THE RESPONSE TO A SUDDEN CHANGE IN INDONESIAN THROUGHFLOW IN A GLOBALOCEAN GCM

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 Indonesi an Throughflow. In one experiment, the model is run to equilibrium wit h the Indonesian passage completely closed off; The passage is then op ened, and the evolution of the system toward a new equilibrium is exam ined. In a second experiment, the model equilibrium solution with pass age open is slightly perturbed by application of a body force to the w ater in the passage. The force is such that the change in throughflow (an increase of about 5%) has vertical profile almost identical to tha t of the original throughflow. The changes that evolve in the second e xperiment are, after appropriate scaling, quantitatively similar to th ose in the first, thereby verifying the approximate linearity of the r esponse. The dynamics of this response are investigated with the aid o f several idealized small-perturbation experiments, in which the model is reconfigured with a flat bottom and to be initially at rest with h orizontally homogeneous density fields. It is shown that the extensive subsurface temperature responses in both the Indian and Pacific Ocean s primarily result from a process of adjustment akin to baroclinic wav e propagation of the first and second internal modes. The model's (app roximate) first internal mode response is fairly similar to that expec ted from viscous linear theory. However, temperature perturbations ass ociated with the second internal mode response are strongly distorted, in part by advection associated with the background currents. Tempera ture advection by the perturbation barotropic mode is unimportant exce pt locally in the Tasman Sea and Agulhas Retroflection regions. Large differences in the patterns of response obtained previously for shallo w and deep Indonesian sills; and for full versus buoyancy-driven-only throughflow, are interpreted in terms of preferential excitement of in ternal modes. Thus the model's baroclinic wave properties, and the spe ctrum of baroclinic modes excited by the throughflow change, appear ve ry important to the pattern and timing of the subsurface (and hence su rface) temperature response.