This study examines the relative contributions of the tropical Indian Ocean variability to the climate over East African during the short rains (October-December, OND) season. Seasonal/intraseasonal composites of circulation and convection anomalies associated with Indian Ocean SST and East African rainfall variability have been investigated using National Centers for Environmental Prediction / National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and outgoing longwave radiation (OLR, proxy for large-scale tropical convection anomalies). Indian Ocean dipole (IOD) is based on multivariate measures of differences in the western and eastern basin for sea surface temperature (SST) and sea level pressure (SLP) anomalies, with near-surface zonal wind anomaly in the central basin.

The IOD teleconnections with tropical circulation and convection anomalies shows strong anomalies across the equatorial Indian Ocean into East African coast, as a result of the enhanced convergence of moisture over the western Indian Ocean. Intraseasonal composites clearly isolated two modes of Indian Ocean variability leading to extended wet spells in East Africa. Positive phase in IOD have strong Indian Ocean composite anomalies about 10 days before the rainfall event, but these structures tend to weaken during the rainfall event. When the large-scale teleconnection is unfavorable (negative phase in IOD), the extended wet spells in East Africa appears more generated by local mechanisms.

The study therefore provides insight into seasonal/intraseasonal rainfall variability over East Africa, in view of equatorial Indian Ocean SST anomalies. These results will feed into real-time monitoring and forecasting to enhance early warning and disaster preparedness activities and hence minimize the climate-related catastrophes that are prevalent in the region.