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secondary pathway of Indonesian Throughflow (ITF) that enters from its inflow portal of the northern Maluku Sea is assumed to recirculate in the Banggai - Maluku Sea, where its upper-layer circulation returns into the Pacific and deeper flows continue southward into Seram/Banda Sea via Lifamatola Strait. However, direct observation in this key region is still limited. Here, using direct CTD (Conductivity Temperature Depth) casts and shipboard ADCP (Acoustic Doppler Curent Profiler) measurement datasets from BUDEE (Banggai Upwelling Dynamics and Ecosystem Experiment) cruise in September 2023, we study stratification and structure of ITF water and its circulation in a meridional section along 125E. The results show the water mass stratification of the mixed layer having a depth of 0-75m with a temperature range of 26-29°C, while the thermocline layer has a depth of 28-407m with a temperature range of 23-26°C, and the deep layer has a depth of 407-1000m with a temperature value of 4-12°C. The current pattern of the Banggai Sea Maluku Sea at a depth of 52-400m is more dominantly influenced by non-tidal currents, the influence of tidal currents only covers a depth of 52m less than non-tidal currents.

The Halmahera Sea is part of the eastern entry route of the Indonesian throughflow (ITF) passages. The characteristics and transformation of the South Pacific water masses are presented based on the HYCOM model (Hybrid Coordinate Ocean Model) and direct observations of CTD (Conductivity, Temperature, Depth) acquired during the East Indonesian Expedition in October 2017 using RV Baruna Jaya VIII. This study is aimed at characterizing the typical water masses observed in the Halmahera Sea and tracking the transformation of the South Pacific water masses in the region. The results show that there was an indication of strong vertical mixing of maximum salinity of the South Pacific thermocline water masses, i.e., South Pacific Subtropical Water (SPSW), from ~35.2 PSU at isopycnal 24 – 25 kg m-3 becomes ~34.8 PSU in the Southern Halmahera Strait (around 200 km from the entry passage, the Northern Halmahera Strait), and rapidly decreases from the Southern Halmahera Strait and Obi Strait to become 34.6 PSU in the Maluku Sea. These preliminary findings suggest a strong mixing in the southern Halmahera Strait and Obi Strait.

Internal solitary wave (ISW) is an underwater wave, formed due to the evolution of internal tides. The presence of breaking ISWs in coastal areas can potentially influence coastal ecosystems through cross-shelf exchange mechanisms. The southern waters of the Kangean Island and the eastern waters of the Madura Island, known also as part of the northern Bali waters, have been an active habitat for ISW propagation. This study aims to identify and characterize ISW which potentially breaking onshore the Southern Kangean Waters (SKW) and the Eastern Madura Waters (EMW) using Sentinel 1 SAR (Synthetic Aperture Radar) imagery. ISW characterization was carried out using the Korteweg-de Vries (KdV) model. The ISW amplitudes which propagate to EMW varies from 1 - 28 m with a propagation speed of 1.60 - 1.98 m/s, while the ISW which propagate to the SKW has an amplitude of 1 - 60 m with a propagation speed of 0.83 to 2.17 m/s. The results show that the ISW propagating to the SKW has stronger amplitude, phase velocity, and horizontal-vertical velocities than the ISW propagating to the EMW. The maximum speed of the horizontal current triggered by ISW leading to the EMW is 0.03 - 0.17 m/s and the vertical velocity 0.11 - 4.53 cm/s; the maximum speed of horizontal velocity leading to the SKW is 0.02 - 1.29 m/s and the maximum speed of the vertical velocity 0.04 - 19.03 cm/s.

