Ac. Hirst et Js. Godfrey, THE RESPONSE TO A SUDDEN CHANGE IN INDONESIAN THROUGHFLOW IN A GLOBALOCEAN GCM, Journal of physical oceanography, 24(9), 1994, pp. 1895-1910
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.