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Stability of North Atlantic Deep Water Formation in a Global Ocean General Circulation Model

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  • 1 Bureau of Meteorology Research Centre, Melbourne, Australia
  • | 2 Centre for The Commonwealth Scientific and Industrial Research Organization, Division of Atmospheric Research, Aspendale, Victoria, Australia
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Abstract

A global ocean general circulation model is forced using mixes boundary conditions (i.e., a restoring condition on the upper-level temperature but using a fixed, specified surface salt flux). Freshwater flux anomalies lasting 5 years are then applied over the western half of the subpolar gyre in the northern North Atlantic.

The current climate is found to be stable to anomalies that have salt deficits equivalent to about seven times that estimated for the “great salinity anomaly” of 1968–1982, although this value is a function of the duration over which the anomaly is imposed. Above this level the thermohaline circulation collapses to a state in which the zonally averaged overturning associated with North Atlantic Deep Water formation is only about half its original value, the sea surface temperatures over the North Atlantic are lowered, and both the subpolar and subtropical gyres have weakened horizontal transports. Various atmospheric feedbacks on the momentum and salt flux are then applied under a restorative condition on temperature. The feedbacks on the momentum flux do not have a significant impact on the overturning, other than to increase the Ekman flow, while a modest recovery is possible if the salt flux feedback includes an enhanced divergence of freshwater out of the Atlantic basin.

In contrast, the collapse is critically dependent upon the restorative condition on temperature. This central role suggests that the heat flux feedback maintains the stability exhibited by the collapsed state modeled by Manabe and Stouffer.

Abstract

A global ocean general circulation model is forced using mixes boundary conditions (i.e., a restoring condition on the upper-level temperature but using a fixed, specified surface salt flux). Freshwater flux anomalies lasting 5 years are then applied over the western half of the subpolar gyre in the northern North Atlantic.

The current climate is found to be stable to anomalies that have salt deficits equivalent to about seven times that estimated for the “great salinity anomaly” of 1968–1982, although this value is a function of the duration over which the anomaly is imposed. Above this level the thermohaline circulation collapses to a state in which the zonally averaged overturning associated with North Atlantic Deep Water formation is only about half its original value, the sea surface temperatures over the North Atlantic are lowered, and both the subpolar and subtropical gyres have weakened horizontal transports. Various atmospheric feedbacks on the momentum and salt flux are then applied under a restorative condition on temperature. The feedbacks on the momentum flux do not have a significant impact on the overturning, other than to increase the Ekman flow, while a modest recovery is possible if the salt flux feedback includes an enhanced divergence of freshwater out of the Atlantic basin.

In contrast, the collapse is critically dependent upon the restorative condition on temperature. This central role suggests that the heat flux feedback maintains the stability exhibited by the collapsed state modeled by Manabe and Stouffer.

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