Lai
Zhigang
Lai
Zhigang
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ArticleReply to comment on “Current separation and upwelling over the southeast shelf of Vietnam in the South China Sea”(John Wiley & Sons, 2013-03-31) Chen, Changsheng ; Lai, Zhigang ; Beardsley, Robert C. ; Xu, Qichun ; Lin, Huichan ; Viet, Nguyen Trung ; Yang, Ding
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ArticleStudies of the Canadian Arctic Archipelago water transport and its relationship to basin-local forcings : results from AO-FVCOM(John Wiley & Sons, 2016-06-25) Zhang, Yu ; Chen, Changsheng ; Beardsley, Robert C. ; Gao, Guoping ; Lai, Zhigang ; Curry, Beth ; Lee, Craig M. ; Lin, Huichan ; Qi, Jianhua ; Xu, QichunA high-resolution (up to 2 km), unstructured-grid, fully coupled Arctic sea ice-ocean Finite-Volume Community Ocean Model (AO-FVCOM) was employed to simulate the flow and transport through the Canadian Arctic Archipelago (CAA) over the period 1978–2013. The model-simulated CAA outflow flux was in reasonable agreement with the flux estimated based on measurements across Davis Strait, Nares Strait, Lancaster Sound, and Jones Sounds. The model was capable of reproducing the observed interannual variability in Davis Strait and Lancaster Sound. The simulated CAA outflow transport was highly correlated with the along-strait and cross-strait sea surface height (SSH) difference. Compared with the wind forcing, the sea level pressure (SLP) played a dominant role in establishing the SSH difference and the correlation of the CAA outflow with the cross-strait SSH difference can be explained by a simple geostrophic balance. The change in the simulated CAA outflow transport through Davis Strait showed a negative correlation with the net flux through Fram Strait. This correlation was related to the variation of the spatial distribution and intensity of the slope current over the Beaufort Sea and Greenland shelves. The different basin-scale surface forcings can increase the model uncertainty in the CAA outflow flux up to 15%. The daily adjustment of the model elevation to the satellite-derived SSH in the North Atlantic region outside Fram Strait could produce a larger North Atlantic inflow through west Svalbard and weaken the outflow from the Arctic Ocean through east Greenland.
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ArticleImpact of high-frequency nonlinear internal waves on plankton dynamics in Massachusetts Bay(Sears Foundation for Marine Research, 2010-03-01) Lai, Zhigang ; Chen, Changsheng ; Beardsley, Robert C. ; Rothschild, Brian ; Tian, RuchengA simple Nutrient-Phytoplankton-Zooplankton (NPZ) model was coupled with the non-hydrostatic Finite-Volume Coastal Ocean Model (FVCOM-NH) to study the impact of high-frequency nonlinear internal waves on plankton dynamics in Massachusetts Bay (MB) during the stratified summer season. The temporal and spatial variability of phytoplankton concentration follows the vertical isopycnal displacement to the lowest order as the waves are generated by the semidiurnal tidal flow over Stellwagen Bank (SB) and propagate westward across MB. The tidally-averaged distribution of phytoplankton is characterized by three distinct zones of low subsurface concentration: (I) the western flank of Stellwagen Bank; (II) the center of Stellwagen Basin; and (III) the upper western flank of Stellwagen Basin. The result of a model dye experiment suggests that these zones are created by the following physical processes which are dominant in each zone: (I) hydraulic jump; (II) strong internal wave-tidal current nonlinear interaction; and (III) energetic internal wave dissipation and subsequent mixing processes. The nonlinear interaction of the internal waves and offshore tidal currents significantly enhances the vertical velocity, and increases wave dissipation, thus causing an onshore transport of phytoplankton in zone II. Although the phytoplankton patchy structure can be produced using the hydrostatic FVCOM, the resulting phytoplankton concentration is overestimated due to the unrealistic intensification of vertical velocity and thus vertical nutrient flux from the deep water. It suggests that non-hydrostatic dynamics should be considered for certain small-scale biological processes that are driven primarily by the physics.
