Near-bottom currents play important roles in the formation and dynamics of deep-water sedimentary systems. This study examined the characteristics and temporal variations of near-bottom currents, especially the tidal components, based on two campaigns (2014 and 2016) of in situ observations conducted southeast of the Dongsha Island in the South China Sea. Results demonstrated near-bottom currents are dominated by tidal currents, the variance of which could account for ~70% of the total current variance. Diurnal tidal currents were found stronger than semidiurnal currents for both barotropic and baroclinic components. The diurnal tidal currents were found polarized with predominantly clockwise-rotating constituents, whereas the clockwise and counterclockwise constituents were found comparable for semidiurnal tidal currents. It was established that diurnal tidal currents could induce strong current shear. Baroclinic tidal currents showed pronounced seasonal variation with large magnitude in winter and summer and weak magnitude in spring and autumn in 2014. The coherent components accounted for ~65% and ~50% of the diurnal and semidiurnal tidal current variances, respectively. The proportions of the coherent and incoherent components changed little in different seasons. In addition to tidal currents, it was determined that the passing of mesoscale eddies could induce strong near-bottom currents that have considerable influence on the deep circulation.

Based on the daily sea surface height and absolute geostrophic velocity data from 1993 to 2015 provided by the AVISO Center of French Space Agency, the surface Kuroshio transport east of Taiwan and its adjacent eddy field (sea surface height anomaly) were analyzed. Four main periods of the surface Kuroshio transport and eddy field east of Taiwan were obtained, which were used to reveal their interactions. The main conclusions are as follows: (1) Based on the wavelet analysis, the surface Kuroshio transport east of Taiwan and its nearby eddy field showed significant seasonal, annual and interannual periods. In addition to the obvious periods of 182 days (0.5 year) and 365 days (1 year), there were also more obvious periods of about 860 days (2.35 years) and 2 472 days (6.8 years) for the surface Kuroshio transport. There were also four more obvious periods corresponding to the eddy field of 200 days (0.55 year), 374 days (1 year), 889 days (2.43 years) and 2 374 days (6.5 years), although there were latitudinal variations. (2) Based on both the correlation and causal analysis, the correlation between the surface Kuroshio transport and the nearby eddy field over the above four periods was analyzed, and different Kuroshio-eddy interactions, with period and latitudinal variability, were revealed.

Using the fuzzy cluster analysis and the temperature-salinity (T-S) similarity number analysis of cruise conductivity-temperature-depth (CTD) data in the upper layer (0–300 m) of the northern South China Sea (NSCS), we classify the upper layer water of the NSCS into six water masses: diluted water (D), surface water (SS), the SCS subsurface water mass (U_S), the Pacific Ocean subsurface water mass (U_P), surface-subsurface mixed water (SU) and subsurface-intermediate mixed water (UI). A new stacked stereogram is used to illustrate the water mass distribution, and to examine the source and the distribution of U_P, combining with the sea surface height data and geostrophic current field. The results show that water mass U_P exists in all four seasons with the maximum range in spring and the minimum range in summer. In spring and winter, the U_P intrudes into the Luzon Strait and the southwest of Taiwan Island via the northern Luzon Strait in the form of nonlinear Rossby eddies, and forms a high temperature and high salinity zone east of the Dongsha Islands. In summer, the U_P is sporadically distributed in the study area. In autumn, the U_P is located in the upper 200 m layer east of Hainan Island.

Based on the high-resolution Eulerian fields of an ocean general circulation model simulation, the heat contribution of the Indonesian throughflow (ITF) to the Indian Ocean is estimated by Lagrangian tracing method. The heat transport of each particle of ITF waters is calculated by tracing temperature change along the trajectory until the particle exits the Indian Ocean. The simulation reveals that the ITF waters flow westward and branch near Madagascar, further showing the ITF waters are redistributed in both northern and southern Indian Ocean. Heat budget analysis indicates that the ITF waters gain 0.41 PW (Petawatts, 10¹⁵ W) in the northern Indian Ocean and lose 0.56 PW in the southern Indian Ocean, respectively. As a result, the ITF waters warm the whole Indian Ocean basin with only 0.15 PW, which shows an “insignificant” role of ITF on the Indian Ocean because of the heat exchange compensation between northern and southern Indian Ocean. Furthermore, the tracing pathways show that the ITF waters mainly flow out the Indian Ocean at both sides of the basin via Agulhas Current and Leeuwin Current. About 89% of the ITF waters leave along western boundary and the rest 11% along eastern boundary. Compared to seeding section, 0.10 PW and 0.05 PW are released to the Indian Ocean, respectively.

