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The interaction between the Antarctic Circumpolar Current (ACC) jets and mesoscale eddies plays a prominent role in the dynamics of the ACC. However, few studies have investigated the influence of the jets on the statistics of the mesoscale turbulence field. This study combines a mesoscale eddy detection algorithm with the jet-detection technique to extract their interaction in the Southern Ocean from 1993 to 2016. It is found that stronger jet filaments can effectively shorten eddy lifetime, thus introducing a negative correlation between eddy lifetime and matched jet filament velocities. It can also be seen along the pathway of the ACC whether the eddy is located upstream or downstream of topography. A series of numerical experiments suggest that the stronger zonal jet plays the role of a waveguide to facilitate the stronger and faster eastward linear Rossby wave field induced by the mesoscale eddy. A stronger eastward Rossby wave phase speed, enabled by the jet-induced Doppler shift, results in a more rapid loss of eddy energy and shortens the eddy lifetime.

An inverse reduced-gravity model is used to simulate the deep South China Sea (SCS) circulation. A set of experiments are conducted using this model to study the influence of the Luzon overflow through the two inlets on the deep circulation in the northern SCS. Model results suggest that the relative contribution of these inlets largely depends on the magnitude of the input transport of the overflow, but the northern inlet is more efficient than the southern inlet in driving the deep circulation in the northern SCS. When all of the Luzon overflow occurs through the northern inlet the deep circulation in the northern SCS is enhanced. Conversely, when all of the Luzon overflow occurs through the southern inlet the circulation in the northern SCS is weakened. A Lagrangian trajectory model is also developed and applied to these cases. The Lagrangian results indicate that the location of the Luzon overflow likely has impacts upon the sediment transport into the northern SCS.

The existence of a deep western boundary current (DWBC) in the South China Sea (SCS) was verified by direct observations from three current moorings deployed from September 2015 to September 2018. The average current speeds observed in the DWBC were around 1 cm s−1 along the northern boundary and 3 cm s−1 along the western boundary. The DWBC demonstrates significant intraseasonal variability in the 30–120-day-period band, which may come from the variability in the Luzon overflow or the eddies in the deep SCS forced by a stable Luzon overflow. In addition, observations found that this DWBC along the northern boundary can reverse its direction meridionally in the spring. Model results suggest that if the Luzon overflow decreases one-third of its typical transport, this current reversal can occur. This behavior can be explained through “relaxation” theory.

Diurnal variation in surface latent heat flux (LHF) and the effects of diurnal variations in LHF-related variables on the climatological LHF are examined using observations from the Global Tropical Moored Buoy Array. The estimated amplitude of the climatological diurnal LHF over the Indo-Pacific warm pool and the equatorial Pacific and Atlantic cold tongues is remarkable, with maximum values exceeding 20.0 W m−2. Diurnal variability of sea surface skin temperature (SSTskin) is the primary contributor to the diurnal LHF amplitude. Because the diurnal SSTskin amplitude has an inverse relationship with surface wind speed over the tropical oceans, an inverse spatial pattern between the diurnal LHF amplitude and surface wind speed results. Resolving diurnal variations in the SSTskin and wind improves the estimate of the climatological LHF by properly capturing the daytime SSTskin and daily mean wind speed, respectively. The diurnal SSTskin-associated contribution is large over the warm pool and equatorial cold tongues where low wind speeds tend to cause strong diurnal SSTskin warming, while the magnitude associated with the diurnal winds is large over the highly dynamic environment of the Inter-Tropical Convergence Zone. The total diurnal contribution is about 9.0 W m−2 on average over the buoy sites. There appears to be a power function (linear) relationship between the diurnal SSTskin-associated (wind-associated) contribution and surface mean wind speed (wind speed enhancement from diurnal variability). The total contribution from diurnal variability can be estimated accurately from high-frequency surface wind measurements using these relationships.

