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The deformation and evolution of internal solitary waves (ISWs) beneath an ice keel can enable potential diapycnal mixing and facilitate upper ocean heat transport, despite a poor understanding of the underlying physics and energetics of ISWs in Polar environments. This study aims to understand the dynamic processes and mixing properties during the evolution of ISWs beneath ice keels in the Arctic Ocean using high‐resolution, non‐hydrostatic simulations. Ice keels can destabilize ISWs through overturning events. Consequently, the initial ISW disintegrates and transfers its energy into secondary smaller‐scale waves. During the ISW‐ice interaction, ISW‐induced turbulent mixing can reach O(10−3) W/kg with a magnitude of resultant heat flux of O(10)W/m. Sensitivity experiments demonstrated that the ISW‐ice interaction weakened as the ice keel depth decreased, and consequently, the resultant turbulent mixing and upward heat transfer also decreased. The ice keel depth was critical to the evolution and disintegration of an ISW beneath the ice keel, while the approximate ice keel shape had little effect. Our results provide an important but previously overlooked energy source for upper ocean heat transport in the Arctic Ocean. Plain Language Summary: Internal wave‐driven mixing plays an important role in both ocean general circulation and climate change. Internal waves (IWs) at mid to low latitudes have attracted much attention; however, our understanding of the evolution and physics of IWs in high‐latitude Polar oceans remains limited. This study aims to explore the evolution and energetics of internal solitary waves (ISWs, a kind of high‐frequency IW) in the ice environment of the Arctic Ocean using non‐hydrostatic 2D simulations. The results reveal the destabilization process of an ISW during its interaction with an ice keel (undersea portion of ice cover). During the ISW‐ice interaction in the Arctic Ocean, ISW‐induced turbulent mixing can reach O(10−3) W/kg with a magnitude of resultant heat flux of O(10) W/m. Thus, the ISW‐ice interaction provides an important, but previously overlooked, energy source for upper ocean heat transport in the Arctic Ocean. Key Points: Destabilization and disintegration of internal solitary waves (ISWs) under ice keels can radiate energy into secondary wavesTurbulent mixing and vertical heat flux are significantly enhanced during ISW‐ice interactionsThe ISW‐ice interaction and resultant heat transfer weaken with the shrinking of ice keels [ABSTRACT FROM AUTHOR]

Ocean currents in the southern East China Sea (ECS) are complex and have long lacked systematic observational data. Observations from coastal radars in northern Taiwan, along with several drifters and satellite data, reveal a detailed ECS surface flow structure and thermal front. Drifter trajectories followed the seasonal flow field and were oscillatory and trapped due to strong tidal currents. Three drifters crossed the oceanic front off the northern coast of Taiwan following the tidal motion with a rapid change in the sea surface temperature by 2.5°C, which was recorded within a small distance of 500 m. A significant seasonal water mass exchange cycle occurred in the southern ECS. Starting in October, the Kuroshio Current intrudes onto the ECS shelf, causing the formation of a southwestward Northern Taiwan Coastal Current that mixes with the China Coastal Current to form a southward flow in the northern Taiwan Strait. Beginning in March, the ocean current off northwestern Taiwan shifts northward and then gradually flows eastward into the Kuroshio region. With satellite‐derived and model‐simulated surface currents contradicting each other over long periods of time, hourly coastal radar data have not only successfully explained the spatiotemporal variations in the sea surface temperature, salinity, and chlorophyll concentration observed by satellites but have also resolved the long‐standing disputes over the surface currents in the southern ECS. Plain Language Summary: In the southern East China Sea (ECS), the ocean surface currents observed by satellite altimeters and estimated based on numerical models have considerably differed for a long time, especially in winter. The data observed by shore‐based high‐frequency radar have yielded new discoveries. Beginning in October, the Kuroshio Current intrudes onto the ECS shelf and pushes Kuroshio waters with high temperatures, high salinities, and low chlorophyll concentrations to the west, causing the current from northern Taiwan to flow southwestward into the Taiwan Strait (TS). Then, starting in March, ocean currents off northwestern Taiwan turn northward, and the water off northern Taiwan is pushed eastward back to the Kuroshio region. These flow field variations cause a significant oceanic front in the 121°E−122°E area in the southern ECS. We deployed four surface velocity program (SVP) drifters to observe the ocean flow and used virtual drifters to simulate the distribution probability of the track in each month. The drifters basically followed the movement direction of the seasonal flow field but could have been affected by the significant tidal currents and become trapped. With this study, we aim to better understand the southern ECS environment from a geophysical perspective. Key Points: Coastal radar data and long‐term multisatellite data are used to observe the formation of oceanic fronts driven by ocean currentsThe Kuroshio Current's intrusion of the shelf causes the Northern Taiwan Coastal Current to flow southward along western Taiwan in winterVirtual drifter experiments present the possible trajectories of and seasonal changes in near‐surface ocean currents [ABSTRACT FROM AUTHOR]

