Seasonal and Interannual Variability of the Subsurface Velocity Profile of the Indonesian Throughflow at Makassar Strait

Although there have been many previous studies that have interpreted the variability of Indonesian Throughflow transport, the processes that determine its vertical structure have rarely been investigated due to limited observations and model inefficiencies. In this study, a regionally coupled model is developed to address this issue. The model adopts an unstructured model grid, with an similar to 3-km resolution within the Indonesian seas and straits, and reveals somewhat inconsistent results compared to previous models with coarser resolutions. The results suggest that the seasonal variability of the depth of the Indonesian Throughflow velocity maximum is partially controlled by the seasonally reversed Karimata throughflow, while the remainder primarily originated from the Mindanao-Sulawesi inflow rather than the Sibutu Strait throughflow. The Mindanao-Sulawesi inflow possesses a subsurface velocity core similar to that of the Makassar Strait, with a deep (shallow) maximum during El Nino (La Nina) years that plays a crucial role in determining the downstream Makassar throughflow profile in the interannual time scale. A sensitivity experiment by fixing Indian Ocean boundary condition shows that the impacts from the Indian Ocean might be significant only within the intraseasonal to seasonal time scales.