Invariance in the Seasonal Median Dates for Mono-Modal Monsoonal Rainfall Distribution over the Semi-Arid Ecotone of Sub-Saharan West Africa

Naraine Persaud,Xining Zhao,Xiaoli Chen,Xiaobo Zhou,Moustafa Elrashidi

doi:10.4236/ijg.2013.46a2001

Abstract

Seasonal distribution of mono-modal, monsoonal rainfall across the semi-arid ecotone of sub-Saharan of West Africa is highly variable and unpredictable. The ever-present risk of drought and crop failure in this environment often results in food shortages that are met by emergency food aid. Humanitarian assistance planners would be better prepared for such interventions in a timely manner if they have reliable indicators that forewarn the impending failure of the rains. A good indicator would be a characteristic of the seasonal rainfall distribution that can be shown to be reasonably invariant over time and space. The objective of this study is to investigate whether such invariance existed for the seasonal median date (meaning the date when 50% of the seasonal total occurs). Such invariance is expected since the sun’s cyclic declination forces the advance and retreat of the Inter-tropical Front over West Africa. We examined the statistical properties of the seasonal median date for 1349 station-years of rainfall records for 30 rainfall stations in Burkina Faso and Niger with coordinates ranging from 9.88° to 18.5° north latitude and -4.77° to 13.2° longitude. The results showed that the median date was quite narrowly distributed over years with rather weak dependence on geographical coordinates. It can therefore be used as a reasonable ex-ante indicator of the success or failure of the rains as the rainy season progress.

Highlights

The climate of sub-Saharan West Africa changes from rainless and arid at latitudes on the fringe of the Sahara desert to wet and humid close to the equator
The objective of this study is to investigate whether such invariance existed for the seasonal median date
We examined the statistical properties of the seasonal median date for 1349 station-years of rainfall records for 30 rainfall stations in Burkina Faso and Niger with coordinates ranging from 9.88 ̊ to 18.5 ̊ north latitude and −4.77 ̊ to 13.2 ̊ longitude

Summary

Introduction

The climate of sub-Saharan West Africa changes from rainless and arid at latitudes on the fringe of the Sahara desert to wet and humid close to the equator. Agro-ecologically, over 50% of sub-Saharan West Africa is semi-arid (climate Bsh according to the Koppen-GeigerPohl classification system) with annual average temperature > 20 ̊C [1] In their landmark study on the point rainfall patterns over sub-Saharan West Africa, Coheme and Franquin [2] showed that the mean annual rainfall isohyets over this region were almost parallel and oriented east-west, with rainfall decreasing progressively with increasing latitude. Their findings were in accordance with those previously suggested by Chevalier [3] and Trochain [4] based on eco-botanical observations. This progressive increase in the mono-modal, monsoonal, rainfall amount and distribution with decreasing latitude south of the Sahara is the primary factor controlling the agriculture, ecology, and demography of the semi-arid ecotone of sub-Saharan West Africa [5,6]

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