Moored Observations of the Timor Passage Currents in the Indonesian Seas

The structure and variability of the currents in the Timor Passage are studied based on the measurements from a subsurface mooring deployed in the center of the passage from September 2017 to December 2019. The annual mean currents are found to flow westward into the Indian Ocean in the upper 1,400 m, with a maximum velocity core at ∼50 m. The mean volume transport through the Timor Passage into the Indian Ocean is estimated to be −9.9 ± 1.0 Sv (1 Sv = 106 m3 s−1) in the upper 1,400 m with much of this transport concentrated in the upper 480 m (−8.9 Sv). This value contrasts with the estimate of −7.5 Sv above 1,890 m based on INSTANT measurements. The transport during the 2019 positive Indian Ocean Dipole event was only slightly larger than that during the 2018 normal year in the upper 480 m. The variability in the along‐strait velocity in the Timor Passage is dominated by an annual cycle in the upper 150 m and a semiannual cycle in the lower layer. The annual cycle in the upper layer is mainly driven by local monsoonal forcing. Downward energy propagation is observed below 200 m, which is shown to be mainly driven by remote Kelvin waves from the Indian Ocean. The transport variability through the Timor Passage is dominated by a semiannual cycle, associated with the semiannual circulation over the equatorial Indian Ocean, with the transport in the upper and lower layers largely canceling each other in the annual cycle. Plain Language Summary Based on the INSTANT mooring measurements, the transport through the Timor Passage contributes a large portion of the transport of the Indonesian Throughflow (ITF). However, due to the scarcity of the observations, the transport and variability of the Timor currents have not been adequately studied. In this study, we show that the currents in the upper 150 m, measured by an acoustic Doppler current profiler moored at the valley center of the Timor Passage, feature strong annual harmonics associated with local monsoonal forcing. Semiannual harmonics below 200 m reveal upward phase propagation associated with remote forcing from the Indian Ocean. The transport variability through the Timor Passage features a stronger semiannual cycle in the upper 480 m instead of an annual cycle, as observed in the local annual winds, due to the mutual cancelation of the annual transports in the upper and lower layers. Thus, the baroclinic processes play an important role in producing the semiannual‐dominant variability of the ITF throug