ABSTRACT
The International Atomic Energy Agency (IAEA), in cooperation with the World Meteorological Organization (WMO), has been conducting since the early sixties a worldwide survey of hydrogen and oxygen isotopic composition of precipitation. The East African region is represented in the IAEA database by six stations: Addis Ababa (Ethiopia), Kericho (Kenya), Entebbe (Uganda), Dar Es Salaam (Tanzania), Ndola (Zambia) and Harare (Zimbabwe). The Indian Ocean, which is the major source of precipitation for this part of the African continent, is represented by two stations: Antananarivo (Madagascar) and Diego Garcia Island (USA). Four out of eight stations listed above are still actively collecting monthly precipitation for isotope analyses.
The isotopic composition of precipitation in the East African region reflects the regional circulation patterns. Seasonal fluctuations of deuterium and 18O content of precipitation are well pronounced at almost all stations of the region and coincide with the seasonal displacement of the intertropical convergence zone (ITCZ). Isotopically depleted rains are in general associated with the rainy period(s). The amplitude of seasonal variations is relatively high in the northern and southern parts of the region (5 to 7‰ for δ18O at Antananarivo, Harare, Ndola and Addis Ababa). The annual weighted mean values of δ18O lie between –6.5% for the stations located in the south, and –1.3‰ for Addis Ababa. The average deuterium excess values vary between 12.5 and 15.5‰ for the inland stations, whereas the maritime stations (Diego Garcia and Dar Es Salaam) reveal deuterium excess values close to 10. Relatively large year-to-year variability of the isotopic composition of precipitation is observed. The region experiences recurrent events of extremely low deuterium and 18O isotope composition of precipitation, often persisting for more than one month, which most probably is associated with unusually strong convective activity within the air column, associated with passage of the ITCZ.
