Natural gas hydrates, recognized as a promising 21st-century energy source, offer significant potential for lowcarbon energy. However, the exploitation of marine natural gas hydrate may pose risks to sediment stability and may induce submarine deformation, necessitating environmental impact assessments. Current methods for monitoring submarine hazards are inadequate for long-term, in-situ assessments of strata stability. This study introduces an integrated submarine deformation monitoring instrument deployed during the second offshore natural gas hydrate production test in the South China Sea. The instrument captured variations in seabed static water pressure, azimuth, and tilt angles over a 183-day production period. By applying empirical mode decomposition and deformation field theory, the study reveals that only two monitoring stations recorded vertical settlements exceeding 1 cm, with a maximum of 1.98 cm, while others remained below 10 mm. This suggests the absence of severe geological disasters such as strata deformation or seabed collapse during the production test. The finding demonstrates the efficacy of the submarine deformation monitoring system combined with multivariate signal analysis for long-term, in-situ strata stability monitoring, which is essential for environmental impact monitoring and safety assessment of gas hydrate production.

Observations of currents and temperatures from four moorings deployed around the deep slope (similar to 2500 m) of Caiwei Guyot in the Pacific Prime Crust Zone were utilized to investigate topographically trapped waves at low-latitude seamounts. Contrasting with commonly reported persistent diurnal seamount-trapped wave cases at middle and high latitudes, the subinertial variability in deep currents and temperatures at the slope of Caiwei Guyot was primarily characterized by two distinct lower-frequency bands (i.e., 13-24 and 3.3-4.7 days). These subinertial variabilities are interpreted as intermittent seamount-trapped waves and topographic Rossby waves (TRWs). During certain time periods, the observations include key signatures of seamount-trapped waves, such as near-opposite phases of azimuthal velocity (and temperature) on opposite flanks of the seamount, and patterns of temporal current rotation consistent with counterrotating cells of horizontal current propagating counterclockwise around the seamount. After comparing these observations to idealized seamount-trapped wave solutions, we conclude that the 13-24-day (3.3-4.7-day) energy is mainly due to radial-vertical mode 5 (3) for azimuthal wavenumber 1 (3). Sometimes the subinertial energy remained pronounced at only one flank of the seamount, primarily explained as TRWs with 192-379-m vertical trapping scale and 14-28-km wavelength. Upper-layer mesoscale perturbations might provide energy for deep seamount-trapped waves and TRWs. This study highlights the role of topographically trapped waves in modulating the deep circulation at low-latitude seamounts.

Internal solitary waves (ISWs) contain great energy and have the characteristics of emergency and concealment. To avoid their damage to offshore engineering, a new generation of monitoring and early warning system for ISWs was developed using technologies of double buoys monitoring, intelligent real-time data transmission, and automatic software identification. The system was applied to the second natural gas hydrates (NGHs) production test in the Shenhu Area, South China Sea (SCS) and successfully provided the early warning of ISWs for 173 days (from October 2019 to April 2020). The abrupt changes in the thrust force of the drilling platform under the attack of ISWs were consistent with the early warning information, proving the reliability of this system. A total of 93 ISWs were detected around the drilling platform. Most of them occurred during the spring tides in October–December 2019 and April 2020, while few of them occurred in winter. As suggested by the theoretical model, the full-depth structure of ISWs was a typical current profile of mode-1, and the velocities of wave-induced currents can reach 80 cm/s and 30 cm/s, respectively, in the upper ocean and near the seabed. The ISWs may be primarily generated from the interactions between the topography and semidiurnal tides in the Luzon Strait, and then propagate westward to the drilling platform. This study could serve as an important reference for the early warning of ISWs for offshore engineering construction in the future.

Material transport caused by mesoscale eddies has been revealed much in the open ocean; however, it is still unclear how much eddy-induced mass transport in the slope region of the northern South China Sea (SCS). Using the LASG/IAP Climate System Ocean Model (LICOM) from 2007 to 2017, we identified 47 anticyclonic eddies and 97 cyclonic eddies that intruded onto the continental slope, termed slope intrusion eddies. The slope intrusion eddies are more horizontally asymmetric and energetic than those without entering the slope. These eddies induced cross-slope heat and salt transport of O (10(12)) W and O (10(4)) kg s(-1) owing to their horizontal asymmetry in both Xisha Islands and Dongsha Islands, where are the intrusion zones of mesoscale eddies. Based on the potential vorticity budget, we found that the horizontal asymmetry of velocity was caused by the asymmetry of potential vorticity, which was mainly generated by eddy-current nonlinear effect in the Dongsha Islands and topographic beta effect in the Xisha Islands, respectively. This study may promote our understanding on the mesoscale dynamics and oceanic energy redistribution in the continental shelf zone of marginal sea.

