Yeqiang Shu - Academia.edu (original) (raw)

Papers by Yeqiang Shu

Research paper thumbnail of The 4-D structure of upwelling and Pearl River plume in the northern South China Sea during summer 2008 revealed by a data assimilation model

Ocean Modelling, 2011

We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South C... more We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South China Sea (NSCS) during the summer of 2008 based on data assimilation. An Ensemble Kalman Smoother scheme is employed in the Princeton Ocean Model. It is found that the Pearl River plume axis extended eastward along with the surface current and swerved offshore twice near (116°E, 22.6°N) and (117.5°E, 22.8°N) before reaching the Taiwan Strait. The vertical transect of salinity along the plume axis indicates that the Pearl River freshwater could affect salinity distribution down to a depth of 10-20 m. Anomalously warm water is found in the upper layer, which could be attributed to the intensified stratification and suppressed vertical mixing caused by the freshwater of the plume capping the upwelling west of 116°E. The varying winds from upwelling favorable to downwelling favorable could induce a low-salinity water lens at the center of the model domain. Upwelling in the NSCS initially occurred at 114.5°E, to the east of the Pearl River Estuary, intensified eastward, and reached its maximum near Shantou (116.7°E, 23.2°N). Since current-induced upwelling appeared mainly in Shantou due to the widened shelf, it is found that even if the wind-induced upwelling was shut down in Shanwei by downwelling favorable wind on July 4, the upwelling still existed in Shantou. Moreover, because the direction of large-scale current was in favor of upwelling in the NSCS that cannot be reversed by varying local winds over a short time period, the upwelling shutdown time is longer for both wind-induced and current-induced upwelling in Shantou than for mainly wind-induced upwelling in Shanwei. The steeper slope in Shanwei also shortens the upwelling shutdown time there.

Research paper thumbnail of Performance of four sea surface temperature assimilation schemes in the South China Sea

Continental Shelf Research, 2009

Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their ... more Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their performances in assimilating the advanced very high resolution radiometer pathfinder best SST data in the South China Sea using the Princeton Ocean Model. Schemes 1 and 2 project SST directly to subsurface according to model-based correlations between SST and subsurface temperature. The difference between these two schemes is related to the order of vertical projection and horizontal optimal interpolation (OI). In Scheme 1, the spatially non-uniform SST observations are first projected to subsurface levels, followed by horizontal OI at each level. While in Scheme 2, the remotely sensed SSTs are first optimally interpolated to all grid points at the surface, followed by projecting gridded SSTs to subsurface levels. Scheme 3 assumes that the mixed layer is well mixed and has a uniform temperature vertically. In Scheme 4, SST is propagated to subsurface levels using a linear relationship of temperature between any two neighboring depths (Scheme 4a) or between surface and subsurface (Scheme 4b), which is derived by empirical orthogonal function (EOF) technique. To verify the results of the four schemes, the authors use the hydrographic data from two cruises during the South China Sea Monsoon Experiment in April and June 1998. It was shown that all four schemes could improve the SST field by reducing about 50% of the root mean square errors (RMSEs). All but Scheme 3 can improve model thermocline structure that is too diffused otherwise, though the RMSEs increase in the thermocline, especially for Scheme 2 when the model has opposite bias between upper layers and lower layers. Scheme 3 fails in the subsurface depth by increasing the thermocline depth, especially when there is a cold model bias. Projecting SST downward by EOF technique can deepen the depth of assimilation especially in Scheme 4a. Both Schemes 4a and b can correct the bias in the mixed layer and do not change the vertical thermal structure.

Research paper thumbnail of Deep-water sedimentary systems and their relationship with bottom currents at the intersection of Xisha Trough and Northwest Sub-Basin, South China Sea

Research paper thumbnail of Observed evidence of the anomalous South China Sea western boundary current during the summers of 2010 and 2011

Journal of Geophysical Research: Oceans, 2016

Research paper thumbnail of Assimilating temperature and salinity profiles using Ensemble Kalman Filter with an adaptive observation error and T-S constraint

Acta Oceanologica Sinica, 2016

Research paper thumbnail of Observed deep energetic eddies by seamount wake

Scientific reports, 2015

Despite numerous surface eddies are observed in the ocean, deep eddies (a type of eddies which ha... more Despite numerous surface eddies are observed in the ocean, deep eddies (a type of eddies which have no footprints at the sea surface) are much less reported in the literature due to the scarcity of their observation. In this letter, from recently collected current and temperature data by mooring arrays, a deep energetic and baroclinic eddy is detected in the northwestern South China Sea (SCS) with its intensity, size, polarity and structure being characterized. It remarkably deepens isotherm at deep layers by the amplitude of ~120 m and induces a maximal velocity amplitude about 0.18 m/s, which is far larger than the median velocity (0.02 m/s). The deep eddy is generated in a wake when a steering flow in the upper layer passes a seamount, induced by a surface cyclonic eddy. More observations suggest that the deep eddy should not be an episode in the area. Deep eddies significantly increase the velocity intensity and enhance the mixing in the deep ocean, also have potential implicati...

