Indonesian Seas Research Articles

Indonesian policy and researches toward 70% reduction of marine plastic pollution by 2025

Marine plastic litter has become an emerging pollution issue in Indonesia. In this review, we examined policies related to plastic litter and their impact on reducing plastic litter in Indonesian seas and analyzed the progress in marine plastic research to contribute to the policy. Then, synthesized gaps to improve policy and possible scientific contributions to reduce marine plastic pollution. Our review showed that Indonesia's policy on reducing marine plastic litter pollution was largely focused on land-based sources, lack of sea-based leakage, and limited community behavior change in plastic use. Most research was done in the central region of Indonesia where a hotspot of marine plastic pollution occurred, and understudied in western and eastern regions. The national policy to reduce 70% of marine plastic litter entering the Indonesian seas by 2025 requires an extended period. The active role of local governments, improvement of solid waste management information system, and establishment of a marine plastic research agenda are among the most crucial for achieving national targets.

Effects of High‐Frequency Flow Variability on the Pathways of the Indonesian Throughflow

AbstractPrevious studies have shown the presence of strong mesoscale eddy activities in the Indonesian Seas and their influence on the transport and water mass properties of the Indonesian Throughflow (ITF), a mean flow from the Pacific Ocean to the Indian Ocean through the Indonesian Archipelago. This study explores the effects of these eddy activities, or high‐frequency flow variability (HFFV), on residence time and pathway of the ITF by conducting Lagrangian particle tracking experiments using a velocity field from an eddy‐resolving ocean general circulation model. Particles are released at key locations in the western and eastern routes of the ITF and tracked both backward and forward in time. To assess the effects of flow variability that has a time scale longer than a day but shorter than a month, the definition of HFFV in this study, we conduct parallel experiments using daily and monthly averaged velocity fields. Particle trajectories reveal the contrasting circulation characteristics of the Sulawesi and Banda Seas. HFFV in the Sulawesi Sea (in the western route) is high, causing water to circulate longer over a broader area. The longer residence time in the Sulawesi Sea helps the upwelling of the inflowing Pacific waters, especially the intermediate water masses, to rise above 300 m at the Makassar Strait, and also has the potential to allow mixing processes to modify the water mass properties of the ITF. In contrast, HFFV is much lower in the Banda Sea and has minimal effects on the ITF.

Seasonal response of coccolithophores and its potential to reconstruct paleomonsoon in the eastern Indonesian seas: an overview

Coccolithophore is a group of marine calcifying phytoplankton and its living distribution is sensitive to ecological change within the photic zone. Their coccoliths contribute majorly to the marine sediments and have become a reliable proxy in paleoceanography and paleoclimatology. The upper hydrology of the eastern Indonesian seas has been strongly influenced by seasonal climate change, the Australian-Indonesian monsoon. We review and discuss the result of previous studies on the response of living coccolithophore to seasonal change and the significance of their coccoliths as a proxy for reconstructing past dynamic of monsoon in the region. Coccolithophores abundance, species assemblages and coccolith malformation change in respond to the shift of photic environment condition modulated by reversing monsoon with particular responds across the region. Coccoliths variations in the marine sediment core archives from off southwest Sumba, eastern Banda Sea and Halmahera Sea show an insignificant control of glacial-interglacial climate variability to paleoproducitivity. Primary productivity based on the relative abundance of Florisphaera profunda has contributed to reconstruction of the past dynamic of Australian-Indonesian monsoon. However, more extensive and continuous observation of coccolithophores and their relationship with the photic environment in the eastern Indonesian seas is necessary for a better understanding of their response to monsoon dynamics in the present, past and future.

