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The Response to a Sudden Change in Indonesian Throughflow in a Global Ocean GCM

Anthony C. HirstCSIRO Division of Atmospheric Research, Aspendale, Victoria, Australia

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J. S. GodfreyCSIRO Division of Oceanography, Hobart, Tasmania, Australia

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Abstract

The timescale and mechanisms of remote response in a global ocean GCM is investigated in the case of a sudden change in the rate of Indonesian Throughflow. In one experiment, the model is run to equilibrium with the Indonesian passage completely closed off. The passage is then opened, and the evolution of the system toward a new equilibrium is examined. In a second experiment, the model equilibrium solution with passage open is slightly perturbed by application of a body force to the water in the passage. The force is such that the change in throughflow (an increase of about 5%) has vertical profile almost identical to that of the original throughflow. The changes that evolve in the second experiment are, after appropriate scaling, quantitatively similar to those in the first, thereby verifying the approximate linearity of the response. The dynamics of this response are investigated with the aid of several idealized small-perturbation experiments, in which the model is reconfigured with a flat bottom and to be initially at rest with horizontally homogeneous density fields. It is shown that the extensive subsurface temperature responses in both the Indian and Pacific Oceans primarily result from a process of adjustment akin to baroclinic wave propagation of the first and second internal modes. The model's (approximate) first internal mode response is fairly similar to that expected from viscous linear theory. However, temperature perturbations associated with the second internal mode response are strongly distorted, in part by advection associated with the background currents. Temperature advection by the perturbation barotropic mode is unimportant except locally in the Tasman Sea and Agulhas Retroflection regions. Large differences in the patterns of response obtained previously for shallow and deep Indonesian sills, and for full versus buoyancy-driven-only throughflow, are interpreted in terms of preferential excitement of internal modes. Thus the model's baroclinic wave properties, and the spectrum of baroclinic modes excited by the throughflow change, appear very important to the pattern and timing of the subsurface (and hence surface) temperature response.

Abstract

The timescale and mechanisms of remote response in a global ocean GCM is investigated in the case of a sudden change in the rate of Indonesian Throughflow. In one experiment, the model is run to equilibrium with the Indonesian passage completely closed off. The passage is then opened, and the evolution of the system toward a new equilibrium is examined. In a second experiment, the model equilibrium solution with passage open is slightly perturbed by application of a body force to the water in the passage. The force is such that the change in throughflow (an increase of about 5%) has vertical profile almost identical to that of the original throughflow. The changes that evolve in the second experiment are, after appropriate scaling, quantitatively similar to those in the first, thereby verifying the approximate linearity of the response. The dynamics of this response are investigated with the aid of several idealized small-perturbation experiments, in which the model is reconfigured with a flat bottom and to be initially at rest with horizontally homogeneous density fields. It is shown that the extensive subsurface temperature responses in both the Indian and Pacific Oceans primarily result from a process of adjustment akin to baroclinic wave propagation of the first and second internal modes. The model's (approximate) first internal mode response is fairly similar to that expected from viscous linear theory. However, temperature perturbations associated with the second internal mode response are strongly distorted, in part by advection associated with the background currents. Temperature advection by the perturbation barotropic mode is unimportant except locally in the Tasman Sea and Agulhas Retroflection regions. Large differences in the patterns of response obtained previously for shallow and deep Indonesian sills, and for full versus buoyancy-driven-only throughflow, are interpreted in terms of preferential excitement of internal modes. Thus the model's baroclinic wave properties, and the spectrum of baroclinic modes excited by the throughflow change, appear very important to the pattern and timing of the subsurface (and hence surface) temperature response.

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