STABILITY OF NORTH-ATLANTIC DEEP-WATER FORMATION IN A GLOBAL OCEAN GENERAL-CIRCULATION MODEL

A global ocean general circulation model is forced using mixed boundar y conditions (i.e., a restoring condition on the upper-level temperatu re but using a fixed, specified surface salt flux). Freshwater flux an omalies lasting 5 years are then applied over the western half of the subpolar gyre in the northern North Atlantic. The current climate is f ound to be stable to anomalies that have salt deficits equivalent to a bout seven times that estimated for the ''great salinity anomaly'' of 1968-1982, although this value is a function of the duration over whic h the anomaly is imposed. Above this level the thermohaline circulatio n collapses to a state in which the zonally averaged overturning assoc iated with North Atlantic Deep Water formation is only about half its original value, the sea surface temperatures over the North Atlantic a re lowered, and both the subpolar and subtropical gyres have weakened horizontal transports. Various atmospheric feedbacks on the momentum a nd salt flux are then applied under a restorative condition on tempera ture. The feedbacks on the momentum flux do not have a significant imp act on the overturning, other than to increase the Ekman flow, while a modest recovery is possible if the salt flux feedback includes an enh anced divergence of freshwater out of the Atlantic basin. In contrast, the collapse is critically dependent upon the restorative condition o n temperature. This central role suggests that the heat flux feedback maintains the stability exhibited by the collapsed state modeled by Ma nabe and Stouffer.