Assessment of the Sea Surface Salinity Simulation and Projection Surrounding the Asian Waters in the CMIP6 Models

Sea surface salinity (SSS) is a crucial indicator that is used to monitor the hydrological cycle in the ocean system. In this study, we evaluated the simulation skill of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models in reproducing the SSS in the Asian Marginal Seas (AMSs). The results show that the AMSs’ SSS simulated by most CMIP6 models is generally in good agreement with the observations in terms of spatial patterns and seasonal variability. However, these models tend to overestimate the SSS in the Eastern Arabian Sea and the Bay of Bengal by up to 1.3 psu, while they underestimate the SSS in the Bohai Sea, the Yellow Sea, the Southern South China Sea, and the Indonesian Seas, with the bias exceeding −1.5 psu. Additionally, the seasonal variations in the Sea of Okhotsk, the Bay of Bengal, and the Arabian Sea exhibit large biases with phase shift or reversal in some CMIP6 models. Notably, the observed magnitudes in the AMSs are significantly higher than the global average of 0.2 psu, ranging from 0.22 to 1.19 psu. Furthermore, we calculated the projected trends in sea surface salinity under different future scenarios by using the CMIP6 models. The results reveal relatively larger SSS freshening trends in the second half of the 21st century compared to the first half. Specifically, the freshening trends for the Shared Socio-Economic Pathway (SSP) of low- (global radiative forcing of 2.6 W/m2 by the year 2100), medium- (global radiative forcing of 4.5 W/m2 by 2100), and high-end (8.5 W/m2 by 2100) pathways are 0.05–0.21, 0.12–0.39, and 0.28–0.78 psu/century, respectively. The most rapid freshening trends of SSS are observed in the East China Seas and the Indonesian Seas, which are over two times greater than the global mean. On the other hand, the SSS freshening trends in the Arabian Sea are slightly lower than the global mean SSS freshening trend.

LAW ENFORCEMENT OF ILLEGAL FISHING IN THE IMMIGRATION PERSPECTIVE IN THE WORKING AREA OF BELAWAN IMMIGRATION OFFICE

The abundance of natural resources and the vast area of ​​the ocean in Indonesia are an attraction for the surrounding countries. This makes the case of Illegal fishing a case that is often found in Indonesian seas. One of the affected areas is Belawan seas. Based on the supervision carried out by the Ministry of Maritime Affairs and Fisheries (KKP), it is stated that illegal fishing violations often occur in EEZ (Exclusive Economic Zone) seas. One of the institutions authorized to enforce the law on illegal fishing is immigration. This study aims to see the implementation of law enforcement and immigration law strategies against foreigners who commit illegal fishing crimes in the work area of ​​the Belawan Immigration Office. This research is structured using a combination of normative and empirical research with qualitative methods using the basic theory of State Sovereignty and Effectiveness (Enforcement) of Law. The results of the study found that the Belawan Immigration Office carried out law enforcement in the form of Immigration Administration Actions namely deportation, some of the obstacles encountered in law enforcement were the lack of infrastructure in the form of ships and the limited detention room at the Medan Immigration Detention Center. The researchers also found that the legal strategy efforts carried out by the Belawan Immigration Office were preventive and repressive strategies. With Preventive Strategies in the form of coordination with Foreign Surveillance Team and Foreign Embassies, Repressive Strategies in the form of strengthening the intensity of Immigration Administrasion Action and Investigations.

Sea level anomalies in the southeastern tropical Indian Ocean as a potential predictor of La Niña beyond one-year lead

Most climate forecast agencies failed to make successful predictions of the strong 2020/2021 La Niña event before May 2020. The western equatorial Pacific warm water volume (WWV) before the 2020 spring failed to predict this La Niña event because of the near neutral state of the equatorial Pacific Ocean in the year before. A strong Indian Ocean Dipole (IOD) event took place in the fall of 2019, which is used as a precursor for the La Niña prediction in this study. We used observational data to construct the precursory relationship between negative sea level anomalies (SLA) in the southeastern tropical Indian Ocean (SETIO) in boreal fall and negative Niño 3.4 sea surface temperature anomalies index one year later. The application of the above relation to the prediction of the 2020/2021 La Niña was a great success. The dynamics behind are the Indo-Pacific “oceanic channel” connection via the Indian Ocean Kelvin wave propagation through the Indonesian seas, with the atmospheric bridge playing a secondary role. The high predictability of La Niña across the spring barrier if a positive IOD should occur in the previous year suggests that the negative SETIO SLA in fall is a much better and longer predictor for this type of La Niña prediction than the WWV. In comparison, positive SETIO SLA lead either El Niño or La Niña by one year, suggesting uncertainty of El Niño predictions.

