Climatological average and seasonal cycle of water, heat and salt budgets 

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SCS circulation

As explained above, the SCS is subjected to forcings of different scales and origins, and its circulation is regulated by a combination of factors: the geometry of the zone, the connection with the Western Pacific Ocean and Indian Ocean and a strong variability of atmospheric forcings, from the daily to the seasonal and interannual scales (Shaw and Chao 1994, Metzger and Hurlburt 1996, Gan et al. 2006). In this section we present a short overview on our current knowledge of the SCS multi-layer circulation.

Surface and subsurface circulation

Earliest studies on the SCS surface circulation were carried out in 1950s, 1960s by Dale (1956) and Wyrtki (1961) using observational data deduced from ship drift data and prevailing wind data. They all pointed out that the monsoon winds are the main factor driving the SCS surface circulation. Since then, several studies on the SCS surface circulation have been conducted, broadening our understanding on this subject. Wyrtki (1961), Xu (1982), Shaw and Chao (1994), Chu and Li (2000), Fang et al. (2002) all agreed on a reversal surface circulation in response to the monsoon winds reversal. In winter, strong northeasterly monsoon winds generate a cyclonic surface circulation over the whole basin with subbasin cyclonic gyres in both northern and southern parts of the SCS (Figure I.9a). In summer, relatively weaker southwesterly monsoon winds blow over the region, leading to a structure with a cyclonic gyre in the North and an anticyclonic gyre in the South (Figure I.9b, Qu 2000, Gan et al. 2016).

Intermediate and deep layer circulation

In the intermediate layer (500 – 1700 m depth), the SCS circulation – deduced from hydrographic data (Gan et al. 2016) and geographic current velocities (Zhu et al. 2017) – is considered to be anticyclonic on average. Previous work suggests that the intermediate layer circulation is characterized by a relatively steady flow pattern, dominated by local gyres and eddies. Most numerical studies show an anticyclonic circulation in yearly mean and in winter (Yuan 2002, Xu and Oey 2014), but results differ in summer: Gan et al. (2016) found a basin wide anticyclonic circulation in both winter and summer (Figure I.8c, d) while Wei et al. (2016) obtained in the southwestern SCS a subbasin cyclonic gyre in winter and a subbasin anticyclonic gyre in summer.
In the deep layer (> 1700 m), limited observations of Wang (1986) and Li and Qu (2006) inferred a possible cyclonic circulation. Later, Qu et al. (2006), Wang.G et al. (2011), Gan et al. (2016), Lan et al. (2015) and Zhu et al. (2017) obtained the same result: a strong inward flow from the Western Pacific enters the SCS basin through Luzon strait, turns northwestward then southwestward off the southern China continental slope, intensifies along the southwestern end of the deep basin, and eventually forms a basin scale cyclonic circulation (Figure I.8e, f).
It is important to note that due to the lack of in situ observations, the intermediate and deep layer circulation within the SCS basin, especially its seasonal variability, stays relatively poorly known until now. In recent years, several numerical studies and geostrophic analyses support the concept proposed by Yuan et al. (2002) of a sandwiched vertical circulation of the SCS basin: cyclonic circulations in the upper and deep layers separated by an anticyclonic circulation in the intermediate layer (Xu and Oey 2004, Gan et al. 2016, Zhu et al. 2017). This alternating cyclonic – anticyclonic circulation would be induced to the alternating inflow – outflow vertical distribution of the water fluxes at Luzon strait, that will be detailed in section I.4.2.1.

