Abstract
The main objective of this study is to examine factors that control the Indonesian throughflow and its variability. The study consists of three parts, an inverse analysis of tracer data along the throughflow pathways, analysis of the flow structure in the Makassar Strait, and dynamical study of the western Pacific. The inverse model analysis reveals aspects of the mixing process that include seasonal and possibly interannually variable backflushing of water from the Banda Sea into the Makassar Strait. Vertical exchanges are significant, both through seasonal upwelling and downwelling, and through turbulence above the sills and rough bottoms. Along the Makassar-Flores-Banda pathway, strong baroclinic flows are observed during the southern monsoon, and small and more barotropic flows occur during the northern monsoon, suggesting different flow patterns for the upper and intermediate layer. Analysis of bottom moored Inverted Echo Sounder and pressure data combined with altimeter measurements in the Makassar Strait show the stretching and squeezing of the intermediate layer in response to ENSO variability. The highly baroclinic flow and the behavior of the thermal structure suggest a minimum of three layers is necessary to explain the dynamics of the throughflow. The seasonal variability of the throughflow, and its complex vertical structures are similar to the current structures in the western Pacific. Surface drifters and hydrographic data show that the flow at the surface there has a different phase compared with the intermediate water. The Mindanao Eddy - Halmahera Eddy interaction provide the mechanism for North Pacific and South Pacific water to enter the Indonesian Seas. The pressure buildup associated with the strengthening of the Halmahera Eddy possibly blocks the throughflow from the North Pacific during northern winter. Interannual variability plays an important role in the throughflow volume and structure. Higher pressure in the western Pacific during La Nina conditions will increase the throughflow in the upper thermocline via the Makassar Strait, and in the lower thermocline via channels east of Sulawesi Island.