Water Mass Variations in the Maluku Channel of the Indonesian Seas During the Winter of 2018–2019

AbstractAn 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.

Government Efforts In Realizing Legal, Reported And Regulated Capture Fisheries In Indonesia

The condition of Indonesia's vast waters with large marine resources has important meaning for Indonesia because it contains fishery resources which have great potential as a source of new economic growth as well as being one of the prime movers of national development. The abundance of fishery resources in Indonesian seas has attracted the attention of foreign parties to enjoy them illegally through illegal fishing activities. In this descriptive qualitative research, the researcher serves as the human instrument. a qualitative descriptive method that involves looking at various published sources and drawing conclusions from them. the results and conclusions of this study is the lack of supervision and enforcement in Indonesia demonstrates that the management of the country's fish resources is not yet at its best. Fishing without a permit, using destructive fishing gear, and fishing from foreign vessels are just a few examples of the IUU fishing problem in Indonesia. Legal, reported, and regulated fishing—fishing that is legal, reported, and regulated—is a requirement for effective fisheries management that is sustainable and responsible. Increasing supervision and law enforcement, establishing international cooperation, establishing a monitoring center for fishing vessels, developing a sophisticated communication system (VMS), and imposing severe penalties on those who engage in IUU fishing are government efforts that can be made to combat the problem in Indonesia.

Shipwrecks of Feasibility in the Field of Investment Business

Indonesian Valuable Objects on the Load of Sinking Ships (BMKT) have a large potential. This makes the ships buried in Indonesian territorial seas a tremendous resource. Illegal treasure hunters have targeted Indonesia's BMKT because to its great potential, causing considerable losses. Realizing this, it is vital to pay attention to Valuable Items on the Load of Sinking Ships (BMKT) and develop rules to maximize their potential. This study aims to analyze the policies governing BMKT to support investors in participating in investing in the business sector of BMKT management and creating BMKT management that is optimal and feasible for all aspects of the values contained in the BMKT itself. The approach method used to achieve this research is an approach focused on regulations or policies (statute approach) and conceptual analysis. The result showed that there is still substantial overlap between regulations controlling BMKT, making BMKT administration feasible. In spite of this, the Economic Analysis of Law is used to explain the expansion of the legal dimension through its studies. In addition, the economic value and cultural legacy of BMKTs that cannot be appointed due to the difficulties of appointment or the impossibility of relocation may be used on-site via marine tourist activities, notably diving tourism. As such, investors who wish to participate in the business sector of BMKT management may use BMKT more properly and optimally, including local and international investors.

Multi-temporal variability forecast of particulate organic carbon in the Indonesian seas.

The current global condition characterized by high levels of CO2 is altering the carbon cycle and elemental biogeochemistry, resulting in subsequent global warming, climate change, ocean acidification, and the indirect response of deoxygenation. The features of Indonesia's coastal ecosystems and continental shelf waters also contribute to spatio-temporal ocean carbon variability. For instance, the level of particulate organic carbon (POC) will change annually, and thus, over a decadal period, ocean dynamics may affect the temporal variability of POC. Motivated by such conditions, future forecasting is needed to envision the productivity of Indonesian seas by predicting vital parameters such as POC. This research aimed to forecast the temporal variability of POC in Indonesian waters. The Seasonal Autoregressive Integrated Moving Average (SARIMA) forecasting model was used by considering the lowest value of the Akaike information criterion (AIC) and the mean absolute percentage error/MAPE (threshold < 10%). Using the highest correlation coefficient (threshold: 0.75), we obtained the best fit for forecasting POC temporal variability. Hindcast POC data (2002-2020/2021) was used to train the forecasting model. The result shows that forecasting of POC temporal variability can be conducted up to 2030. The validity of prediction is ensured for less than 5years forward after 2020 with correlation coefficients of 0.65 and 0.83 for seasonal and monthly POC, respectively. The hindcast and forecast estimates of POC in the Indonesian seas show a decreasing trend. The present study emphasizes the different forecasting results obtained using the different approaches of annual versus inter-annual variability. A sustained research effort is still required to assess POC forecasting for its potential benefits in marine system monitoring and assessment.

