A Three Dimensional Circulation Model of the South China Sea

doi:10.1016/S0422-9894(08)70451-3

Elsevier Oceanography Series

Volume 45, 1987, Pages 245-268

https://doi.org/10.1016/S0422-9894(08)70451-3 Get rights and content

Abstract

Up to now there is a great lack of observational data in the South China Sea. The best available information about the general hydrography of the region was the Naga Report compiled by Wyrtki already in 1961.

The South China Sea is an equatorial region with a complex topography. It is a regime dominated by the monsoon and stratification is of enormous importance.

A prognostic baroclinic circulation model was applied in order to increase our present knowledge and our understanding of this region. The gridsize of this 12- layer model is about 50 km in the horizontal. The layers have a thickness of 10 m to 3000 m, increasing with depth.

Simulations were carried out for the winter- and for the summer monsoon, respectively. The calculation of temperature and salinity distributions which are consistent with the circulation provide insight into new features like deep-reaching up- and downwelling phenomena. A first validation of the model results was carried out in comparison with the observational data compiled by Klaus Wyrtki.

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Cited by (56)

Long-term changes of South China Sea surface temperatures in winter and summer
2017, Continental Shelf Research
Citation Excerpt :
They suggested that the summer monsoon weakened, thus reducing the upwelling east of Vietnam and warming the area. The upwelling is a localized phenomenon due to the southwesterly wind along the coast (Chen et al., 2012; Pohlmann, 1987), so scrutiny of the local wind is necessary. More importantly, as we will display later, another part of the SCS showed stronger warming, but this phenomenon was not investigated.
Utilizing available atmospheric and oceanographic reanalysis data sets, the long-term trend in South China Sea (SCS) sea surface temperature (SST) between 1950 and 2008 and the governing processes are investigated. Both winter and summer SST increased by comparable amounts, but the warming patterns and the governing processes were different. Strong warming in winter occurred in a deep central area, and during summer in the southern region. In winter the net heat flux into the sea increased, contributing to the warming. The spatial pattern of the heat flux, however, was different from that of the warming. Heat flux increased over the coastal area where warming was weaker, but decreased over the deeper area where warming was stronger. The northeasterly monsoon wind weakened lowering the shoreward Ekman transport and the sea surface height gradient. The cyclonic gyre which transports cold northern water to the south weakened, thereby warming the ocean. The effect was manifested more strongly along the southward western boundary current inducing warming in the deep central part. In summer however, the net surface heat flux decreased and could not contribute to the warming. Over the southern part of the SCS, the weakening of the southwesterly summer monsoon reduced southeastward Ekman transport, which is parallel to the mean SST gradient. Southeastward cold advection due to Ekman transport was reduced, thereby warming the surface near the southeastern boundary of the SCS. Upwelling southeast of Vietnam was also weakened, raising the SST east of Vietnam contributing to the southern summer warming secondarily. The weakening of the winds in each season was the ultimate cause of the warming, but the responses of the ocean that lead to the warming were different in winter and summer.
Assessment and analysis of the chlorophyll-a concentration variability over the Vietnamese coastal waters from the MERIS ocean color sensor (2002–2012)
2017, Remote Sensing of Environment
Citation Excerpt :
The tidal energy is dissipated in the ESV by the bottom friction in shallow areas and by the scattering of surface tide into internal tides (Zu et al., 2008). The ESV is strongly influenced by the periodic semi-annual reversing circulation of the atmosphere associated to the monsoon regime (Pohlmann, 1987). In winter northeasterly wind dominate with an average velocity of 9 m s− 1 over Vietnam (October–April).
Spatio-temporal patterns of the chlorophyll-a concentration, Chla, have been assessed from the MEdium Resolution Imaging Spectrometer (MERIS) over the whole Vietnamese coastal waters from 2002 to 2012. For that purpose, six bio-optical algorithms already documented and based on different approaches have been tested over a large in situ data set collected at different seasons and locations along the Vietnamese coast. The OC5 algorithm (Gohin, Druon, and Lampert, 2002) presents the best performances and has been selected to assess Chla in the studied region. The notion of optimal bio-optical environment associated with the best performance of OC5 has been introduced. For suspended particulate matter concentration, SPM, lower than 100 g·m^− 3, and colored dissolved organic matter, a_cdom(443), lower than 0.5 m^− 1, Chla is estimated with an uncertainty of 36% and a bias of 5%. A Chla climatology has been generated and the temporal patterns (seasonal variability, long term trend, and irregular component) have been described using the Census-X-11 time series decomposition method. Three-dimensional hydrodynamical numerical simulations have been used to analyze the spatio-temporal patterns of Chla. The seasonal contribution dominates the variance of the signal, in good relationship with the dynamic of the mixed layer depth as well as with the occurrence of a seasonal upwelling induced by the summer monsoon. The irregular variability of Chla which may reach up to 35% of the total variance in the central part of the Vietnamese coast, can be explained through the surface kinetic energy and its standard deviation which is associated with small scales processes. A long term monotonic trend from about 2 to > 5%·yr^− 1 (that is 20 to > 50% from 2002 to 2012) have been noticed in different coastal areas where aquaculture activities exhibit a concurrence increase, ranging from 31% to 113% (in production weight) over the same time period. In situ measurements, and especially of nutrients, are however necessary to confirm the link between aquaculture activities and phytoplankton biomass long term evolutions.
