Research Highlight

Dynamics of intensified downwelling circulation over a widened shelf in the northeastern South China Sea

Prof. Jianping Gan’s research group of ocean dynamics and modeling has been involving in research of ocean circulation and ecosystem dynamics and modeling; coastal and estuarine physical oceanography, coastal marine environment modeling and atmosphere-ocean interaction. We have developed a multi-scale observation-modeling system for ocean circulation and coupled physics-biochemistry processes in China Seas. The system links estuaries, East China Sea, South China Sea, Yellow Sea, Bohai Sea and western Pacific Ocean. By combining with extensive in situ field measurements, we have conducted kinematic and dynamic interdisciplinary investigations of the physical, and associated bio-geochemical processes, based on synthesized model and observed data to provide understanding of the role of ocean in climate change and environmental pollution in the China Seas.

Our recent study entitled “Dynamics of intensified downwelling circulation over a widened shelf in the northeastern South China Sea” (Gan et al., 2013, Journal of Physical Oceanography) has, for the first time, provided a new understanding for the mechanism that controls the upslope/downslope cross-isobath water transport in the shelf sea. We found that the transport can be largely contributed by the balance between the pressure gradient force induced by bottom stress curl and earth rotation, or geostrophic balance, rather than by the balance between bottom frictional effect and earth rotation alone in the previously studies. The bottom stress curl is analytically and numerically proved to be formed by the naturally sheared velocity (or vorticity) over variable shelf topography, as shown in Equation (1) and Figure 1.



where Py* is pressure gradient along the isobath coordinate y* and H is water depth, τs and τb are surface and bottom stress; ψ is streamfunction; ξ is vorticity and v is velocity. The study provided a new insight into the physical origin of the geostrophic cross-isobath transport and is critical important for the interpretation of enrichment/depletion of nutrients, biological productivity, dissolve inorganic carbon and thus ecosystem in the shelf sea. Furthermore, the study connects the cross-isobath transport with variable bathymetry, stratification and other physical parameters in the ocean.







Figure 1. Depth-integrated along-isobath pressure gradient force (top, Py*, 10-6 m2 s-2) and bottom vorticity (bottom, 10-6 s-1) demonstrate the dynamic origin of geostrophic cross-isobath transport. The red lines are the 50 m, 100 m and 200 m isobaths.

Modeling South China Sea circulation: Response to seasonal forcing regimes

Jianping Gan, H. Li, E. N. Curchitser, and D. B. Haidvogel

A three-dimensional ocean model has been utilized to study circulation and its seasonal variation in the South China Sea (SCS) in response to the forcing of the Asian monsoon and the Kuroshio intrusion. This study reports on the analysis of the mean seasonal circulation and dynamic processes in response to monsoonal wind stress, the Kuroshio intrusion, and other intrinsic forcing processes. It is found that the seasonal circulation in the SCS is mainly driven by the monsoonal wind stress and greatly influenced by the inflow from the Kuroshio intrusion. Strong currents along the continental margin of the SCS form mean basin-wide cyclonic and anticyclonic circulations in the winter and summer, respectively. Multiscale eddies are embedded in the general circulation across the basin. While mainstream of the Kuroshio passes through the Luzon Strait without intruding into the SCS, partial intrusion occurs in the upper 200 m near the shelf margin southwest of Taiwan at times when winter dynamic conditions prevail in the north SCS. The intrusion of the Kuroshio into the SCS also occurs at depths in all seasons, mainly along the continental slope. The coastal current separation to the east off southern Vietnam and the associated eddy formations characterize the circulation in the south SCS. The simulated results compare well with the corresponding observed fields. Dynamical processes involved in the forced flow fields are investigated by examination of the momentum balances. The analyses reveal that the circulation in the SCS is generally dominated by the geostrophic currents. North of the Luzon Strait, positive nonlinearity in the zonal direction is locally intensified, which leads to the formation of centripetal acceleration for the mainstream of the Kuroshio to turn eastward. The Kuroshio intrusion at depths is governed by the ageostrophic flows and highly associated with the net westward pressure gradient force. Coastal jet separation to the east off Vietnam is mainly associated with the local wind stress field and with the shelf topography in the summer and winter, respectively. Sensitivity study reveals that the weakening of the Kuroshio markedly enhances Kuroshio’s intrusion and forms an anticyclonic eddy west of the Luzon Strait.Figure. Mean velocity vectors (ms-1) averaged in the upper 200 m in (left) winter and (right) summer, respectively.

