomes largely sensitive to the seasonal cycle in Zimbabwe, and in South-Western Zambia. A high sensitivity to the interannual rainfall variability is only observed on the Southern African Plateau, around the Kalahari basin. Multivariate analyses show that geographical conditions of seasonal and interannual rainfall-NDVI associations strongly di er. While a sensitivity to seasonal rainfall is observed in areas where mean annual rainfall varies from 300 to 900 mm and where the contrast between wet and dry seasons is strong, the sensitivity to interannual rainfall anomalies is observed only for relative dry areas, where mean annual rainfall varies from 300 to 500 mm. In both cases, the relation is observed whatever the soil type or vegetation formation.

No abstract

The standardised precipitation index (SPI) is an index that allows monitoring the intensity and spatial extension of droughts at different time scales (3, 6, 12 and 24 months). The SPI is linked to the probability occurrence of dry or wet events. The SPI allows monitoring operationally any location with a 30-year time series. It is also used here to do a retrospective analysis of the spatial extension and intensity of droughts in South Africa since 1921. According to this index, the 8 most severe droughts at the 6-month time scale for the summer rainfall region of South Africa happened in 1926, 1933, 1945, 1949, 1952, 1970, 1983 and 1992. There is considerable decadal variability and an 18 to 20 year cycle is only found in the number of dry districts. The total number of wet and dry districts per decade seems to have increased since the 1960s. Drought lasting 3 years is not uncommon for each of the 8 South African rainfall regions defined by the South African Weather Service. Combining the retrospective analysis with real time monitoring could be extremely beneficial in the development of response, mitigation strategies and awareness plans.

No abstract

No abstract

No abstract

Southern African rainfall does not show any trend to desiccation during the 20th century. However, the subcontinent experienced particularly severe droughts in the 1980s and at the beginning of the 1990s and the magnitude of the interannual summer rainfall variability shows significant changes. Modifications of the intensity and spatial extension of droughts is associated with changes in ocean-atmosphere teleconnection patterns.This paper focuses mostly on the well-documented 1950 -1988 period and on late summer season (January-March). A principal component analysis on southern African rainfall highlights modifications of the rainfall variability magnitude. The 1970-1988 period had more variable rainfall, and more widespread and intense droughts than the 1950-1969 period.To investigate the potential modifications of the associated ocean -atmosphere teleconnection patterns, a composite analysis is performed on sea-surface temperature (SST) and National Center for Environmental Protection (NCEP) atmospheric parameters, according to the 5 driest years of both sub-periods. Significant changes are shown in ocean-atmosphere anomaly patterns coincident with droughts for both sub-periods. The 1950 -1969 droughts were associated with regional ocean-atmosphere anomalies, mainly over the southwest Indian Ocean region. In contrast, during the 1970-1988 droughts near-global anomalies were observed in the tropical zone, corresponding to El Niñ o-Southern Oscillation (ENSO) phenomenon.Within the whole century, significant correlations between Southern Oscillation Index (SOI) and southern African Rainfall Index (SARI) were found in the periods (1900 -1933 and 1970 -1998) when SOI and SARI experienced high variability, and when southern Africa was affected by intense and extended droughts. During periods of low SOI (1934SOI ( -1969, correlations became less significant and droughts were less intense and widespread.

The relationship between sea-surface temperature (SST) inter-annual variability at the subtropical and midlatitudes of the southern Atlantic and Indian Oceans and its links with the atmospheric circulation in the Southern Hemisphere are investigated over the 1950-1999 period. Exploratory analysis using singular value decomposition and further investigations based on simple indices show that a large part of regional SST variability is common between the southwestern parts of both basins at subtropical and midlatitudes during the austral summer. Interestingly, these areas are also significantly associated with the far southwestern Pacific (Tasman Sea area). The patterns and time series of covariability between the southern Atlantic and Indian Oceans are shown to correspond to SST modes previously described in the literature as 'subtropical dipoles', independently for the Atlantic and Indian Oceans. Composite analyses show that austral summers characterized by simultaneous warm (and to a lesser extent cold) SST anomalies in the southwestern (northern) part of both southern oceans are related to atmospheric anomalies mainly involving a southward shift and a strengthening of the subtropical high-pressure systems over both basins. These anomalies are embedded in a hemispheric signal associating two cores of positive pressure anomalies within the South Pacific anticyclone. The global picture appears to have a wave number 4 spatial structure. The associated low-level wind and latent heat-flux anomalies and the lags between atmospheric variables and SST anomalies are consistent with an atmospheric forcing on the ocean. Potential links of these patterns with large-scale modes of climate variability in the Southern Hemisphere are discussed.