Ping-Tung Shaw

Monsoon-forced chlorophyll distribution and primary production in the South China Sea: Observations and a numerical study

Although the South China Sea (SCS) exchanges water constantly with the western Philippine Sea, its nutricline is much shallower and its chlorophyll level in surface waters is twice as high. Analysis of CZCS-SeaWiFS data and shipboard data reveals a strong seasonality of chlorophyll in the SCS in three upwelling regions. A three-dimensional numerical model with coupled physics and biogeochemistry is developed to study the effect of monsoonal forcing on nutrient upwelling and phytoplankton growth in the SCS. The model has a horizontal resolution of 0.41 in the domain 2–24.81N and 99–124.61E and 21 layers in the vertical. The circulation is driven by monthly climatological winds. The nitrogen-based ecosystem model has four compartments: dissolved inorganic nitrogen (DIN), phytoplankton, zooplankton and detritus. The chlorophyll-to-phytoplankton ratio depends on light and DIN availability. The biological equations and parameters are taken from previous modeling studies of the Sargasso Sea. The model simulates the nitrate profile, the strong subsurface chlorophyll maximum, and the primary production in the central basin with reasonable success. It also generates intense chlorophyll patches in the monsoon-driven upwelling regions northwest of Luzon and north of the Sunda Shelf in winter and off the east coast of Vietnam in summer. The results are in reasonable agreement with shipboard observations and CZCS-SeaWiFS data. The primary production derived from SeaWiFS data shows a strong peak in winter and weak peak in summer with an annual mean of 354 mg C m � 2 d � 1 for the whole basin. The modeled primary production displays seasonal variation resembling the trend derived from SeaWiFS data, but the magnitude (280 mg C m � 2 d � 1 ) is smaller by 20%. The model also predicts an export fraction of 12% from the primary production in the euphotic zone. r 2002 Elsevier Science Ltd. All rights reserved.

Deep water ventilation in the South China Sea

Abstract As the western Pacific intermediate water funnels into the deep basin of the South China Sea through the Luzon Strait, how does it eventually upwell? This question is examined by releasing a passive tracer, which is uniformly distributed in the deep basin initially, in a three-dimensional, climatology-driven circulation model. Subsequent advection and diffusion of the tracer are studied in relation to the circulation in the deep basin. Two primary regions of deep upwelling responsible for deep water renewal are identified: southwest of Taiwan and off Vietnam. The former is associated with inflow of water at the sill depth of the Luzon Strait, while the latter is induced by orographic lifting of currents over the continental margin. Upwelling off Vietnam begins in August near a topographic bump at 16°N, 114°E, expands southwestward until December, and is weakened by downward motion during the rest of the year. Simultaneous occurrence of deep downwelling off Luzon and Palawan with deep upwelling off Vietnam suggests that the upward tracer dispersion is modulated by basin-wide circulation at mid-depths. At shallow depths, upwelling is present off Vietnam in summer and off northwest Luzon in winter, and shelf break upwelling appears on the edge of Sunda Shelf from October to December, when the southward coastal jet impinges on the shelf. However, these shallow upwelling processes have little effect on the deep water renewal. These findings are supported by climatological data.

The seasonal variation of the intrusion of the Philippine sea water into the South China Sea

The seasonal variation of the intrusion of the Philippine Sea Water into the South China Sea was studied by analyzing the historical hydrographic station data in the northern South China Sea and the Philippine Sea. Water masses at 150, 200, and 250 m were classified by discriminant analysis according to their temperature-salinity characteristics. At each depth, most water in the study region was classified into two groups representing the Philippine Sea Water and the South China Sea Water, respectively. The geographic distribution of water masses in the South China Sea shows that the Philippine Sea Water was present along the continental margin south of China between October and January. A westward current in the northern South China Sea in winter was inferred from the distribution of the intrusion water.

Sea surface height variations in the South China Sea from satellite altimetry

Abstract Sea surface elevation in the South China Sea is examined in the Topex/Poseidon altimeter data from 1992 to 1995. Sea level anomalies are smoothed along satellite tracks and in time with tidal errors reduced by harmonic analysis. The smoothed data are sampled every ten days with an along-track separation of about 40 km. The data reveal significant annual variations in sea level. In winter, low sea level is over the entire deep basin with two local lows centred off Luzon and the Sunda Shelf. In summer, sea level is high off Luzon and off the Sunda Shelf, and a low off Vietnam separates the two highs. The boundary between the Vietnam low and Sunda high coincides with the location of a jet leaving the coast of Vietnam described in earlier studies. Principal component analysis shows that the sea level variation consists mainly of two modes, corresponding well to the first two modes of the wind stress curl. Mode 1 represents the oscillation in the southern basin and shows little inter-annual variation. The mode 2 oscillation is weak in the southern basin and is strongest off central Vietnam. During the winters of 1992–1993 and 1994–1995 and the following summers, the wind stress curl is weak, and the mode 2 sea level variation in the northern basin is reduced, resulting in weaker winter and summer gyres. Weakening of the Vietnam low in summer implies diminishing of the eastward jet leaving the coast of Vietnam. The results are consistent with model simulations.