Intensified upwelling over a widened shelf in the northeastern South China Sea

Jianping Gan, Anson Cheung, Xiaogang Guo, and Li Li

Observational and three-dimensional modeling studies reveal that the intensified upwelling in the northeastern South China Sea (NSCS) is formed as a result of intensified upslope advection of dense deep waters that cross the middle shelf toward the inner shelf over a distinctly eastward widened shelf. The strongest advection occurs over the converging isobaths near the head of the widened shelf. As these dense deep waters advance shoreward, they are advected downstream by the quickly developed upwelling current over the inner shelf and eventually outcropped at the lee of a coastal cape. Dynamically, the shoreward cross-isobath transport over the widened shelf is geostrophically enhanced by a quasi-barotropic negative (westward) along-isobath pressure gradient force as a result of the net rate of the momentum influx and by an intensified bottom frictional transport owing to the flow confluence near the head of the widened shelf. A negative pressure gradient also exists at the lee of the coastal cape over the inner shelf and locally amplifies shoreward motion. Induced by the respective widened shelf and the coastal promontory, the along-isobath variations of cross-isobath transport in the water column over the middle and inner shelves interactively characterize intensified upwelling in the NSCS.

Figure. Schematic showing the wind-driven upwelling processes and forcing mechanism over the middle and inner shelves of a widened shelf.

Biological response to intensified upwelling and to a river plume in the northeastern South China Sea: A modeling study

Jianping Gan, Zhongming Lu, Minhan Dai, Anson Y. Y. Cheung, Hongbin Liu, and Paul Harrison

A coupled three dimensional physical model and a nitrogen based dissolved inorganic nitrogen, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model was used to study the ecosystem responses to the wind driven summer upwelling and to the Pearl River plume over a distinctly widened shelf in the northeastern South China Sea (NSCS). Forced with an idealized, but representative, upwelling favorable wind and the river discharge for the purpose of process oriented study, we identified two high chlorophyll centers that are typically observed over the NSCS shelf and stimulated by nutrient enrichment from intensified upwelling over the widened shelf and from the river plume. The nutrient enrichment has strong along shore variability involving the variable cross isobath nutrient transport between the middle and the inner widened shelf during the upwelling and an eastward expansion of the nutrient rich plume. About 20% of the upwelled nutrient rich deep water from the outer shelf reaches the inner shelf where algal blooms occur. Nutrient enrichment in the plume stretches over a broad extent of the shelf and produces significant biomass on the NSCS shelf. The plume is physically governed by intensified surface Ekman dynamics that leads to a strong offshore nutrient transport and eventually offsets the shoreward transport caused by the upwelling in the NSCS. Biological forcing and circulation dynamics of the surface Ekman layer jointly form the spatial dislocation and temporal variation of NO3, phytoplankton, and zooplankton biomasses in the upwelled and plume waters. The simulated results qualitatively resemble field and satellite measurements and demonstrate the physically modulated biological responses to the intensified upwelling and plume influenced NSCS shelf.

Figure. Surface NO3, phytoplankton, and zooplankton (mmol/m3) greater than their respective thresholds in the plume on day 30.

Interaction of a river plume with coastal upwelling in the northeastern South China Sea

Jianping Gan, Li Li, Dongxiao Wang, Xiaogang Guo

Observational and modeling studies were conducted to investigate the Pearl River plume and its interaction with the southwesterly driven upwelling circulation in the northern South China Sea during the summer. After exiting the Pearl River Estuary, the discharged freshwater generates a nearly stationary bulge of freshwater near the entrance of the estuary. Forced by the wind-driven coastal upwelling current, the freshwater in the outer part of the bulge flows downstream at the speed of the current and forms a widening and deepening buoyant plume over the shelf. The plume axis gradually shifts offshore of the current maximum as a result of currents induced by the contrasting density at the nose of plume and by the intensified Ekman drift in the plume. In this plume–current system, the fraction of the discharged freshwater volume accumulated in the bulge reaches a steady state and the volume of newly discharged freshwater is transported downstream by the upwelling current. Enhancement of stratification by the plume thins the surface frictional layer and enhances the cross shelf circulation in the upper water column such that the surface Ekman current and compensating flow beneath the plume are amplified while the shoaling of the deeper dense water in the upwelling region changes minimally. The pressure gradient generated between the buoyant plume and ambient seawater accelerates the wind-driven current along the inshore edge of the plume but retards it along the offshore edge. Along the plume, downward momentum advection is strong near the highly nonlinear source region and a weaker upward momentum advection occurs in the far field over the shelf. Typically, the plume is shaped by the current over the shelf while the current itself is adjusting to a new dynamic balance invoked by the plume-induced changes of vertical viscosity and the horizontal pressure gradient. The spatial variation of this new balance leads to a coherent change in the cross isobath transport in the upper water column during upwelling.

