Feedbacks between climate and surface water in northern Africa during the middle Holocene

Observations indicate that the area of lakes and wetlands in northern Africa was considerably greater during the middle Holocene than at present. Simulations are designed to examine whether expanded surface waters may have had a significant impact on the strength of the summer monsoon of northern Africa. Three experiments with the National Center for Atmospheric Research community climate model (NCAR CCM3) are analyzed, a modern and two middle Holocene (6000 years before present) simulations, one with and one without prescribed expanded surface water. There is a significant increase in the strength of the summer monsoon in the middle Holocene simulation due to the enhanced seasonal insolation cycle. The addition of surface waters result in a June, July, and August mean increase in the net surface radiation (5%), an increase in the latent heat flux (30%), a decrease in the sensible heat flux (10%), and an increase in the near-surface specific humidity (>5%) compared to the middle Holocene simulation without surface water changes. The changes in these simulated climate variables are comparable in scale to changes due to orbital forcing alone. The expanded surface waters result in a cooling of the atmosphere and anticyclonic flow over the large water bodies in summer relative to the simulation without surface water changes. The combination of increased atmospheric moisture and altered circulation results in significant changes to the precipitation distribution in northern Africa including a small increase in the zonal mean July precipitation to the north of the lakes and a decrease to the south. The geographic distribution of the precipitation with surface water changes is qualitatively in better agreement with observations than the distribution with orbital forcing alone but still does not fully match the expansion implied by observations nor the expansion required to produce the simulated middle Holocene surface waters used in this study. The results of this study suggest that surface waters were an important factor in the climate of northern Africa during the middle and early Holocene and that they must be included for accurate simulation of this climate.