Sensitivity of the Global Thermohaline Circulation to Interbasin Freshwater Transport by the Atmosphere and the Bering Strait Throughflow

Sensitivity of the global thermohaline circulation to interbasin freshwater transport by the atmosphere and the Bering Strait throughflow is investigated by using a free-surface, coarse-resolution ocean general circulation model. The model is run by prescribing freshwater flux at the sea surface without restoring the sea surface salinity to climatology in order that effects of salinity advection are properly represented. Comparison of experiments with the open and closed Bering Strait shows that the throughflow reduces the intensity of the Atlantic deep circulation by ∼17%, while minimally affecting the Pacific deep circulation. Increase in the atmospheric freshwater transport from the Atlantic to the Pacific intensifies both the Atlantic deep circulation and the Bering Strait throughflow. On the other hand, changes in the throughflow transport under a fixed amount of atmospheric interbasin freshwater transport are found to have a minor impact on the global thermohaline circulation. This insensitivity is realized because increased volume transport leads to increased salinity advection from low to high latitudes in the North Pacific and hence causes a salinity increase at the strait. Reducing net freshwater export from the Atlantic sea surface to nearly zero results in shutdown of the Atlantic deep circulation. The actual atmospheric freshwater transport anomaly required to shut the circulation down depends on the configuration of the Bering Strait, and the Atlantic deep circulation shows high sensitivity to the atmospheric freshwater transport around the shutdown.