Chenwu Fan

Dynamical mechanisms for the South China Sea seasonal circulation and thermohaline variabilities

Abstract The seasonal ocean circulation and the seasonal thermal structure in the South China Sea (SCS) were studied numerically using the Princeton Ocean Model (POM) with 20-km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography. A 16-month control run was performed using climatological monthly mean wind stresses, restoring-type surface salt and heat, and observational oceanic inflow/outflow at the open boundaries. The seasonally averaged effects of isolated forcing terms are presented and analyzed from the following experiments: 1) nonlinear dynamic effects removed, 2) wind effects removed, and 3) open boundary inflow/outflow set to zero. This procedure allowed analysis of the contribution of individual parameters to the general hydrology and specific features of the SCS: for example, coastal jets, mesoscale topographic gyres, and countercurrents. The results show that the POM model has the capability of simulating seasonal variations of the SCS circulation and thermoha...

An airborne expendable bathythermograph survey of the South China Sea, May 1995

An extensive airborne expendable bathythermograph survey of the South China Sea (SCS) conducted in May 1995 and historical data are used to analyze and infer the upper layer (300 m) synoptic structure and general circulation. The primary thermal feature observed was a central SCS warm pool surrounded by several cool pools. The size of the warm pool decreased with depth from approximately 200,000 km2 at 50 m depth to about 70,000 km2 at 300 m depth. The maximum temperature of the warm pool was 30°C, appearing near the surface. At the depth of 50 m, the temperature of the central SCS warm pool was 29°C, and the temperature of the five surrounding cool pools ranged from 26°C to 22°C. A three-dimensional estimate of the absolute velocity field was obtained from the observed temperature field and a climatological salinity field using the β spiral method. Striking circulation features were the existence of dual anticyclonic eddies in the central SCS warm pool and the existence of cyclonic eddies associated with the cool pools. In the upper layer the tangential velocity of the dual central SCS anticyclonic warm-core eddies is around 30–40 cm/s and that of the five cyclonic cool-core eddies varies from 10 cm/s to 40 cm/s. The tangential velocity of all the eddies decreased with depth. At 300 m depth, it became less than 5 cm/s for all the eddies.

Response of the South China Sea to Tropical Cyclone Ernie 1996

A moving tropical cyclone is an intense localized source of surface wind stress and wind stress curl that produces a significant response in the ocean environment, especially in the ocean thermal structure, the upper ocean currents, and the sea surface elevation. Such a response has been well identified in the open-ocean region, but not in the coastal ocean region. In this study we use the Princeton Ocean Model with 20 km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography to identify the response of the South China Sea to Tropical Cyclone Ernie 1996. Results show strong similarities in the responses between open ocean and coastal regions, including near-surface strong asymmetric response such as divergent currents with near-inertial oscillations, significant sea surface temperature cooling, biase to the right of the storm track, sea surface depressions in the wake of the storm, and subsurface intense upwelling and cooling at the base of the mixed layer to the right of the storm track. The unique features of the SCS response to Ernie are also discussed.

Sixth-Order Difference Scheme for Sigma Coordinate Ocean Models

Abstract How to reduce the horizontal pressure gradient error is a key issue of using σ-coordinate ocean models, especially of using primitive equation models for coastal regions. The error is caused by the splitting of the horizontal pressure gradient term into two parts and the subsequent incomplete cancellation of the truncation errors of those parts. Due to the fact that the higher the order of the difference scheme, the less the truncation error and the more complicated the computation, a sixth-order difference scheme for the σ-coordinate ocean models is proposed in order to reduce error without increasing complexity of the computation. After the analytical error estimation, the Semi-spectral Primitive Equation Model is used to demonstrate the benefit of using this scheme. The stability and accuracy are compared with those of the second-order and fourth-order schemes in a series of calculations of unforced flow in the vicinity of an isolated seamount. The sixth-order scheme is shown to have error red...

Japan Sea Thermohaline Structure and Circulation. Part I: Climatology

Abstract In this study, the U.S. Navy’s Generalized Digital Environmental Model (GDEM) climatological temperature and salinity data on a 0.5° × 0.5° grid is used to investigate the seasonal variabilities of the Japan/East Sea (JES) thermohaline structure and circulations. The GDEM for the JES was built up on historical (1930–97) 136 509 temperature and 52 572 salinity profiles. A three-dimensional estimate of the absolute geostrophic velocity field was obtained from the GDEM temperature and salinity fields using the P-vector method. The seasonal variabilities of the thermohaline structure and the inverted currents such as the Subpolar Front, the salinity minimum and maximum in the Japan Sea Intermediate Water, and the Tsushima Warm Current and its bifurcation are identified.

