Guohong Fang

Seasonal structures of upper layer circulation in the southern South China Sea from in situ observations

[1] Conductivity-temperature-depth and acoustic Doppler current profiler data from six cruises spanning 1989 to 1999 are used to investigate the seasonal structure of the upper layer circulation in the southern South China Sea (SCS). The surveys were made during winter, late spring, summer, and late fall. More detailed structures not presented in previous studies are found. In summer the upper layer circulation of the southern SCS is dominated by an anticyclonic gyre with a strong eastward flow on its northern border, which originates off the southeast coast of Vietnam: the Summer Southeast Vietnam Offshore Current. In winter a stronger cyclonic gyre exists in the western portion of the southern SCS and a weaker anticyclonic circulation in the eastern portion. At the juncture of these two gyres, there is a strong northward upwind flow, called the Winter Natuna Off-Shelf Current. In late spring the anticyclonic gyre begins to form in the northwest, and a trace of the Natuna Off-Shelf Current can still be observed, especially subsurface in the southern part of the study area. Mesoscale eddies are active in this period. In late fall the winter-type pattern begins to appear. The Natuna Off-Shelf Current and the cyclonic gyre in the northwest part of the southern SCS emerge, and the summer anticyclonic gyre vanishes. In the southeast the currents are basically toward the northeast with an anticyclonic trend, revealing a multi-eddy feature. These upper ocean currents appear consistent with the wind forcing.

Empirical cotidal charts of the Bohai, Yellow, and East China Seas from 10 years of TOPEX/Poseidon altimetry

[1] Harmonic analysis of 10 years of TOPEX/Poseidon (TP) along-track altimetry is performed to derive the semidiurnal, diurnal, long-period, and quarter-diurnal tides in the Bohai, Yellow, and East China Seas. The TP solutions are evaluated through intercomparison for crossover points and comparison with the ground truth, showing that the accuracy of TP solutions in the study area has achieved levels of 2 - 4 cm in amplitudes and 5degrees in phase lags for principal constituents (M-2, S-2, K-1, O-1, and S-a). The TP-derived S-a amplitudes have a systematic bias of about - 10% as compared with the ground truth, indicating the possible importance of loading effect of this constituent in the study area, which is generally not considered in geodetic surveys. The tidal harmonics from TP altimetry and at coastal and island stations are used to give a new set of empirical cotidal charts for principal constituents (M-2, S-2, K-1, O-1, and S-a). The accuracy of these new charts is believed to be significantly higher than the previous charts for the offshore area.

A review on the South China Sea western boundary current

The advances in understanding the South China Sea (SCS) western boundary current (SCSwbc) have been reviewed since the works of Dale (1956) and Wyrtki (1961) in the middle of the 20th century. The features of the pattern of SCSwbc and the oceanic phenomena associated with it are focused on. The current is driven mainly by monsoon over the SCS and partially by winds over the tropical Pacific governed by the island rule. The SCSwbc exhibits strong seasonal variation in its direction and patterns. In winter, the current is strong and flows southwestward along the South China shelf and slope from the east of Dongsha Islands to the northern central Vietnamese coast, then turns to the south along the central and southern Vietnamese coast, and finally partially exits the SCS through the Karimata Strait. In summer and early fall, the SCSwbc can be divided into three segments based on their characteristics. The southern segment is stable, flowing northward from the Karimata Strait up to about 11°N, where it separates from the coast forming an eastward offshore current. The separation of the current from Vietnamese coast induces some striking features, such as upwelling and cold sea-surface temperature. The middle segment off the central Vietnamese coast may have a bimodal behavior: northward coastal current and meandering current in early summer (June–July), and cyclonic gyre in later summer and early fall (August–September). The northern segment is featured by the summer SCS Warm Current on the South China shelf and a southwestward subsurface current along the continental slope.

