What is driving the ITF? An illumination of the Indonesian throughflow with a numerical nested model system

A nested numerical model system has been set up to realistically simulate more than 30 years of the Indonesian throughflow (ITF). A global circulation model delivered the boundary values for sea level, temperature and salinity distributions to a local model covering the region of the ITF. Both models were forced with NCEP data. Results of the regional model are in good agreement with measured data regarding velocity distribution and stratification, as well as transported water masses. Model results show a highly variable and very complex current system. The presence of a realistic throughflow has been simulated even with a barotropic pressure gradient directed from the Indian towards the Pacific Ocean. Furthermore, model experiences indicate that the intensity of the ITF is correlated with the seasonal wind system. It is concluded that the ITF is neither driven by a barotropic or baroclinic pressure gradient nor by local winds. The ITF seems to be, rather, the extension of the very strong tropical Pacific Ocean circulation system westward into the Indonesian seas, where the western boundary is not fully closed due to the passages between the Indonesian islands. A hypothesis for the physical reason is given to explain the existence of the Indonesian throughflow.