Kuroshio intrusion and Luzon strait transport

The Kuroshio current is the western boundary current of the North Pacific subtropical gyre. The Kuroshio originates from the North Equatorial Current (NEC), which bifurcates at the east coast of Luzon (Philippines) into the southward flowing Minadao Current (MC) and the northward Kuroshio current (Nitani et al. 1972), forming the so-called NMK current system shown in Figure I.10. As the Kuroshio flows northward passing the Luzon Strait, its path makes a slight detour intruding into the SCS with only a small fraction actually entering the interior, mostly during winter (dashed black line, Figure I.10). According to the overview of Hu et al. (2000), different types of the Kuroshio intrusion into the SCS occur: Kuroshio direct branch, Kuroshio Current Loop (KCL), Kuroshio extension and Anticyclonic rings. As explained above, this intrusion acts against the effect of the northeastward summer monsoon wind and contributes to maintain a cyclonic circulation in the northern SCS in summer. The Kuroshio intrusion contributes the most to the Luzon strait transport (LST) – the Western Pacific water masses entering the SCS through Luzon strait. Kuroshio intrusion into the SCS is seasonally varying. Historical hydrographic data (Wyrtki 1961, Qu 2000, Xue et al. 2004) as well as numerical studies (Qu et al. 2004, Metzger and Hurlburt 1996, Hsin et al.
2012) showed similar seasonal cycle: the LST is larger in winter and smaller in summer under the influences of the seasonal reversing monsoon. On an interannual time scale, the LST from the Pacific into the SCS tends to be higher during El Niño years and lower during La Niña years, and play a key role in conveying the ENSO signal from the Pacific into the SCS (Qu et al. 2004, Liu et al. 2008). Estimates of the Kuroshio intrusion or LST, by observation and numerical methods, are detailed in the section I.4.2.2.

Surface characteristics and mixed layer depth

The temperature and salinity patterns of the SCS are strongly influenced by the alternating seasonal monsoons and the exchanges of water masses with the Western Pacific, Indian Ocean and surrounding seas (the SCSTF, sea above).
Seasonal averages of SST, SSS and Mixed Layer Depth (MLD) (Figure I.15) over the SCS zone reveal a strong spatial and seasonal variability. In winter, under the influences of NE monsoon associated with cold and dry atmospheric fluxes (see I.2.1) and the intrusion of the Kuroshio into the SCS (see section 1.4.1), the northern SCS basin has lower SST and higher SSS compared to the southern basin. The strong dry and cold NE monsoon generates an intense western boundary current flowing southward along the continental slope, transporting cold and salty water to the south of the SCS and generating a cold and salty tongue in the southern basin and a temperature – salinity gradient over the SCS (Figure I.15a, b). Lowest SST zones are the Chinese southern coasts and the Gulf of Tonkin, whereas highest SST is observed in the southeast basin and over the Gulf of Thailand. SSS is maximum in the northern part and minimum in the Gulf of Thailand. In terms of MLD, we also observe a north – south gradient in winter, with higher MLD in the northern SCS (~80 m). SST, SSS and MLD ranges in winter are large, respectively at about 10-12°C, 3 – 4 psu and 50-70 m.
In summer, the SCS SST is relatively uniform, with minor variations over the whole basin (~2 °C, Figure I.15d). The warm and humid SW summer monsoon coming from the Southern Pacific and Indian Ocean associated with warmer atmospheric heat fluxes are the principal factors that increase the SST over the basin. Lowest SST are observed at the northern boundary (Chinese coasts), around Hainan island and the SVU while highest SST zones are located over the Gulf of Tonkin, Gulf of Thailand and southeastern boundary. Similarly, the SSS in summer becomes more homogeneous than in winter, with variations between 1.5 – 2 psu. The deep basin zones have higher SSS than coastal zones, partly due to strong freshwater discharges in summer. We also observe higher salinity along the central Vietnam coast due to upwelling. With a warmer and fresher upper layer, the SCS basin is much more stratified in summer, leading to a shallower MLD barely exceeding 40 m. The thickest MLD zone is no longer the northern basin but the southern basin, with around 40-50 m of depth at the center of eddies.

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Biogeochemistry and pelagic ecosystem of the SCS