Research paper thumbnail of SST assimilation experiment in the northern South China Sea using ensemble Kalman filter

Research paper thumbnail of Upwelling off East Guangdong: observation, simulation and data assimilation

Research paper thumbnail of Contrasting dynamic characteristics of shear turbulence and Langmuir circulation in the surface mixed layer

Acta Oceanologica Sinica, 2015

Research paper thumbnail of Thermal variations in the South China Sea associated with the eastern and central Pacific El Niño events and their mechanisms

Journal of Geophysical Research: Oceans, 2014

In this study, we investigate the interannual variability of the sea surface temperature (SST) in... more In this study, we investigate the interannual variability of the sea surface temperature (SST) in the South China Sea (SCS) associated with two types of El Niño, namely, the eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño. First, double warm peaks can occur during both types of El Niño events in the SCS. However, the strong warm basin mode can only develop in the EP El Niño, while the warm semibasin mode exists during the CP El Niño. Associated with an anomalous positive (negative) net surface heat flux in the EP (CP) El Niño, along with a shallower thermocline with weaker (stronger) northeasterly wind anomalies, the SST anomalies become warmer (cooler) in the developing autumn. Over the background of cooling SST in autumn of CP El Niño, therefore, only a weak warming can occur in the subsequent years, which is limited in the western boundary area under the forcing of warm ocean advection. Second, the SST oscillation periods are different in these two types of El Niño. The SST evolution in the EP El Niño is negative-positive with a quasi-biennial oscillation, but that in the CP El Niño is positive-negative-positive-negative with an annual oscillation. It seems that the double cooling in the CP El Niño is phase-locked to the late autumn season.

Research paper thumbnail of Effects of the Pearl River plume on the vertical structure of coastal currents in the Northern South China Sea during summer 2008

Ocean Dynamics, 2014

Analyses of moored current meter data from the South China Sea (SCS) Coastal Oceanographic Proces... more Analyses of moored current meter data from the South China Sea (SCS) Coastal Oceanographic Process Experiment Pilot Project (SCOPE-PP) reveal that strong spatial variation in the vertical structure of coastal currents in the northern SCS (NSCS) was caused by the interaction between the Pearl River plume (PRP) and the alongshore coastal jet. Observations indicate that the NSCS coastal current was generally eastward, spreading the PRP water eastward. The lighter brackish plume residing on the top of the saline, coastal water strengthened the stratification near the surface and blocked the downward momentum transport, resulting in a strong vertical shear of the velocity at the mooring site S205. Meanwhile, the plume water raised the sea level. Because of the geostrophic balance in the across-shore direction, the eastward coastal current in the upper layer strengthened on the onshore side (near S205) and weakened on the offshore side (near S206 and S305) due to the plume water intrusion. Therefore, the vertical shear of the velocity was enhanced at S205 and reduced at S206 and S305. Moreover, because the plume varied substantially with the wind, the vertical shear of the velocity responding to the wind was different between S205 and S206. The numerical simulation during summer 2008 confirms that the PRP enhances (weakens) the vertical shear of the velocity on the onshore (offshore) side by increasing (decreasing) the upper-layer current by about 0.2 m s -1 and decreasing (increasing) the middle-layer current slightly.

Research paper thumbnail of Meridional overturning circulation in the South China Sea envisioned from the high-resolution global reanalysis data GLBa0.08

Journal of Geophysical Research: Oceans, 2014

The pattern of meridional overturning circulation (MOC) in the South China Sea (SCS) is studied u... more The pattern of meridional overturning circulation (MOC) in the South China Sea (SCS) is studied using a numerical Lagrangian tracing method with the HYCOM1NCODA Global 1/12 Analysis (GLBa0.08) data. The SCS MOC has a ''sandwich'' structure, which consists of a layer of stronger clockwise circulation above 500 m depth, a counterclockwise layer in the mid layer between 500 and 1000 m depth, and a weaker clockwise layer below 1000 m. The deep (below 1000 m depth) clockwise layer is divided into three cells, namely, the deep southern MOC cell, DSMOC; the deep middle MOC cell, DMMOC; and the unclosed deep northern MOC cell, DNMOC. The inflow through the Luzon Strait is the main source for the SCS MOCs. The upper layer Luzon Strait inflow dominates the upper SCS MOC structure but has relatively less contribution to the DNMOC, whereas the deep layer Luzon Strait inflow mainly influences the DNMOC and it mostly rises near 18 N. The inflow through the Taiwan Strait mainly contributes to the upper layer MOC. Moreover, inflows from the Mindoro and Karimata straits contribute negatively to the upper MOC but play a significant role on the DSMOC. The backward integration of Lagrangian trajectories further validates that the SCS deep water comes not only from the deep inflow but also from the entrainment of the middle and upper layer inflow through the Luzon Strait. In the SCS basin, there are three northwest-southeast tilted zones where tracers upwell, which correspond to the three deep MOC cells. One possible mechanism for these upwelling zones is the interaction between the continental slope-trapped waves and the westward planetary Rossby waves.

