Competing effects of sea ice change control the pace and amplitude of millennial-scale climate oscillations

Despite an increasing number of climate simulations showing millennial-scale oscillatory regimes, such as Dansgaard-Oeschger cycles, the mechanisms behind past abrupt climate changes remain elusive. Based on previous experiments that simulated such variability under Last Glacial Maximum boundary conditions forced with fixed freshwater snapshots derived from the early last deglaciation ice sheet history, this paper investigates the AMOC oscillatory mechanisms under different climate forcings (i.e., different levels of CO2 concentrations and varying orbital parameters). Our results show that sea ice plays a key role as a pacer, regulating AMOC transitions between strong/interstadial and weak/stadial modes. At lower CO2 levels, sea-ice volume increases and the warm-mode duration is reduced through enhanced summer sea ice melt. In contrast, higher levels of CO2 lead to thinner sea ice and in turn, cooler North Atlantic subsurface temperatures and suppressed oscillations. Orbital changes influence seasonality and localised sea ice dynamics, shortening or lengthening strong AMOC periods based on obliquity variations. These simulations, performed with the HadCM3 general circulation model, show that small climate changes can impact the shape and existence of oscillations in glacial climates, potentially explaining the variability in the periodicity and amplitude of Dansgaard-Oeschger cycles and transitions from weak to strong AMOC states.