The thermohaline circulation during the Last Glacial Maximum and in the Present-Day climate

Abstract: The thermohaline circulation (THC) corresponds to the large time- and spatial-scales ocean circulation associated with the transport of heat and salt, and is known to be an important factor controlling the climate variability. The large scales involved in the THC make it difficult to observe, and therefore the synergy of numerical models and climate proxy reconstructions is particularly relevant to study the characteristics of this circulation in the present and past climates.In this doctoral thesis, the THC during the Last Glacial Maximum (LGM) and the Present-Day (PD) is explored using a state-of-the-art Ocean General Circulation Model in its high- and low-resolution regimes. By comparing the LGM model outputs with the paleo-proxy reconstructions, it is shown that the high-resolution simulation improves the representation of the sea surface tem- peratures in the regions where the current structures appear to be complex, i.e., the western boundary currents (Agulhas, Kuroshio, Gulf Stream) and the Antarctic Circumpolar Current, although statistical comparisons with paleo- proxy reconstructions are not significantly improved on a global scale.The THC involves a superposition of processes acting at widely different spatial and temporal scales, from the geostrophic large-scale and slowly-varying flow to the mesoscale turbulent eddies and at even smaller-scale, the mixing generated by the internal wave field. Not all these processes can be properly resolved in numerical models, and thus need to be parameterized. Analyzing the THC in an eddy-permitting numerical model, it was found that the temporal scales required for diagnosing the Southern Ocean circulation should not exceed 1 month and the spatial scales needed to be taken into account must be smaller than 1°. Important changes in the nature and intensity of the THC were observed between the LGM and PD simulations. An estimation of the turnover times (i.e., the time it takes for the water parcel to make and entire loop on the Conveyor Belt) revealed that the LGM THC could be more vigorous than under the PD conditions. As a result, the ocean transports of heat and freshwater, the oceanic uptake of CO2, the ventilation of the deep ocean and the reorganization of the passive and active tracers (e.g., temperature, salinity, greenhouse gases, nutrients) can be altered in these different regimes.