Research paper thumbnail of Relative contributions of local wind and topography to the coastal upwelling intensity in the northern South China Sea

J. Geophys. Res. Oceans, 2014

ABSTRACT Topographically-induced upwelling caused by the interaction between large-scale currents... more ABSTRACT Topographically-induced upwelling caused by the interaction between large-scale currents and topography was observed during four cruises in the northern South China Sea (NSCS) when the upwelling favorable wind retreated. Using a high-resolution version of the Princeton Ocean Model (POM), we investigate relative contributions of local wind and topography to the upwelling intensity in the NSCS. The results show that the topographically-induced upwelling is sensitive to alongshore large-scale currents, which have an important contribution to the upwelling intensity. The topographically-induced upwelling is comparable with the wind-driven upwelling at surface, and has a stronger contribution to the upwelling intensity than the local wind does at bottom in the near-shore shelf region. The widened shelf to the southwest of Shanwei and west of the Taiwan Banks intensifies the bottom friction especially off Shantou, which is a key factor for topographically-induced upwelling in terms of bottom Ekman transport and Ekman pumping. The local upwelling favorable wind enhances the bottom friction as well as net onshore transport along the 50-m isobath, whereas it has less influence along the 30-m isobath. This implies the local wind is more important in upwelling intensity in the offshore region than in the nearshore region. The contribution of local upwelling favorable wind on upwelling intensity is comparable with that of topography along the 50-m isobath. The effects of local upwelling favorable wind on upwelling intensity are twofold: on one hand, the wind transports surface warm water offshore, and as a compensation of mass the bottom current transports cold water onshore; on the other hand, the wind enhances the coastal current, and the bottom friction in turn increases the topographically-induced upwelling intensity.

Research paper thumbnail of Hydrographic field investigations in the Northern South China Sea by open cruises during 2004–2013

Science Bulletin, 2015

ABSTRACT In the past 10 years (2004-2013), annual open cruise during late summer provided new opp... more ABSTRACT In the past 10 years (2004-2013), annual open cruise during late summer provided new opportunities for comprehensive studies in the Northern South China Sea (NSCS). The 10-year field investigation program was carried out by the South China Sea Institute of Oceanology, Chinese Academy of Sciences (SCSIO, CAS). Measurements included water mass property, ocean circulation, atmospheric structure, and chemical and biological elements. The observation data collected during these open cruises have been intensively used in the studies of marine oceanographic, meteorological, chemical, and biological processes in the NSCS. In this study, comprehensive assessment of data application in oceanographic and meteorological studies is provided: (1) the property and variability of water masses in different layers; (2) the distribution of main currents and three-dimensional structure of mesoscale eddies; and (3) atmospheric structure and its feedback to the ocean. With the continuance of open cruises, it is feasible to construct high-quality, gridded climatological marine meteorological datasets in the NSCS in the near future.

Research paper thumbnail of Marine Vehicle Sensor Network Architecture and Protocol Designs for Ocean Observation

Sensors, 2012

The micro-scale and meso-scale ocean dynamic processes which are nonlinear and have large variabi... more The micro-scale and meso-scale ocean dynamic processes which are nonlinear and have large variability, have a significant impact on the fisheries, natural resources, and marine climatology. A rapid, refined and sophisticated observation system is therefore needed in marine scientific research. The maneuverability and controllability of mobile sensor platforms make them a preferred choice to establish ocean observing networks, compared to the static sensor observing platform. In this study, marine vehicles are utilized as the nodes of mobile sensor networks for coverage sampling of a regional ocean area and ocean feature tracking. A synoptic analysis about marine vehicle dynamic control, multi vehicles mission assignment and path planning methods, and ocean feature tracking and observing techniques is given. Combined with the observation plan in the South China Sea, we provide an overview of the mobile sensor networks established with marine vehicles, and the corresponding simulation results.

Research paper thumbnail of The 4-D structure of upwelling and Pearl River plume in the northern South China Sea during summer 2008 revealed by a data assimilation model

Ocean Modelling, 2011

We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South C... more We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South China Sea (NSCS) during the summer of 2008 based on data assimilation. An Ensemble Kalman Smoother scheme is employed in the Princeton Ocean Model. It is found that the Pearl River plume axis extended eastward along with the surface current and swerved offshore twice near (116°E, 22.6°N) and (117.5°E, 22.8°N) before reaching the Taiwan Strait. The vertical transect of salinity along the plume axis indicates that the Pearl River freshwater could affect salinity distribution down to a depth of 10-20 m. Anomalously warm water is found in the upper layer, which could be attributed to the intensified stratification and suppressed vertical mixing caused by the freshwater of the plume capping the upwelling west of 116°E. The varying winds from upwelling favorable to downwelling favorable could induce a low-salinity water lens at the center of the model domain. Upwelling in the NSCS initially occurred at 114.5°E, to the east of the Pearl River Estuary, intensified eastward, and reached its maximum near Shantou (116.7°E, 23.2°N). Since current-induced upwelling appeared mainly in Shantou due to the widened shelf, it is found that even if the wind-induced upwelling was shut down in Shanwei by downwelling favorable wind on July 4, the upwelling still existed in Shantou. Moreover, because the direction of large-scale current was in favor of upwelling in the NSCS that cannot be reversed by varying local winds over a short time period, the upwelling shutdown time is longer for both wind-induced and current-induced upwelling in Shantou than for mainly wind-induced upwelling in Shanwei. The steeper slope in Shanwei also shortens the upwelling shutdown time there.

