Peter C. Chu

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...

South China Sea Isopycnal-Surface Circulation

This paper investigates the seasonal variabilities of the South China Sea isopycnal-surface circulations and of the Kuroshio intrusion through the Luzon Strait using the U.S. Navy’s climatological temperature and salinity dataset (public domain) with ‰ 83 ‰8 resolution by the P-vector method. The representative pattern is a persistent basin-scale cyclonic circulation away from the surface, and a seasonally varying circulation with a weak anticylonic gyre in the summer and a strong cyclonic gyre in the winter near the surface. This pattern is consistent with a classical view of mean cyclonic circulation in large stratified lakes and semienclosed marginal seas by Emery and Csanady and with a recent numerical simulation using the navy’s Layered Ocean Model by Metzger and Hurlburt. The computed monthly volume transport through the Luzon Strait is negative (inflow) all year round with a minimum value of 213.7 Sv in February (strongest intrusion) and a maximum value of 21.4 Sv in September (weakest intrusion). The annual mean transport is 26.5 Sv (intrusion).

Wind-Driven South China Sea Deep Basin Warm-Core/Cool-Core Eddies

The formation of the South China Sea (SCS) deep basin warm-core and cool-core eddies was studied numerically using the Princeton Ocean Model (POM) with 20 km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography. Numerical integration was divided into pre-experimental and experimental stages. During the pre-experimental stage, we integrated the POM model for three years from zero velocity and April temperature and salinity climatological fields with climatological monthly mean wind stresses, restoring type surface salt and heat fluxes, and observational oceanic inflow/outflow at the open boundaries. During the experimental stage, we integrated the POM model for another 16 months under three different conditions: one control and two sensitivity runs (no-wind and no lateral transport). We take the fields of the last 12 months for analysis. The simulation under control run agrees well with earlier observational studies on the South China Sea surface thermal variabilities. In addition, the sensitivity study further confirms that the wind effect is the key factor for generation of the SCS deep basin warm/cool eddy and that the lateral boundary forcing is the major factor for the formation of the strong western boundary currents, especially along the southeast Chinese coast during both summer and winter monsoon seasons.

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.

Seasonal Variability of the Yellow Sea/East China Sea Surface Fluxes and Thermohaline Structure

We use the U.S. Navy’s Master Oceanographic Observation Data Set (MOODS) for the Yellow Sea/East China Sea (YES) to investigate the climatological water mass features and the seasonal and non-seasonal variabilities of the thermohaline structure, and use the Comprehensive Ocean-Atmosphere Data Set (COADS) from 1945 to 1989 to investigate the linkage between the fluxes (momentum, heat, and moisture) across the air-ocean interface and the formation of the water mass features. After examining the major current systems and considering the local bathymetry and water mass properties, we divide YES into five regions: East China Sea (ECS) shelf, Yellow Sea (YS) Basin, Cheju bifurcation (CB) zone, Taiwan Warm Current (TWC) region, Kuroshio Current (KC) region. The long term mean surface heat balance corresponds to a heat loss of 30 W m−2 in the ESC and CB regions, a heat loss of 65 W m−2 in the KC and TWC regions, and a heat gain of 15 W m−2 in the YS region. The surface freshwater balance is defined by precipitation minus evaporation. The annual water loss from the surface for the five subareas ranges from 1.8 to 4 cm month−1. The fresh water loss from the surface should be compensated for from the river run-off. The entire water column of the shelf region (ECS, YS, and CB) undergoes an evident seasonal thermal cycle with maximum values of temperature during summer and maximum mixed layer depths during winter. However, only the surface waters of the TWC and KC regions exhibit a seasonal thermal cycle. We also found two different relations between surface salinity and the Yangtze River run-off, namely, out-of-phase in the East China Sea shelf and in-phase in the Yellow Sea. This may confirm an earlier study that the summer fresh water discharge from the Yangtze River forms a relatively shallow, low salinity plume-like structure extending offshore on average towards the northeast.

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...

A parametric model for the Yellow Sea thermal variability

A thermal parametric model has been developed for analyzing observed regional sea temperature profiles based on a layered structure of temperature fields (mixed layer, thermocline, and deep layers). It contains three major components: (1) a first-guess parametric model, (2) high-resolution profiles interpolated from observed profiles, and (3) fitting of high-resolution profiles to the parametric model. The output of this parametric model is a set of major characteristics of each profile: sea surface temperature, mixed-layer depth, thermocline depth, thermocline temperature gradient, and deep layer stratification. Analyzing nearly 15,000 Yellow Sea historical (1950-1988) temperature profiles (conductivity-temperature-depth station, 4825; expendable bathythermograph, 3213; bathythermograph, 6965) from the Naval Oceanographic Offices Master Oceanographic Observation Data Set by this parametric model, the Yellow Sea thermal field reveals dual structure: one layer (vertically uniform) during winter and multilayer (mixed layer, thermocline, sublayer) during summer. Strong seasonal variations were also found in mixed-layer depth, thermocline depth, and thermocline strength.

Particulate air pollution in Lanzhou China

Concentrations of total suspended particles (TSP) and PM(10) in Lanzhou China have been kept high for the past two decades. Data collected during the intensive observational period from October 1999 to April 2001 show high TSP and PM(10) concentrations. Starting from November, the PM(10) pollution intensifies, and reaches mid to high alert level of air pollution, continues until April next year, and is at low alert level in the summer. In the winter and spring, the TSP concentration is 2-10 times higher than the third-level criterion of air quality (severe pollution). Effects of intrinsic factors (sources of pollution) and remote preconditions (propagation of dust storms) for severe PM(10) and TSP pollution in Lanzhou are analyzed.

Simulation of More Realistic Upper-Ocean Processes from an OGCM with a New Ocean Mixed Layer Model

A new ocean mixed layer model (OMLM) was embedded into an ocean general circulation model (OGCM) with the aim of providing an OGCM that is ideal for application to a climate model by predicting the sea surface temperature (SST) more accurately. The results from the new OMLM showed a significant improvement in the prediction of SST compared to the cases of constant vertical mixing and the vertical mixing scheme by Pacanowski and Philander. More accurate prediction of the SST from the new OMLM reduces the magnitude of the restoring term in the surface heat flux and thus provides a simulated ocean that can be coupled to the atmospheric general circulation model more naturally. The new OMLM was also shown to improve various other features of the OGCM such as the mixed layer depth and the equatorial circulation.