Search

Spatio-temporal variability of the suspended sediment concentration (SSC) in the Pearl River Estuary (PRE) is analyzed over 13 years (2003–2015) using MODIS-Aqua Level-1B data. A unique seasonal SSC variability was found. During the wet season, the SSC in the northwestern estuary (NWE) is high, which could reach up to 100 mg/L, while during the dry season it suffers from a markedly decrease. Over the southeastern estuary (SEE), however, the pattern is reversed. This seasonality is mainly affected by freshwater discharge, winds and water column stratification. During the wet season, freshwater discharge dominates the SSC in the NWE while stratification and water discharge control the SSC variation in the SEE. During the dry season, freshwater discharge also plays a more important role in changing the SSC in the NWE, while the combined effect of wind mixing and tidal mixing dominates the SSC in the SEE. The interannual variability of the SSC is revealed by an EOF analysis. The first mode is highly related to water discharge of the Pearl River, indicating that the interannual variability is mainly subject to the rainfall over the Pearl River basin. The long-term SSC trend is characterized by a general decrease across the entire estuary and proved to be closely related to human activity. The reduction rate of the SSC varies spatially and seasonally. Spatially, the northern West Shoal and upper eastern part of the PRE suffer from a much greater SSC decrease, while the rest of the estuary, especially the southern West Shoal, is relatively less influenced. Seasonally, the decreasing rate in the dry season (49.3%) is higher than that in the wet season (30.1%).

Knowledge of horizontal transport pathways is important for the protection of the marine ecosystem in coastal areas. In this paper, we develop a 3D model to simulate hydrodynamics and particle transport in the Pearl River Estuary (PRE), Southern China, to study the barriers to transport in the PRE. Specifically, we use the flow velocity produced by the model to locate Lagrangian coherent structures (LCSs) hidden in ocean surface currents. Our findings show that a remarkable LCS begins upstream near the Humen inlet, extends to the Wanshan Islands via Neilingding Island, and can act as a transport barrier in the estuary. This LCS appeared 1–2 h after high tide and was persistent for 6–7 h during every ebb tide. Particles released on the west side of the LCS moved downstream, exited the estuary by Daxi Channel, and seldom spread to the east side, especially the Hong Kong Sea area. An analysis of several scenarios suggested that the formation of this LCS was due to topography restrictions and tidal forces.

Ocean current is one of the major drivers of water mass, energy, and biogeochemical cycles in the global ocean–atmosphere boundary layer and also a key variable in the formation of extreme climate events (e.g., El Nino). Direct measurement of the global ocean surface current is of great scientific interest and application value for understanding multiscale ocean dynamics, air-sea interaction, ocean mass and energy balance, and ocean carbon budget, as well as their variabilities under climate change. This paper reviews the state-of-the-art developments of ocean dynamics and technology on the observation of multiscale ocean circulation and related scientific frontiers. Presently, measurements of global ocean surface currents, which are mainly geostrophically derived from satellite altimeter data, are only available to resolve quasi-geostrophic current at large- to meso-scale in the off-equatorial open ocean. This becomes a bottleneck impeding the application and the development of ocean circulation dynamics. With the ambition of breaking the bottleneck, Ocean Surface Current multiscale Observation Mission (OSCOM) will launch a satellite equipped with a Doppler Scatterometer to directly measure ocean surface currents for the first time with a very high horizontal resolution of 5–10 km and a 3-day global coverage. Through carrying a Surface Temperature Infrared Radiometer and a Surface Temperature Microwave Radiometer, the OSCOM satellite is supposed to have the capability of observing ocean surface current, ocean surface vector wind, and sea surface temperature simultaneously. With a cutting-edge design, OSCOM will provide an in-depth picture of non-equilibrium ocean state and air-sea interaction from mesoscale to submesoscale, and helps to construct the fine structure of deep ocean current through a combination with Array for Real-time Geostrophic Oceanography profiling. Those direct measurements and derived dynamic parameters will further facilitate analyses of ocean carbon budget and ocean biogeochemical cycle, and provide a novel and improved pathway to data assimilation, coupling of General Circulation Models, and the Earth System Modelling for ocean prediction and climate change.

The deterioration of dissolved oxygen conditions in the Pearl River Estuary (PRE) in summer has recently attracted considerable-scientific and political-attention. This paper documents the development, calibration, and verification of a coupled three-dimensional hydrodynamic and water quality model for the PRE. A comparison of the model's performance against field observations indicated that the model is capable of reproducing key hydrodynamic and water quality characteristics of the estuary within an acceptable range of accuracy. Furthermore, a scenario analysis showed that the extent of the hypoxic zone responds differently to changes in the river discharge at different inlets. Moreover, the hypoxic zone also changes in response to variations in the southwest wind in summer; specifically, a larger hypoxic zone develops as southwest winds blow in a more southward direction. However, the hypoxic conditions are much more sensitive to changes in the wind speed than changes in the wind direction.