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ArticleObservational and model studies of the circulation in the Gulf of Tonkin, South China Sea(John Wiley & Sons, 2013-12-03) Ding, Yang ; Chen, Changsheng ; Beardsley, Robert C. ; Bao, Xianwen ; Shi, Maochong ; Zhang, Yu ; Lai, Zhigang ; Li, Ruixiang ; Lin, Huichan ; Viet, Nguyen TrungMoored current measurements were made at one mooring site in the northern Gulf of Tonkin for about 1 year during 1988–1989. Analyses were performed to examine characteristics and variability of tidal and subtidal flows. Rotary spectra showed two peaks at diurnal and semidiurnal periods, with higher diurnal energy. Complex demodulations of diurnal and semidiurnal tidal currents indicated that the tidal current magnitudes varied significantly with seasons: more energetic in the stratified summer than in the vertically well-mixed winter. The observed subtidal currents were highly correlated with the surface wind in winter but not in summer; challenging the conceptual summertime anticyclonic circulation pattern derived using wind-driven homogenous circulation theory. The computed currents from a global ocean model were in good agreement with the observed currents. Similar to the current observations, the model-computed flow patterns were consistent with the conceptual wind-driven circulation pattern in winter but opposite in summer. Process-oriented experiments suggest that the summertime cyclonic circulation in the northern Gulf of Tonkin forms as a result of the combination of stratified wind-driven circulation and tidal-rectified inflow from Qiongzhou Strait. The interaction between the southwest monsoon and buoyancy-driven flow from Hong River can significantly intensify the cyclonic circulation near the surface, but its contribution to the vertically averaged flow of the cyclonic circulation is limited.
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ArticleImpact of multichannel river network on the plume dynamics in the Pearl River estuary(John Wiley & Sons, 2015-08-21) Lai, Zhigang ; Ma, Ronghua ; Gao, Guangyin ; Chen, Changsheng ; Beardsley, Robert C.Impacts of the multichannel river network on plume dynamics in the Pearl River estuary were examined using a high-resolution 3-D circulation model. The results showed that during the dry season the plume was a distinct feature along the western coast of the estuary. The plume was defined as three water masses: (a) riverine water (<5 psu), (b) estuarine water (12–20 psu), and (c) diluted water (>22 psu), respectively. A significant amount of low-salinity water from Hengmen and Hongqimen was transported through a narrow channel between the QiAo Island and the mainland of the Pearl River delta during the ebb tide and formed a local salinity-gradient feature (hereafter referred to as a discharge plume). This discharge plume was a typical small-scale river plume with a Kelvin number K = 0.24 and a strong frontal boundary on its offshore side. With evidence of a significant impact on the distribution and variability of the salinity and flow over the West Shoal, this plume was thought to be a major feature of the Pearl River plume during the dry season. The upstream multichannel river network not only were the freshwater discharge sources but also played a role in establishing an estuarine-scale subtidal pressure gradient. This pressure gradient was one of the key dynamical processes controlling the water exchange between discharge and river plumes in the Pearl River estuary. This study clearly showed the role of the river network and estuary interaction on river plume dynamics.
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ArticleCurrent separation and upwelling over the southeast shelf of Vietnam in the South China Sea(American Geophysical Union, 2012-03-21) Chen, Changsheng ; Lai, Zhigang ; Beardsley, Robert C. ; Xu, Qichun ; Lin, Huichan ; Viet, Nguyen TrungThe high-resolution, unstructured grid Finite-Volume Community Ocean Model (FVCOM) was used to examine the physical mechanisms that cause current separation and upwelling over the southeast shelf of Vietnam in the South China Sea (SCS). Process-oriented experiments suggest that the southwesterly monsoon wind is a key physical mechanism for upwelling in that area but not a prerequisite to cause current separation. With no wind forcing, current separation in summer can occur as a result of the encounter of a southward along-shelf coastal current from the north and northeastward buoyancy-driven and stratified tidal-rectified currents from the southwest. The southward current can be traced upstream to the Hong River in the Gulf of Tonkin. This current is dominated by semigeostrophic dynamics and is mostly confined to the narrow shelf along the northern Vietnamese coast. The northeastward currents are generated by tidal rectification and are intensified by the Mekong River discharge and southwesterly monsoon wind forcing. The dynamics controlling this current are fully nonlinear, with significant contributions from advection and vertical turbulent mixing. Upwelling in the current separation zone can be produced by a spatially uniform constant wind field and can be explained using simple wind-induced Ekman transport theory. This finding differs from previous theory in which the regional dipole wind stress curl is claimed as a key mechanism for current separation and upwelling in this coastal region. Our SCS FVCOM, driven by the wind stress, river discharge, and tides, is capable of reproducing the location and tongue-like offshore distribution of temperature as those seen in satellite-derived sea surface temperature imagery.
