Hydraulics and mixing in the deep branch of the Indonesian Throughflow

The Indonesian Throughflow (ITF) is a conduit for mass, heat and freshwater transports from the Pacific to the Indian Ocean, playing an important role in regional and global climate. In addition to the well-known surface branch of the Indonesian Throughflow, there is a deep branch that carries water from the Philippine Sea into the Maluku Sea, through the narrow Lifamatola Passage, and then into the 5000 m deep Seram and Banda Seas. These waters then must ascend and exit into the Indian Ocean through the Timor Passage or Ombi Strait, each of which have sills of around 1000m depth. With its 2000m deep sill, the Lifamatola Passage is the main choke point in this path, and therefore an advantageous spot to measure the flow and perform long term monitoring. It is also a site of intense mixing (~10^-2-10^-1m²s^-1) and entrainment (0.6-1.2 Sv) due to the presence of a hydraulically controlled overflow with hydraulic jump-like features in the lee. The volume transport there has previously been estimated at 2.5 Sv by van Aken et al. (2009) and updated to 3.0-3.6 Sv (120-140%) by including the entrained overlying warmer water. We combine recent hydrographic observations at the Lifamatola sill in order to establish the nature and dynamics of the hydraulic control, to quantify the mixing and entrainment, and to suggest a strategy for long term monitoring. Estimates of heat flux are used to infer values of turbulent diffusivity (~1.3-1.6×10^-3m²s^-1) necessary to produce upwelling in the Banda Sea sufficient to close the mass budget of the hydraulically controlled overflow and downstream entrainment.