Inter-annual variability of upwelling off the South-Vietnamese coast and its relation to nutrient dynamics
2013, Global and Planetary Change
Citation Excerpt :
Both deduced the surface currents from ship drift data and prevailing wind data. Pohlmann (1987) applied the first prognostic baroclinic three-dimensional circulation model to simulate the SCS circulations during the winter and summer monsoons, more or less the same did Mao et al. (1992). Li et al. (1992a,b,1994), Liu and Su (1992), Zeng et al. (1989) and Zeng et al. (1992) used two-dimensional numerical models to calculate the monthly or seasonal mean SCS circulation.
It is the general consensus that the South China Sea (SCS) is influenced by the El Niño southern Oscillation (ENSO). Off southern Vietnam, coastal upwelling is one of the major features of the SCS, which is consequently influenced by the ENSO. However, the processes behind this influence are under discussion. It is the general thesis that the inter-annual changes of the monsoon wind cause the changes of the strength of the upwelling. This thesis has been tested and it is discarded. Instead it can be shown that the wind stress is not the main cause for the impact of the ENSO. Therefore, also nutrient dynamics must be completely re-evaluated.
This study investigates in the circulation and the hydrographical situation during the summer 2003 and 2004, an El Niño and a normal year, respectively. The influence of the circulation on the new biological production based on vertical nutrient fluxes is estimated. Numerical modelling is used to study the circulation inside the SCS. The results of the simulation discover two different states of the horizontal circulation: During the first state an anticyclonic circulation dominates the region. During the second state the cyclonic circulation in the north and the anticyclonic circulation in the south build the dipole. Two cruises during the summer 2003 and summer 2004 verify the two states of the simulated circulations. The ENSO influence on the summer circulation is by tending towards the first state in the summer after an El Niño, while during normal summers the second state is dominant.
Against earlier findings, this study features one new major key of the ENSO influence on the temperature distribution and primary production in the western SCS: This influence is not via the vertical velocities, but due to the different horizontal circulations. In the dipole circulation pattern, the decreased horizontal velocities cause longer residence times which come along with the increase in the efficiency of the upwelling. In this study, for the first time, the advection of the pronounced different water masses of the northern and southern SCS is considered. The different water masses support an important preconditioning of the nutrient concentrations in the water column. An experiment, which compares primary production with and without this preconditioning proves that the originating water masses moving into the upwelling region control the primary production.
Horizontal advection explains the substantial contributions of the strong ENSO signal in the region. Not the regional Ekman drift currents, but the basin wide horizontal circulation explains the inter-annual changes of the vertical nutrient uplift during the summer. It can be shown that the dipole circulation is an almost perfect nutrient pump for new production.
Modelling the circulation on the continental shelf of the province Khanh Hoa in Vietnam
2009, Journal of Marine Systems
A model simulation of the circulation on the continental shelf of the Khanh Hoa province in central Vietnam during the year 2004 is presented. The model, a three-dimensional baroclinic z-coordinate model (the Hamburg Shelf Ocean Model), is implemented with a horizontal resolution of about 1 km. It is initialised with temperature and salinity fields taken from the Levitus data, and by the two main tidal constituents. The model is forced by daily fields of wind stress, air temperature, wind speed, and cloudiness taken from NCEP, and by monthly mean river runoff values. At the open boundary sea surface displacements are prescribed by the tidal variation and by the steric height determined from the density anomalies determined from climatological values of temperature and salinity.
The modelled circulation reflects the monsoonal forcing fields, and reveals downwelling during winter and upwelling during summer. The modelled hydrography is compared with measured profiles, and some biases are found. The flushing times of three bays along the Khanh Hoa coast are calculated. The relative influence of river runoff, tides, and weather on the flushing times is discussed.
Evaluation of CORDEX-SEA Models on Wind Simulation during the Southwest Monsoon in the Southwestern Part of the South China Sea
2022, Atmosphere
Impact of climate variability of the Western Tropical Pacific on maximum salinity water in the South China Sea
2021, Ocean Dynamics

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A Three Dimensional Circulation Model of the South China Sea

Abstract

Continent. Shelf Res.

Journal of Computational Physics

A Three-Dimensional Model for the Simulation of Shelf Sea Dynamics

Dt. hydrogr. Z.