CLIMATIC AND OCEANIC RESPONSE TO YOUNGER DRYAS AGE FORCINGS IN THE COMMUNITY EARTH SYSTEM MODEL VERSION 1.2

The Younger Dryas was one of the last periods of abrupt climate cooling for the northern hemisphere at approximately 12.9 kya, and the mechanisms by which the event occurred remain under investigation. Utilizing the fully coupled Community Earth System Model version 1.2 at a moderate-resolution, this study examines the climatic and oceanic response to the application of a 13 kya ICE-5G ice sheet, a 0.3 Sv maximum freshwater forcing scheme, and orbital and atmospheric pCO₂ forcings. With these additions, sea surface temperatures and sea surface salinity in the region of deep water formation in the Northern Atlantic Ocean decreases by approximately 12°C and 6 psu respectively. The freshening of the area of deep water formation leads to a decrease in the strength of Atlantic meridional overturning circulation (AMOC) by approximately 20 Sv, which corresponds with proxy reconstructions for the AMOC. With the reduction in the strength of the AMOC, the surface air temperature over Summit, Greenland decreases by approximately 15°C compared to the Holocene, which is in agreement with proxy reconstructions from the GISP2 ice core. The sensitivity of the AMOC resumption time to the rate of freshwater input decrease was also tested, one at a 50-year reduction and one at a 100-year reduction, and both induce an AMOC resumption time of approximately 65 years, which is in agreement with reconstructions that suggest the AMOC resumption was on the order of decades following the Younger Dryas. This also indicates that the AMOC is not sensitive to the rate of freshwater input reduction. The AMOC overshoots its initial strength by approximately 5 Sv, which is also seen in previous climate modeling studies. The re-strengthening of the AMOC is supported by an increase in sea surface temperature and sea surface salinity by approximately 5°C and 6 psu respectively. Surface air temperature increases by approximately 10°C once the AMOC begins to strengthen. Model output was also compared to temperature and oxygen isotope reconstructions for select time slices, one at the beginning of the freshwater input, one at the maximum, one at the end of the freshwater reduction, and one 100 years after the freshwater was turned off. Each of the time slices match agreeably with the temperature and oxygen isotope observations.