A long CCSM3 transient simulation of the centennial to millennial variability of the hydrologic cycle in Africa

Proxy records from Africa identify large and abrupt changes in the hydrological cycle during the last deglaciation. Explaining the complex spatial and temporal variations in African hydrology faces two challenges. First, orbital forcing of local summer insolation should have reduced precipitation in the southern African tropics during the early Holocene in contrast to proxy evidence that indicates that the African Humid Period (AHP) encompassed this region. A second issue is that hydrologic changes tended to be abrupt, in contrast to the more gradual orbital forcing, and several of the changes occurred at the same time as the abrupt changes in Northern Hemisphere deglacial climate.

To address these issues, we use a transient simulation of the climate evolution from the LGM (21,000 years ago) to the early Holocene (10,000 years ago) with the coupled atmosphere-ocean general circulation model, the Community Climate System Model version 3 (CCSM3). With realistic time-varying boundary conditions and forcings, the model successfully simulates the deglacial climate evolution of the North Atlantic region, including the collapse of North Atlantic Deep Water (NADW) and Greenland cooling in response to increased meltwater flux into the North Atlantic, and the abrupt resumption and overshoot of NADW and Greenland warming at the onset of the Bolling-Allerod associated with the sudden decrease in meltwater flux.

Our simulation simulates an AHP in both the Sahel and the equatorial southeast African Great Lakes region, with increased precipitation during their respective rainy seasons. In the Sahel region, the orbital summer insolation changes explain the increase in precipitation, with the deglacial increase in CO₂ having a secondary influence. In contrast, orbital forcing does not contribute to the simulated precipitation increase in the Great Lakes region. Rather, CCSM3 indicates that the deglacial increase in CO₂ is responsible for the AHP in this region. The start of the AHP is abrupt in the Sahel, correlating well and thus suggesting a strong influence of the North Atlantic region. Subsequent millennial variations in precipitation also occur synchronously with variations in NADW and North Atlantic climate. A less clear connection between North Atlantic climate and precipitation in the Great Lakes region of Africa is apparent in our simulation.