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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]

Lombok Strait is one of the main exit passages of the Indonesian throughflow (ITF), connecting the interior Indonesian seas with the Indian Ocean. Satellite images have shown that this strait is also the most remarkable zone of the Indonesian seas for the generation of internal tides, which frequently evolve into internal solitary waves (ISWs). Here we present the first comprehensive high-frequency observations of the internal tide in the strait. The observations were carried out at a fixed position around 8 km from the western coast of Lombok Island at a depth of ~300 m. We confirmed the appearance of the non-linear internal tide with an amplitude of ~90 m at a fairly close distance (~30 km) from its generation site in the Nusa Penida Sills (NPS), with a dominant M 2 tidal frequency. High-frequency internal waves associated with vertical velocities reaching 20 cm s-1 were observed within the internal tidal trough. We estimated the turbulent kinetic energy dissipation rate using Thorpe and Ellison scales and found a mean dissipation rate of the order of ~ [10-6-10-7] m2 s-3, with enhanced values in the thermocline layer, possibly driven by shear instabilities. We underlined that the use of the Ellison scale is a promising alternative for indirectly estimating the dissipation rate from high-resolution time-series data sets. • CTD profiles and high frequency temperature measurements were used to estimate the turbulent dissipation rate. • Non-linear internal tide with dominant M 2 frequency appeared in the Lombok Strait. • High frequency internal waves were observed continuously within internal tidal troughs. • The improved Thorpe and the Ellison methods are utilized to estimate the dissipation rate. • Enhanced dissipation is induced when internal waves interact with the shore. [ABSTRACT FROM AUTHOR]

• A large database of CTD profiles was used to estimate the turbulent dissipation rate. • The improved Thorpe method is validated against VMP measurements. • Higher mixing is found in the eastern route and exit passages. • Bias in turbulent dissipation rate are identified in regional models. • M 2 internal tide is the dominant factor driving the turbulent kinetic dissipation rates. Turbulent kinetic energy dissipation rates and vertical diffusivities in the Indonesian seas are inferred from historical CTD measurements gathering for the first time data from Indonesian and international cruises. Dissipation rates are inferred from the CTD using an improved Thorpe scale method, which is validated against microstructure measurements. Elevated dissipation rates ~[10−6–10−7 m2 s−3, were observed in the near field stations, such as in the straits, narrowing passages and shallowing topography where internal tides are generated and Indonesian throughflow (ITF) is intense, while lower dissipation rates ~[10−8–10−10 m2 s−3 were observed in the far field stations and below the pycnocline. The main mixing hot spots are located in the Labani Channel and shallowing topography of the Dewakang waters for the western route of ITF, i.e. the passage that connects the north Pacific source via Sulawesi Sea, Makassar Strait and Flores Sea; in the straits of Halmahera, Lifamatola, and Buru for the eastern route of ITF, i.e. the passage that connects the south Pacific source via Halmahera Sea, Maluku Sea, Seram Sea; and in the ITF exit passages, i.e. the Lombok, Sape and Ombai Straits. The eastern route is more dissipative than the western route, which is consistent with the stronger erosion of the salinity peak of the Pacific waters along the eastern route. We found that tidal variations influence the dissipation rates and diffusivities as has been suggested from the yoyo profiling datasets. The spatial pattern of dissipation rates inferred from the high-resolution 3D hydrodynamics model output of Nagai and Hibiya (2015) shows a general agreement with the observations in the location of the mixing hot spots and suggests that the M 2 internal tide is the dominant factor driving the turbulent kinetic energy dissipation rates in the Indonesian seas. Yet the model also shows a bias toward lower dissipation rate in the pycnocline, that we attribute to the lack of representation of the ITF and mesoscale circulation and a bias toward higher dissipation rate in the weak mixing region, suggesting an overestimation of the background dissipation rate in calm waters. [ABSTRACT FROM AUTHOR]