Microplastic Contamination in Marine Fish and Shells in the Coastal Areas of Jember Regency, Indonesia

Highlight Research Fish and shellfish on the coast of Payangan and Puger Several types of microplastics were detected in their gastrointestinal tract There were microplastic types of fiber, fragments, granules, and filaments Abstract Every year, it is estimated that the Indonesian seas receive 100,000-400,000 tons of plastic waste used for human consumption. Indiscriminate disposal of plastic waste will have an impact in the future. The problem of microplastics is an illustration that the use of plastic in daily activities will cause environmental ecological damage. The purpose of the study was to describe microplastic contamination in marine fish and shells in the coastal areas of Jember Regency, Indonesia. The method used is to detect and identify the type and numbers of microplastic particles in the gastrointestinal tract content of sea fish and shells obtained from fishermen around Payangan and Puger coastal Jember, Indonesia. The gastrointestinal tract was extracted with peroxide oxidation method (WPO). A light microscope was used to examine microplastic particles of types and numbers. The results showed that marine fish and shells in the coastal area of Jember Regency, Indonesia have been contaminated with microplastic. The microplastic in each marine fish and shells sample has a different type. There were microplastic types of fiber, fragments, granules, and filaments in the shells sample, while in the marine fish samples, there were all these types except granules. Fish and shellfish on the coast of Payangan and Puger had several types of microplastics detected in their gastrointestinal tract.

Fortnightly variability of Chl a in the Indonesian seas

Abstract. Twenty years of daily MODIS-Aqua ocean color observations (2002–2022) are used to identify periodic variability of near-surface chlorophyll (Chl a) in the Indonesian seas. The frequency spectrum of Chl a is dominated by the mean and low-frequency monsoonal variability; however, a prominent peak around the fortnightly tidal period, MSf, is present. Harmonic analysis is used to quantify and map the fortnightly Chl a signal, which is discovered to be significant along the continental shelves of NW Australia and at several sites associated with narrow passages between the Lesser Sunda Islands, within the Sulu Archipelago, and at a few other sites in the Philippines Archipelago. Fortnightly variability at the shallow coastal sites is attributed to the spring–neap cycle of barotropic ocean currents, while we hypothesize that the variability in deeper water near the island passages is due to the modulation of vertical nutrient fluxes by baroclinic tidal mixing. The results document the significance of tidal mixing and highlight the heterogeneous character of biophysical processes within the Indonesian seas.

Tinjauan Yuridis Penangkapan Ikan Secara Illegal (Illegal Fishing) Menurut Undang Undang Nomor 45 Tahun 2009 Tentang Perikanan

Indonesia is a country where most of its territory consists of oceans with a population whose development continues to increase and can be expressed in meeting the needs of life, especially in coastal areas, such as fishermen who live in coastal areas generally using a source of life from the marine and fisheries sector. Indonesia as an archipelagic country surrounded by oceans has a strategic position and extraordinary potential with territorial waters, both archipelagic waters and inland waters. In addition, the Indonesian seas also include the sea waters of the Exclusive Economic Zone (EEZ). The economic loss caused by the practice of illegal fishing does not only result in loss of state revenues but also loss of opportunities for catching fish by Indonesian fishermen. If examined more deeply, it can be said that illegal fishing causes a lot of losses, not just counting how many fish have been stolen, but the impact is even wider, the financial burden is borne by the state, such as fuel subsidies (BBM) which are ultimately enjoyed by ships. -illegal vessels, reduced non-tax revenue (PNBP), reduced Indonesian fishermen's job opportunities, because illegal vessels are foreign ships that use their own crew (ABK), catches are generally brought directly abroad (country from ships), threats to the sustainability of fish resources because catches are not detected, both in type, size and quantity.

Phylogeography of Long-spined Sea Urchin Diadema setosum Across the Indo-Malay Archipelago.

