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A revised picture of the structure of the “monsoon” and land ITCZ over West Africa

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Abstract

This article presents an overview of the land ITCZ (Intertropical Convergence Zone) over West Africa, based on analysis of NCAR–NCEP Reanalysis data. The picture that emerges is much different than the classic one. The most important feature is that the ITCZ is effectively independent of the system that produces most of the rainfall. Rainfall linked directly to this zone of surface convergence generally affects only the southern Sahara and the northern-most Sahel, and only in abnormally wet years in the region. A second feature is that the rainbelt normally assumed to represent the ITCZ is instead produced by a large core of ascent lying between the African Easterly Jet and the Tropical Easterly Jet. This region corresponds to the southern track of African Easterly Waves, which distribute the rainfall. This finding underscores the need to distinguish between the ITCZ and the feature better termed the “tropical rainbelt”. The latter is conventionally but improperly used in remote sensing studies to denote the surface ITCZ over West Africa. The new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift, which in turn is controlled by the characteristics of the African Easterly Jet and Tropical Easterly Jet, rather than by moisture convergence. This new picture also includes a circulation feature not generally considered in most analyses of the region. This feature, a low-level westerly jet termed the African Westerly Jet, plays a significant role in interannual and multidecadal variability in the Sahel region of West Africa. Included are discussions of the how this new view relates to other aspects of West Africa meteorology, such as moisture sources, rainfall production and forecasting, desertification, climate monitoring, hurricanes and interannual variability. The West African monsoon is also related to a new paradigm for examining the interannual variability of rainfall over West Africa, one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north–south displacements of this feature. The new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the African Easterly Waves, the disturbances that are linked to rainfall over West Africa and spawn hurricanes over the Atlantic, and with our knowledge of the prevailing synoptic and dynamic features. This article demonstrate a new aspect of the West Africa monsoon, a bimodal state, with one mode linked to dry conditions in the Sahel and the other linked to wet conditions. The switch between modes appears to be linked to an inertial instability mechanism, with the cross-equatorial pressure gradient being a critical factor. The biomodal state has been shown for the month of August only, but this month contributes most of the interannual variability. This new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability, but also to the multidecadal variability evidenced in the region between the 1950s and 1980s.

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Acknowledgments

This work was supported by the Climate Dynamics Program of NSF, grant ATM-0004479. Figures were produced by Mr. Douglas Klotter of Florida State.

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    Sharon E. Nicholson

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Nicholson, S.E. A revised picture of the structure of the “monsoon” and land ITCZ over West Africa. Clim Dyn 32, 1155–1171 (2009). https://doi.org/10.1007/s00382-008-0514-3

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