Indonesian seas have been known as the hotspot for internal waves (IWs) generation sites. The sinking tragedy of the Indonesian Naval Submarine, KRI Nanggala 402 on 21 April 2021 in the northern Bali Sea was highlights the need of detecting, mapping, and characterizing internal waves in Indonesian waters are compulsory mainly in the Indonesian Archipelago Sea Lanes (IASLs). IASLs is a gateway for sea crossings which can be used by civil navigations, intercontinental trade, and foreign military. This study focused on mapping the distribution and characteristics of internal waves qualitatively using EA 600 Single beam Echosounder (SBES) of the Indonesian Naval Vessel, KRI Rigel 933 during The Jala Citra Expedition 2-2022 Banda. The measurements were conducted around the IASLs route in the Bali Waters, Banda Waters and Buru Waters. The results show that the typical highfrequency non-linear internal waves (HFNWs) with an amplitude of less than 10 m were observed intermittently during the expedition, detected mostly in the narrow passages, rough topography and shallowing waters. Typical characteristics of internal solitary waves (ISWs) were also observed with typical amplitudes of ~20 - 65 m. An ISWs extreme events found in this study verified the "turbulent water" that was reported in in the IASLs around the Underwater Seamount (US) Nieuwerkerk waters, in 1925 during the Snellius Expedition. The detection of internal wave events using acoustic equipment, the single beam echosounder is considered effective which can later be used as the guidance for marine activities in the water column. [ABSTRACT FROM AUTHOR]

An autonomous underwater glider was deployed in the Maluku Channel during 1 December 2018 through 21 February 2019 to measure temperature and salinity profiles of the upper ∼1,000 m continuously. The measurements suggest intraseasonal variations of the Pacific water mass intrusions into the Maluku Channel, which has not been reported before. The North Pacific Tropical and Intermediate Waters are found to intrude episodically into the eastern Maluku Channel at the depths of 130–180 m and 250–300 m, respectively, during January through February of 2019, each lasting for a few days to a week. The Antarctic Intermediate Water was present in the middle and western Maluku Channel between 600 and 800 m with intraseasonal variability. The South Pacific Subsurface Water was only found to intrude into the eastern Maluku Channel between 250 and 300 m during the first two transections in early December of 2018. Concurrent meter data from three moorings in the Maluku and Talaud‐Halmahera Channels are used to verify the water mass movement. The subsurface and intermediate water mass variations here are found to be closely related to the swing of the Mindanao Current near the Maluku Channel. These intraseasonal water mass intrusions are important aspects of water exchange between the western Pacific Ocean and the Indonesian seas. Plain Language Summary: Indonesian Throughflow (ITF) carries a large amount of mass, heat and salt from the Pacific to the Indian Ocean, playing an important role in global ocean circulation and climate. The Maluku Channel is one of the main passages of the eastern Indonesian seas, the ocean circulation of which has been poorly observed in history. The water exchange between the Pacific and Indian Ocean through the eastern Indonesian seas is not clear so far. An autonomous underwater glider was used to collect salinity and temperature profiles of the upper 1,000 m in the Maluku Channel for nearly 3 months in the winter of 2018–2019. Three moorings were also deployed to measure the movement of the water masses. These measurements have shown for the first time that the North Pacific Intermediate Water, the Antarctic Intermediate Water, and the South Pacific Subsurface Water have intruded into the Maluku Channel, sometimes episodically. The presence of the Pacific water masses in the Indonesian seas has shed light on the pathway of the ITF, the importance of which is profound on global ocean circulation and climate change. Key Points: The North Pacific Intermediate Water intruded into the Maluku Channel at the depths of 250–300 m in the winter 2018/2019The Antarctic Intermediate Water was present in the middle and western Maluku Channel between 600 and 800 m with intraseasonal variabilityThe distribution of the water masses in the Maluku Channel is different and closely related to the retroflection of the Mindanao Current [ABSTRACT FROM AUTHOR]

The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multiyear current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60–450 m is northward toward the Pacific Ocean with a mean transport of 2.07–2.60 Sv (1 Sv ≡ 106 m3 s−1). In the depth range of 450–1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anticlockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83–2.25 Sv, which is balanced by three transports: 1) 0.62–0.93 Sv southward transport into the Seram–Banda Seas through the Lifamatola Passage, 2) 0.97–1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and 3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semienclosed basin. Significance Statement: The Indonesian Throughflow plays an important role in the global ocean circulation and climate variations. Existing studies of the Indonesian Throughflow have focused on the upper thermocline currents. Here we identify, using mooring observations, an intermediate western boundary current with the core at 800–1000-m depth in the Maluku Sea, transporting intermediate waters from the Pacific into the Seram–Banda Seas through the Lifamatola Passage. Potential vorticity balance suggests an anticlockwise gyre circulation in the intermediate Maluku Sea, which is evidenced by the mooring and model data. Transport estimates suggest northward countercurrent in the upper Maluku Sea toward the Pacific, supplied by the Lifamatola Passage transport and upwelling from the intermediate layer in the Maluku Sea. Our results suggest the importance of the intermediate Indonesian Throughflow in global ocean circulation and overturn. More extensive investigations of the Indo-Pacific intermediate ocean circulation should be conducted to improve our understanding of global ocean overturn and heat and CO2 storages. [ABSTRACT FROM AUTHOR]

