Zonal circulations over the Indian and Pacific Oceans and the level of Lakes Victoria and Tanganyika

Level records of two East African Great Lakes, Lake Victoria and Lake Tanganyika, which are considered as hydro-climatic proxies, are analysed. Comparisons between the two lake signals show synchronisms, which can only be accounted for by large-scale mechanisms. Lake-level variations associated with the short rains season (October-January) appear to have a prominent effect on the annual lake levels. The relations between lake-level variations and atmospheric circulation indexes are then investigated. Over the period 1946-2000, four indexes are selected to characterize the October-December zonal circulation over the Pacific and Indian Oceans. Over the Indian Ocean two surface zonal wind indexes (ZWIs) are used. For the Pacific, the southern oscillation index (SOI) and the Nino3 index are held to account for the El Nino-southern oscillation (ENSO). It is shown that significant overall negative correlations of level fluctuations are preferentially obtained with Indian Ocean circulation indexes. Although ZWI is highly correlated with the ENSO indexes, the latter display weaker relations with East African lake levels. It is shown that, for the 1946-2000 period, the October-December zonal circulation cell over the Indian Ocean plays a key role in the equatorial lake-level anomalies, thus demonstrating their influence on the hydro-climatic interannual variability of a large region. However, lake-level variation is a function not only of regional hydro-climatic conditions, but also of the initial (October) absolute lake level. Higher correlations are evident in a multiple correlation approach taking into account the initial lake level status in addition to the ZWI and ENSO indexes. Copyright (C) 2004 Royal Meteorological Society.