Sea surface cooling induced by tropical cyclones (TCs) is an important component of air-sea interactions. Using coordinate transformation and composite analysis methods, we examined the interannual variability in TC-induced sea surface cooling (TCSSC) in the South China Sea (SCS). The frequency of surface cooling cases was over 86% and that of surface warming cases was less than 14%. The magnitude of TCSSC was defined as the absolute value of TCSSC. The maximum magnitude of TCSSC occurred on the right side of the TC track, and the mean magnitude of TCSSC decreased by 0.04°C/a from 2006 to 2018. The interannual variability in TCSSC was highly correlated with the TC translation speed and pre-TC mixed layer depth. Notably, TCSSC got enhanced in El Nino years of 2007, 2010, and 2015. The El Nino types were suggested to determine the occurring periods of strong TCSSC via controlling the positions of SCS anticyclones, which brought pre-TC shallow mixed layer and caused strong TCSSC via vertical mixing process during El Nino events. To quantify how the anticyclone influences TCSSC, we need to use mixed layer heat balances model in the next study.

海底甲烷渗漏可能会影响海洋环境,乃至影响全球气候和碳循环,但目前人们对渗漏甲烷在海水中的运动行为了解有限.本文基于水体甲烷释放室内实验,研究了不同甲烷渗漏条件下的甲烷气泡行为及其运动特征.结果显示,甲烷气泡在上浮过程中发生合并、分离和破碎,运动轨迹呈"S"形;气泡尺寸及上浮速度随释放气体流量增大而增大;气泡运动还受水中障碍物影响,具体作用与释放气体流量、障碍物表面粗糙度及形态有关.采用 日本水合物试采数据开展甲烷喷发式释放模拟实验发现,短期释放大量甲烷会引起水流速度和动压力显著增大.此外,水体甲烷吸附实验表明,经白炭黑疏水处理后,活性炭吸附甲烷能力可提高5％.

SST fronts at the mesoscale eddy edge (ME fronts) were investigated from 2007–2017 in the northern South China Sea (NSCS) based on an automatic method using satellite sea level anomaly (SLA) and SST data. The relative probabilities between the number of anticyclonic/cyclonic ME fronts (AEF/CEF) and the number of anticyclones/cyclones reached 20%. The northeastern and southwestern parts of these anticyclones had more fronts than the northwestern and southeastern parts, although CEFs were nearly equally distributed in all directions. The number of ME fronts had remarkable seasonal variations, while the eddy kinetic energy (EKE) showed no seasonal variations. The total EKE at the ME fronts was three times of that within the MEs, and it was much stronger in AEFs than in CEFs. The interannual variability in the number of ME fronts and EKE had no significant correlation with the El Niño-Southern Oscillation (ENSO) index. Possible mechanisms of ME fronts were discussed, but the contributions of mesoscale eddies to SST fronts need to be quantified in future studies.

Mesoscale eddies regulate oceanic material transport and atmospheric current. The area around Xisha Islands is an important sink region for mesoscale eddies. We observed two cyclones during September to October 2019, and found that these two cold eddies had split from one local cold eddy. Using a Chinese Autonomous Underwater Vehicle(AUV), called 'Sea-Whale 2000 degrees, for field experiment, we successfully observed the fine structures at the interface of the two cyclones, which showed that two vertical layers of water developed into three layers. The AUV was operated at 300 m and captured the temperature, salinity and velocity oscillation at the bottom of the two cyclonic eddies. A simple barotropic quasi-geostrophic numerical experiment showed that the slope gradient effect played a major role in eddy splitting. Through energy conservation analysis, we confirmed that the barotropic instability was a major contributor to the development of the cold eddies.

Some mesoscale eddies intrude over the continental slope in the northern South China Sea, supporting cross-shelf matter transport. We investigated the characteristics, the intruding tracks and formation mechanisms, of slope intrusion mesoscale eddies using satellite altimeter data and model outputs. In total, 36 and 22 slope intrusion anticyclonic and cyclonic eddies (SAEs/SCEs) are found, respectively. Slope intrusion eddies have longer lifetimes (similar to 58 days), smaller size (similar to 110 km), and greater eddy kinetic energy and vorticity compared to ordinary eddies but are more unstable and more easily deformed during their life cycles. The statistical results show that more slope intrusion eddies are generated during winter than other seasons. It is found that slope intrusion eddies mainly propagate westward/northwestward, and southwestward along the continental slope and shelf. Eddy intrusions occur mainly near the Dongsha Islands, east of Hainan, and north of the Xisha Islands. SAEs continue to propagate onshore after arrival at the continental slope, while SCEs dissipate more quickly. Using mooring data, we found that eddy-ambient flow interaction could cause the differences between SAEs and SCEs around the Dongsha Islands. Energy conversion was analyzed in these three regions using numerical products. During intrusion, eddies lose eddy kinetic energy and ambient flows gain energy.

