Indonesian Throughflow in a coupled climate model and the sensitivity of the heat budget and deep overturning

The circulation of the Indian Ocean in the Hadley Centre Climate Model is characterized by a very strong Indonesian Throughflow of O(24 Sv) compared with observational estimates of O(10 Sv) and very weak deep inflow across 32 degreesS compared with observational estimates. The model throughflow has an upper core and a lower core of approximately equal magnitudes. We examine the driving terms for the throughflow and find that the upper core is driven by the wind stress curl and the lower core is driven by bottom pressure torques. The heat gained by the Indian Ocean from the atmosphere is approximately zero in the model, which is at odds with the observational estimates, which indicate: a net heating of the Indian Ocean of between 0.4 and 1.2 PW. We hypothesize that the small heat nux is a direct consequence of the large throughflow and weak overturning. Two sensitivity experiments with different throughflows are described. We find that the magnitude of the deep overturning in the coupled model is strongly related to the magnitude of the lower core of the throughflow. The deep overturning increases from 3 to 9 Sv, as the magnitude of the lower core is reduced from 12 to 0 Sv. We find that the sensitivity of the heat gain of the Indian Ocean to the magnitude of the throughflow is O(0.005 PW) per sverdrup. This weak sensitivity may be due to the fact that the changes in the throughflow in the sensitivity experiments take place in the lower (colder) core.