The surface waves are the most energetic motions, which have great contribution to the turbulent mixing in the upper ocean. In this study, a novel turbulent mixing scheme is proposed in terms of the non‐breaking wave velocity shear module with the buoyancy flux. In the scheme, the mixing coefficients can be calculated empirically from the significant wave height, the wave number, the wave frequency, the buoyancy frequency, and the turbulence Prandtl number. The buoyancy fluxes, as well as the non‐breaking wave velocity shear production, are important for the upper‐ocean turbulent mixing, and make the calculated turbulence dissipation rate closer to in situ observations, which are provided by the "Responses of Marine Hazards to Climate Change in the Western Pacific (ROSE)" Project. The effects of the buoyancy flux on the turbulent mixing are examined based on three numerical experiments using the MASNUM ocean model. In the experiments, the topography, the lateral boundaries, initialization conditions, and the surface forcing fluxes are taken from the GEBCO, HYCOM/NCODA, and ERA5 data, respectively. Comparing to the AVHRR remote sensing sea surface temperature (SST) products and in situ observations, the simulated results with the non‐breaking wave‐generated turbulent mixing gain significant improvement in the SST, upper‐ocean temperature structure, and the mixed layer compared with the classic Mellor‐Yamada scheme. The results show that the buoyancy flux is able to suppress the enhanced non‐breaking wave‐generated turbulent mixing, so that the improved model simulates the observations better than that without the buoyancy effects. Plain Language Summary: The sea surface waves can transfer the energy from the wave fields to turbulent mixing in the upper ocean. A novel turbulent mixing scheme is proposed in terms of non‐breaking wave velocity shear module with buoyancy flux. The simulated results from the ocean circulation model with the novel scheme gain significant improvement in the SST and the upper‐ocean temperature structure compared with the classic Mellor‐Yamada scheme. Buoyancy flux slightly suppresses the enhanced vertical mixing induced by non‐breaking wave‐generated turbulence, so that the improved model simulates the observations much better than that without the buoyancy effects. Key Points: A novel turbulent mixing scheme is proposed in terms of non‐breaking wave velocity shear module with buoyancy fluxBuoyancy flux suppresses the over enhanced vertical mixing induced by non‐breaking wave‐generated turbulenceNon‐breaking wave‐induced mixing with buoyancy flux improves simulated upper‐ocean temperature structure [ABSTRACT FROM AUTHOR]

Tidal current‐induced cyclonic eddies cause cold‐water upwelling and periodic sea surface temperature (SST) drops around the coral reef area in Nanwan Bay, which is located at the southern tip of Taiwan. This study used Himawari‐8 satellite data and tide gauge and coastal ocean dynamics application radar (CODAR) data to analyze the characteristics of the SST drops and cyclonic eddy propagation and used an oceanic general circulation model (OGCM) to simulate the tidal current flowing process. According to the CODAR data analysis, the mixed primary semidiurnal tide had an average current velocity of 0.3–0.4 m s−1 throughout the bay, and the average life cycle of a cyclonic eddy is 6.6 hr, with a propagation speed of 0.35 m s−1. The SST drop during the spring tide period was greater than that during the neap tide period, and the SST dropped in both summer and winter. The average daily SST drop in the summer reached 2 °C with a maximum observed value of 4.7 °C, and the SST drop rate was 0.3–0.5 °C hr−1. The annual mean chlorophyll‐a concentration was 0.25 mg m−3. This study explored the special properties of the Nanwan Bay coral reef area from the perspective of ocean physics to allow ecologists to facilitate the implementation of long‐term conservation and monitoring programs. Plain Language Summary: In recent years, the increased global sea temperatures have caused severe coral bleaching; however, Nanwan Bay has suffered a comparatively smaller impact. To understand why, we analyzed satellite and radar data. We found that Nanwan Bay experienced an almost daily sea surface temperature (SST) drop during the summer and that this periodic variation was caused by a cyclonic eddy upwelling cold water to the surface. In the summer, the SST drop was 2 °C on average and dropped at a rate of 0.3–0.5 °C hr−1. The observational record indicated that the water cooled as much as 4.7 °C in 1 day. These eddies were caused by the unique topography in this area of the bay; they survived an average of 6.6 hr and moved at a rate of 0.35 m s−1. The formed eddies disappeared in the bay or flowed southward out of the bay. With this study, we hope to help marine ecologists understand the Nanwan Bay environment from a geophysical perspective so that they have sufficient background knowledge when investigating coral biology and coral reef ecology in the future. Key Points: Himawari‐8 satellite and coastal radar data are used to analyze the characteristics of eddy‐induced sea surface temperature drop in a bayIn summer, ebb current‐induced cyclonic eddies cause 2 °C sea surface temperature drops that reduce thermal stressThe average life cycle of eddies was 6.6 hr with propagation speed of 0.35 m s−1, which was close to the tidal currents around the bay [ABSTRACT FROM AUTHOR]