In this study, subsurface eddies near the Vietnam coast of the South China Sea were observed with in situ observations, including Argo, CTD, XBT and some processed and quality controlled data. Based on temperature profiles from four Argo floats near the coast of Vietnam, a subsurface warm eddy was identified in spring and summer. The multi-year Argo and Global Temperature and Salinity Profile Programme (GTSPP) data were merged on a seasonal basis based on the data interpolating variational analysis (DIVA) method to reconstruct the three-dimensional temperature structure. There is a warm eddy in the central subsurface at 12.5°N, 111°E below 300 m depth in spring, which does not exist in autumn and is weak in winter and summer. From CSIRO Atlas of Regional Seas (CARS) and Generalized Digital Environment Model (GDEM) reanalysis data, this subsurface warm eddy is also verified in spring.

Submesoscale eddies are often seen in high‐resolution satellite‐derived ocean colour images. To efficiently identify these eddies from surface chlorophyll data, here we develop an automatic submesoscale eddy detection method and apply it to the South China Sea (SCS). The detected submesoscale eddies are found to have a radius of 13 5 km and an aspect ratio of 0.5 0.2, with a notable predominance of cyclones. Further investigation reveals that the surface structure of these eddies displays a unique ‘cat's‐eye' pattern and the eddies become more circular with increasing eddy radius. Submesoscale eddies can strongly regulate surface chlorophyll via horizontal advection while they have less coherent signatures in sea surface temperature. These findings may help to improve submesoscale parameterizations in Earth system models.

Both observations and empirical model results show that the responses of the diurnal amplitude of sea surface temperature (DSST) to the Madden-Julian Oscillation (MJO) in the tropical regions are opposite on the two sides of a pivot longitude, which varies seasonally. This pivot longitude occurs at 180° in boreal winter, but moves to 150°E in boreal summer. When the active convection center of the MJO is located to the west of the pivot longitude, reduced solar insolation and enhanced wind speed decrease DSST; when the active convection center moves to the east of the pivot longitude, reduced wind speed increases DSST. Due to the opposing wind speed anomalies across the pivot longitude, opposite responses to the MJO can be seen in latent heat flux anomalies, evaporation rate anomalies and surface roughness anomalies. These findings suggest that opposite responses in the tropical regions on the west and east of the pivot longitude to the MJO be common for variables that are closely related to wind speed.

We analyze the projected sea level rise (SLR) for the 21st century for the China Seas (the Bohai Sea, Yellow Sea, East China Sea, and South China Sea) using the Coupled Model Intercomparison Project Phase 5 dataset. We find that the projected SLR over the shallow continental shelves of the China Seas is nearly the same as the global mean sea level change in all future emission scenarios, with a magnitude of 43.6 cm (20.8–67.7 cm, 90% confidence interval) in RCP2.6 and 74.5 cm (41.7–112.8 cm, 90% confidence interval) in RCP8.5 by the year 2100 relative to 1986–2005. We further analyze the causes of SLR and find that more than 90% of the total projected SLR over the continental shelves of the China Seas will result from mass contributions and only a minor contribution will result from local steric height adjustments. This increase in water mass over the continental shelves is not only caused by the loss of land ice, but also from the change in sterodynamic, which tends to push water mass onto the continental shelves from the open oceans.

Based upon a suite of satellite and hydrology data along with numerical model simulations, the Kuroshio is found to be separated into two branches. The two branches are located on the western and eastern sides of the Batanes Islands. The western branch, which is the main branch of the Kuroshio, is estimated to carry roughly 68% of the transport, while the eastern branch, which has not been reported before, carries the remainder. Both branches bring warmer water northward, producing two separate warm tongues east of the Luzon Strait. The western warm tongue has an obvious seasonal variation due to the seasonal variation of the Kuroshio in the northern Luzon Strait, while the eastern warm tongue is associated with Pacific mesoscale eddies. As an anticyclonic (a cyclonic) eddy approaches the Batanes Islands from the east, the eastern branch of the Kuroshio is strongly intensified (weakened), and a more (less) pronounced warm tongue is induced. Consequently, interannual variability of Pacific mesoscale eddies affects the strength of the eastern branch.