Figure. (a) Surface velocity vectors (ms-1); (b) surface salinity (psu); (c) the differences of surface alongshore velocities (Du, ms-1); and (d) cross-shore velocities (Dv, ms-1) for the cases with and without the river discharge on day 30. In (c) and (d), the color contours are the differences, and the black contour lines and labeled numbers refer to the percentage change in the respective velocities caused by the buoyant plume.

Observed three-dimensional structure of a cold eddy in the southwestern South China Sea

Jianyu Hu, Jianping Gan, Zhenyu Sun, Jia Zhu, and Minhan Dai

The dynamic structure of an ocean eddy in the eddy-abundant South China Sea has rarely been captured by measurements and has seldom been discussed in the literature. In the present study, in situ current and hydrographic measurements from a weeklong cruise and concurrent satellite altimeter observations were utilized to examine the three-dimensional structure and physical properties of a cold eddy in the southwestern South China Sea. The underlying forcing mechanism for the formation of this cyclonic cold eddy was found to be tightly associated with the recirculation in a coastal baroclinic jet that had separated off the Vietnamese coast. The eddy was significantly influenced by a coexisting, anticyclonic warm eddy in the separated jet. With relatively steady intensity and radius, the cold eddy endured for two weeks after its swift formation in late August and prior to its quick dissipation in mid-September. This cold eddy was horizontally and vertically heterogeneous. Asymmetric currents with much stronger magnitude were found on its southeastern flank, next to the warm eddy, where a front in the pycnocline was

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responsible for the sharp decrease in the cold eddy’s intensity in the water below. The distributions of temperature, vorticity, and vertical velocity in the cold eddy were spatially asymmetric and not overlapping. The

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intensity of the cold eddy gradually decreased with the depth and the eddy extended downward for more than 250 m with a vertically tilted central axis. The upward velocities around the center of the eddy and the downward velocities to the southwest and to the east of the center jointly formed the upward domes of isotherms and isohalines in the central part of the cold eddy.

Figure. Three dimensional structures of (a) temperature (°C), (b) vorticity (10-5s-1), and (c) vertical velocity (10-5m/s, positive upward). The contour lines in black are those of the −0.2σw that defines the scale of the eddy at each depth.

Coastal jet separation and associated flow variability in the southwest South China Sea

Jianping Gan, Tangdong Qu

A three-dimensional, high-resolution regional ocean model, forced with high-frequency wind stress and heat flux as well as time- and space-dependent lateral fluxes, is utilized to investigate the coastal jet separation and associated variability of circulation in the southwest South China Sea (SSCS). It is found that the circulation and its variability in the SSCS are dominated by the flow fields and eddies associated with the southward and northeastward wind-driven coastal jet separation from the coast of central Vietnam in the winter and summer, respectively. As a result of the coastal jet separation, cyclonic and anticyclonic eddies with strong flow variability are generated in the regions to the southeast of the Vietnam in the winter and to the east off central Vietnam in the summer. The separation of the wind-driven coastal jet is largely associated with the formation of adverse pressure gradient force over the shallow shelf topography around the coastal promontory off central Vietnam, balanced mainly by wind stress in the summer and by both wind stress and nonlinear advection in the winter. In the vorticity balance, a bottom pressure torque, the force exerted on the wind-driven current by the shelf topography, tends to yield an adverse vorticity favorable for the separation of coastal jet. The results suggest that the interaction between wind-driven coastal currents and shelf topography in the nearshore waters plays a crucial role in controlling the separation of the coastal jet.

Figure. (a) Three-year mean value wind stress curl (10-6 Pa m-1), (b) the std of wind stress curl vector amplitudes (10-6 Pa m-1) and (c) the std of wind stress vector amplitudes (Pa) in the SSCS.