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ArticleDownwelling wind, tides, and estuarine plume dynamics(John Wiley & Sons, 2016-06-24) Lai, Zhigang ; Ma, Ronghua ; Huang, Mingfen ; Chen, Changsheng ; Chen, Yong ; Xie, Congbin ; Beardsley, Robert C.The estuarine plume dynamics under a downwelling-favorable wind condition were examined in the windy dry season of the Pearl River Estuary (PRE) using the PRE primitive-equation Finite-Volume Community Ocean Model (FVCOM). The wind and tide-driven estuarine circulation had a significant influence on the plume dynamics on both local and remote scales. Specifically, the local effect of downwelling-favorable winds on the plume was similar to the theoretical descriptions of coastal plumes, narrowing the plume width, and setting up a vertically uniform downstream current at the plume edge. Tides tended to reduce these plume responses through local turbulent mixing and advection from upstream regions, resulting in an adjustment of the isohalines in the plume and a weakening of the vertically uniform downstream current. The remote effect of downwelling-favorable winds on the plume was due to the wind-induced estuarine sea surface height (SSH), which strengthened the estuarine circulation and enhanced the plume transport accordingly. Associated with these processes, tide-induced mixing tended to weaken the SSH gradient and thus the estuarine circulation over a remote influence scale. Overall, the typical features of downwelling-favorable wind-driven estuarine plumes revealed in this study enhanced our understanding of the estuarine plume dynamics under downwelling-favorable wind conditions.
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ArticleApplication and comparison of Kalman filters for coastal ocean problems : an experiment with FVCOM(American Geophysical Union, 2009-05-13) Chen, Changsheng ; Malanotte-Rizzoli, Paola ; Wei, Jun ; Beardsley, Robert C. ; Lai, Zhigang ; Xue, Pengfei ; Lyu, Sangjun ; Xu, Qichun ; Qi, Jianhua ; Cowles, Geoffrey W.Twin experiments were made to compare the reduced rank Kalman filter (RRKF), ensemble Kalman filter (EnKF), and ensemble square-root Kalman filter (EnSKF) for coastal ocean problems in three idealized regimes: a flat bottom circular shelf driven by tidal forcing at the open boundary; an linear slope continental shelf with river discharge; and a rectangular estuary with tidal flushing intertidal zones and freshwater discharge. The hydrodynamics model used in this study is the unstructured grid Finite-Volume Coastal Ocean Model (FVCOM). Comparison results show that the success of the data assimilation method depends on sampling location, assimilation methods (univariate or multivariate covariance approaches), and the nature of the dynamical system. In general, for these applications, EnKF and EnSKF work better than RRKF, especially for time-dependent cases with large perturbations. In EnKF and EnSKF, multivariate covariance approaches should be used in assimilation to avoid the appearance of unrealistic numerical oscillations. Because the coastal ocean features multiscale dynamics in time and space, a case-by-case approach should be used to determine the most effective and most reliable data assimilation method for different dynamical systems.
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ArticleInitial spread of 137Cs from the Fukushima Dai-ichi Nuclear Power Plant over the Japan continental shelf : a study using a high-resolution, global-coastal nested ocean model(Copernicus Publications on behalf of the European Geosciences Union, 2013-08-14) Lai, Zhigang ; Chen, Changsheng ; Beardsley, Robert C. ; Lin, Huichan ; Ji, Rubao ; Sasaki, J. ; Lin, JianThe 11 March 2011 tsunami triggered by the M9 and M7.9 earthquakes off the Tōhoku coast destroyed facilities at the Fukushima Dai-ichi Nuclear Power Plant (FNPP) leading to a significant long-term flow of the radionuclide 137Cs into coastal waters. A high-resolution, global-coastal nested ocean model was first constructed to simulate the 11 March tsunami and coastal inundation. Based on the model's success in reproducing the observed tsunami and coastal inundation, model experiments were then conducted with differing grid resolution to assess the initial spread of 137Cs over the eastern shelf of Japan. The 137Cs was tracked as a conservative tracer (without radioactive decay) in the three-dimensional model flow field over the period of 26 March–31 August 2011. The results clearly show that for the same 137Cs discharge, the model-predicted spreading of 137Cs was sensitive not only to model resolution but also the FNPP seawall structure. A coarse-resolution (∼2 km) model simulation led to an overestimation of lateral diffusion and thus faster dispersion of 137Cs from the coast to the deep ocean, while advective processes played a more significant role when the model resolution at and around the FNPP was refined to ∼5 m. By resolving the pathways from the leaking source to the southern and northern discharge canals, the high-resolution model better predicted the 137Cs spreading in the inner shelf where in situ measurements were made at 30 km off the coast. The overestimation of 137Cs concentration near the coast is thought to be due to the omission of sedimentation and biogeochemical processes as well as uncertainties in the amount of 137Cs leaking from the source in the model. As a result, a biogeochemical module should be included in the model for more realistic simulations of the fate and spreading of 137Cs in the ocean.
