Global Thermohaline Circulation and Ocean - Atmosphere Coupling

A global ocean general circulation model (GCM) with idealized geometry (two basins of equal size, Marotzke and Willebrand, 1991) is coupled to an energy balance atmospheric model with nonlinear parameterizations of meridional atmospheric transports of heat and moisture. With the coupled model that prescribes the atmospheric heat and moisture transports, the North Atlantic meridional mass overturning rates at equilibrium increases as the global hydrological cycle strength increases. Furthermore, the equilibrium overturning rate is primarily controlled by the hydrological cycle of the Southern Hemisphere, whereas the Northern Hemispheric hydrological cycle has little impact. The transition of the thermohaline circulation from the conveyor belt to the southern sinking state is controlled by two factors, the hydrological cycle in Northern Hemisphere, and the ratio of hydrological cycle strengths between the Northern Hemisphere and the Southern Hemisphere. Increasing either of them destabilizes the thermohaline circulation . The large-scale dynamics of the North Atlantic overturning is mainly interhemispheric, with the bulk of the overturning rising in the Southern Hemisphere. Multiple intermediate states exist that are only quantitatively different, under very small salinity perturbations. The coupled feedbacks between the thermohaline circulation and the atmospheric heat and moisture transports are demonstrated to exist in the coupled model, and all of them are positive. In addition, it is identified that the coupled feedbacks associated with the atmospheric transports in the Southern Hemisphere are also positive. Two different flux adjustments are used in the coupled model, with one adjusting the atmospheric transports efficiencies, the other adjusting the surface fluxes. Different flux adjustments influence the coupled feedback intensities, and hence the stability of the thermohaline circulation. Thesis Supervisor: Peter H. Stone Title: Professor of Meteorology Thesis Supervisor: Jochem Marotzke Title: Assistant Professor of Physical Oceanography