Introduction to special section: Dynamics and Circulation of the Yellow, East, and South China Seas

. The seas aresurrounded by 12 countries: China, North Korea, SouthKorea, Japan, the Philippines, Indonesia, Brunei, Malaysia,Singapore, Thailand, Cambodia, and Vietnam, which ac-count for over 2.0 billion of the human inhabitants (mid-2005 estimate), nearly one third of the world population.These populations are impacted by all coastal manifesta-tions of global climate change, such as rising sea level andmore frequent and severe storms [Anderson et al., 2001].Since ancient times, the seas have served as a convenientnavigation waterway for the East Asian and Southeast Asiannations to communicate with each other and with nations ofthe outside world [Guo et al., 2004; Su and Yuan, 2005].Even today, the seas are among the busiest waterways in theworld because of the size and the high growth rates of theregion in the world economy and trade.[

Observations of the Karimata Strait througflow from December 2007 to November 2008

In order to quantitatively estimate the volume and property transports between the South China Sea and Indonesian Seas via the Karimata Strait, two trawl-resistant bottom mounts, with ADCPs embedded, were deployed in the strait to measure the velocity profile as part of the South China Sea-Indonesian Seas transport/exchange (SITE) program. A pair of surface and bottom acoustic modems was employed to transfer the measured velocity without recovering the mooring. The advantage and problems of the instruments in this field work are reported and discussed. The field observations confirm the existence of the South China Sea branch of Indonesian throughflow via the Karimata Strait with a stronger southward flow in boreal winter and weaker southward bottom flow in boreal summer, beneath the upper layer northward (reversal) flow. The estimate of the averaged volume, heat and freshwater transports from December 2007 to March 2008 (winter) is (−2.7±1.1)×106 m3/s, (−0.30±0.11) PW, (−0.18±0.07)×106 m3/s and from May to September 2008 (summer) is (1.2±0.6)×106 m3/s, (0.14±0.03) PW, (0.12±0.04)×106 m3/s and for the entire record from December 2007 to October 2008 is (−0.5±1.9)×106 m3/s, (−0.05±0.22) PW, (−0.01±0.15)×106 m3/s (negative/positive represents southward/northward transport), respectively. The existence of southward bottom flow in boreal summer implies that the downward sea surface slope from north to south as found by Fang et al. (2010) for winter is a year-round phenomenon.

A fresh look at the deepwater overflow in the Luzon Strait

On the basis of the latest version of a U.S. Navy generalized digital environment model (GDEM-V3.0) and World Ocean Atlas (WOA13), the hydraulic theory is revisited and applied to the Luzon Strait, providing a fresh look at the deepwater overflow there. The result reveals that: (1) the persistent density difference between two sides of the Luzon Strait sustains an all year round deepwater overflow from the western Pacific to the South China Sea (SCS); (2) the seasonal variability of the deepwater overflow is influenced not only by changes in the density difference between two sides of the Luzon Strait, but also by changes in its upstream layer thickness; (3) the deepwater overflow in the Luzon Strait shows a weak semiannual variability; (4) the seasonal mean circulation pattern in the SCS deep basin does not synchronously respond to the seasonality of the deepwater overflow in the Luzon Strait. Moreover, the deepwater overflow reaches its seasonal maximum in December (based on GDEM-V3.0) or in fall (October–December, based on the WOA13), accompanied by the lowest temperature of the year on the Pacific side of the Luzon Strait. The seasonal variability of the deepwater overflow is consistent with the existing longest (3.5 a) continuous observation along the major deepwater passage of the Luzon Strait.

Seasonal variability of the isopycnal surface circulation in the South China Sea derived from a variable-grid global ocean circulation model

In this study, we develop a variable-grid global ocean general circulation model (OGCM) with a fine grid (1/6)° covering the area from 20°S–50°N and from 99°–150°E, and use the model to investigate the isopycnal surface circulation in the South China Sea (SCS). The simulated results show four layer structures in vertical: the surface and subsurface circulation of the SCS are characterized by the monsoon driven circulation, with basin-scaled cyclonic gyre in winter and anti-cyclonic gyre in summer. The intermediate layer circulation is opposite to the upper layer, showing anti-cyclonic gyre in winter but cyclonic gyre in summer. The circulation in the deep layer is much weaker in spring and summer, with the maximum velocity speed below 0.6 cm/s. In fall and winter, the SCS deep layer circulation shows strong east boundary current along the west coast of Philippine with the velocity speed at 1.5 m/s, which flows southward in fall and northward in winter. The results have also revealed a fourlayer vertical structure of water exchange through the Luzon Strait. The dynamics of the intermediate and deep circulation are attributed to the monsoon driving and the Luzon Strait transport forcing.