For acoustic detection of internal waves, the core issue is to obtain the temporal and spatial distribution of the sound speed profile (SSP). In the inversion process, the SSP is usually expressed by a few parameters through expansion. However, information about internal waves may sometimes be hard to read directly from the inversion results. The aim of this paper is to characterize the internal waves directly though expansion coefficients. By deducing the dynamic equations of the internal waves, an orthogonal basis called the hydrodynamic normal modes (HNMs) can be extracted from a certain number of SSP samples. Unlike the existing widely used empirical orthogonal functions (EOFs), the HNMs have a more explicit physical meaning that is directly related to internal wave activity. The HNMs are then used to expand the SSP time series, and the expansion coefficients are derived. Eventually, information about internal waves can be read directly from the time derivative of the expansion coefficients of the first two modes. In this study, this method is applied to thermistor string profiles from the northern shelf of the South China Sea, where the SSP shows evident spatial and temporal variations due to internal waves. The results show that the SSP can be described approximately by the first two modes with adequate precision. The special oscillation structure of the time derivative of the expansion coefficients can be used to detect internal solitary waves. The expansion coefficients can also give information on internal solitary wave amplitude and width. According to theoretical and experimental analysis, it can be concluded that the internal waves monitoring method introduced in this paper is effective. The HNMs method is simple to apply and depends less on sample data than EOFs. It could be used as an efficient alternative to EOFs to expand the use of the SSP in highly variable areas, where internal waves are intensive.

A 1.5-layer quasi-geostrophic reduced gravity model is used to study the hysteresis of a periodic or leaking western boundary current (WBC) flowing by a gap. When the periods of the WBC variations are much longer than the Rossby adjustment time scales of the circulation in the vicinity of the gap, the Hopf bifurcations during the Re-increase and Re-decrease loops are delayed to produce a new domain of hysteresis of the Reynolds numbers, and the critical Reynolds numbers of the WBC regime transitions change significantly, with the domain of the hysteresis Reynolds number larger for shorter periodic forcing. When the periods of the WBC variations are comparable to those of the Rossby adjustment time scales of the circulation in the vicinity of the gap, the WBC path inside the gap becomes periodic without hysteresis. The intrusion of the WBC into the western basin generally gets smaller as the period decreases. In addition, the partial leakage of the WBC transport through the gap into the western basin is found to have significant impact on the hysteresis loop of the WBC path when the leaked transport is larger than 1/2 of the WBC. Both the intrusion extent and the critical Reynolds numbers of the WBC regime transition are changed, and the larger the throughflow transport, the larger the change.

After validated by the in-situ observation, the slab model is used to study the wind-generated near-inertial energy flux (NIEF) in the South China Sea (SCS) based on satellite-observed wind data, and its dependence on calculation methods and threshold criteria of the mixed layer depth (MLD) is investigated. Results illustrate that the total amount of NIEF in the SCS could be doubled if different threshold criteria of MLD are adopted. The NIEF calculated by the iteration and spectral solutions can lead to a discrepancy of 2.5 GW (1 GW=1×10⁹ W). Results also indicate that the NIEF exhibits spatial and temporal variations, which are significant in the boreal autumn, and in the southern part of the SCS. Typhoons are an important generator of NIEF in the SCS, which could account for approximately 30% of the annual mean NIEF. In addition, deepening of the MLD due to strong winds could lead to a decrease of NIEF by approximately by 10%. We re-estimate the annual mean NIEF in the SCS, which is (10±4) GW and much larger than those reported in previous studies.

