Coherent Changes of Northern and Eastern Equatorial Africa Rainfall during the Last Deglaciation

Abstract:

During the last deglaciation, proxy records indicate that wetter conditions developed abruptly at about 14,800 years ago in both northern and eastern equatorial Africa and continued into the Holocene. Explaining the abrupt beginning and coherent regional expression of the African Humid Period has remained challenging in view of opposing seasonal insolation patterns north and south of the equator. Our model-data comparison provides a mechanistic understanding of deglacial tropical precipitation change, and strengthens understanding of how tropical African precipitation responds to increased GHG forcing.

Using the CESM coupled climate model, we show that meltwater-induced reduction in the Atlantic Meridional Overturning Circulation (AMOC) during the early deglaciation suppressed the precipitation response to orbital and greenhouse gas (GHG) forcing. Once the AMOC reestablished, the subsequent persistence of wetter conditions north of the equator was a response to high summer insolation and increasing GHG concentrations, whereas wetter conditions south of the equator were a response primarily to the GHG increase.

Simulated changes in sea-surface temperature (SST) contributed to the deglacial precipitation increases in North Africa (NA) and Eastern Equatorial Africa (EEA) by decreasing the north-south SST gradient in the tropical Atlantic and the west-east gradient in the tropical Indian Ocean, which favor moisture convergence into and, in turn, precipitation in NA and EEA, respectively. These simulated changes agree with proxy records of deglacial SSTs in both the patterns and amplitudes of SST change. CESM sensitivity experiments indicate that the increases in GHGs are primarily responsible for the overall SST increases and their regional patterns.