Widely distributed, broadcast-spawning Diadema sea urchins have been used as model invertebrate species for studying the zoogeography of the tropical Indo-Pacific. So far, the Indo-Malay archipelago, a wide and geographically complex maritime region extending from the eastern Indian Ocean to the western Pacific Ocean, has been under-sampled. This study aims to fill this sampling gap and uncover the phylogeographic structure of the long-spined sea-urchin D. setosum in the central Indo-West pacific region. D. setosum samples (total N = 718) were collected in 13 sites throughout the Indo-Malay archipelago. We sequenced over 1157 bp of COI gene. The Phylogeographic structure was derived from pairwise ФST estimates using multidimensional scaling and hierarchical clustering analysis; biogeographic hypotheses were tested by AMOVA; genetic relationships between haplotypes were summarised in the form of a minimum-spanning network; and pairwise mismatch distributions were compared to the expectations from demographic and spatial expansion models. All samples from the Indo-West Pacific were of the previously uncovered D. setosum-a lineage. Phylogeographic structure was evident: the Andaman Sea population and the northern New Guinea population were genetically distinct. Subtler but significant haplotype-frequency differences distinguished two populations within the Indonesian seas, distributed in a parapatric-like fashion. The phylogeographic partition observed was insufficiently explained by previous biogeographic hypotheses. The haplotype network showed a series of closely related star-shaped haplogroups with a high proportion of singletons. Nucleotide-pairwise mismatch patterns in the two populations from the Indonesian seas were consistent with both demographic and spatial expansion models. While geographic barriers to gene flow were inferred at the western and eastern extremities of the Indo-Malay archipelago, the subtler parapatric pattern observed within the Indonesian seas indicated restriction in gene flow, in a fashion that can hardly be explained by geographic isolation given the dynamic current systems that cross this region. Our results thus raise the hypothesis of subtle reproductive isolation between ecologically incompatible populations. While the coalescence pattern of the Andaman-Sea population suggested demographic stability over evolutionary timescales, that of the two populations from the Indonesian seas indicated recent population expansion, possibly linked to the rapid changes in available D. setosum habitat caused by sea-level oscillations in the late Pleistocene. The phylogeographic patterns observed in this study point to likely allopatric differentiation in the central Indo-West Pacific region. Genetic differences between populations were likely reinforced during interglacials by some form of reproductive isolation.

Simulated circulation and trajectory analysis in the Makassar Strait

Makassar Strait conveys the main Indonesian Throughflow (ITF) pathway, as part of the global thermohaline circulation transferring heat and freshwater fluxes from the Pacific Ocean to the Indian Ocean. Dynamics and variability of the ITF and the monsoon winds system influence significantly marine ecosystem in the Indonesian Seas. The study aims to analyze sea surface circulation patterns and particle trajectories in the Makassar Strait by developing a simulation of the CROCO’s 1/36º regional general circulation model between 2017-2018. The results revealed the dynamics of near-surface layer (0 – 50 m depth) varied seasonally. The magnitude of surface current was about 0.6 m/s during the peak of northwest monsoon (NWM) flowing northward and reverse southward during the peak of southeast monsoon (SEM). The magnitude of the current intensified up to 1 m/s in the subsurface at Labani Channel and persistent flows to the southward. The thermocline transport volume was maximum during SEM with average transport about -11.00 (±1.54) Sv. The analysis shows that the particles near Balikpapan coast spreaded slowly to the north, then rotating at the center of the strait and spreaded according to the NWM and SEM.

High-Frequency Observations of Oceanic Internal Waves from Geostationary Orbit Satellites

The Geostationary Orbiting Satellite (GOS) offers extensive opportunities for the study of oceanic internal waves (IWs) through high-frequency observations. In this study, the spatial and temporal distributions of sunglint from 3 GOSs (Himawari-8, FY-4A, and GK-2A) were calculated, and the observation times of IWs in various seas were also recorded. The GOS can continuously observe IWs at a frequency of 10 min for 2 to 3 h. As demonstrated by the application to IWs in the Andaman Sea, the GOS effectively captures the surface features of IWs, including soliton number, the length and wavelength of the leading wave, and the speed and direction of propagation. Furthermore, the GOS can be used to track the dynamic processes of IWs within a short duration and provide more accurate “instantaneous” phase speeds. In the case of the Indonesian Seas, the average error of the GOS-derived phase speeds is 0.13 m/s compared to the Korteweg–de Vries phase speeds. Additionally, a 7-day observation from FY-4A suggests the possibility of diurnal IWs in the Sulu Sea. The advent of high-temporal-resolution GOS provides an enriched dataset for oceanic IW studies, which will contribute greatly to a more comprehensive understanding of IW mechanisms.

