Modes of Variability and Projections on to Recent Flooding and Severe drought Events over Africa
Wassila M. Thiaw, NOAA/NWS/NCEP/CPC, Camp Springs, MD
The continent of Africa experienced extremely heavy rains over portions of West Africa during the period from June to October 2005 and during the 2005-06 southern Africa rainfall season. At the same time, severe drought occurred in the Greater Horn of Africa. The rainfall seasons in the far western end of the Sahel and in the areas encompassing Namibia and Botswana were the wettest in over 30 years, while Kenya and Uganda experienced one of the worst droughts in the historical record. This paper examines the physical processes associated with the unusually wet West Africa and southern Africa rainfall seasons and the extreme dryness in East Africa with emphasis on the role of the dominant modes of variability on the synoptic features. First, we investigate West Africa, where rainfall amounts were over 50% above the climatological mean in many areas resulting in flooding mainly over Senegal and an increase in infectious disease outbreaks mainly cholera and malaria. The rainy season in this region was marked by highly frequent heavy rainfall episodes resulting from very active African wave disturbances. This marks a sharp contrast from the long term drying trend that started in the mid-70s and continued into the 80s and the 90s. Over the past 3 years, though the coupled ocean-atmosphere system featured circulation patterns similar to those observed in the 1950s and more favorable to enhanced convection. This so called “rainfall recovery” in the Sahel is discussed. In particular, during the period from May to August, the prominent Atlantic dipole mode characterized by a cold Gulf of Guinea and warm north Atlantic was well in place. The physical processes associated with this mode of variability which is more prominent on the decadal time scale and its influence on the synoptic features that resulted in a very active West African monsoon season is discussed. Next, we study East Africa and southern Africa, where the 2005-06 rainfall seasons featured a prominent rainfall dipole variability mode. This mode is well known to be associated with ENSO such that cold episodes are conducive to dry/wet East Africa/southern Africa rainfall seasons and vice versa. An examination of SSTs over the Pacific did not reveal a clearly defined ENSO event. Hence, the atmospheric circulation associated with the rainfall dipole is examined. We also discuss the role of synoptic systems mainly “rain bands” in the enhanced southern Africa rainfall season and the westward shift in these cloud bands that accounted for flooding in Namibia and Botswana, an area that is climatologically dry. The role of the ocean-atmosphere coupling over the Indian Ocean in the frequent dry spells during the Oct-Dec 2005 East Africa rainfall is also discussed. Finally an evaluation of the NCEP coupled forecast system (CFS) in depicting the 2005-2006 extreme events is examined.
Session 4B, The African Monsoon
Tuesday, 16 January 2007, 1:30 PM-3:00 PM, 214C
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