Empirical Analysis of the Recent Rainfall Recovery in West Africa

Abstract

In this study, daily and monthly rainfall data from 167 and 254 stations, respectively, across West Africa with at least 80% data availability for the 31-year period 1980-2010 and the gridded African Rainfall Climatology- Version 2 (ARC2) data for the period 1983-2013, are used to investigate the monthly, annual and inter-annual rainfall variability over West Africa. Precipitation-related indices of the Expert Team on Climate Change Detection Indices (ETCCDI) are used to investigate the implication of the recovery in terms of the occurrence of precipitation extremes, intensity and frequency. Also, trends in the rainy season onset and retreat dates are analysed to assess the implication of the recovery on monsoon season length. The Standardised Precipitation Index (SPI) for various running time scales is used to assess the consistency of the rainfall recovery with the change from the drought states toward wet or normal conditions. Using the joint global observational dataset from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR), the study further examined the physical mechanisms and teleconnections that led to the observed recovery in the West African rainfall. This analysis involved the influence of Sea Surface Temperature (SST) Anomaly on West African rainfall variability and the feedbacks from land surface condition changes. A projection for future scenarios of the recovery in West African rainfall is proposed based on the Empirical-Statistical Downscaling (ESD) applied to the output of the Hadley Global Environment Model 2 (HadGEM2) projections. The study reveals that the majority of stations in the Sahel between the West Coast and 15°E show statistically significant positive (increasing) rainfall trend for annual totals. The August-October period shows the largest rainfall recovery in the Sahel and the date of the retreat of the rainy season significantly moved later into the year by 2 days per decade over that region. The Sahel rainfall recovery is reflected in more rainy days associated with longer wet spell duration and more extreme rainfall events. In contrast, stations along the Guinea Coast show constant or weak trends generally statistically non-significant. However, a tendency toward a more intense 2nd rainy season indicates a later retreat of rains from the Guinea Coast. Results also establish that the Atlantic Multidecadal Oscillation (AMO) changes its sign at the same time that the drought in West Africa started to recover. Evidence is found for a significant feedback between land surface variables (i.e., soil moisture, vegetation index and surface albedo) and rainfall variability at monthly and annual time scales. The feedback of soil moisture increases the rainfall variation by as much as 30% of the variability in annual precipitation in several areas in the Sahel. Based on the climate projection scenarios of the Coupled Model Intercomparison Project Phase 5 (CMIP5) used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR5), the Empirical-Statistical Downscaling (ASD) applied to data from HadGEM2 underscores a projection of future wetter rainy season over West Africa with a small delay to both rainy season onset and retreat in the ongoing 21st Century for the Representative Concentration Pathways 4.5 (RCP4.5).