Role of Paleotides for the ocean circulation during Termination I

Tides, as one of the principal drivers of the Global Meridional Overturning Circulation (GMOC), play a critical role in the global climate system, yet their dynamic representation within climate models remains inadequate, often neglecting the temporal evolution of tidal mixing and its interaction with background climate states. This thesis investigates the overarching hypothesis that the influence of paleo-tides on large-scale ocean circulation is not dictated by tidal strength alone, but is fundamentally modulated by the state of oceanic stratification. By explicitly incorporating temporally evolving tidal processes into an ocean model, we analyze this interplay across two distinct climatic intervals: the Last Glacial Maximum (LGM) and the Bølling-Allerød (BA) interstadial. Our results show that during the LGM, when barotropic tidal processes were nearly three times stronger than today, the pronounced oceanic stratification of the glacial epoch restricted this enhanced tidal energy from strengthening the Atlantic Meridional Overturning Circulation (AMOC). Instead, under these highly stratified conditions, strong tides primarily facilitated the formation of Antarctic Bottom Water (AABW). In sharp contrast, during the Bølling-Allerød interstadial, the combination of still-robust tidal mixing with significantly reduced stratification—owing to global warming—allowed tides to substantially amplify the AMOC through positive feedback mechanisms. This interplay not only contributed to the rapid intensification of the AMOC but also increased its resilience to freshwater perturbations from deglaciation. Collectively, these findings reveal a crucial state-dependency of tidal influence, underscoring the importance of incorporating the dynamic coupling between tidal mixing and oceanic stratification in paleoclimate simulations, as this interaction is a key mechanism governing the stability and response of large-scale ocean circulation during major climate transitions.