Research paper thumbnail of Performance of four sea surface temperature assimilation schemes in the South China Sea

Continental Shelf Research, 2009

Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their ... more Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their performances in assimilating the advanced very high resolution radiometer pathfinder best SST data in the South China Sea using the Princeton Ocean Model. Schemes 1 and 2 project SST directly to subsurface according to model-based correlations between SST and subsurface temperature. The difference between these two schemes is related to the order of vertical projection and horizontal optimal interpolation (OI). In Scheme 1, the spatially non-uniform SST observations are first projected to subsurface levels, followed by horizontal OI at each level. While in Scheme 2, the remotely sensed SSTs are first optimally interpolated to all grid points at the surface, followed by projecting gridded SSTs to subsurface levels. Scheme 3 assumes that the mixed layer is well mixed and has a uniform temperature vertically. In Scheme 4, SST is propagated to subsurface levels using a linear relationship of temperature between any two neighboring depths (Scheme 4a) or between surface and subsurface (Scheme 4b), which is derived by empirical orthogonal function (EOF) technique. To verify the results of the four schemes, the authors use the hydrographic data from two cruises during the South China Sea Monsoon Experiment in April and June 1998. It was shown that all four schemes could improve the SST field by reducing about 50% of the root mean square errors (RMSEs). All but Scheme 3 can improve model thermocline structure that is too diffused otherwise, though the RMSEs increase in the thermocline, especially for Scheme 2 when the model has opposite bias between upper layers and lower layers. Scheme 3 fails in the subsurface depth by increasing the thermocline depth, especially when there is a cold model bias. Projecting SST downward by EOF technique can deepen the depth of assimilation especially in Scheme 4a. Both Schemes 4a and b can correct the bias in the mixed layer and do not change the vertical thermal structure.

Research paper thumbnail of Assimilating remote sensing and in situ observations into a coastal model of northern South China Sea using ensemble Kalman filter

Continental Shelf Research, 2011

Major forecast errors on the background error covariance from initial conditions, atmospheric for... more Major forecast errors on the background error covariance from initial conditions, atmospheric forcing, model open boundary conditions, and the river discharges are examined in a coastal model of northern South China Sea. The analysis of background error covariance matrix produced by model ensemble shows that the perturbations of the initial conditions and atmospheric forcing play major roles in producing and maintaining the amplitude of ensemble spread except for the sea surface height (SSH) field. The perturbation of model open boundary conditions can influence ensemble spread of all variables and covariance between temperature and velocity or between temperature and SSH. The perturbation of river discharge mainly affects the covariance of salinity in river estuary. A data assimilation experiment of northern South China Sea is conducted using ensemble Kalman filter (EnKF) in the Princeton Ocean Model. In the experiment the ensemble model forecasts are made by perturbing the above mentioned four major model inputs. The assimilated data include sea-surface temperature (SST) and conductive-temperature-depth (CTD) observations. The assimilation experiment suggests that assimilating SST and CTD data can effectively improve the model simulation that has a shallower thermocline and weaker plume comparing to the observations. Moreover, consistent with these improvements of temperature and salinity, the along-shore velocity, cross-shore velocity, and characters of water mass are also corrected, respectively. Crown

Research paper thumbnail of Progress of regional oceanography study associated with western boundary current in the South China Sea

Chinese Science Bulletin, 2013

Research paper thumbnail of Coastal upwelling in summer 2000 in the northeastern South China Sea

Journal of Geophysical Research, 2012

1] Using a combination of hydrographic, tide-gauge, near-bottom mooring, and satellite observatio... more 1] Using a combination of hydrographic, tide-gauge, near-bottom mooring, and satellite observations; and a numerical circulation model, we investigate the coastal upwelling in the northeastern South China Sea (NSCS) off the coast of Fujian and Guangdong Provinces, China, in the summer of 2000. Subsurface upwelling phenomenon exists mainly near the bottom boundary in the whole region investigated. It is closely related to the coastal sea level fluctuations, which are evidently modulated by both the local wind-forcing and the large-scale circulation. The northeastward interior flow following the bathymetry is accelerated by the drop of coastal sea level and leads to onshore transport and subsequent cooling in the bottom boundary layer (BBL) over the shelf west of Shantou. To the east of Shantou, the near-bottom flow veers more eastward, parallel to the coastline, and transports the nearshore cold water mass farther to the southern Fujian coast. The cross-shelf advected cold water does not always penetrate through the stratification and reach the surface. The local wind exhibits considerable synoptic variability. The decrease in sea surface temperature (SST) is mostly significant near Dongshan-Shantou, intermittent in time and intensifies preferably during weather events that bring southwesterly alongshore wind. To the west a freshwater tongue originating from the Pearl River forms a barrier layer, which results in high surface temperature in the freshwater plume. The observational evidences and modeled results shown in this study provide important information for further understanding the ecological effects associated with the upwelling processes in the NSCS.

