Hysteresis of the Glacial Atlantic Meridional Overturning Circulation Controlled by Thermal Feedbacks

The multiple equilibrium behavior of the Atlantic meridional overturning circulation (AMOC) is related to the mode transition of the AMOC, which played an important role in abrupt glacial climate changes. However, the difference between modern and glacial climates is not well understood. Here, we examined the structure of the multiple equilibria of the AMOC under the modern and glacial climates through hysteresis experiments using an earth system model of intermediate complexity. The glacial AMOC exhibited a shallower AMOC mode but not the off mode, and a narrower range of multiple equilibria than the modern AMOC. This glacial hysteresis behavior occurred associated with a shift of deep water formation due to changes in surface air temperature and sea ice, whereas that of the modern AMOC is attributed to the basin-scale salinity feedback. These features were commonly found in two glacial simulations with different strengths of the AMOC at the Last Glacial Maximum.