The biogeochemical conditions and marine ecosystems in the SCS are subjected to strong climate forcing as well as anthropogenic impacts. As the SCS is bordered by the world’s most densely populated coastal communities, changes in these ecosystems may threaten the livelihood of a large population of humans over the region. Therefore, it is important to study the functioning and variability of planktonic ecosystems – the first level in the ocean food web. Ocean dynamics, by mixing and transporting nutrients and the other compartments of the planktonic ecosystems, are major factors of spatial and temporal variability for those ecosystems. Previous studies revealed that the biogeochemistry and ecosystems in the SCS are sensitive to atmospheric forcing, land-to-ocean fluxes and the Kuroshio intrusion (Liu et al. 2002, Tang et al. 2004, Liu and Chai. 2009, Du et al. 2013, Liu. K et al. 2014, Zhang et al. 2016). Since biogeochemical in-situ measurements are limited over the domain, numerical models and satellite observations are widely used to investigate the seasonal and interannual variability of the SCS planktonic ecosystem. Studies based on satellite remote sensing data of Suhung et al. 2008 and Yu.Y et al. (2019) showed that the Chl-a levels reached their maximum in winter and minimum in summer over most of the SCS, except southeast of Vietnam where the surface Chla becomes maximum in summer and minimum in winter (Figure I.18). The monsoon winds and SST were the most important factors impacting the distribution and variability of surface Chl-a along with other associated and influential environmental drivers, e.g., wind stress curl, frontal activity, and sea level anomalies (Liu et al. 2013, Yu.Y et al. 2019). Liu et al. (2002), using a coupled physical/biogeochemical model together with shipboard data and the CZCS-SeaWiFs satellite data, revealed a strong seasonality of surface chlorophyll in the SCS in monsoon-driven upwelling regions, with maximum concentrations developing northwest of Luzon in winter and off the east coast of Vietnam in summer (Figure I.18). Wang et al. (2010) indicated that the winter phytoplankton bloom off the northwest of Luzon is primarily induced by both Ekman pumping-driven upwelling and upper mixed layer entrainment. Over the southern Vietnamese coasts in summer, the wind stress curl and cyclonic eddies induce the so-called SVU (see I.4.1), which brings nutrients to the surface waters, leading to a phytoplankton bloom in this region (Tang et al. 2004, Da et al. 2019).

Table of contents :

Chapter I: Introduction – Context and objectives
I.1. Physical geography of the South China Sea
I.2. Atmospheric forcing
I.2.1. The monsoon wind system
I.2.2. Atmospheric heat and freshwater fluxes
I.2.3. Tropical cyclones
I.2.4. Climate phenomena
I.3. River discharges
I.4. Ocean dynamics characteristics
I.4.1. SCS circulation
I.4.2. The South China Sea Throughflow (SCSTF)
I.4.3. Thermohaline structure
I.4.4. Tides
I.5. Biogeochemistry and pelagic ecosystem of the SCS
I.6. Scientific questions and objectives of the thesis
Chapter II: Material & Methods
II.1. The hydrodynamic ocean model SYMPHONIE
II.1.1. The governing equation
II.1.2. Turbulence closure scheme
II.1.3. Boundary conditions
II.1.4. Discretization of equations
II.2. The South China Sea configuration
II.3. Observation data for model evaluation
II.3.1. Satellite observations
II.3.2. In – situ observations
II.3.3. Climate indices
II.4. Analysis and statistical methods
II.4.1. Flux and budget calculations
II.4.2. Statistical methods
Chapter III: Model evaluation
III.1. Surface characteristics
III.1.1. Annual cycle
III.1.2. Interannual variations
III.1.3 Spatial seasonal surface patterns
III.2. Water masses characteristics
III.2.1. Comparison with Argo, glider and in situ profiles
III.2.2. Representation of SCS water masses
Chapter IV: Climatological average and seasonal cycle of water, heat and salt budgets 
IV.1. Volume fluxes
IV.1.1. Lateral fluxes through the interocean straits
IV.1.2 Contributions of atmospheric, river and lateral water fluxes to the SCS budget
IV.2 Heat fluxes
IV.2.1 Lateral heat fluxes at interocean straits
IV.2.2. Atmospheric, river and lateral heat fluxes
IV.3. Salt fluxes
IV.4. Vertical structure of strait fluxes
IV.5. Summary and discussion
Chapter V: Interannual variability of the water, heat and salt budgets
V.1. Interocean straits fluxes
V.2. Interannual variability of water, heat and salt budgets over the SCS
V.2.1. Water budget
V.2.2. Heat Budget
V.2.3. Salt budget
V.5. Discussion and conclusion
Chapter VI: Conclusion & Perspectives
VI.1. Conclusion
VI.1.1. Numerical methods and model evaluation
VI.1.2. Water, heat and salt fluxes and budgets: climatological averages
VI.1.3. Water, heat and salt budgets: seasonal cycle
VI.1.4. Water, heat and salt budgets: interannual variability
VI.2. Limitations and Perspectives
VI.2.1. Long-term in-situ measurements
VI.2.2. Sensitivity studies
VI.2.3 Studies of other temporal scales and regions
VI.2.4. Impact on ecosystems: coupled physical/biogeochemical model studies
Conclusion Générale
A.1. The biogeochemical model (Eco3M-S)
A.2. The implementation of the Eco3M-S model on the SCS basin
A.3. Case study 2016 – 2017
A.4. Discussions & Conclusions


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