Research paper thumbnail of The 4-D structure of upwelling and Pearl River plume in the northern South China Sea during summer 2008 revealed by a data assimilation model

Ocean Modelling, 2011

We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South C... more We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South China Sea (NSCS) during the summer of 2008 based on data assimilation. An Ensemble Kalman Smoother scheme is employed in the Princeton Ocean Model. It is found that the Pearl River plume axis extended eastward along with the surface current and swerved offshore twice near (116°E, 22.6°N) and (117.5°E, 22.8°N) before reaching the Taiwan Strait. The vertical transect of salinity along the plume axis indicates that the Pearl River freshwater could affect salinity distribution down to a depth of 10-20 m. Anomalously warm water is found in the upper layer, which could be attributed to the intensified stratification and suppressed vertical mixing caused by the freshwater of the plume capping the upwelling west of 116°E. The varying winds from upwelling favorable to downwelling favorable could induce a low-salinity water lens at the center of the model domain. Upwelling in the NSCS initially occurred at 114.5°E, to the east of the Pearl River Estuary, intensified eastward, and reached its maximum near Shantou (116.7°E, 23.2°N). Since current-induced upwelling appeared mainly in Shantou due to the widened shelf, it is found that even if the wind-induced upwelling was shut down in Shanwei by downwelling favorable wind on July 4, the upwelling still existed in Shantou. Moreover, because the direction of large-scale current was in favor of upwelling in the NSCS that cannot be reversed by varying local winds over a short time period, the upwelling shutdown time is longer for both wind-induced and current-induced upwelling in Shantou than for mainly wind-induced upwelling in Shanwei. The steeper slope in Shanwei also shortens the upwelling shutdown time there.

Research paper thumbnail of Performance of four sea surface temperature assimilation schemes in the South China Sea

Continental Shelf Research, 2009

Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their ... more Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their performances in assimilating the advanced very high resolution radiometer pathfinder best SST data in the South China Sea using the Princeton Ocean Model. Schemes 1 and 2 project SST directly to subsurface according to model-based correlations between SST and subsurface temperature. The difference between these two schemes is related to the order of vertical projection and horizontal optimal interpolation (OI). In Scheme 1, the spatially non-uniform SST observations are first projected to subsurface levels, followed by horizontal OI at each level. While in Scheme 2, the remotely sensed SSTs are first optimally interpolated to all grid points at the surface, followed by projecting gridded SSTs to subsurface levels. Scheme 3 assumes that the mixed layer is well mixed and has a uniform temperature vertically. In Scheme 4, SST is propagated to subsurface levels using a linear relationship of temperature between any two neighboring depths (Scheme 4a) or between surface and subsurface (Scheme 4b), which is derived by empirical orthogonal function (EOF) technique. To verify the results of the four schemes, the authors use the hydrographic data from two cruises during the South China Sea Monsoon Experiment in April and June 1998. It was shown that all four schemes could improve the SST field by reducing about 50% of the root mean square errors (RMSEs). All but Scheme 3 can improve model thermocline structure that is too diffused otherwise, though the RMSEs increase in the thermocline, especially for Scheme 2 when the model has opposite bias between upper layers and lower layers. Scheme 3 fails in the subsurface depth by increasing the thermocline depth, especially when there is a cold model bias. Projecting SST downward by EOF technique can deepen the depth of assimilation especially in Scheme 4a. Both Schemes 4a and b can correct the bias in the mixed layer and do not change the vertical thermal structure.

Research paper thumbnail of Deep-water sedimentary systems and their relationship with bottom currents at the intersection of Xisha Trough and Northwest Sub-Basin, South China Sea

Research paper thumbnail of Observed evidence of the anomalous South China Sea western boundary current during the summers of 2010 and 2011

Journal of Geophysical Research: Oceans, 2016

Research paper thumbnail of Assimilating temperature and salinity profiles using Ensemble Kalman Filter with an adaptive observation error and T-S constraint

Acta Oceanologica Sinica, 2016

Research paper thumbnail of Observed deep energetic eddies by seamount wake

Scientific reports, 2015

Despite numerous surface eddies are observed in the ocean, deep eddies (a type of eddies which ha... more Despite numerous surface eddies are observed in the ocean, deep eddies (a type of eddies which have no footprints at the sea surface) are much less reported in the literature due to the scarcity of their observation. In this letter, from recently collected current and temperature data by mooring arrays, a deep energetic and baroclinic eddy is detected in the northwestern South China Sea (SCS) with its intensity, size, polarity and structure being characterized. It remarkably deepens isotherm at deep layers by the amplitude of ~120 m and induces a maximal velocity amplitude about 0.18 m/s, which is far larger than the median velocity (0.02 m/s). The deep eddy is generated in a wake when a steering flow in the upper layer passes a seamount, induced by a surface cyclonic eddy. More observations suggest that the deep eddy should not be an episode in the area. Deep eddies significantly increase the velocity intensity and enhance the mixing in the deep ocean, also have potential implicati...