Cotidal charts and tidal power input atlases of the global ocean from TOPEX/Poseidon and JASON-1 altimetry

The global distributions of eight principal tidal constituents, M2, S2, K1, O1, N2, K2, P1, and Q1, are derived using TOPEX/Poseidon and JASON-1(T/P-J) satellite altimeter data for 16 a. The intercomparison of the derived harmonics at 7000 subsatellite track crossover points shows that the root mean square (RMS) values of the tidal height differences of the above eight constituents range from 1.19 cm to 2.67 cm, with an average of about 2 cm. The RMS values of the tidal height differences between T/P-J solutions and the harmonics from ground measurements at 152 tidal gauge stations for the above constituents range from 0.34 cm to 1.08 cm, and the relative deviations range from 0.031 to 0.211. The root sum square of the RMS differences of these eight constituents is 2.12 cm, showing the improvement of the present model over the existing global ocean tidal models. Based on the obtained tidal model the global ocean tidal energetics is studied and the global distribution of the tidal power input density by tide-generating force of each constituent is calculated, showing that the power input source regions of semidiurnal tides are mainly concentrated in the tropical belt between 30°S and 30°N, while the power input source regions of diurnal tides are mainly concentrated off the tropic oceans. The global energy dissipation rates of the M2, S2, K1, O1, N2, P1, K2 and Q1 tides are 2.424, 0.401, 0.334, 0.160, 0.113, 0.035, 0.030 and 0.006 TW, respectively. The total global tidal dissipation rate of these eight constituents amounts to 3.5 TW.

Interbasin exchanges and their roles in global ocean circulation：A study based on 1 400 years＇ spin up of MOM4p1

A global prognostic model based on MOM4p1, which is a primitive equation nonBoussinesq numerical model, has been integrated with 1 400 years from the state of rest based on the realistic topography to study the long-term pattern of combined wind-driven and thermodynamically-driven general circulation. The model is driven by monthly climatological mean forces and includes 192×189 horizontal grids and 31 pressure-based vertical levels. The main objective is to investigate the mass and heat transports at inter-basin passages and their compensations and roles in the global ocean circulation under equilibrium state of long-term spin up. The kinetic energy analysis divides the spin up process into three stages: the quasi-stable state of wind driven current, the growing phase of thermodynamical circulation and the equilibrium state of thermohaline circulation. It is essential to spin up over a thousand years in order to reach the thermohaline equilibrium state from a state of rest. The Arctic Throughflow from the Bering Strait to the Greenland Sea and the Indonesian Throughflow (ITF) are captured and examined with their compensations and existing data. Analysis reveals that the slope structures of sea surface height are the dynamical driving mechanism of the Pacific-Arctic-Atlantic throughflow and ITF. The analysis denotes, in spite of O (1.4×106 m3/s) of the southward volume transport in the northern Atlantic, that there is still O (1 PW) of heat transported northward since the northward currents in the upper layer carrymuch higher temperature water than the southward flowing northern Atlantic deep water (NADW). Meridional volume and heat transports are focused on the contributions to NADW renewals and Atlantic meridional overturning circulation (AMOC). Quantitative descriptions of the interbasin exchanges are explained by meridional compensations and supported by previous observations and numerical modeling results. Analysis indicates that the volume and heat exchanges on the interbasin passages proposed in this article manifest their hub roles in the Great Ocean Conveyor System.

Data assimilation of tides in the Bohai Sea using the adjoint method

The two-dimensional data assimilation model of the tides in the Bohai Sea was established using the ajoint method to study the tides of the Bohai Sea. The adjoint equations were derived from the two-dimensional tidal equations by Lagrange multiplier method. The observation data were from the tidal gauge stations and the satellite altimetry data. To evaluate the role of the altimetry data and the tidal gauge data in the data assimilation of the tides in the Bohai Sea, three numerical experiments were carried out. Experiment 1 only assimilates the tidal gauge data, Experiment 2 only assimilates the altimetry data, and Experiment 3 assimilates the combined data of the tidal gauge stations and the satellite altimetry. The tidal constituents M2, S2, K1 and O1 of the Bohai Sea were simulated. The results show that it is better when assimilating the tidal gauge data and the altimetry data simultaneously than assimilating any one separately. The mean absolute differences of the amplitude and phase-lag are 1.52 cm and 2.22° for M2, 1.26 cm and 3.30° for S2, 1.19 cm and 2.72° for K1, 0.55 cm and 1.93° for O1, respectively. The results reflect the tidal system of the Bohai Sea well, especially for the outer sea area since the altimetry data were assimilated.