СЕЗОННАЯ ИЗМЕНЧИВОСТЬ ВОДООБМЕНА МЕЖДУ ТИХИМ И ИНДИЙСКИМ ОКЕАНАМИ ПО ДАННЫМ АРГО И ЧИСЛЕННОГО МОДЕЛИРОВАНИЯ

Circulation associated with the Indonesian Troughflow (ITF) is studied using the Argo-based Model for Investigation of the Global Ocean (AMIGO). The model consists of a block for variational interpolation to a regular grid of Argo floats data and a block for model hydrodynamic adjustment of variationally interpolated fields. The seasonal and monthly mean temperature, salinity and velocity fields were calculate for the period of 2005–2014 using such an aproach. The mean ITF mass, heat, and salt transports over a period of 2005–2014 based on the AMIGO data is diagnosed as 15.2 ± 2.3 Sv. The maximum value of mass transport was diagnosed in August as 18.8 ± 1.4 Sv, the minimum value of 11.8 ± 2.1 Sv was found in January. The detailed analisys of seasonal variability of the mass transport through the Makassar Strait (Makassar throughflow or MT), the primary inflow path of Pacific water into the Indonesian Seas, were performed. The maximum and minimum values of MT were also diagnosed in August and January as 22.1 ± 1.2 Sv and 9.8 ± 2.1 Sv respectively. It was found significant changes in the MT sutface layer flow during boreal winter (with total reverse in January) associated with the intrusion of the low surface salinity waters of the Java and South China Seas. These modeling results agree very well with the direct velocity measurements.

Impact of ENSO on the Entrance of the Indonesian Throughflow: The Oceanic Wave Propagation

AbstractA regional ocean model simulation for the period from 2000 through 2016 is used to study oceanic wave propagation in the Indonesian seas. The model simulation compares favorably with the interannual variability of the satellite derived sea level anomaly and the observed currents in the entrance passages of ITF. Lag correlation analysis suggests that the interannual sea level anomaly and opposite responses of the velocity above and below the thermocline in the western (the Sulawesi Sea and the Makassar Strait) and eastern (the Halmahera Sea) pathways of ITF are associated with the ENSO induced Rossby waves from the equatorial Pacific rather than the local wind forcing. In particular, during La Niña years ENSO induced anomalies traveling through the western pathway are comparable to those through the traditional eastern waveguide, suggesting the importance of both pathways in the propagation of ENSO signals in the Indonesian seas. Different roles of the western and eastern pathways in transmitting the ENSO signals into the Indonesian seas are associated with the nonlinear processes in the Sulawesi Sea, which appears to be linked with different states of the western boundary currents during the warm and cold ENSO phases.

Remote energy sources for mixing in the Indonesian Seas

The role of the Indonesian Seas in climate is attributed to the intense mixing observed throughout the region. Mixing cools the surface temperature and hence modifies the atmospheric convection centered over the region. Mixing also controls the heat exchange between the Pacific and Indian Oceans by transforming water-mass properties while they transit through the region. Mixing in the Indonesian Seas has long been identified to be driven locally by tides. Here we show that the observed mixing can also be powered by the remotely generated planetary waves and eddies. We use a regional ocean model to show that the Indonesian Seas are a sink of the energy generated in the Indian and Pacific Oceans. We estimate that 1.7 GW of the remotely generated energy enters the region across all straits. The energy flux is surface intensified and characterized by a convergence, implying dissipation and mixing, within the straits and along topography. Locally, energy convergence associated with this process is comparable in magnitude to tidal energy dissipation, which dominates the deep ocean.

Moored Observations of the Timor Passage Currents in the Indonesian Seas

AbstractThe 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.