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The Effect of Cold Climate upon North Atlantic Deep Water Formation in a Simple Ocean-Atmosphere Model

The sensitivity of North Atlantic Deep Water formation to variations in mean surface temperature is explored with a meridional-vertical plane ocean model coupled to an energy balance atmosphere. It is found that North Atlantic Deep Water formation is favored by a warm climate, while cold climates are more likely to produce Southern Ocean deep water or deep-decoupling oscillations (when the Southern sinking region is halocline covered). This behavior is traced to a cooling-induced convective instability near the North Atlantic sinking region, that is, to unstable horizontal spreading of a halocline that stratifies part of the region. Under the convective instability it is found that climate cooling is generally equivalent to increased freshwater forcing. This is because in a cold climate, high-latitude water masses approach the temperature of maximum density and the convectiondriving, upward thermal buoyancy flux induced by surface cooling becomes insufficient to overcome the stratifying effect of surface freshening (a downward buoyancy flux). An extensive halocline is then formed and this halocline interferes with the heat loss necessary for the steady production of North Atlantic Deep Water.