The currents and water mass properties at the Pacific entrance of the Indonesian seas are studied using measurements of three subsurface moorings deployed between the Talaud and Halmahera Islands. The moored current meter data show northeastward mean currents toward the Pacific Ocean in the upper 400 m during the nearly 2-yr mooring period, with the maximum velocity in the northern part of the channel. The mean transport between 60- and 300-m depths is estimated to be 10.1–13.2 Sv (1 Sv ≡ 106 m3 s−1) during 2016–17, when all three moorings have measurements. The variability of the along-channel velocity is dominated by low-frequency signals (periods > 150 days), with northeastward variations in boreal winter and southwestward variations in summer in the superposition of the annual and semiannual harmonics. The current variations evidence the seasonal movement of the Mindanao Current retroflection, which is supported by satellite sea level and ocean color data, showing a cyclonic intrusion into the northern Maluku Sea in boreal winter whereas a leaping path occurs north of the Talaud Islands in summer. During Apri–July, the moored CTDs near 200 m show southwestward currents carrying the salty South Pacific Tropical Water into the Maluku Sea. [ABSTRACT FROM AUTHOR]

The Maluku Channel is a major opening of the eastern Indonesian Seas to the western Pacific Ocean, the upper-ocean currents of which have rarely been observed historically. During December 2012-November 2016, long time series of the upper Maluku Channel transport are measured successfully for the first time using subsurface oceanic moorings. The measurements show significant intraseasonal-to-interannual variability of over 14 Sv (1 Sv ≡ 106m3 s-1) in the upper 300m or so, with a mean transport of 1.04-1.31 Sv northward and a significant southward interannual change of over 3.5 Sv in the spring of 2014. Coincident with the interannual transport change is the Mindanao Current, choked at the entrance of the Indonesian Seas, which is significantly different from its climatological retroflection in fall-winter. A highresolution numerical simulation suggests that the variations of theMaluku Channel currents are associated with the shifting of the Mindanao Current retroflection. It is suggested that the shifting of the Mindanao Current outside the Sulawesi Sea in the spring of 2014 elevates the sea level at the entrance of the Indonesian Seas, which drives the anomalous transport through the Maluku Channel. The results suggest the importance of the western boundary current nonlinearity in driving the transport variability of the Indonesian Throughflow. [ABSTRACT FROM AUTHOR]

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Chlorophyll-a is the predominant phytoplankton pigment responsible for determining primary productivity. In the present study, we used satellite-based data of chlorophyll-a, surface wind, and precipitation from 2003 to 2019 to investigate the variability of chlorophyll-a in the northern Maluku Sea and the Sulu Sill and examine its generating mechanism. We found that the chlorophyll-a bloom in the northern Maluku Sea occurs during the southeast monsoon season, while in the Sulu Sill, the chlorophyll-a concentration is higher than that in the northern Maluku Sea and occurs throughout the year. In the northern Maluku Sea, the chlorophyll-a bloom is generated by coastal upwelling. The maximum southerly wind during the southeast monsoon generates the strongest offshore Ekman Mass Transport (EMT) in the northern Maluku Sea triggering coastal upwelling. However, the power spectra analysis of satellite-derived chlorophyll-a shows strong peaks and amplitudes at both fortnightly (MSf) and monthly (Mm) frequencies, indicating that tidal mixing is an important generating mechanism for chlorophyll-a blooms in the Sulu Sill. Shallow bathymetry in the Sulu Sill may aid tidal mixing in effectively transporting nutrients from the near bottom to the sea surface, increasing chlorophyll-a concentration. • Chlorophyll-a bloom in the northern Maluku Sea (Sulu Sill) occurs during the southeast monsoon season (throughout year). • In the northern Maluku Sea, the chlorophyll-a bloom is generated by coastal upwelling due to strongoffshore Ekman Transport. • In the Sulu Sill, tidal mixing is an important mechanism for chlorophyll-a blooms which has strong MSf and Mm signals. [ABSTRACT FROM AUTHOR]