Research paper thumbnail of SST assimilation experiment in the northern South China Sea using ensemble Kalman filter

Research paper thumbnail of Upwelling off East Guangdong: observation, simulation and data assimilation

Research paper thumbnail of Contrasting dynamic characteristics of shear turbulence and Langmuir circulation in the surface mixed layer

Acta Oceanologica Sinica, 2015

Research paper thumbnail of Thermal variations in the South China Sea associated with the eastern and central Pacific El Niño events and their mechanisms

Journal of Geophysical Research: Oceans, 2014

In this study, we investigate the interannual variability of the sea surface temperature (SST) in... more In this study, we investigate the interannual variability of the sea surface temperature (SST) in the South China Sea (SCS) associated with two types of El Niño, namely, the eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño. First, double warm peaks can occur during both types of El Niño events in the SCS. However, the strong warm basin mode can only develop in the EP El Niño, while the warm semibasin mode exists during the CP El Niño. Associated with an anomalous positive (negative) net surface heat flux in the EP (CP) El Niño, along with a shallower thermocline with weaker (stronger) northeasterly wind anomalies, the SST anomalies become warmer (cooler) in the developing autumn. Over the background of cooling SST in autumn of CP El Niño, therefore, only a weak warming can occur in the subsequent years, which is limited in the western boundary area under the forcing of warm ocean advection. Second, the SST oscillation periods are different in these two types of El Niño. The SST evolution in the EP El Niño is negative-positive with a quasi-biennial oscillation, but that in the CP El Niño is positive-negative-positive-negative with an annual oscillation. It seems that the double cooling in the CP El Niño is phase-locked to the late autumn season.

Research paper thumbnail of Effects of the Pearl River plume on the vertical structure of coastal currents in the Northern South China Sea during summer 2008

Ocean Dynamics, 2014

Analyses of moored current meter data from the South China Sea (SCS) Coastal Oceanographic Proces... more Analyses of moored current meter data from the South China Sea (SCS) Coastal Oceanographic Process Experiment Pilot Project (SCOPE-PP) reveal that strong spatial variation in the vertical structure of coastal currents in the northern SCS (NSCS) was caused by the interaction between the Pearl River plume (PRP) and the alongshore coastal jet. Observations indicate that the NSCS coastal current was generally eastward, spreading the PRP water eastward. The lighter brackish plume residing on the top of the saline, coastal water strengthened the stratification near the surface and blocked the downward momentum transport, resulting in a strong vertical shear of the velocity at the mooring site S205. Meanwhile, the plume water raised the sea level. Because of the geostrophic balance in the across-shore direction, the eastward coastal current in the upper layer strengthened on the onshore side (near S205) and weakened on the offshore side (near S206 and S305) due to the plume water intrusion. Therefore, the vertical shear of the velocity was enhanced at S205 and reduced at S206 and S305. Moreover, because the plume varied substantially with the wind, the vertical shear of the velocity responding to the wind was different between S205 and S206. The numerical simulation during summer 2008 confirms that the PRP enhances (weakens) the vertical shear of the velocity on the onshore (offshore) side by increasing (decreasing) the upper-layer current by about 0.2 m s -1 and decreasing (increasing) the middle-layer current slightly.

Research paper thumbnail of Meridional overturning circulation in the South China Sea envisioned from the high-resolution global reanalysis data GLBa0.08

Journal of Geophysical Research: Oceans, 2014

The pattern of meridional overturning circulation (MOC) in the South China Sea (SCS) is studied u... more The pattern of meridional overturning circulation (MOC) in the South China Sea (SCS) is studied using a numerical Lagrangian tracing method with the HYCOM1NCODA Global 1/12 Analysis (GLBa0.08) data. The SCS MOC has a ''sandwich'' structure, which consists of a layer of stronger clockwise circulation above 500 m depth, a counterclockwise layer in the mid layer between 500 and 1000 m depth, and a weaker clockwise layer below 1000 m. The deep (below 1000 m depth) clockwise layer is divided into three cells, namely, the deep southern MOC cell, DSMOC; the deep middle MOC cell, DMMOC; and the unclosed deep northern MOC cell, DNMOC. The inflow through the Luzon Strait is the main source for the SCS MOCs. The upper layer Luzon Strait inflow dominates the upper SCS MOC structure but has relatively less contribution to the DNMOC, whereas the deep layer Luzon Strait inflow mainly influences the DNMOC and it mostly rises near 18 N. The inflow through the Taiwan Strait mainly contributes to the upper layer MOC. Moreover, inflows from the Mindoro and Karimata straits contribute negatively to the upper MOC but play a significant role on the DSMOC. The backward integration of Lagrangian trajectories further validates that the SCS deep water comes not only from the deep inflow but also from the entrainment of the middle and upper layer inflow through the Luzon Strait. In the SCS basin, there are three northwest-southeast tilted zones where tracers upwell, which correspond to the three deep MOC cells. One possible mechanism for these upwelling zones is the interaction between the continental slope-trapped waves and the westward planetary Rossby waves.

