Abstract
Using a regional climate model asynchronously coupled to a dynamic vegetation model, this study examines future climate predictions for the West Africa region and how dynamic vegetation feedback may influence such predictions. Without accounting for the impact of vegetation dynamics, the model predicts a future decrease of annual rainfall over Sahel. Dynamic vegetation feedback reverses this trend, leading to a substantial increase of annual rainfall. Regardless of how vegetation is treated, the predicted future trend of precipitation in the Sahel region follows a specific seasonal pattern, with a decrease during the pre- and early-monsoon season (May-June and early July) due to the warming-induced enhancement of spring convective barrier and an increase after the monsoon is fully established (typically in July-August-September) due to enhanced moisture import from a warmer ocean. Dynamic vegetation feedback reduces the magnitude of the predicted rainfall reduction in the early season and increases the magnitude of the predicted rainfall increase later in the rainy season. The future decrease of early-season rainfall has significant agronomic implications.
Highlights
Climate variability in West Africa is characterized by a megadrought in the second half of the 20th century and the alternating occurrence of multidecadal dry and wet spells in recent history
For the 20th century climate, Xue and Shukla [4] found that desertification in the Sahel region causes a decrease of precipitation that resembles the megadrought observed in the Sahel region from the late 1960s on; Wang and Eltahir [5] found that feedback from natural vegetation dynamics enhances the impact of both oceanic forcing and man-made desertification in causing the 20th century Sahel drought
The focus of the following analysis is on future changes of precipitation, its seasonal cycle, the West African monsoon onset, and on the impact of vegetation dynamics in these predictions
Summary
Climate variability in West Africa is characterized by a megadrought in the second half of the 20th century and the alternating occurrence of multidecadal dry and wet spells in recent history (e.g., the fluctuation of lake level in [1]). Given the strong latitudinal gradient of precipitation in West Africa and the importance of monsoon onset for West African agriculture, it is necessary to assess future changes of precipitation seasonality using RCMs that have a finer spatial resolution than most GCMs. While still uncommon among the IPCC projections, some GCMs have included dynamic vegetation in future prediction experiments [25, 26]. Compared with the prediction assuming static vegetation, vegetation dynamics acts to increase precipitation amount over most of West Africa except for the narrow band along the coast This current study is based on the same set of experiments used in Alo and Wang [8], but focuses on changes in precipitation seasonality and monsoon onset in West Africa and how vegetation dynamics may influence them
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