The Indonesian Throughflow plays an important role in the global ocean circulation and climate. Existing studies of the Indonesian Throughflow have focused on the Makassar Strait and the exit straits, where the upper thermocline currents carry North Pacific waters to the Indian Ocean. Here we show, using mooring observations, that a previous unknown intermediate western boundary current (with the core at ~1000 m depth) exists in the Maluku Sea, which transports intermediate waters (primarily the Antarctic Intermediate Water) from the Pacific into the Seram-Banda Seas through the Lifamatola Passage above the bottom overflow. Our results suggest the importance of the western boundary current in global ocean intermediate circulation and overturn. We anticipate that our study is the beginning of more extensive investigations of the intermediate circulation of the Indo-Pacific ocean in global overturn, which shall improve our understanding of ocean heat and CO2 storages significantly.

The ocean salinity is an important climate variable that plays an important role in surface mixed layer dynamics (Lagerloef, 2002;Ren et al., 2011). In the eastern tropical Indian Ocean (ETIO), the spatial distribution of sea surface salinity (SSS) shows a stark east-west contrast with freshwater concentrated near the eastern boundary (Donguy & Meyers, 1996). Precipitation (P) and evaporation (E) are major forcings of the SSS distribution over the ETIO. Low salinity in the ETIO and the Bay of Bengal (BOB) has been attributed to large amounts of precipitation and river runoff, while high salinity in the Arabian Sea is caused by excessive evaporation (Kumar & Prasad, 1999).Regional ocean circulation plays an essential role in SSS distributions (Nyadjro et al., 2011;Yu, 2011). In the northern ETIO, the currents are dominated by the Indian monsoon, and salinity variations are mainly associated with the semiannual monsoons (D'Addezio et al., 2015). The water exchanges between the Arabian Sea and BOB maintain a long-term salinity balance. The salty water from the Arabian Sea is carried by the Southwest Monsoon Current into the BOB in summer, and the fresh water from the BOB is conveyed by the Northeast Monsoon Current into the