In this study, we for the first time observed two subsurface cyclonic eddies (SCEs) in the southeastern Tropical Indian Ocean. One SCE (SCE‐C) was observed along 8°S transect during the cruise conducted in July 2016. Another SCE (SCE‐A) was also observed near 8°S but by an Argo float in November 2005. The SCE‐C had a maximum thickness of about 500 m (300–800 m) and a radius of ~90 km, while the SCE‐A has a vertical extent of 700 m (200–900 m) and a radius larger than 110 km. The water trapped by SCE shows a three‐compartment structure. Both the two SCEs are characterized by stratified water with negative lens structure and cause prominent temperature/salinity/spiciness anomalies in a wide density range. As the water carried by SCEs is stratified, we can trace the SCE origin by finding an area sharing the same water property with the SCE on different density surfaces. The result suggests that the two SCEs originate from the southwest coast of the Sumatra Island. Considering the topography and hydrologic characteristics of the SCE origin, we speculate that the SCEs are generated due to topography‐induced South Java Undercurrent instability. Plain Language Summary: Subsurface eddies usually have long lifespans and can carry fluid parcels and transport them far away from the eddy origin. However, compared with surface eddies, subsurface eddies are poorly observed because they usually have small or no surface signatures. In this study, we for the first time observed two subsurface cyclonic eddies (SCEs) in the southeastern tropical Indian Ocean. The SCEs were characterized by stratified water with negative lens structure. Further analysis suggested that the SCEs should be generated off southwest coast of the Sumatra Island due to topography‐induced South Java Undercurrent instability and move southwestward. These results provide new insight into the SCEs in the southeastern tropical Indian Ocean. Key Points: Two subsurface cyclonic eddies are observed in the southeastern Tropical Indian OceanThe subsurface cyclonic eddies are characterized by stratified water and cause prominent water property anomalies in a wide density rangeThe subsurface cyclonic eddies should be generated off southwest coast of the Sumatra due to topography‐induced undercurrent instability [ABSTRACT FROM AUTHOR]

Using high‐resolution full‐water column velocity data at a site of 60‐m depth on the northeastern shelf of the South China Sea, we observed high‐frequency internal waves (HIWs) occurring continuously on the continental shelf during the 15‐hr observation on 13‐14 July 2011. This phenomenon is in contrast to the regular occurrence of internal solitary waves (ISWs) that are generally phase‐locked to the tides. We hypothesize that the continuously occurring HIWs are generated from shoaling ISWs via fission. Analysis of the Moderate Resolution Imaging Spectroradiometer (MODIS) true‐color images supports this hypothesis. The mechanism of ISW fission is then investigated using an analytical model as well as a two‐layer weakly nonlinear numerical model based on the regularized long‐wave (RLW) equation. The results are also compared with those from a fully nonlinear continuously stratified numerical model. Simulation results are generally consistent with the field measurements, explaining the observations of the continuous occurrence of HIWs. Sensitivity experiments are conducted to investigate the role and effects of the initial wave amplitude, pycnocline depth and thickness, bathymetry, and viscosity on ISW fission. Although all these factors impact the properties of the shoaling ISWs, both the two‐layer and continuously stratified models suggest that fission mainly occurs near the critical point region, highlighting the importance of pycnocline depth. However, simulations using the two‐layer model suggest that fission occurs after ISWs pass through critical points, whereas the fission location predicted by the continuously stratified model is generally offshore of critical points. This points to the deficiency of the two‐layer model in simulating small‐amplitude short trailing waves, large‐amplitude ISWs, wave breaking, and higher‐mode waves. Key Points: High‐frequency internal waves were continuously recorded from moored observations in July 2011 in the northeastern South China SeaSatellite images indicate that the observed high‐frequency internal wave trains were generated from fission of internal solitary wavesThe ISW fission characteristics are investigated using an analytical model, a two‐layer weakly nonlinear numerical model, and a continuously stratified fully nonlinear numerical model [ABSTRACT FROM AUTHOR]