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ArticleObserved wintertime tidal and subtidal currents over the continental shelf in the northern South China Sea(John Wiley & Sons, 2014-08-19) Li, Ruixiang ; Chen, Changsheng ; Xia, Huayong ; Beardsley, Robert C. ; Shi, Maochong ; Lai, Zhigang ; Lin, Huichan ; Feng, Yanqing ; Liu, Changjian ; Xu, Qichun ; Ding, Yang ; Zhang, YuSynthesis analyses were performed to examine characteristics of tidal and subtidal currents at eight mooring sites deployed over the northern South China Sea (NSCS) continental shelf in the 2006–2007 and 2009–2010 winters. Rotary spectra and harmonic analysis results showed that observed tidal currents in the NSCS were dominated by baroclinic diurnal tides with phases varying both vertically and horizontally. This feature was supported by the CC-FVCOM results, which demonstrated that the diurnal tidal flow over this shelf was characterized by baroclinic Kelvin waves with vertical phase differences varying in different flow zones. The northeasterly wind-induced southwestward flow prevailed over the NSCS shelf during winter, with episodic appearances of mesoscale eddies and a bottom-intensified buoyancy-driven slope water intrusion. The moored current records captured a warm-core anticyclonic eddy, which originated from the southwestern coast of Taiwan and propagated southwestward along the slope consistent with a combination of β-plane and topographic Rossby waves. The eddy was surface-intensified with a swirl speed of >50 cm/s and a vertical scale of ∼400 m. In absence of eddies and onshore deep slope water intrusion, the observed southwestward flow was highly coherent with the northeasterly wind stress. Observations did not support the existence of the permanent wintertime South China Sea Warm Current (SCSWC). The definition of SCSWC, which was based mainly on thermal wind calculations with assumed level of no motion at the bottom, needs to be interpreted with caution since the observed circulation over the NSCS shelf in winter included both barotropic and baroclinic components.
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ArticleA nonhydrostatic version of FVCOM : 1. Validation experiments(American Geophysical Union, 2010-11-13) Lai, Zhigang ; Chen, Changsheng ; Cowles, Geoffrey W. ; Beardsley, Robert C.The unstructured grid finite volume coastal ocean model (FVCOM) system has been expanded to include nonhydrostatic dynamics. This addition uses the factional step method with both split mode explicit and semi-implicit schemes. The unstructured grid finite volume method, combined with a correction of the final free surface from its intermediate value with inclusion of nonhydrostatic effects, efficiently reduces numerical damping and thus ensures second-order accuracy of the solutions with local/global volume conservation. Numerical experiments have been made to fully validate the nonhydrostatic FVCOM, including surface standing and solitary waves in idealized flat- and sloping-bottomed channels in homogeneous conditions, the density adjustment problem for lock exchange flow in a flat-bottomed channel, and two-layer internal solitary wave breaking on a sloping shelf. The model results agree well with the relevant analytical solutions and laboratory data. These validation experiments demonstrate that the nonhydrostatic FVCOM is capable of resolving complex nonhydrostatic dynamics in coastal and estuarine regions.
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ArticleA nonhydrostatic version of FVCOM : 2. Mechanistic study of tidally generated nonlinear internal waves in Massachusetts Bay(American Geophysical Union, 2010-12-21) Lai, Zhigang ; Chen, Changsheng ; Cowles, Geoffrey W. ; Beardsley, Robert C.The generation, propagation, and dissipation processes of large-amplitude nonlinear internal waves in Massachusetts Bay during the stratified season were examined using the nonhydrostatic Finite-Volume Coastal Ocean Model (FVCOM-NH). The model reproduced well the characteristics of the high-frequency internal waves observed in Massachusetts Bay in August 1998. The model experiments suggested that internal waves over Stellwagen Bank are generated by the interaction of tidal currents with steep bottom topography through a process of forming a large-density front on the western slope of the bank by the release of an initial density perturbation near ebb-flood transition, nonlinear steepening of the density front into a deep density depression, and disintegrating of the density depression into a wave train. Earth's rotation tends to transfer the cross-bank tidal kinetic energy into the along-bank direction and thus reduces the intensity of the density perturbation at ebb-flood transition and density depression in the flood period. The internal wave packet propagates as a leading edge feature of the internal tidal wave, and the faster propagation speed of the high-frequency internal waves in Massachusetts Bay is caused by Earth's rotation. The model experiments suggested that bottom friction can significantly influence the cross-bank scale of the density perturbation and thus the density depression during wave generation and the dissipation during the wave's shoaling. Inclusion of vertical mixing using the Mellor-Yamada level 2.5 turbulence closure model had only a marginal effect on wave evolution. The model results support the internal wave theory proposed by Lee and Beardsley (1974) but are in disagreement with the lee-wave mechanism proposed by Maxworthy (1979).