East African hydroclimatic variability: 1950-1999

The interannual variability in precipitation over East Africa is well-understood. Many studies have identified the factors influencing the interannual variability of precipitation such as El Niño – Southern Oscillation (ENSO), the Indian Ocean Dipole Mode (IODM), and Atlantic Ocean sea surface temperature and pressure variations. The relatively arid conditions in much of the East African region are not understood fully. The objective of present study is to determine the meteorological association of aridity over East Africa with regional hydroclimatic variables as well as to find global teleconnections affecting spatial distribution of aridity over East Africa. The East Africa Aridity index is calculated as the ratio of the mean seasonal precipitation to the mean seasonal potential evapotranspiration (after Budyko, 1974), and is used as a measure of aridity over East Africa. Principal components analysis was performed on the aridity index to identify characteristic modes of the temporal variability of the aridity index across East Africa. Correlation analysis was performed to identify the meteorological association in the interannual variability of aridity over East Africa and to find the global teleconnections, such as with ENSO, IODM, North Atlantic Oscillations (NAO), Tropical Atlantic SST Dipole (TASD), and Quasi Biennial Oscillations (QBO) in it. The first principal component of the aridity index was used for the correlation analysis. Correlations of the normalized difference vegetation index (NDVI) and Palmer Drought Severity Index (PDSI) with the first principal component of aridity index were calculated. The aridity index over East Africa is driven by precipitation rather than potential evapotranspiration (PET). The PET over East Africa is driven by precipitation rather than temperature. Aridity over East Africa is well correlated with the NDVI and PDSI. The ENSO influence on interannual variability of precipitation and hence on aridity is very much evident in all the seasons, while IODM influence is evident in the June – September season, the driest season for East Africa. Influence of NAO, TASD, and QBO was observed to be very small compared to that of ENSO and IODM. The teleconnections influencing the rainfall variability of East Africa also influenced variability in aridity