Investigating the Role of the Tibetan Plateau in the Formation of Pacific Meridional Overturning Circulation

The effects of the Tibetan Plateau (TP) on the Pacific Ocean circulation are investigated using a fully coupled climate model. Sensitivity experiments are designed to demonstrate that the presence of the TP is the reason for the lack of strong deep water formation in the subpolar North Pacific, because removing the TP in the model would enable the establishment of the Pacific meridional overturning circulation (PMOC). The processes involved are described in detail as follows. Removing the TP in the model would excite an anomalous high pressure over the subpolar North Pacific, causing anomalous Ekman downwelling that enhances surface water subduction north of 40 degrees N. Removing the TP would also lead to less freshwater flux into the western Pacific, increasing sea surface salinity over the region. The high-salinity surface water can then be advected northward and eastward by the Kuroshio and its extension, subducting along the 26-27 sigma(theta) isopycnal surfaces to the deeper ocean, which enables the formation of deep water in the North Pacific and the setup of the PMOC. Afterward, more high-salinity warm water would be transported from the tropics to the extratropics by the Kuroshio, leading to the establishment of the PMOC. The role of the Rocky Mountains is also examined in this study. We conclude that the Rocky Mountains may play a trivial role in modulating the meridional overturning circulations in both the Pacific and Atlantic Oceans.