Late Quaternary Hydrologic Changes in the Arid and Semiarid Belt of Northern Africa

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The zonal climate pattern associated with the Hadley cell circulation is best exemplified in northern Africa, with its Mediterranean northern tip, subtropical Sahara desert, and belts of monsoonal and equatorial climates related to the seasonal migration of the Intertropical Convergence Zone (ITCZ). In the past, astronomical forcing has been the prime factor driving the meridional shifts of these climate belts, but feedback processes from oceans and land surfaces have amplified and modified the direct effects of insolation changes. This chapter uses selected groundwater, paleolake, and paleobotanic data to illustrate changes in precipitation, moisture sources and trajectories, and wind intensity over the Sahara and its southern semiarid margins (~30°N-10°N) over the past 25,000 years. This time interval spans a wet Late Pleistocene phase, followed by two periods of extreme dry and wet conditions, the last glacial maximum (LGM) and the early to mid-Holocene, respectively. During the cool Late Pleistocene wet period, data indicate a strengthening and a southward displacement of extratropical cyclonic disturbances associated with an equatorial shift of the subtropical westerly jet and of the Saharan anticyclone. During the LGM, the generally dry conditions are in good agreement with model simulations. Northern Africa climates responded to reduced summer insolation over the Northern Hemisphere, associated with a stronger northern branch of the Hadley cell circulation, tropical cooling, and a global decrease in water vapor. Data from the early to mid-Holocene wet period show a northward migration of the tropical rainfall belts as far as 20°N-24°N. They suggest a strengthening of the ITCZ, a northward migration of 5°-6° of the core of the upper-level easterly jets, and a recycling of water vapor along the West African Monsoon (WAM) flow toward the southeastern Sahara. North of 20°N-24°N, moisture was most likely of extratropical origin, as is the case today. The best agreement between observations and simulations is found with coupled ocean-atmosphere-vegetation models. Orbital forcing enhanced the land-sea pressure gradient; feedbacks from the vegetation and the oceans amplified the intensity and the northward penetration of WAM rainfalls and the length of the monsoon season.