Rainfall variability over the East African coast

Abstract

The coastal region of Kenya and Tanzania experiences two rainy seasons per year (October-November-December (OND) and March-April-May (MAM)) and has an economy that is highly dependent on and vulnerable to the amounts and timing of rainfall during these seasons. Most of the interannual variability in OND seasonal rainfall totals relate to El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events. While these relationships are fairly well documented and understood, there is a relatively poor understanding of the timing and intensity of the rainfall during ENSO/IOD seasons. In an attempt to improve understanding on this topic, daily rainfall station data, dekad and seasonal satellite rainfall estimates and Normalized Difference Vegetation Index (NDVI) imagery are analyzed for two recent OND seasons with El Nino conditions. These are OND 2006 which was characterized by devastating floods over the region and 2009 when the magnitude and spatial extent of the above average rainfall patterns were smaller. Daily rainfall data for the Tanzanian coastal stations showed that Tanga and Dar es Salaam (north and central coast) experienced few dry spells and several relatively intense wet spells during OND 2006 whereas at Mtwara, on the south coast, there were two very intense wet spells and a number of dry spells during the season. In OND 2009, only the north coast (Tanga) experienced above average rainfall, comprised of three wet spells with the one about a month after the beginning of the season being very intense. These data highlight the complexity of the rainfall distributions in the coastal region. A shift of the Walker circulation over coastal East Africa with strong uplift there seemed to be responsible for the very wet conditions during OND 2006. The marine air mass being advected from the western tropical Indian Ocean towards East Africa contained more moisture than average. Similar, but weaker, horizontal circulation anomalies occurred in OND 2009 along with increased moisture over the coastal zone although there was no obvious shift in the Walker circulation in this season.