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Satellite-tracked Lagrangian drifters are used to investigate the transport pathways of near-surface water around the Luzon Strait. Particular attention is paid to the intrusion of Pacific water into the South China Sea (SCS). Results from drifter observations suggest that except for the Kuroshio water, other Pacific water that carried by zonal jets, Ekman currents or eddies, can also intrude into the SCS. Motivated by this origin problem of the intrusion water, numerous simulated trajectories are constructed by altimeter-based velocities. Quantitative estimates from simulated trajectories suggest that the contribution of other Pacific water to the total intrusion flux in the Luzon Strait is approximately 13% on average, much smaller than that of Kuroshio water. Even so, over multiple years and many individual intrusion events, the contribution from other Pacific water is quite considerable. The interannual signal in the intrusion flux of these Pacific water might be closely related to variations in a wintertime westward current and eddy activities east of the Luzon Strait. We also found that Ekman drift could significantly contribute to the intrusion of Pacific water and could affect the spreading of intrusion water in the SCS. A case study of an eddy-related intrusion is presented to show the detailed processes of the intrusion of Pacific water and the eddy-Kuroshio interaction.

The instrument used for ocean colour remote-sensing works in the visible wavelengths, and the presence of clouds frequently lead to invalid observations. The formation of clouds is known to be infl...

The concentration of chlorophyll-a (CHL) is an important proxy for the amount of phytoplankton biomass in the ocean. Characterizing the variability of CHL in the Pearl River Plume (PRP) is therefore of great importance for the understanding of the changes in oceanic productivity in the coastal region. By applying quantile regression analysis on 21-year (1998&ndash, 2018) near-surface CHL data from satellite observations, this study investigated the long-term trend of CHL in the PRP. The results show decreasing trends (at an order of 10&minus, 2 mg m&minus, 3 year&minus, 1) for all percentiles of the CHL in the PRP, suggesting a decrease in productivity in the past two decades. The trends differ fundamentally from those in the open regions of the northern South China Sea with mixed signs and small magnitudes (10&minus, 4 mg m&minus, 1). The magnitudes of the trends in high quantiles (&gt, 80th) are larger than those in low quantiles (&lt, 50th) in the PRP, indicative of a decrease in the variance of the CHL. The area with apparent decreasing trends is restricted to the PRP in summer and extends to the entire coastal region in winter. This decrease in CHL is possibly attributed to the decrease in nutrient input from the river runoff and the weakening of wind-forced mixing rather than the changes in sea surface temperature. This study extends our knowledge on the variability of CHL in the PRP and provides references to the investigation of the changes of the coastal ecological environment.

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.

Zhang, T.; Tang, S., and Zhan, H., 2018. Using satellite sensors and in situ observations to monitor phytoplankton blooms in the Pearl River Estuary. Eutrophication in the Pearl River Estuary (PRE) has become severe with the rapid urbanization of the Pearl River Delta in recent years. As a result, large-scale phytoplankton blooms occur frequently because of eutrophication. This study investigated a massive Cochlodinium geminatum bloom near the mouth of the Pearl River in August 2011 based on data from both in situ measurements and satellites. Both methods provided a useful approach for monitoring phytoplankton blooms. The in situ measurements illustrated a high concentration of algal cells (1.5 × 107 cells L−1) in the study area, and the bloom reached dimensions of approximately 10 to 15 km in length, 8 to 10 km in width, and 100 km2 in area based on continuous field measurements. A chlorophyll algorithm for satellite sensors was developed based on in situ chlorophyll concentrations and remote se...

This study investigated boreal wintertime phytoplankton blooms in the western equatorial Indian Ocean (WEIO) and the underlying physical mechanisms. The Sea viewing Wide field of View sensor (SeaWiFS) chlorophyll-a (Chla) concentrations show that phytoplankton blooms occur in the WEIO during December-March. The development of these blooms is not only a seasonal process but also consists of 2-3 intraseasonal events induced by the Madden-Julian Oscillation (MJO). During a typical intraseasonal event, enhanced cross-equatorial wind induces strong upwelling and ocean mixing, thus increasing the supply of nutrients to the surface in equatorial regions. Argo profiles clearly show various responses to the intraseasonal wind bursts, including shoaling of the thermocline and deepening of the mixed layer. Further analysis reveals that the former is the dominant mechanism for the blooms along the equator, while the latter controls the high Chla concentrations off the coast of Somalia. Surface ocean circulations not only account for the blooms south of the equator but also modulate the thermocline depth in the WEIO. The shallower thermocline during the early period of the northeast monsoon season provides favorable conditions for a stronger Chla response to intraseasonal forcing.

Recent composite based researches reveal that anticyclonic eddies facilitate the growth of phytoplankton in the subtropical gyres. Two dynamical mechanisms, eddy-Ekman pumping and winter mixing, have been examined individually, but their relative and combined effects remain unclear. Using satellite observations and model simulations, this study investigated the process of a distinct phytoplankton bloom generated in an anticyclonic eddy in the nutrient-depleted southeastern Indian Ocean. The bloom propagated westward along with the eddy for more than 600 km from late April to August in 2010. The peak of surface chlorophyll concentration in the eddy is 2.2 times larger than the mean value of the ambient. The development of the bloom is dominated by the winter deepening of mixed layer, whose velocity in vertical nutrient flux is on average 3 times larger than that of eddy-Ekman pumping. The results of a 1-D physical-biogeochemical model demonstrate that the role of eddy-Ekman pumping is also indispensable, because it not only transports extra nutrients into the mixed layer, but also results in significant chlorophyll enrichment in subsurface water. The superposition of eddy-Ekman pumping on winter mixing triples the chlorophyll both at the surface and in the upper layer, and the entrainment of subsurface phytoplankton into the mixed layer contributes significantly to the surface bloom, especially in its initial stage. Both the satellite observations and model simulation show that eddy-Ekman pumping can lead to an early occurrence of the bloom for more than 2 weeks.

