Simulated range of mid-Holocene precipitation changes from extended lakes and wetlands over North Africa

Abstract

Abstract. Enhanced summer insolation over North Africa induced a monsoon precipitation increase during the mid-Holocene, about 6000 years ago, and led to a widespread expansion of lakes and wetlands in the present-day Sahara. This expansion of lakes and wetlands is documented in paleoenvironmental sediment records, but the spatially sparse and often discontinuous sediment records provide only a fragmentary picture. Previous simulation studies prescribed either a small lake and wetland extent from reconstructions or focused on documented mega-lakes only to investigate their effect on the mid-Holocene climate. In contrast to these studies, we investigate the possible range of mid-Holocene precipitation changes in response to a small-lake extent and a potential maximum lake and wetland extent. Our study shows that during the summer monsoon season, the African rain belt is shifted about 2 to 7∘ farther north in simulations with a maximum lake or wetland extent than in simulations with a small lake extent. This northward extent is caused by a stronger and prolonged monsoon rainfall season over North Africa which is associated with an increased monsoon precipitation over the southern Sahara and an increased precipitation from tropical plumes over the northwestern Sahara. Replacing lakes with vegetated wetlands causes an enhanced precipitation increase, which is likely due to the high surface roughness of the wetlands. A moisture budget analysis reveals that both lakes and wetlands cause a local precipitation increase not only by enhanced evaporation but also by a stronger inland moisture transport and local moisture recycling to the south of Lake Chad and the west Saharan lakes. Analysis of the dynamic response shows that lakes and wetlands cause a circulation response inverse to the one associated with the Saharan heat low. Depending on the latitudinal position of the lakes and wetlands, they predominantly cause a northward shift or a decay of the African Easterly Jet. These results indicate that the latitudinal position of the lakes and wetlands strongly affects the northward extension of the African summer monsoon.

Contribution of Lakes in Sustaining Greening of the Sahara during the Mid-Holocene

“…Our study confirmed that Megalake Chad does not influence the northward monsoon movement when western lakes are missing (Broström et al, 1998;Carrington et al, 2001;Chandan & Peltier, 2020). We also confirmed the influence of Western Sahara lakes on the northward monsoon movement (Specht et al, 2022), but further stressed that Megalake Chad could extend the westerly monsoon eastward when accompanied by Western Sahara lakes. Chandan and Peltier (2020) underestimated the contribution of lakes by supposing that the weakened SHL induced by the surface cooling effect would reduce precipitation.…”

“…However, we found that such SHL weakening effect can be offset by the adjacent cyclonic circulation response in the lower atmosphere, which promotes precipitation. Moreover, we found that the northward WAM movement goes with a weakened AEJ and a strengthened TEJ, in agreement with Specht et al (2022). Therefore, we emphasized the importance of how the climate model represents the AEJ and TEJ behaviors in reproducing the MH climate (Claussen et al, 2017;Bercos-Hickey et al, 2020;Ngoungue et al, 2021).…”

“…Therefore, we emphasized the importance of how the climate model represents the AEJ and TEJ behaviors in reproducing the MH climate (Claussen et al, 2017;Bercos-Hickey et al, 2020;Ngoungue et al, 2021). Furthermore, regarding the lake position (Chandan & Peltier, 2020;Specht et al, 2022), we suggest that western lakes and Megalake Chad should be located in the WAM regions to induce the monsoon movement. Limited by the model integration and uncertainty, especially the dynamic lake or vegetation modules coupled with MIROC5-iso, the model-dependent findings of this study only focused on how the changes in the presence of lakes in terms of surface boundary conditions influence the GS hydroclimatic conditions without considering the climate reinforcement on lake expansion or shrinkage.…”

“…Chandan and Peltier (2020) further stressed that such a cooling effect could weaken the SHL and local convection, reducing the precipitation. On the contrary, Specht et al (2022) found that a weakened AEJ enhanced inland moisture transportation, leading to a northward and prolonged rain belt. Owing to these discrepancies, the mechanisms of lake-climate interaction in the NAf monsoon system remain unclear.…”

“…Tegen et al (2002) further indicated the presence of larger lakes and wetlands over the western Sahara based on dust emission simulations (hereinafter potential maximum-lake map; MH_02 in Tables 1 and S1). Based on the MH_98 and MH_02 lake maps, Specht et al (2022) investigated the impacts of the latitudinal position of lakes and wetlands on changes in precipitation, and initially highlighted the influence of western lakes on the northward WAM shift. These studies suggested that western lakes and Megalake Chad may play different roles in humidifying the Sahara, and this aspect requires further investigation.…”