Mesoscale eddies are important for transporting oceanic energy and matter. We investigated the three-dimensional structure of an irregularly shaped warm eddy using three Chinese underwater gliders and satellite data during May 2015 in the northern South China Sea. The warm eddy lasted for 2 months, remained quasi-steady, and had a mean radius of ~ 70 km from May 10 to May 31. The heat contents observed along the two glider tracks differed markedly, by 2 × 109 J/m2, which reflected an imbalance in the geostrophic and tangential velocity distributions of the eddy. The geostrophic/tangential velocity decreased/increased with depth within the warm eddy. The maximum tangential velocities calculated using the datasets from the two gliders were 0.8 and 0.25 m/s, respectively, confirming that the shape of the warm eddy was horizontally asymmetrical. Large errors can arise when the heat, energy, and matter transport for an irregularly shaped eddy are estimated using a regular circular model. We suggest that more intersecting glider tracks should be used to retrieve the three-dimensional eddy structure, and that those tracks should be better designed. The irregular shape of the warm eddy was likely induced by oceanic currents such as the wind-induced Ekman current. Further study is needed to elucidate the eddy–current interactions and the mechanisms thereof.

Mesoscale eddies are important for regulating oceanic energy. Variation in eddy shapes leads to uncertainty in calculations of heat or energy content. In this study, we investigated the deformation of a warm eddy in the northern South China Sea from April to June 2018 and elucidated the mechanism governing the deformation. Satellite altimetry images showed that the warm eddy originated from Luzon Strait (LS eddy), migrated westward, and then moved along 500-m isobaths until it approached the east of Hainan Island. Thereafter, the LS eddy deformed, moved southward, merged with other eddies, and finally dissipated. Using ship and virtual-mooring Chinese underwater glider observations, we examined the three-dimensional structure of the LS eddy. The warm eddy had a low-density core that reached a depth of 250 m. The LS eddy gave rise to a front along its eastern edge, and two associate submesoscale eddies with horizontal radii of approximately 10 km were found at the front. The warm eddy was circular before deformation but morphed into an egg shape after deformation. This shape change allowed the eddy to entrain additional water mass (approximately 10(14) kg). The deformation event was able to be forecasted by the vorticity and deformation index, as the eddy deformed by leaking into the zone with a high vorticity and deformation index. We used modeling and energy transformation calculations to analyze the mechanism of the warm eddy deformation. Our results revealed that baroclinic instability played a primary role in the deformation event.

Vertical structures of the water column, especially the surface mixed layer and the pycnocline layer, are important in the mediation of vertical transportation of oceanic matter, momentum, heat and salt. Field observations were conducted to investigate vertical structures and mixed layer depth (MLD) in the shelf sea of the northern South China Sea, where the Pearl River plume enters with distinct extents in different seasons. A total of six cruises were performed in spring, summer and autumn from 2006 to 2016. Four different types of vertical structures, A-alpha, A-beta, B and C types, were identified in the upper water layer. The A-alpha type was characterized by a gradual increase in density with depth, while the A-beta type was characterized by an increase in density in a very thin surface layer ( < 10 m) with a sub-layer of density uniformity. The B type was defined as an almost uniform layer over the surface water column. The C type was defined as an entirely uniform layer throughout the whole water column. Both the A-alpha and A-beta types occurred in spring and summer. The B type appeared in all three seasons, and the C type mostly appeared in spring and autumn. The MLD was shallower in summer and deeper in autumn. The A-alpha type could change to the B type under strong wind energy. Two 24-h mooring surveys, one within the Pearl River plume and the other beyond the plume zone, were conducted to examine the MLD temporal variations. The A-alpha and B types prevailed in and beyond the plume zone, respectively. The A-alpha type resulted from the strong stratification formed by freshwater and sea water, while the B type was mainly induced by air-sea interface buoyancy fluxes. We proposed that the A-alpha type should be considered in studies of the mixing and transporting of matter, salt, heat and energy in all the global costal seas.

We observed the structure of the Pearl River plume and its turbulent characteristics, and investigated the turbulent effect on total suspended matter (TSM) within its "far-field" region, based on in situ and satellite data collected in June 2015. A significant northeastward plume was created under southern monsoonal conditions. The in situ data provided the width, depth, and velocity of the plume, as inferred by salinity. Weaker turbulence occurred at the front surface position than in the plume zone. Stronger turbulence induced greater turbidity in the bottom boundary layer; however, the surface mixed layer differed. By estimating the turbidity budget, we found the lateral fluxes term was the largest term in the plume, turbulent fluxes comprised the second largest term, and the settling terms comprised the smallest term. We quantified the turbulent mechanisms and found that stronger river discharge induced greater TSM turbidity. Tidal and buoyancy fluxes had minor regulatory effects on TSM. Our observations suggest that TSM in the "far field" region originated from the Pearl River and the coastal region.