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.

Using satellite-derived and in situ data, the wind-driven potential new production (nitrate supply) for the 300 km wide coastal band in two upwelling regions of the western Arabian Sea (AS) during the southwest monsoon is estimated. The upward nitrate flux to the euphotic zone is generally based on the physical processes of coastal transport (Ekman transport and geostrophic transport) and offshore Ekman pumping. The coastal geostrophic current in the western AS influences the upwelling intensity and latitudinal distributions of nitrate supply. The Oman and Somalia upwelling regions have similar level of potential new production (nitrate supply) during the summer monsoon, while the satellite estimates of primary production off Oman are 2 times greater than those off Somalia. The much higher potential f-ratio in the Somalia upwelling region indicates that the primary production could be limited by availability of other macronutrients (e.g., silicate). The correlation analysis of the primary production and the aerosol optical thickness shows that the Oman upwelling region displays a stronger coupling between the atmospheric deposition and the phytoplankton abundance. The high summertime dust levels in the atmosphere are suggested to contribute to the high primary production in the Oman upwelling region.

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.

The Finite Time Lyapunov Exponent technique is a highly effective way to visualize and analyze flow fields. Its ridges are referred to as Lagrangian Coherent Structures which reveal the transport barriers of fluid systems, or their hidden skeletons. This allows us to investigate the complex mesoscale and submesoscale ocean processes on a Lagrangian view. In this article, we use the Lagrangian technique to investigate one chlorophyll bloom event in the northeastern South China Sea and discuss the impact of horizontal transport on the distribution of surface chlorophyll. Our results demonstrate that horizontal transport barriers uncovered by Lagrangian Coherent Structures can shape the vortex and filament in a simple and distinct way.

The effects of interannual variability in chlorophyll concentrations on sea surface temperatures in the east tropical Indian Ocean (ETIO) during two positive Indian Ocean Dipole (IOD) events (in the boreal fall of 1997 and 2006) are investigated; this is done through two ocean model experiments, one with and one without the interannual variability of chlorophyll concentrations. A comparison of the two chlorophyll perturbation experiments reveals that, in contrast to the cooling effects at the seasonal timescale, increased chlorophyll concentrations during positive IOD events leads to an increase in the sea surface temperatures within the ETIO. Although upward velocity is enhanced (with a cooling effect), this does not counterbalance the increase in solar heating caused by the increased chlorophyll concentrations. This interannual variability of chlorophyll concentrations in the ETIO could reduce the amplitude of the IOD by about 6%. This article is protected by copyright. All rights reserved.

The effects of biological heating on upper-ocean temperature and circulation in the Arabian Sea are investigated using an ocean general circulation model. We find that the change of sea surface temperature (SST) is not only dependent on the variation of chlorophyll concentration, but also the dynamic processes, e.g., mixing and upwelling. Biological heating can warm the SST in the north Arabian Sea during spring and the central Arabian Sea during autumn when the mixed layer depth is shallow. However, the situation is quite different during winter and summer. Although the chlorophyll concentration is high in the north Arabian Sea during winter and in the western Arabian Sea during summer, the SSTs become significantly cool instead of warm. The heat budget analyses indicate that the cold SSTs result from both the strong convective mixing during the winter and the strong upwelling during the summer, which bring the cold water below the mixed layer to the surface.

Chlorophyll-a (Chla) concentration derived from the Sea viewing Wide field of View sensor (SeaWiFS) data (January 1998 to December 2010) shows phytoplankton blooms in the western south equatorial Indian Ocean (WSEIO) during the summer monsoon. The mechanism that sustains the blooms is investigated with the high-resolution Ocean General Circulation Model for the Earth Simulator (OFES) products. The summer blooms in the WSEIO are separated from the coast; they occur in June, reach their maximum in August, and decay in October. With summer monsoon onset, cross-equatorial wind induces open-ocean upwelling in the WSEIO, uplifting the nutricline. The mixed layer heat budget analysis reveals that both thermal forcing and ocean processes are important for the seasonal variations of SST, especially wind-driven entrainment plays a significant role in cooling the WSEIO. These processes cause nutrient enrichment in the surface layer and trigger the phytoplankton blooms. As the summer monsoon develops, the strong wind deepens the mixed layer; the entrainment thus increases the nutrient supply and enhances the bloom. Horizontal advection associated with the Southern Gyre might also be an important process that sustains the bloom. This large clockwise gyre could advect nutrient-rich water along its route, allowing Chla to bloom in a larger area.

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.

The Java-Sumatra upwelling is one of the most important upwelling systems in the Indian Ocean, with maximum upwelling intensity in July through August. To estimate the nitrate supplied by upwelling, we developed a three-dimensional hydrodynamic model to calculate the mean vertical speed and determine the depth of upwelling. We used in-situ vertical nitrate profiles to assess nitrate concentration in the upwelled waters, and calculated the nitrate supply as the product of nitrate concentration and vertical transport obtained from the numerical model. The calculated result represents potential new production generated in the upwelling region. We found that on the event time scale (monthly) of Java-Sumatra upwelling, water brought to the surface originated from locations 100-m deep, giving a nitrate supply of 93.77×103 mol/s and potential new production of 1.02×1014 gC/a.