Multiple equilibria of the Late Eocene ocean circulation and their role in the Eocene-Oligocene transition

Abstract

One of the largest shifts in climate in the Cenozoic (last 66 million years) occurred around the Eocene-Oligocene transition (EOT, 33.9 Ma) and is marked by global cooling and the sudden appearance of a large continental ice sheet on Antarctica. The two main proposed explanations of this climatic shift are a decline in atmospheric CO2 and changes in ocean gateways influencing ocean circulation, though the exact mechanisms behind this are still debated. This thesis focusses on the idea that the rapid cooling could have been initiated by a shift in the Meridional Overturning Circulation (MOC) from one equilibrium state to another. To study this, equilibrium ocean states are obtained with the ocean-only fully implicit thermohaline circulation model (THCM), using two different paleobathymetries (40 Ma and 30 Ma) and two different patterns of surface forcing representing different CO2 levels. Under restoring boundary conditions a strong southern sinking MOC-pattern is found, which is in line with fully coupled Global Circulation Model (GCM) simulations, only stronger. When the upper layer salinity forcing is mirrored at the equator, a MOC-pattern with only northern sinking is found. Starting from these solutions, bifurcation diagrams of the equilibrium ocean circulation under changing freshwater flux forcing are calculated. In these bifurcation diagrams a range of different MOC-states are found to occur, including southern sinking, bipolar sinking and northern sinking states, most of which are connected. There is a range of boundary conditions for which multiple MOC-states occur, suggesting that a shift between two of these patterns, induced by e.g. random atmospheric fluctuations, could be possible. Specifically, a southern sinking MOC-state with weak positive (northern) overturning in the Atlantic is found under the same boundary conditions as the strong fully southern sinking pattern. A shift between these two MOC-states seems to agree with the proposed onset of the Atlantic Meridional Overturning Circulation (AMOC) around time of EOT, which could have induced southern hemisphere cooling. Furthermore, the observed meridional heat transport suggests that this shift in MOC-states would result in a cooler southern and warmer northern hemisphere. Since the results were found to be relatively similar for both paleobathymetries and surface forcings used, the possibility of a shift in MOC-pattern around the time of the EOT seems to be relatively independent of the exact timing of gateway openings and of CO2 levels.