Anomalously wet and dry rainy seasons in Equatorial East Africa and associated differences in intra-seasonal characteristics

Abstract

Differences between anomalously wet and dry rainy seasons over the period 1961–2001 in Equatorial East Africa (Kenya, northern Tanzania) are studied at local scale using a 53 rain-gauge network and analysing six intra-seasonal characteristics (ISCs): the onset and end dates, the wet spells number and length, daily mean rainfall amount and the dry spells length. As compared to anomalously dry Long Rains, anomalously wet ones record earlier onsets, more wet spells with higher daily mean rainfall amounts (above 14 mm/day). For the Short Rains, the end date is also widely modulated across the rain gauge network, and daily mean rainfall amounts above (below) 10 mm/day are more (less) frequent during anomalously wet (dry) seasons. The differences in the number, length and daily mean rainfall amount of wet spells between anomalously wet and anomalously dry rainy seasons is also analysed in an ensemble of 24 sea surface temperature-forced runs of the ECHAM4.5 Atmospheric General Circulation Model. For the Long Rains, results are inconsistent between runs which suggests a lack of reproducibility of the ISCs in the model. For the Short Rains, the ISCs distributions between anomalously wet and anomalously dry years are significantly different in most of the runs/locations. In particular, daily mean rainfall amounts above 8 mm/day are more (less) frequent in the model during anomalously wet (dry) years.