The Role of Indonesian Throughflow in a Global Ocean GCM

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  • 1 CSIRO Division of atmospheric Research, Aspendale, Victoria, Australia
  • | 2 CSIRO Division of Oceanography, Hobart, Tasmania, Australia
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Abstract

The effect of the Indonesian Throughflow on the World Ocean circulation is examined by a series of experiments with a global ocean GCM. The principal objective is to gain an understanding of how ocean flows respond to the throughflow, and how these changes result in changes in the pattern of surface heat flux and sea surface temperature. Four model runs are conducted. The first run features an open Indonesian passage through which a nonzero net throughflow is permitted. The second run features a complete blockage of the Indonesian passage. The third run is designed to isolate the effects of purely buoyancy-driven throughfiow: the Indonesian passage is open but the net volume transport is required to be zero. The fourth run is designed to isolate the effects of nonzero net throughflow on the Indian Ocean, independent of interocean buoyancy differences: the Indonesian passage is open but the throughflow water is cooled and salted toward profiles characteristic of the east Indian Ocean in the absence of throughflow.

Comparison of the first and second runs shows that the throughflow generally warms the Indian Ocean and cools the Pacific. However, large changes in the surface temperature and heat flux are restricted to certain well-defined regions: the Agulhas Current/outflow, the Leeuwin Current region off western Australia, the Tasman Sea, the equatorial Pacific, and two bands in the midlatitude South Pacific. In contrast, large subsurface temperature changes are widespread across both oceans. Heat budget analysis indicates that the large surface responses are dependent on the subsurface temperature change being brought to the surface, either by strong wind-forced upwelling (as in the equatorial Pacific) or by vigorous mixing in convective mixed layers (as in the other regions). Over most of both oceans, such mechanisms are absent and surface heat-flux changes are small (a few W m−2). There, subsurface temperature perturbations are largely insulated from the surface and may extend via direct advection or baroclinic wave propagation. The additional beat is released upon encounter with upwelling or a convective mixed layer, which may be far removed from the source of the perturbation. Atlantic and far Southern Ocean effects are mostly very small, possibly because of our use of restoring upper boundary conditions. The third and fourth runs break the throughflow into its baroclinic and barotropic components. The baroclinic (buoyancy-driven) component affects surface beat flux strongly in the Leeuwin Current region but relatively weakly in the Agulhas Current and Tasman Sea. The barotropic component has the converse effect. Interocean heat exchange is discussed; the full throughflow transports a net 0.63 petawatts out of the Pacific Ocean, which represents about one-third of the total heat input into the model's equatorial Pacific.

Abstract

The effect of the Indonesian Throughflow on the World Ocean circulation is examined by a series of experiments with a global ocean GCM. The principal objective is to gain an understanding of how ocean flows respond to the throughflow, and how these changes result in changes in the pattern of surface heat flux and sea surface temperature. Four model runs are conducted. The first run features an open Indonesian passage through which a nonzero net throughflow is permitted. The second run features a complete blockage of the Indonesian passage. The third run is designed to isolate the effects of purely buoyancy-driven throughfiow: the Indonesian passage is open but the net volume transport is required to be zero. The fourth run is designed to isolate the effects of nonzero net throughflow on the Indian Ocean, independent of interocean buoyancy differences: the Indonesian passage is open but the throughflow water is cooled and salted toward profiles characteristic of the east Indian Ocean in the absence of throughflow.

Comparison of the first and second runs shows that the throughflow generally warms the Indian Ocean and cools the Pacific. However, large changes in the surface temperature and heat flux are restricted to certain well-defined regions: the Agulhas Current/outflow, the Leeuwin Current region off western Australia, the Tasman Sea, the equatorial Pacific, and two bands in the midlatitude South Pacific. In contrast, large subsurface temperature changes are widespread across both oceans. Heat budget analysis indicates that the large surface responses are dependent on the subsurface temperature change being brought to the surface, either by strong wind-forced upwelling (as in the equatorial Pacific) or by vigorous mixing in convective mixed layers (as in the other regions). Over most of both oceans, such mechanisms are absent and surface heat-flux changes are small (a few W m−2). There, subsurface temperature perturbations are largely insulated from the surface and may extend via direct advection or baroclinic wave propagation. The additional beat is released upon encounter with upwelling or a convective mixed layer, which may be far removed from the source of the perturbation. Atlantic and far Southern Ocean effects are mostly very small, possibly because of our use of restoring upper boundary conditions. The third and fourth runs break the throughflow into its baroclinic and barotropic components. The baroclinic (buoyancy-driven) component affects surface beat flux strongly in the Leeuwin Current region but relatively weakly in the Agulhas Current and Tasman Sea. The barotropic component has the converse effect. Interocean heat exchange is discussed; the full throughflow transports a net 0.63 petawatts out of the Pacific Ocean, which represents about one-third of the total heat input into the model's equatorial Pacific.

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