Role of the biosphere in the mid‐Holocene climate of West Africa

Abstract

[1] In previous studies, a zonally symmetric, synchronously coupled biosphereatmosphere model (ZonalBAM), which includes explicit representation of ecosystem dynamics, has been developed and validated based on current conditions over the region of West Africa. Here, we use ZonalBAM to study the response of the coupled biosphereatmosphere system to changes in the Earth's orbital forcing during the Middle Holocene (6K yrs BP) and the relative contribution of vegetation feedbacks. Simulations in which vegetation conditions were fixed to the current distribution, show that an orbitally induced increased seasonality in insolation for the Middle Holocene, by itself, results in a 1.1°n orthward shift in the location of the southern margin of the Sahara as compared to current solar forcings. When vegetation is allowed to be dynamic, a 2.4°northward shift is simulated. However, when dynamic vegetation is initialized to palaeovegetation, a 5.1°n orthward shift is simulated, bringing results more consistent with palaeoevidence. Based on previous studies on the role of the gradient of moist static energy on the dynamics of large-scale tropical circulations, a mechanism for the enhancement of the summer monsoon circulation has been developed. Our results suggest that multiple equilibria could have coexisted over the region of West Africa during the Middle Holocene. Furthermore, based on previous studies on the current climate over the region, we hypothesize that transitions between the different equilibria could have taken place during the Middle Holocene causing the southern desert margin to migrate between 18.1°N and 21.4°N and shaping climate variability.

Changes in Precipitation Seasonality in West Africa Predicted by RegCM3 and the Impact of Dynamic Vegetation Feedback

“…Following Sultan and Janicot [20], here we define the monsoon onset as the timing of the sudden shift of rain maximum from [5][6] • N to [8][9][10] • N. The monsoon onset is characterized by an abrupt increase of rainfall at about 10…”

“…Studies on paleoclimate indicated that vegetation feedback plays an important role in sustaining the extended grassland cover during the mid-Holocene and in the subsequent abrupt transition from a green Sahara to a desert later during the mid-Holocene [6,[9][10][11][12]. Specifically, vegetation dynamics enhances the response of the West African climate system to changes in solar insolation, leading to the drastic differences in climate and vegetation distribution between mid-Holocene and the present day [10].…”

“…Specifically, vegetation dynamics enhances the response of the West African climate system to changes in solar insolation, leading to the drastic differences in climate and vegetation distribution between mid-Holocene and the present day [10]. For the 20th century climate, Xue and Shukla [4] found that desertification in the Sahel region (in the form of land cover change from natural vegetation to desert) 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 (and sustains) the impact of both oceanic forcing and man-made desertification in causing the 20th century Sahel drought.…”

Contribution of oceanic and vegetation feedbacks to Holocene climate change in monsoonal Asia

“…Claussen and Gayler, 1997;Broström et al, 1998;Irizarry-Ortiz et al, 2003;Levis et al 2004;Hales et al, 2006). Nevertheless, some studies include or even focus on the Asian monsoon (e.g.…”

Detecting hotspots of atmosphere–vegetation interaction via slowing down – Part 2: Application to a global climate model

“…North Africa is a region where atmospherevegetation interactions have been particularly important during the Holocene (Claussen, 1998). In a number of climate models rapid transitions due to strong feedbacks (Claussen et al, 1999;Renssen et al, 2003) and multiple equilibria (Claussen, 1994(Claussen, , 1997(Claussen, , 1998Zeng and Neelin, 2000;Wang and Eltahir, 2000;Irizarry-Ortiz et al, 2003) have been found.…”