Effects of glacial meltwater in the GISS coupled atmosphere-ocean model - 2. A bipolar seesaw in Atlantic Deep Water production

The relationship between the Northern Hemisphere and Southern Hemisphere de ep water circulation systems is explored in experiments with gradual and im pulsive freshwater input through the St. Lawrence. With sufficient freshwat er volume input (50 Sv years), North Atlantic Deep Water (NADW) cessation o ccurs, as does substantial cooling in the Northern Hemisphere. The colder t emperatures are accompanied by increased mass and sea level pressure in the Northern Hemisphere, with corresponding lower pressure in the Southern Hem isphere. The low-pressure response occurs at high southern latitudes, consi stent with the Antarctic Annular Mode, the leading mode of variability in t he current climate. Stronger winds, associated with this increased cyclonic ity, intensify the Antarctic Circumpolar Current (ACC) with heat flux diver gences in the South Atlantic and convergences and warming in the Indian Oce an. Weddell Sea Bottom Water production increases in response to the South Atlantic high-latitude cooling and sea ice growth, hence acting as a "seesa w" with the decreasing NADW, and even global Antarctic Bottom Water increas es, although not as strongly. The initial "seesaw" response occurs within a few years, although it takes some 100 years to maximize due to the respons e time of the ACC. The South Atlantic cooling occurs approximately in phase with the North Atlantic, so the "seesaw" is not in temperature within that ocean basin; however, warming in portions of the southern Indian Ocean occ urs out of phase with the Northern Hemisphere cooling. NADW does not resume of its own accord once complete cessation occurs even when freshwater inpu t is stopped, but when increased evaporation is used to force NADW formatio n temporarily, Weddell Sea Bottom Water decreases accordingly.