Effects of Mountain Height on the Atlantic and Pacific Thermohaline Circulations

Geological evidence indicates that mountain uplift, such as those in southern Asia and western North America, occurred on large scales during the middle and late Cenozoic. Models have shown that mountain uplift is associated with changes in monsoon due to alteration of wind patterns and shifting of areas of high precipitation. These changes are accompanied by drier conditions in the continental interiors, as seen in the central Asian desert, and SST decreases over the western Indian Ocean. In our experiments, we use a low-resolution, fully- coupled GFDL ocean-atmosphere-land surface model to investigate the effects of mountain height on the deeper ocean and focus on the thermohaline circulation of the Atlantic and Pacific Oceans. The series of experiments consists of the global orography being fixed at a certain fraction of the present day values (control case), ranging from 0% (ie flat orography) up to 140%. Land distribution and initial conditions are kept the same for all experiments. Depending on the mountain heights, the model in each experiment is integrated for 1000-2000 years. Results show that, in the extreme case of flat orography, there is a complete switch-over from a strong meridional overturning in the Atlantic to one in the Pacific. Moreover, the Pacific overturning, which reaches an equilibrium state within 800 years, is much stronger than that of the Atlantic in the control case (27Sv compared to 19Sv). The Pacific overturning is accompanied by deep downwelling, especially in the northwest Pacific, and a stronger Kuroshio current. Heat exchange between the atmosphere and ocean appears to be important to the maintenance of the Pacific overturning. As mountain height is increased, the maximum Pacific overturning strength attainable is reduced gradually. The scenario in which mountain height is set to 50% of that in the control case represents a threshold whereby strong overturning in the Atlantic starts to develop and after which there is no longer significant activity in the Pacific.