Research paper thumbnail of Relative contributions of local wind and topography to the coastal upwelling intensity in the northern South China Sea

J. Geophys. Res. Oceans, 2014

ABSTRACT Topographically-induced upwelling caused by the interaction between large-scale currents... more ABSTRACT Topographically-induced upwelling caused by the interaction between large-scale currents and topography was observed during four cruises in the northern South China Sea (NSCS) when the upwelling favorable wind retreated. Using a high-resolution version of the Princeton Ocean Model (POM), we investigate relative contributions of local wind and topography to the upwelling intensity in the NSCS. The results show that the topographically-induced upwelling is sensitive to alongshore large-scale currents, which have an important contribution to the upwelling intensity. The topographically-induced upwelling is comparable with the wind-driven upwelling at surface, and has a stronger contribution to the upwelling intensity than the local wind does at bottom in the near-shore shelf region. The widened shelf to the southwest of Shanwei and west of the Taiwan Banks intensifies the bottom friction especially off Shantou, which is a key factor for topographically-induced upwelling in terms of bottom Ekman transport and Ekman pumping. The local upwelling favorable wind enhances the bottom friction as well as net onshore transport along the 50-m isobath, whereas it has less influence along the 30-m isobath. This implies the local wind is more important in upwelling intensity in the offshore region than in the nearshore region. The contribution of local upwelling favorable wind on upwelling intensity is comparable with that of topography along the 50-m isobath. The effects of local upwelling favorable wind on upwelling intensity are twofold: on one hand, the wind transports surface warm water offshore, and as a compensation of mass the bottom current transports cold water onshore; on the other hand, the wind enhances the coastal current, and the bottom friction in turn increases the topographically-induced upwelling intensity.

Research paper thumbnail of Hydrographic field investigations in the Northern South China Sea by open cruises during 2004–2013

Science Bulletin, 2015

ABSTRACT In the past 10 years (2004-2013), annual open cruise during late summer provided new opp... more ABSTRACT In the past 10 years (2004-2013), annual open cruise during late summer provided new opportunities for comprehensive studies in the Northern South China Sea (NSCS). The 10-year field investigation program was carried out by the South China Sea Institute of Oceanology, Chinese Academy of Sciences (SCSIO, CAS). Measurements included water mass property, ocean circulation, atmospheric structure, and chemical and biological elements. The observation data collected during these open cruises have been intensively used in the studies of marine oceanographic, meteorological, chemical, and biological processes in the NSCS. In this study, comprehensive assessment of data application in oceanographic and meteorological studies is provided: (1) the property and variability of water masses in different layers; (2) the distribution of main currents and three-dimensional structure of mesoscale eddies; and (3) atmospheric structure and its feedback to the ocean. With the continuance of open cruises, it is feasible to construct high-quality, gridded climatological marine meteorological datasets in the NSCS in the near future.

Research paper thumbnail of Marine Vehicle Sensor Network Architecture and Protocol Designs for Ocean Observation

Sensors, 2012

The micro-scale and meso-scale ocean dynamic processes which are nonlinear and have large variabi... more The micro-scale and meso-scale ocean dynamic processes which are nonlinear and have large variability, have a significant impact on the fisheries, natural resources, and marine climatology. A rapid, refined and sophisticated observation system is therefore needed in marine scientific research. The maneuverability and controllability of mobile sensor platforms make them a preferred choice to establish ocean observing networks, compared to the static sensor observing platform. In this study, marine vehicles are utilized as the nodes of mobile sensor networks for coverage sampling of a regional ocean area and ocean feature tracking. A synoptic analysis about marine vehicle dynamic control, multi vehicles mission assignment and path planning methods, and ocean feature tracking and observing techniques is given. Combined with the observation plan in the South China Sea, we provide an overview of the mobile sensor networks established with marine vehicles, and the corresponding simulation results.

Research paper thumbnail of The 4-D structure of upwelling and Pearl River plume in the northern South China Sea during summer 2008 revealed by a data assimilation model

Ocean Modelling, 2011

We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South C... more We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South China Sea (NSCS) during the summer of 2008 based on data assimilation. An Ensemble Kalman Smoother scheme is employed in the Princeton Ocean Model. It is found that the Pearl River plume axis extended eastward along with the surface current and swerved offshore twice near (116°E, 22.6°N) and (117.5°E, 22.8°N) before reaching the Taiwan Strait. The vertical transect of salinity along the plume axis indicates that the Pearl River freshwater could affect salinity distribution down to a depth of 10-20 m. Anomalously warm water is found in the upper layer, which could be attributed to the intensified stratification and suppressed vertical mixing caused by the freshwater of the plume capping the upwelling west of 116°E. The varying winds from upwelling favorable to downwelling favorable could induce a low-salinity water lens at the center of the model domain. Upwelling in the NSCS initially occurred at 114.5°E, to the east of the Pearl River Estuary, intensified eastward, and reached its maximum near Shantou (116.7°E, 23.2°N). Since current-induced upwelling appeared mainly in Shantou due to the widened shelf, it is found that even if the wind-induced upwelling was shut down in Shanwei by downwelling favorable wind on July 4, the upwelling still existed in Shantou. Moreover, because the direction of large-scale current was in favor of upwelling in the NSCS that cannot be reversed by varying local winds over a short time period, the upwelling shutdown time is longer for both wind-induced and current-induced upwelling in Shantou than for mainly wind-induced upwelling in Shanwei. The steeper slope in Shanwei also shortens the upwelling shutdown time there.

