North African vegetation–precipitation feedback in early and mid-Holocene climate simulations with CCSM3-DGVM

R Rachmayani,M Schulz,M Prange

doi:10.5194/cp-11-175-2015

Abstract

Abstract. The present study analyses the sign, strength, and working mechanism of the vegetation–precipitation feedback over North Africa in middle (6 ka BP) and early Holocene (9 ka BP) simulations using the comprehensive coupled climate–vegetation model CCSM3-DGVM (Community Climate System Model version 3 and a dynamic global vegetation model). The coupled model simulates enhanced summer rainfall and a northward migration of the West African monsoon trough along with an expansion of the vegetation cover for the early and middle Holocene compared to the pre-industrial period. It is shown that dynamic vegetation enhances the orbitally triggered summer precipitation anomaly by approximately 20% in the Sahara–Sahel region (10–25° N, 20° W–30° E) in both the early and mid-Holocene experiments compared to their fixed-vegetation counterparts. The primary vegetation–rainfall feedback identified here operates through surface latent heat flux anomalies by canopy evaporation and transpiration and their effect on the mid-tropospheric African easterly jet, whereas the effects of vegetation changes on surface albedo and local water recycling play a negligible role. Even though CCSM3-DGVM simulates a positive vegetation–precipitation feedback in the North African region, this feedback is not strong enough to produce multiple equilibrium climate-ecosystem states on a regional scale.

Highlights

At present, North Africa is much drier than during the early and middle Holocene when a higher orbitally induced summer insolation triggered more humid and “greener” conditions in the Sahel and Saharan regions (e.g. Kutzbach and Street-Perrot, 1985; Street-Perrott and Perrott, 1993; Jolly et al, 1998; Kohfeld and Harrison, 2000; Prentice et al, 2000; Bartlein et al, 2011; Collins et al, 2013)
Even though these model studies have abundantly documented that an intensification and northward shift of the North African summer monsoon was induced by the enhanced seasonal insolation cycle during the early to mid-Holocene, rainfall anomalies simulated by the models turned out to be significantly smaller than those reconstructed from lake level or pollen data (e.g. Jolly et al, 1998; Kohfeld and Harrison, 2000)
The North African vegetation cover from the 0k(OAV) control run is shown in Fig. 1, where the 10 plant functional types (PFTs) simulated by the model are combined into two groups

Summary

Introduction

North Africa is much drier than during the early and middle Holocene when a higher orbitally induced summer insolation triggered more humid and “greener” conditions in the Sahel and Saharan regions (e.g. Kutzbach and Street-Perrot, 1985; Street-Perrott and Perrott, 1993; Jolly et al, 1998; Kohfeld and Harrison, 2000; Prentice et al, 2000; Bartlein et al, 2011; Collins et al, 2013). Following pioneering modelling work by Kutzbach (1981), numerous numerical climate model experiments have been conducted in order to examine climate sensitivity to Holocene orbital forcing at low latitudes, where insolation variations are strongly dominated by the precessional cycle. Even though these model studies have abundantly documented that an intensification and northward shift of the North African summer monsoon was induced by the enhanced seasonal insolation cycle during the early to mid-Holocene, rainfall anomalies simulated by the models turned out to be significantly smaller than those reconstructed from lake level or pollen data The notion of a positive vegetation–precipitation feedback has received the greatest attention in the literature (see Claussen, 2009).

Results

Discussion

Conclusion