For investigation of internal solitary waves (ISWs) in the South China Sea (SCS), most cruise observations are concentrated from Luzon Strait to Dongsha Atoll in the northeastern SCS but few on the continental slope far away from the west of Dongsha Atoll. In this study, we use 1‐year long mooring data to determine dynamic and statistical features of the ISWs on the shelf slope of the northwestern SCS. The analysis results of the mooring data reveal that the ocean internal waves on the shelf slope of the northwestern SCS have physical properties of highly nonlinear waves, which are well described by the solutions of the Korteweg‐de Vries equation. The mean nonlinear phase speeds of mode‐1 and mode‐2 ISWs are 1.38 ± 0.14 and 0.66 ± 0.12 m/s, respectively. The major direction of mode‐1 ISWs is northwestward 305° ± 21°. Strong ISW currents force the major direction of total current velocities to turn 67.5° in the upper layer and 135° in the lower layer. The monthly occurrence frequency distribution of ISWs shows a peak in July with a maximum frequency of 16.2% and a trough in March with a minimum frequency of 3.3%. Mode‐2 ISWs appear most in December, accounting for 50% of total mode‐2 ISWs. The largest mode‐2 ISW on record up today with the depressed amplitude as large as 91 m, and the elevated amplitude of 73 m was observed at mooring station. These new findings and new data are of significance to local internal wave prediction model development. Plain Language Summary: Unlike the sea surface waves, the ocean internal waves are a sort of wave motions occurring in the ocean interior. Strong internal waves carry strong currents and huge power, which may greatly impact the normal ocean and cause harm to underwater navigation and ocean engineering facilities. Thus, it is a necessary work to clarify the internal wave activities in an area of interest. We deployed a chain of instrument system moored at the sea floor on the continental slope of the northwestern South China Sea, where is rich in oil and gas resources, to observe the internal waves for 1 year from June 2016 to July 2017. Using the data of observed 684 internal waves, we analyze their physical properties and statistical features. The results indicate that (1) the observed waves have typical features of internal solitary waves, (2) the maximum amplitude reaches near 100 m, and (3) strong internal wave currents force the major direction of normal current to turn about 70° and 135° in the upper and lower layers, respectively, and March and July are trough and peak months for internal wave occurrence. These data and results are of significance for underwater navigation safety and ocean engineering designs. Key Points: Nonlinear properties of mooring‐measured mode‐1 and mode‐2 IWs and wave packets in SCS are verified by KdV equation solutionsStatistical features of occurrence frequency, amplitudes, and phase speeds of ISWs are derived from the 1‐year long mooring dataModulation of the internal waves to background current is measured for ocean engineering applications [ABSTRACT FROM AUTHOR]

Field observations were conducted along three west‐east transects in the East China Sea Kuroshio Front in July 2017. The microstructure observations results show that the surface turbulence kinetic energy (TKE) dissipation rates in the northern and southern transects are about 2 orders larger than those in the middle transect. The sea surface temperature, mixed layer depth, and the submesoscale motions (with horizontal scales smaller than 30 km) also suggest the surface turbulences are more active in the northern and southern transects. The current velocity data and sea level anomaly show that the middle transect was located at the northern edge of a cyclonic eddy. The eddy‐generated cross‐front geostrophic current was observed to flow toward the opposite direction of the cross‐front Ekman transport and believed to counteract the Ekman buoyancy flux (EBF)‐induced turbulence enhancement. The TKE dissipation rates can be well scaled by a scaling considering the EBF and the wind in the northern and southern transects but wind scaling only in the middle transect, suggesting the EBF does not contribute to the surface turbulence in the middle transect. An improved empirical scaling considering the cross‐front geostrophic current is defined and can scale the TKE dissipation rates better, indicating an eddy plays a key role in surface turbulent mixing in the East China Sea Kuroshio front. Plain Language Summary: Ocean surface boundary layer is an important mediator for air‐sea interaction. Recent field observations have shown that the turbulence mixing within the ocean surface boundary layer can be significantly elevated at the front when the cross‐front Ekman transport carries dense cold water over less dense warm water. Our observations show that the mesoscale eddy also plays an important role in the surface turbulence at the front by generating cross‐front geostrophic current. This result may have important implications on the ocean model and climate forecast. Key Points: Expected Ekman buoyancy flux‐induced surface turbulence enhancement was not observed in the Kuroshio FrontEddy‐generated cross‐front geostrophic current counteracted the Ekman buoyancy flux‐induced surface turbulence enhancementNewly defined empirical scaling considering geostrophic current can better scale observed turbulent dissipation rates [ABSTRACT FROM AUTHOR]