Research paper thumbnail of Performance of four sea surface temperature assimilation schemes in the South China Sea

Continental Shelf Research, 2009

Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their ... more Four existing sea surface temperature (SST) assimilation schemes are evaluated in terms of their performances in assimilating the advanced very high resolution radiometer pathfinder best SST data in the South China Sea using the Princeton Ocean Model. Schemes 1 and 2 project SST directly to subsurface according to model-based correlations between SST and subsurface temperature. The difference between these two schemes is related to the order of vertical projection and horizontal optimal interpolation (OI). In Scheme 1, the spatially non-uniform SST observations are first projected to subsurface levels, followed by horizontal OI at each level. While in Scheme 2, the remotely sensed SSTs are first optimally interpolated to all grid points at the surface, followed by projecting gridded SSTs to subsurface levels. Scheme 3 assumes that the mixed layer is well mixed and has a uniform temperature vertically. In Scheme 4, SST is propagated to subsurface levels using a linear relationship of temperature between any two neighboring depths (Scheme 4a) or between surface and subsurface (Scheme 4b), which is derived by empirical orthogonal function (EOF) technique. To verify the results of the four schemes, the authors use the hydrographic data from two cruises during the South China Sea Monsoon Experiment in April and June 1998. It was shown that all four schemes could improve the SST field by reducing about 50% of the root mean square errors (RMSEs). All but Scheme 3 can improve model thermocline structure that is too diffused otherwise, though the RMSEs increase in the thermocline, especially for Scheme 2 when the model has opposite bias between upper layers and lower layers. Scheme 3 fails in the subsurface depth by increasing the thermocline depth, especially when there is a cold model bias. Projecting SST downward by EOF technique can deepen the depth of assimilation especially in Scheme 4a. Both Schemes 4a and b can correct the bias in the mixed layer and do not change the vertical thermal structure.

Research paper thumbnail of Assimilating remote sensing and in situ observations into a coastal model of northern South China Sea using ensemble Kalman filter

Continental Shelf Research, 2011

Major forecast errors on the background error covariance from initial conditions, atmospheric for... more Major forecast errors on the background error covariance from initial conditions, atmospheric forcing, model open boundary conditions, and the river discharges are examined in a coastal model of northern South China Sea. The analysis of background error covariance matrix produced by model ensemble shows that the perturbations of the initial conditions and atmospheric forcing play major roles in producing and maintaining the amplitude of ensemble spread except for the sea surface height (SSH) field. The perturbation of model open boundary conditions can influence ensemble spread of all variables and covariance between temperature and velocity or between temperature and SSH. The perturbation of river discharge mainly affects the covariance of salinity in river estuary. A data assimilation experiment of northern South China Sea is conducted using ensemble Kalman filter (EnKF) in the Princeton Ocean Model. In the experiment the ensemble model forecasts are made by perturbing the above mentioned four major model inputs. The assimilated data include sea-surface temperature (SST) and conductive-temperature-depth (CTD) observations. The assimilation experiment suggests that assimilating SST and CTD data can effectively improve the model simulation that has a shallower thermocline and weaker plume comparing to the observations. Moreover, consistent with these improvements of temperature and salinity, the along-shore velocity, cross-shore velocity, and characters of water mass are also corrected, respectively. Crown

Research paper thumbnail of Progress of regional oceanography study associated with western boundary current in the South China Sea

Chinese Science Bulletin, 2013

Research paper thumbnail of Coastal upwelling in summer 2000 in the northeastern South China Sea

Journal of Geophysical Research, 2012

1] Using a combination of hydrographic, tide-gauge, near-bottom mooring, and satellite observatio... more 1] Using a combination of hydrographic, tide-gauge, near-bottom mooring, and satellite observations; and a numerical circulation model, we investigate the coastal upwelling in the northeastern South China Sea (NSCS) off the coast of Fujian and Guangdong Provinces, China, in the summer of 2000. Subsurface upwelling phenomenon exists mainly near the bottom boundary in the whole region investigated. It is closely related to the coastal sea level fluctuations, which are evidently modulated by both the local wind-forcing and the large-scale circulation. The northeastward interior flow following the bathymetry is accelerated by the drop of coastal sea level and leads to onshore transport and subsequent cooling in the bottom boundary layer (BBL) over the shelf west of Shantou. To the east of Shantou, the near-bottom flow veers more eastward, parallel to the coastline, and transports the nearshore cold water mass farther to the southern Fujian coast. The cross-shelf advected cold water does not always penetrate through the stratification and reach the surface. The local wind exhibits considerable synoptic variability. The decrease in sea surface temperature (SST) is mostly significant near Dongshan-Shantou, intermittent in time and intensifies preferably during weather events that bring southwesterly alongshore wind. To the west a freshwater tongue originating from the Pearl River forms a barrier layer, which results in high surface temperature in the freshwater plume. The observational evidences and modeled results shown in this study provide important information for further understanding the ecological effects associated with the upwelling processes in the NSCS.