Freshwater transport estimates and the global overturning circulation: Shallow, deep and throughflow components

Meridional ocean freshwater transports and convergences are calculated from absolute geostrophic velocities and Ekman transports. The freshwater transports are analyzed in terms of mass-balanced contributions from the shallow, ventilated circulation of the subtropical gyres, intermediate and deep water overturns, and Indonesian Throughflow and Bering Strait components. The following are the major conclusions:1.Excess freshwater in high latitudes must be transported to the evaporative lower latitudes, as is well known. The calculations here show that the northern hemisphere transports most of its high latitude freshwater equatorward through North Atlantic Deep Water (NADW) formation (as in [Rahmstorf, S., 1996. On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics 12, 799-811]), in which saline subtropical surface waters absorb the freshened Arctic and subpolar North Atlantic surface waters (0.45+/-0.15Sv for a 15Sv overturn), plus a small contribution from the high latitude North Pacific through Bering Strait (0.06+/-0.02Sv). In the North Pacific, formation of 2.4Sv of North Pacific Intermediate Water (NPIW) transports 0.07+/-0.02Sv of freshwater equatorward. In complete contrast, almost all of the 0.61+/-0.13Sv of freshwater gained in the Southern Ocean is transported equatorward in the upper ocean, in roughly equal magnitudes of about 0.2Sv each in the three subtropical gyres, with a smaller contribution of