A 47-year record of gridded data covering Africa south of the Sahara was used to document the spatial and seasonal patterns of the correlation between precipitation and sea-surface temperatures (SST) in key tropical areas, as depicted by the NIN 0 O3, South Atlantic and North Atlantic indices. El Niñ o -Southern Oscillation (ENSO) is confirmed as playing a dominant part in northeastern, eastern and southern Africa. However, its impact is also found over the Sahel during the northern summer, and other parts of the Gulf of Guinea region outside this season, a hitherto poorly documented feature. Over these two areas, ENSO and Atlantic SST (predominantly South Atlantic) contribute to different parts of the rainfall variance. The correlation with South Atlantic SST appears as a south -north dipole (positive/negative correlation) which shifts northward following the Inter-tropical Convergence Zone (ITCZ) translation between the northern low-sun and high-sun periods. A typing of the seasonal correlation patterns and a mapping of the multiple correlation coefficients are carried out in order to synthesize the space-time impacts of the three SST indices. Decadal-scale changes affect the strength of the teleconnections with both Atlantic and East Pacific SST, as reflected for instance by a small rise of the correlation with the NIN 0 O3 index since [1970][1971][1972][1973][1974][1975] in the Sahel and southern Africa, and additional shifts for the Atlantic Ocean, but the main patterns remain generally apparent over the whole period.The circulation anomalies associated with the teleconnections were assessed using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. A study of the dataset accuracy in depicting long-term climatic variations revealed that a major shift, mainly artificial, is found in 1967-1968 in the time-series of most of the variables. The rest of the work thus concentrated on the 1968-1997 period. A number of changes in east-west circulation patterns have been found to be associated to ENSO variations. Over West Africa, El Niñ o events tend to result in enhanced northeasterlies/reduced monsoon flow, coupled to weakened upper easterlies, and hence dry conditions over West Africa close to the surface position of the ITCZ, in July -September, as well as January-March. Over the southwestern Indian Ocean, the positive equatorial temperature/geopotential height anomalies, which at 200 hPa accompany El Niñ o events, are conducive to an eastward shift of the mid-latitude upper troughs, thus being detrimental to summer rainfall over South Africa. Abnormally wet 'short rains' in East Africa can be accounted for by an ENSO-forced weakening of the equatorial Walker-type (east -west) cell which is found over the Indian Ocean during that season. By contrast, the impact of South Atlantic warmings is mostly shown in low-level dynamics, as exemplified by the weakened trades and monsoon flow which directly result in a southward shift of the ITCZ. The com...

No abstract

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.

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.

No abstract

This is an exploratory study to investigate the spatial and temporal characteristics of east China’s (EC) river runoff and their relationship with precipitation and sea surface temperature (SST) at the continental scale. Monthly mean data from 72 runoff stations and 160 precipitation stations in EC, covering a period between 1951 and 1983, are used for this study. The station river runoff data have been spatially interpolated onto 1° grid boxes as runoff depth based on an extracted drainage network. Comparing runoff depth with precipitation shows that seasonal variation in runoff is consistent with the development of the summer monsoon, including the delayed response of runoff in several subregions. The dominant spatial scales and temporal patterns of summer runoff and precipitation are studied with empirical orthogonal function (EOF) analysis and wavelet analyses. The analyses show interannual, biennial, and longer-term variations in the EOF modes. South–north dipole anomaly patterns for the first two runoff EOF’s spatial distributions have been identified. The first/second runoff principal components (PCs) are highly correlated with the second/first precipitation PCs, respectively. The summer runoff’s EOF PCs also show significant correlations with the multivariate El Niño–Southern Oscillation index (MEI) of the summer and winter months, while the summer precipitation PCs do not. Statistic analysis shows that EOF1 of runoff and EOF2 of precipitation are related to El Niño, while EOF2 of runoff and EOF1 of precipitation are related to a dipole SST anomaly over the northwestern Pacific. The interdecadal relationship between summer runoff, precipitation, and SST variability is further studied by singular value decomposition (SVD) analysis. Pronounced warming (SST) and drying (runoff) trends in first SVD PCs have been identified. These SVDs are used to reconstruct a decadal anomaly pattern, which produces flooding in part of the Chang Jiang River basin and dryness in the northern EC, consistent with observations.

The climate impacts of leaf area index (LAI) and fractional vegetation cover (FVC) on the West African summer monsoon in 1987 and 1988 were investigated through a series of numerical experiments conducted with satellite products and the Simplified Simple Biosphere land surface model (SSiB). The SSiB was run in the offline mode as well as in the coupled mode with the National Centers for Environmental Prediction General Circulation Model (GCM). Prescribed monthly LAI and FVC from a look-up table based on limited ground surveys and those derived from satellite remote sensing were employed in the control and test runs, respectively.Compared with the control runs, both the GCM and offline test runs with satellite measured LAI/ FVC produce higher soil moisture and lower surface temperature in tropical West Africa to the south of about 15 N, with the maximum deviations being located around 12 N. This leads to the northward shift of the maximum of positive latitudinal temperature gradient. The associated easterly shear in the lower troposphere results in the African easterly jet and summer rainfall band in West Africa shifting to the north, which partially corrects the dry bias in the GCM control runs. In addition, the GCM test run simulates a relatively stronger West African summer monsoon in 1988, with a northward shifted African easterly jet, a stronger tropical easterly jet, and more rainfall than in 1987, consistent with observations. However, due to small differences in satellite measured LAI/FVC between 1987 and 1988, the model fails to produce interannual variations of precipitation as large as seen in the observation. Water balance analysis has also been carried out to investigate the dominant processes affecting the changes in precip-