Interannual variability of thermal front west of Luzon Island during the winter of 1993–2013 is examined with the method of singular value decomposition (SVD) and a suite of satellite measurements in this paper. It is found that both the area and intensity of the thermal front west of Luzon Island show apparent interannual variability. Further study based on SVD shows that the interannual variability of the thermal front is highly associated with El Nino and Southern Oscillation (ENSO), and the correlation coefficient between Nino3.4 index and the first Principal Component (PC1) of thermal front can reach–0.65. The mechanism can be described as follows. In El Nino (La Nina) years, the East Asian winter monsoon (EAWM) is weakened (enhanced), inducing weaker (stronger) local wind stress curl (WSC) west of Luzon Island, and resulting in weakened (enhanced) Luzon cold eddy, which finally leads to the weakening (enhancement) of the thermal front.

Winter coastal upwelling off northwest Borneo in the South China Sea (SCS) is investigated by using satellite data, climatological temperature and salinity fields and reanalysis data. The upwelling forms in December, matures in January, starts to decay in February and almost disappears in March. Both Ekman transport induced by the alongshore winter monsoon and Ekman pumping due to orographic wind stress curl are favorable for the upwelling. Transport estimates demonstrate that the month-to-month variability of Ekman transport and Ekman pumping are both consistent with that of winter coastal upwelling, but Ekman transport is two times larger than Ekman pumping in January and February. Under the influence of El Nino-Southern Oscillation (ENSO), the upwelling shows remarkable interannual variability: during winter of El Nino (La Nina) years, an anticyclonic (a cyclonic) wind anomaly is established in the SCS, which behaves a northeasterly (southwesterly) anomaly and a positive (negative) wind stress curl anomaly off the northwest Borneo coast, enhancing (reducing) the upwelling and causing anomalous surface cooling (warming) and higher (lower) chlorophyll concentration. The sea surface temperature anomaly (SSTA) associated with ENSO off the northwest Borneo coast has an opposite phase to that off southeast Vietnam, resulting in a SSTA seesaw pattern in the southern SCS in winter.

Due to orographic blockage, a weak wind wake occurs in summer off northeast Vietnam in the South China Sea. Under the wind wake, warm water is observed from both high-resolution satellite data and hydrographic observations. The wake of warm water forms in June, continues to mature in July and August, starts to decay in September, and disappears in October. The warm water wake also shows robust diurnal variation — it intensifies during the day and weakens in the night. Warm water wakes can be generated through wind-induced mixing and thermal (latent heat flux) processes. In this paper, a mixed layer model is used to evaluate the relative importance of the two processes on seasonal and diurnal timescales, respectively. The results demonstrate that thermal processes make a greater contribution to the wake than wind-induced mixing processes on a seasonal timescale, while the warm water wake is dominated by wind-induced mixing processes on a diurnal timescale.

The interannual variability of the eastward current in the western South China Sea (SCS) during the summer of 1993–2012 is examined with satellite altimeter data and Regional Ocean Modeling System (ROMS) model output. It is found that the meridional location of the eastward current displays apparent interannual variability. The core of the eastward current shifts between 10.7°N and 17.6°N with a standard deviation of 1.6°. Results from Sverdrup theory and ROMS experiments demonstrate a close dynamic linkage between the north-south migration of the eastward current and the SCS summer monsoon anomaly on the interannual time scale. When the summer monsoon has southwesterly (northeasterly) anomaly, the eastward current moves southward (northward). With the southward (northward) shift of the eastward current, the summer cold filament in the SCS moves southward (northward) as well.

