A proxy compatible model for the YD and the subsequent greening of the Sahara

Throughout the Quaternary, northern Africa has experienced recurring periods of intensified precipitation, known as African Humid Periods. The most recent such period began after the termination of the Younger Dryas (YD), which was a dramatic reversion to ice-age temperatures during the deglaciation. One intriguing explanation for the timing of this most recent greening of the Sahara is that the rapid recovery of the Atlantic Meridional Overturning Circulation (AMOC) after the YD caused a northward shift of the Inter-Tropical Convergence Zone (ITCZ), resulting in increased precipitation in Northern Africa. In previous attempts to model the YD (Peltier et al., 2006; Peltier, 2007), and the subsequent transition to a Green Sahara, (Menviel et al. 2021, 2011), the total volume of freshwater forcing applied to the Arctic Ocean was quite large. The Eustatic Sea Level (ESL) increase associated with the freshwater influx in these studies is not compatible with proxy inferred ESL constraints. Furthermore, the increase in precipitation at the end of the YD was not nearly as abrupt as that which was simulated for the Green Sahara period at the end of the penultimate deglaciation, a fact that the authors attributed to the misalignment of the timing of AMOC maximum and the maximum in insolation forcing at the YD.

Here we present a model of the YD, in which only 0.15 Sv forcing for 100 years, applied to the Beaufort Gyre, was needed to collapse the AMOC and keep it in a collapsed state for nearly 1000 additional years. As the YD is approximately 1000 years long, we are able to achieve this interval of AMOC shutdown without continuously hosing the model with freshwater. As a result, the ESL rise in our model is physically plausible. Furthermore, unlike the results of Menviel et al. (2021, 2011), the simulated precipitation over North Africa in our model increases abruptly, in step with the abrupt resumption of the AMOC. The AMOC itself recovers due to a reinvigorated ocean-atmosphere flux exchange that occurs following the opening of a polynya in the Irminger Sea.