The coherent variability of African river flows : composite climate structure and the Atlantic circulation
AbstractThe composite structure of the ocean and atmosphere around Africa is studied in the context of river flow variability. Annual streamflows are analysed for the Blue and White Nile, Congo, Niger, Senegal, Zambezi, and Orange Rivers, and inflow to Lake Malawi. Spectral energy is concentrated in 6.6- and 2.4-year bands. The interannual variability of many river flows is significantly cross-correlated (p < .05) over the period 1950-1995, following removal of the mean trend. A combined river flow index indicates that flows were highest in 1961, 1962, 1968, 1977, 1978 and lowest in 1971, 1972, 1982, 1983, 1991.
Forming a composite of differences between high- and low-flow years, SST anomalies and other atmospheric fields are investigated to better understand climatic factors driving hydrological extremes. During high flows, NCEP reanalysis data reveal a composite La Nina event off Peru-Ecuador. SST patterns reveal an inter-hemispheric dipole in the Atlantic (eg. warm - north) and below normal SST in the west Indian Ocean during years with high flow. The equatorial east Atlantic undergoes warming through the \'composite year\' in a manner consistent with its opposing response to the Indo-Pacific La Nina. Tropical upper winds are easterly and symmetrical about the equator, and may explain why inter-annual variability of river flows south of the equator are correlated with those of the north at 6-month lag. Low level westerly winds are greater during high flow years, particularly along the Guinea coast. Differences of OLR and upper velocity potential demonstrate two distinct centers of action either side of the tropical Atlantic. It is concluded that hydrological events over Africa and South America are sensitive to tropical Atlantic coupling with the global El Nino - Southern Oscillation signal.
Water SA Vol.29(1) 2003: 1-10