Long-term variability of the East African short rains' and its links to large-scale factors

This study utilizes a new 139-year rainfall record for East Africa to evaluate the relationship between the short rains' of October-November and four tropical indices. These indices include the zonal winds at the surface and 200 mb over the central equatorial Indian Ocean, Nino 3.4 and the Indian Ocean Zonal Mode (IOZM). The relationships with these indices are time dependent, as are the relationships among the indices. These change markedly on a decadal timescale, consistent with regime changes indicated by other authors, and the links to El Nino-Southern Oscillation (ENSO) and the IOZM appear to be weaker than those suggested by previous studies. The zonal winds show the strongest and most consistent relationships with October-November rainfall. However, the relationships are very different for wet and dry years, and the zonal winds play a stronger role in producing wet conditions. Further, several factors appear to act in tandem to produce extremely wet years, but appear to act largely independently in producing drought. The links to drought have been markedly weaker since 1982. These links were also very weak roughly between 1920 and 1960, when apparently the Walker cell over the Indian Ocean was very weak and the Pacific Walker cell particularly strong. At that time, ENSO appeared to drive most of the variability of October-November rainfall, interannual variability was weak, and the rainfall was below average during most of that period. When the zonal circulation in the Indian Ocean sector became well-developed c. 1961 and the Pacific cell weakened, both the rainfall and its interannual variability markedly increased. Overall, this study stresses the time dependence of the various relationships with East African October-November rainfall. This has strong implications for seasonal forecasting.