Determination of Vertical Thermal Structure from Sea Surface Temperature

A recently developed parametric model by P. C. Chu et al. is used in this paper for determining subsurface thermal structure from satellite sea surface temperature observations. Based on a layered structure of temperature fields (mixed layer, thermocline, and lower layers), the parametric model transforms a vertical profile into several parameters: sea surface temperature (SST), mixed layer depth (MLD), thermocline bottom depth (TBD), thermocline temperature gradient (TTG), and deep layer stratification (DLS). These parameters vary on different timescales: SST and MLD on scales of minutes to hours, TBD and TTG on months to seasons, and DLS on an even longer timescale. If the long timescale parameters such as TBD, TTD, and DLS are known (or given by climatological values), the degree of freedom of a vertical profile fitted by the model reduces to one: SST. When SST is observed, one may invert MLD, and, in turn, the vertical temperature profile with the known long timescale parameters: TBD, TTG, and DLS. The U.S. Navy’s Master Oceanographic Observation Data Set (MOODS) for the South China Sea in May 1932‐94 (10 153 profiles) was used for the study. Among them, there are 40 data points collocating and coappearing (same week) with the weekly daytime NASA multichannel SST data in 1986‐94. The 40 MOODS profiles were treated as a test dataset. The MOODS dataset excluding the test data is the training dataset, consisting of 10 113 profiles. The training dataset was processed into a dataset consisting of SST, MLD, TBD, TTG, and DLS using the parametric model. SST from the test dataset was used for the inversion based on the known information on TBD, TTG, and DLS. The 40 inverted profiles agreed quite well with the corresponding observed profiles. The rms error is 0.728C, and the correlation between the inverted and observed profiles is 0.79. This is much better than the simple method of estimating subsurface temperature anomaly from SST anomaly by correlating the two in the training dataset. The possibility of using this method globally is also discussed.

Low Salinity, Cool-Core Cyclonic Eddy Detected Northwest of Luzon during the South China Sea Monsoon Experiment (SCSMEX) in July 1998

To detect eddies, intensive surveys of the northeast South China Sea (SCS) (114°30′–121°30′ E, 17°–22°N) were conducted in July 1998 during the international SCS Monsoon Experiment (SCSMEX), the U.S. Navy using Airborne Expendable Bathythermograph and Conductivity-Temperature-Depth sensors (AXBT/AXCTD), and the Chinese Academy of Sciences using Acoustic Doppler Current Profilers (ADCP). The hydrographic survey included 307 AXBT and 9 AXCTD stations, distributed uniformly throughout the survey area. The ADCP survey had two sections. The velocity field inverted from the AXBT/AXCTD data and analyzed from the ADCP data confirm the existence of a low salinity, cool-core cyclonic eddy located northwest of Luzon Island (i.e., the Northwest Luzon Eddy). The radius of this eddy is approximately 150 km. The horizontal temperature gradient of the eddy increases with depth from the surface to 100 m and then decreases with depth below 100 m. The cool core was evident from the surface to 300 m depth, being 1°–2°C cooler inside the eddy than outside. The tangential velocity of the eddy is around 30–40 cm/s above 50 m and decreases with depth. At 300 m depth, it becomes less than 5 cm/s.

Triple Coordinate Transforms for Prediction of Falling Cylinder Through the Water Column

Triple coordinate systems are introduced to predict translation and orientation of falling rigid cylinder through the water column: earth-fixed coordinate (E-coordinate), cylinders main-axis following coordinate (M-coordinate), and hydrodynamic force following coordinate (F-coordinate). Use of the triple coordinate systems and the transforms among them leads to the simplification of the dynamical system. The body and buoyancy forces and their moments are easily calculated rising the E-coordinate system. The hydrodynamic forces (such as the drag and lift forces) and their moments are easily computed using the F-coordinate. The cylinders moments of gyration are simply represented using the M-coordinate Data collected from a cylinder-drop experiment at the Naval Postgraduate School swimming pool in June 2001 show great potential of using the triple coordinate transforms.

Japan Sea Thermohaline Structure and Circulation. Part II: A Variational P-Vector Method

Abstract The second part of this work investigates the seasonal variabilities of the Japan/East Sea (JES) circulation using the U.S. Navy Generalized Digital Environmental Model (GDEM) climatological temperature and salinity dataset (public domain) on a 0.5° × 0.5° grid. A variational P-vector method was developed to invert the velocity field. The GDEM for the JES was built up on historical (1930–97) 136 509 temperature and 52 572 salinity profiles. The climatological mean and seasonal variability of the current systems are well inverted, especially the Tsushima Warm Current and its bifurcation, the East Korean Warm Current (EKWC), the Japan nearshore branch, the confluence of the EKWC, and the North Korean Cold Current near the Korean coast and flows northeastward along the subpolar front, and a mesoscale anticyclonic eddy in the Ulleng/Tsushima Basin. Furthermore, this method has the capability to invert flow reasonably well across the shallow straits such as the Tsushima/Korea, Tsugaru, and Soya Strait...

Evidence of a Barrier Layer in the Sulu and Celebes Seas

Variability in the surface isothermal and mixed layers of the Sulu and Celebes Seas is examined using the conductivity‐temperature‐depth data from the Navy’s Master Oceanographic Observational Data Set (MOODS). Vertical gradient is calculated to determine isothermal layer depth with a criterion of 0.05 8 Cm 21 for temperature profiles and mixed layer depth with a criterion of 0.015 kg m24 for density profiles. When the isothermal layer depth is larger than the mixed layer depth, the barrier layer occurs. This study shows that the barrier layer occurs often in the Sulu and Celebes Seas. In the Sulu Sea, the barrier layer has seasonal variability with a minimum occurrence (38%) and a minimum thickness (3 m) in May and a maximum occurrence (94%) and a maximum thickness (36.5 m) in September. In the Celebes Sea, the barrier layer thickness changes from a maximum (49.7‐ 62.0 m) in March‐April to a minimum (9.6 m) in June. Possible mechanisms responsible for the barrier layer formation are discussed. In the Sulu Sea, the barrier layer may be formed by both rainfall and stratification; in the Celebes Sea, a rain-formed mechanism seems a major factor.