Remote forcing of East African rainfall and relationships with fluctuations in levels of Lake Victoria

This study investigates the climatological variables responsible for fluctuations in Lake Victoria levels, in particular the causal mechanism responsible for a major anomaly that occurred in 1961. A Lake Victoria rainfall series (LVRS) correlates significantly (5%) with Lake Victoria levels and is utilized for subsequent analysis of rainfall variability. Relationships between annual and seasonal (March-May and October-December (OND)) LVRS and a number of tropical and extra-tropical series were analyzed. The results illustrate the dominance of the El Nino-Southern Oscillation phenomenon in modulating LVRS. The greatest correlation is found between LVRSOND and the Southern Oscillation index (r = -0.39, significant at the 1% level), although the relationship is non-linear over the course of the century. Velocity potential chi is employed as the principal diagnostic variable. Seasonal composite maps and empirical orthogonal function (EOF) analysis of Indian Ocean region X-fields are utilized to test the hypothesis that an Indian Ocean Walker cell is responsible for the anomalous 1961 rainfall episode. Subsequent analysis leads to the rejection of this hypothesis. EOF analysis of OND 200 hPa chi-fields reveals a number of modes of tropical variability. EOF analysis of the Indian Ocean basin illustrates the emergence of a meridional circulation directed over the Indian Ocean (EOF5). This time series correlates significantly with LVRSOND (r = -0.40, 1% level), although the spatial pattern only explains a small proportion (2.35%) of the total variance. chi analysis for OND over the broader tropical region reveals significant relationships (5%) between OND EOF3* and July-September Sahel rainfall for the preceding season and subsequent year. This relationship may assist future long-lead seasonal forecast schemes for the Sahel region. Copyright (C) 2003 Royal Meteorological Society.