Re-Examining the ITCZ as a Control on Rainfall and Its Variability in Equatorial Africa

The common explanation for the progression of the rainy season over Africa is the seasonal excursion of the ITCZ. Consequently, many studies examining climatic variability and change over Africa relate it to shifts in the location or intensity of the ITCZ. This is particularly true for studies of rainfall in equatorial Africa. However, the ITCZ paradigm stems from a time when tropical rainfall was assumed to be associated mainly with localized convection and this paradigm has numerous shortcomings, especially over equatorial Africa. A historical look at the concept also shows that its use over Africa has long been controversial. Nevertheless, the seasonal excursion of the ITCZ is still consistently cited as the explanation for the seasonal cycle throughout tropical Africa and as a major factor in interannual variability, particularly on paleoclimatic and historical time scales. This paper challenges that idea. It examines the structure of the motion field during the rainy seasons of the boreal spring and autumn over both eastern and western equatorial Africa. In neither location does the structure show much similarity to the classic ITCZ paradigm. For example, low-level subsidence underlies much of the region of maximum rainfall. Over central equatorial Africa the subsidence results from the divergence of the mountain breezes over the surrounding highlands. Ascent commences higher up as part of a meridional cell with weak convergence at 600 hPa, divergence at 200 hPa, and a descending branch around 10 N. A progressive southward movement of the rainfall maximum within the course of the seasons is also lacking. Over East Africa the progression is strongly controlled by the topography and by the existence of a low-level jet stream in the Turkana Channel. Further research is needed to provide a salable explanation for the equatorial African rainy seasons. This is required in order to truly understand the factors governing climatic change in the region on modern, historical, and paleo-timescales.

The Sahel region is known for the multi-decadal occurrence of severe drought that commenced in the late 1960s. The question as to whether or not the region's rainfall has returned to "normal" is a controversial one. This paper considers that question by examining the longest and most comprehensive gauge series for the region ever published. It extends from 1854 to 2014 and is based on 602 gauge records. A comparative series for the Guinea Coast region to the south is also presented. These two regions collectively provide insight into the long-term variability of the West African monsoon. These analyses suggest that full recovery has not occurred. They also suggest that a major change in the rainfall regime occurred around 1968. Since that time characteristics of the rainfall regime and large-scale teleconnections have changed markedly. The impact of ENSO in regime has also changed since that time. Recovery has been greater in the east than in the west, creating a change in the climatological east-west rainfall gradient. The drier post-1968 conditions appear to be associated with a general weakening of the intensity of the West African monsoon and only a small southward displace of the rainfall maximum. These changes have strong implications for the future of this region and for seasonal prediction.