The Effects of Younger Dryas Orbital Parameter and Atmospheric pCO₂ Changes on Radiative Forcing and African Monsoonal Circulation

Abstract. Differences in the Atlantic meridional overturning circulation (AMOC) from the Younger Dryas (YD) to the Holocene can be explained by, but not limited to, factors relating to atmospheric greenhouse gas concentrations, discharge of freshwater into the surface ocean, and changes in Earth's orbital parameters. Utilizing the Community Earth System Model (CESM1.0.5) with moderate resolution, this study evaluates how Younger Dryas seasonal and annual radiative forcing affect the climate change and variability. The Younger Dryas to Holocene changes in radiative forcing are mostly attributed to change in orbital parameters and to lesser extent to the relatively small rise in atmosphere pCO₂, which is supported by a comparison of model simulations with proxy reconstructions of sea surface temperature and oceanic δ¹⁸O. These factors led to increased precipitation and reduced transport of water masses in the North Atlantic Ocean. Atmospheric pCO₂ and orbital parameter changes are not substantial enough to explain the transition to the Younger Dryas northern hemispheric cooling. Younger Dryas to Holocene changes in the Monsoonal circulation over the African continent appears to be more affected by changes in orbital parameters than in atmospheric pCO₂ but underestimated compared to observed reconstructions from ice and sediment cores. Thus, additional mechanisms such as fresh water hosed-cooling and/or ice sheet-albedo effect need to be considered to explain the Younger Dryas to Holocene climate change and variability.