The effects of eddies on surface chlorophyll (CHL) in the northern South China Sea are investigated by analyzing the long-term composite averages of CHL anomalies between 1998 and 2011 that were associated with eddies. CHL anomalies that are modulated by different dynamical processes are distinguished and discussed separately. The positive CHL anomalies within cyclonic eddies are generally larger in magnitude than the negative CHL anomalies within anticyclonic eddies, which implies a net increase in chlorophyll induced by eddy activities. At least two dynamical mechanisms, eddy pumping and eddy stirring, are found to play important roles in the CHL anomalies. The influence of eddy pumping is restricted by the mixed layer depth and its distance from deep nutriclines. Thus, the associated CHL anomalies are most apparent in winter and diminish in the following summer for both cyclonic and anticyclonic eddies. The inter-annual variation in the CHL anomalies within cyclonic eddies is modulated by the eddy intensity and exhibits a close relationship with the El Nino-Southern Oscillation. Meanwhile, the variation within anticyclonic eddies might be affected by the background CHL, with a downward trend during the study period. This work provides an overview to the nature and temporal variations in CHL anomalies that are associated with eddies in the northern South China Sea.

Mesoscale eddies are ubiquitous features of the World Ocean and have dramatic impacts on the distribution of surface chlorophyll (CHL). It has been reported that CHL anomalies (CHLAs) associated with eddies are dominantly controlled by eddy stirring on a global scale. However, the resultant CHLAs show asymmetric dipoles, indicating that other mechanisms may also play important roles. By analyzing the composite averages of CHLAs associated with 28,293 individual eddies in the southeastern Pacific from 1998 to 2011, we showed that the asymmetry of the CHLAs is mainly attributed to eddy-Ekman pumping. Eddy-Ekman pumping could generate a positive (negative) CHL monopole centered on the core of an anticyclonic (cyclonic) eddy, and the superposition of such a monopole would modify the dipole structure of CHLAs formed by eddy stirring thus result in an asymmetric pattern. The asymmetry exhibits pronounced seasonality, depending on the relative contributions of eddy stirring and eddy-Ekman pumping. In austral winter, the CHL enrichment brought by the upwelling in anticyclonic eddies is more dominant than the CHL diminishment forced by the downwelling in cyclonic eddies, suggesting that eddy-Ekman pumping may represent important sources of new production during this period.

Internal solitary waves (ISWs) were observed by mooring data in the South China Sea in August and September 2014. Normally the arrival pattern of ISWs is relatively regular and the ISW amplitudes have a positive correlation with the magnitude of semidiurnal tidal currents near the Luzon Strait. However, the ISW amplitudes observed in the second spring tide of September are significantly large when an anti-cyclonic eddy is passing the mooring. When an ISW passes the eddy center, its amplitude reduces to nearly zero, but when an ISW passes the eddy edge, its amplitude increases by 50%. A deepened thermocline always damps the ISW amplitudes, whilst the eddy-induced background currents at different locations may have varied effects on the ISW amplitudes, e.g., the background current at the eddy center tends to damp the ISW amplitude but that at the eddy edge amplifies the ISW amplitude. The different effects of a deepened thermocline and eddy-induced background currents near the eddy edge may damp or amplify the ISW amplitude.

Using Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color data, we investigated the biological responses to oceanic Rossby waves in the southern tropical Indian Ocean (TIO) during 1997-2007. The findings indicate that, during the developing phase of El Nino/La Nina events, usually in boreal fall, and triggered by anomalous anticyclonic/cyclonic wind circulations in the southeast TIO, downwelling/upwelling Rossby waves form and then propagate westward. After a few months, downwelling/upwelling Rossby waves interface with the thermocline dome in the southern TIO, and suppress/enhance the upwelling. Correspondingly, less/more nutrient-rich waters enter the mixed layer and result in lower/higher chlorophyll concentrations. Due to the asymmetric effects on the thermocline dome between downwelling and upwelling Rossby waves, higher chlorophyll concentrations appear earlier and in the eastern part of the dome, whereas lower chlorophyll concentrations appear later and in the central part of the dome. Moreover, when El Nino/La Nina-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events coincide, the biological responses are stronger. (C) 2014 Elsevier B.V. All rights reserved.

As an unwanted by-product in the power plants, cooling water (CW) discharge may induce harmful effects on the coastal environment. In this paper, to accurately predict the CW transport in the Daya Bay, China, we develop a three-dimensional hydrodynamic and temperature model to simulate the transport of the CW from nuclear power plant into coastal waters. Specifically, we use the flow velocity produced by the model to locate Lagrangian coherent structures (LCSs) hidden in ocean surface currents. Result show that the transport of the CW is quite strongly tied to the simulated LCSs. The LCSs constitute fluid barriers that accurately demarcate potential pathway for CW transport. Thus, LCSs in velocity fields from a coastal model is an extremely useful way to monitor and interpret the transport of CW.

Satellite imagery of SeaWiFS from October 1997 to November 2007 is used to investigate the dominant seasonal and interannual variations of the surface light absorption due to Colored Dissolved Organic Materials (CDOM) in the South China Sea (SCS). Results show that the spatial distribution of CDOM mimics the major features of the SCS basin-scale circulation. High values of CDOM are found in upwelling regions like southeast of Vietnam in summer and northwest of Luzon in winter. At a basin scale, CDOM is high in winter when upwelling is strong, solar shortwave radiation and stratification weak, and vertical mixing intense. Opposite conditions exist in spring and summer. Interannual variability of the basin-wide CDOM is characterized by abnormal troughs during the El Nino events. A strong relationship exists between the time series of the first EOF mode (for both winter and summer) and Nino 3.4 Index. Associations of these events with climatic and hydrographic properties (i.e. wind forcing, solar shortwave radiation, Ekman pumping, vertical mixing, sea surface height and temperature) are discussed.