A 72-h fine-resolution atmosphere-wave-ocean coupled forecasting system was developed for the South China Sea and its adjacent seas. The forecasting model domain covers from from 15°S to 45°N in latitude and 99°E to 135°E in longitude including the Bohai Sea, the Yellow Sea, the East China Sea, the South China Sea and the Indonesian seas. To get precise initial conditions for the coupled forecasting model, the forecasting system conducts a 24-h hindcast simulation with data assimilation before forecasting. The Ensemble Adjustment Kalman Filter (EAKF) data assimilation method was adopted for the wave model MASNUM with assimilating Jason-2 significant wave height (SWH) data. The EAKF data assimilation method was also introduced to the ROMS model with assimilating sea surface temperature (SST), mean absolute dynamic topography (MADT) and Argo profiles data. To improve simulation of the structure of temperature and salinity, the vertical mixing scheme of the ocean model was improved by considering the surface wave induced vertical mixing and internal wave induced vertical mixing. The wave and current models were integrated from January 2014 to October 2015 driven by the ECMWF reanalysis 6 hourly mean dataset with data assimilation. Then the coupled atmosphere-wave-ocean forecasting system was carried out 14 months operational running since November 2015. The forecasting outputs include atmospheric forecast products, wave forecast products and ocean forecast products. A series of observation data are used to evaluate the coupled forecasting results, including the wind, SHW, ocean temperature and velocity. The forecasting results are in good agreement with observation data. The prediction practice for more than one year indicates that the coupled forecasting system performs stably and predict relatively accurate, which can support the shipping safety, the fisheries and the oil exploitation.

So far, large uncertainties of the Indonesian throughflow (ITF) reside in the eastern Indonesian seas, such as the Maluku Sea and the Halmahera Sea. In this study, the water sources of the Maluku Sea and the Halmahera Sea are diagnosed at seasonal and interannual timescales and at different vertical layers, using the state-of-the-art simulations of the Ocean General Circulation Model (OGCM) for Earth Simulator (OFES). Asian monsoon leaves clear seasonal footprints on the eastern Indonesian seas. Consequently, the subsurface waters (around 24.5σ_θ and at ~150 m) in both the Maluku Sea and the Halmahera Sea stem from the South Pacific (SP) during winter monsoon, but during summer monsoon the Maluku Sea is from the North Pacific (NP), and the Halmahera Sea is a mixture of waters originating from the NP and the SP. The monsoon impact decreases with depth, so that in the Maluku Sea, the intermediate water (around 26.8σ_θ and at ~480 m) is always from the northern Banda Sea and the Halmahera Sea water is mainly from the SP in winter and the Banda Sea in summer. The deep waters (around 27.2σ_θ and at ~1 040 m) in both seas are from the SP, with weak seasonal variability. At the interannual timescale, the subsurface water in the Maluku Sea originates from the NP/SP during El Niño/La Niña, while the subsurface water in the Halmahera Sea always originates from the SP. Similar to the seasonal variability, the intermediate water in Maluku Sea mainly comes from the Banda Sea and the Halmahera Sea always originates from the SP. The deep waters in both seas are from the SP. Our findings are helpful for drawing a comprehensive picture of the water properties in the Indonesian seas and will contribute to a better understanding of the ocean-atmosphere interaction over the maritime continent.

In this study, the sectional characteristics of temperature, salinity and density off the central Zhejiang coast were analyzed using three sections of observational data in the spring of 2016. The results are as follows: (1) a cold water patch was observed in the middle layer of sections from 10 to 25 m, and a weak upwelling was observed at the upper layer near the central Zhejiang coast; (2) several thermoclines, inverted thermoclines, and haloclines were observed in the survey area; (3) the Taiwan Warm Current Water (TWCW) climbing from the slope towards the survey area affected the thermocline, making it thinner and intensified; however, the TWCW was not strong enough to break through the thermocline to reach the sea surface.