The horizontal velocity structure of mesoscale eddies is investigated using data from 18-year altimetry data in the South China Sea (SCS). A total of 2,433 cyclonic eddies (CEs) and 3,132 anticyclonic eddies (AEs) are detected in this data set. This analysis shows that the eddy velocity structure is strongly linked to the eddy amplitude, with the band of maximum velocity moving closer to the eddy core as the eddy amplitude increases. By combining the records of Argo float profiles and satellite altimetry data, the eddy feature found in sea level anomaly (SLA) data can also be seen at the subsurface. This feature is related to sub-mesoscale processes accompanying with water exchange and energy cascade at the outer edge of mesoscale eddy. A modified Taylor vortex model is built including this new finding to describe the eddy velocity structure. This vortex model is found to be useful as it better describes the eddy’s life history, the knowledge of which is important for better understanding physical, biological and chemical processes in the ocean.

The present study investigates the impact of the Madden–Julian oscillation (MJO) on the South China Sea (SCS) in summer with three types of models: a theoretical Sverdrup model, a 1.5-layer reduced gravity model, and a regional ocean model [Regional Ocean Modeling System (ROMS)]. Results show that the ocean circulation in the SCS has an intraseasonal oscillation responding to the MJO. During its westerly phase, the MJO produces positive (negative) wind stress curl over the northern (southern) SCS and thus induces an enhanced cyclonic (anticyclonic) circulation in the northern (southern) SCS. This not only cools sea surface temperature (SST) but also decreases (increases) subsurface temperature in the northern (southern) SCS. During its easterly phase, the MJO basically produces a reversed but weaker influence on SCS ocean circulation and temperature. Thus, the MJO can have an imprint on the summer climatology of SCS circulation and temperature. The authors' analysis further indicates that the MJO's dynamic effect associated with wind is generally more important than its thermodynamic effect in modulating the regional ocean circulation and temperature. The present study suggests that the MJO is important for summer ocean circulation and temperature in the SCS.

The spatial distribution of the energy flux, bottom boundary layer (BBL) energy dissipation, surface elevation amplitude and current magnitude of the major semidiurnal tidal constituents in the Bering Sea are examined in detail. These distributions are obtained from the results of a three-dimensional numerical simulation model (POM). Compared with observation data from seven stations, the root mean square errors of tidal height are 2.6 cm and 1.2 cm for M2 and N2 respectively, and those of phase-lag are 21.8° and 15.8° respectively. The majority of the tidal energy flux off the deep basin is along the shelf edge, although some of this flux crosses the shelf edge, especially in the southeast of the shelf break. The total M2 energy dissipation in the Bering Sea is 30.43 GW, which is about 10 times of that of N2 and S2. The semidiurnal tidal energy enters mainly to the Bering Sea by Samalga Pass, Amukta Pass and Seguam Pass, accounting more than 60% of the total energy entering the Being Sea from the Pacific.

Based on the three-dimensional ECOM model, the tide, tide-induced residual current, wind-driven and density currents in the Beibu (Tonkin) Gulf and Qiongzhou Strait are diagnostically computed in fine grid. The tides and tidal currents in the Beibu Gulf and Qiongzhou Strait are well reproduced. The model results show that the semidiurnal tidal wave propagates eastward from the Beibu Gulf through Qiongzhou Strait, while diurnal tidal waves enter the strait from both the eastern and western sides and interact on the southeast coast of the strait. The formation processes of the residual currents in Qiongzhou Strait in summer (August) and winter (January) are mainly discussed. It is shown that the total residual currents (coupling effect of wind, tide and density) in the strait are westward in both summer and winter. The water volume transported from the east to west into the Beibu Gulf is 0.026 Sv in summer and 0.116 Sv in winter. Numerical experiments indicate that the dominant factor affecting the residual currents in the strait is not the wind stress but the tidal rectification. The westward tide-induced residual current, driven by the tidal rectification, is rather strong in the strait with water volume transport of 0.063 Sv. The wind in summer only reduces the westward tide-induced residual current to a certain extent, and the wind in winter approximately doubles the westward tide-induced residual current through the strait. The density current contributes little to the total residual current in both summer and winter.