The influence of coastal upwelling and a river plume on the subsurface chlorophyll maximum over the shelf of the northeastern South China Sea

Zhongming Lu, Jianping Gan, Minhan Dai, Anson Y.Y. Cheung

Frequently observed subsurface chlorophyll maximum (SCM) contributes substantial biomass to the waters over the continental shelf of the northeastern South China Sea (NSCS) but it has not been sufficiently investigated. In this study, observations and a three-dimensional coupled physical–biological numerical model were utilized to investigate the characteristics of the SCM under strong controls of coastal upwelling circulation and the Pearl River plume in the NSCS. The model captures the observed characteristics of the SCM in the NSCS reasonably well. Both the depth and intensity of the SCM are spatially variable and regulated by the variable upwelling circulation and associated plume distribution over the complex shelf topography. In nearshore waters, the SCM shoals and weakens towards the coast as a result of the upwelling of high nutrient low chlorophyll deep water while surface productivity is enhanced. The intensity of the SCM weakens when the surface layer is covered by the river plume because of the substantial reduction of photosynthetic active radiation (PAR). Strengthening upwelling-favorable wind weakens the intensity of the SCM due to dilution by the enhanced mixing, but the SCM depth remains relatively stable in the offshore water as a result of no apparent shift of the nutricline. Changing incident PAR leads to an interactive response in chlorophyll concentration, nutricline, and the depth of the SCM.

Figure. Vertical profiles of (a)NO3, (b)chlorophyll, (c)phytoplankton, and (d)ratio of chlorophyll to phytoplankton biomass in section B at 21.8°N under different incident PAR.

Numerical study of the tide and tidal dynamics in the South China Sea

Tingting Zu, Jianping Gan, Svetlana Y. Erofeeva

Tides and their dynamic processes in the South China Sea (SCS) are studied by assimilating Topex/Poseidon altimetry data into a barotropic ocean tide model for the eight major constituents (M2 S2 K1 O1 N2 K2 P1 Q1) using a tidal data inversion scheme. High resolution (~10 km) and large model domain are adopted to better resolve the physical processes involved and to minimize the uncertainty from the open boundary

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condition. The model results, which are optimized by an inversion scheme, compare well with tidal gauge measurements. The study reveals that the amplitude of the semi-diurnal tide, M2, decreases, while the amplitude of the diurnal tide, K1, increases similar to the Helmholtz resonance after the tidal waves propagate from the western Pacific into the SCS through the Luzon Strait (LS). Analyses of the energy studies show that the LS is a place where both M2 and K1 tidal energy dissipates the most, and strong M2 tidal dissipation also occurs in the Taiwan Strait (TS). The work rate of the tidal generating force in the SCS basin is negative for M2 and positive for K1. It is found that the responses of tides in the SCS are largely associated with the propagating directions of the tides in the Pacific, the tidal frequency, the wavelengths, the local geometry and bottom topography.

Figure. The tidal energy flux at the Luzon Strait, (a) the inverse and (b) the prior results for M2. Similarly, (c) and (d) are for K1. Contours denote the magnitude of P (W/m).

On the warm/cold regime shift in the South China Sea: Observation and modeling study

P. Swapna , Jianping Gan, Alexis Lau, Jimmy Fung

Remote sensing data sets and a high-resolution three-dimensional regional ocean model were utilized to investigate the shifting of warm/cold regime and the associated sea level variation in the South China Sea (SCS) during 2000-2003. Both the altimetry data and the model results showed an increase in the sea level (warm phase) followed by an abrupt decrease (cold phase) in the SCS during 2000-2003. Heat budget calculations performed with the model revealed excess heat advection from the western Pacific warm pool into the SCS during the warm phase than the cold phase. The warm phase, which occurred during La Nina episodes, resulted from the intrusion of abnormally warmer western Pacific water that increased the heat content and thus sea level in the SCS. The cold phase, which occurred during El Nino episodes, was triggered by a reduction in the net atmospheric heat flux followed by cold water advection into the SCS. Decrease in the rate of precipitation minus evaporation (P-E) also accounted for the falling of sea level during cold phase. The present study integrated the available remote sensing data and advanced numerical model to identify the time-dependent three-dimensional dynamic and thermodynamic forcing that are important in governing the warm/cold regime shift in the SCS.

Figure. (a) Anomalies of heat content (×109Jm-2) during warm phase (September 2000-August 2001). (b) Difference between warm and cold (September 2001-August 2002) phase. (c,d) Same as above except for steric height anomalies. The data are from SODA.









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