Late Cretaceous Ocean: Coupled Simulations with the NCAR CSM

Deep ocean circulation may be a significant factor in determining climate. Here, we describe two long, fully coupled atmosphere-ocean simulations with the NCAR Climate System Model (CSM) for the late Cretaceous (80 Ma). Our results suggest that higher levels of atmospheric CO2 and the altered paleogeography of the late Cretaceous resulted in a surface ocean state temperature, salinity, and circulation, significantly different than present. This, in turn, resulted in deep-water features, although formed by mechanisms similar to present, that were quite different from present. The simulations exhibit large overturning cells in both hemispheres extending from the surface to the ocean bottom and with intensity comparable to the present-day North Atlantic simulated overturning. In the Northern Hemisphere, the sinking takes place in the Pacific due to cooling of the much warmer and saltier waters compared to present-day. In the Southern Hemisphere, the sinking occurs primarily in the southern Atlantic and Indian Oceans. For a simulation with atmospheric CO2 reduced from 6 times to 4 times pre-industrial concentrations, the southern branch is reduced by 35% due to less poleward transport of salty waters in the South Atlantic Ocean. Warm waters inferred from proxy data in deep-sea cores can be explained by the high-latitude sites of overturning. These results contradict the traditional hypothesis that warm Cretaceous ocean bottom waters must have formed by sinking in shallow low-latitude seas.