Winter upwelling off Luzon in the northeastern South China Sea

A major upwelling region centered about 100 km offshore between 16° and 19°N off northwest Luzon in the South China Sea is revealed in the distributions of temperature, salinity, and dissolved oxygen concentration during a cruise in December 1990. Upward movement of water low in dissolved oxygen can be identified to a depth of 300 m. Climatological distributions of temperature and upper ocean heat content and tracer distribution obtained from a recent numerical experiment lend support to the existence of this upwelling region. Results from a general circulation model of the South China Sea further show that upwelling off Luzon occurs between October and January, in agreement with the climatological data. Current structures obtained from a simple two-layer model and the general circulation model both suggest that this upwelling is not driven by local winds but contains a large, remotely forced component arising from the basin circulation. The upwelling is sustained by offshore Ekman drift from above and a converging northward undercurrent from below. The would-be downwind coastal jet in the surface layer is largely offset by currents associated with the winter circulation gyre. Upwelled water with high nutrient content spreads southwestward in January and could be an important nutrient source for the surface water in the interior South China Sea in spring.

El Niño modulation of the South China Sea circulation

Abstract The South China Sea circulation during the 1982–1983 ENSO (El Nino and Southern Oscillation) event was examined through a combined effort of data analysis and numerical modelling. Monthly wind stress and sea surface temperature fields derived from COADS (Comprehensive Ocean-Atmosphere Data Set) were used to drive a three-dimensional primitive-equation ocean model with a free surface. The mean and anomalous circulation fields during 1981–1984 were derived from the model and climatological data. The 1982–1983 El Nino event in the South China Sea began with increased evaporative cooling in late 1982, followed by a weak northeast monsoon in the winter of 1982–1983 and a weak southwest monsoon in the following summer. The surface waters showed persistently higher-than-normal heat content from late 1982 to late 1983. The increased heat content could not be explained by a decrease in latent heat flux. Instead, the surface circulation, which was weaker than normal, played a major role in the increase in sea surface temperature. During maximum warming in October-November 1982, the strengths of both upwelling in the central basin and downwelling around the perimeter of the South China Sea were reduced, resulting in weaker vertical advection of heat and warming of surface waters. Cessation of the year-long warming began with the late arrival of the northeast monsoon in October 1983. The record-strength northeast monsoon in November produced anomalous upwelling in the central basin and anomalous downwelling around the perimeter of the marginal sea below the mixed layer. The abnormal vertical motions accelerated vertical advection of heat and returned the circulation and heat flux to normal conditions by December. A similar sequence of events appeared during 1986–1987 with minor variations, suggesting that the scenario may be typical for most ENSO occurrences.

Circulation and biogeochemical processes in the East China Sea and the vicinity of Taiwan: an overview and a brief synthesis

The East China Sea shelf (including the Yellow Sea and the Bohai Sea) is a very challenging system for hydrodynamic and biogeochemical studies due to its complicated physical and chemical forcing. It receives much attention because of its capacity for absorbing atmospheric CO2 in spite of large riverine fluxes of terrigenous carbon. This volume reports field observations and modeling studies during the Kuroshio Edge Exchange Processes and ensuing projects, which are a part of the continental margins study in the Joint Global Ocean Flux Study. A 3-D numerical model has been developed to simulate the climatological circulation in the East China Sea. The model result is supported by observations in the seas around Taiwan. The significance of inflow from the Taiwan Strait is emphasized. Geochemical tracers prove useful in understanding the water and material transport. Biogeochemical studies suggest very efficient recycling of organic carbon by bacterial and protozoan consumption in the shelf water, but a finite amount of particulate organic carbon with a significant terrigenous fraction is exported from the shelf. The fine-grained sediments in the inner shelf appear to be an important source of organic carbon for export. Future studies are needed to improve our understanding of key physical and biogeochemcial processes, to develop coupled physical–biogeochemical models, and to catch and survey the elusive spring algal bloom. A tantalizing goal of our ongoing effort is to document or even to predict future changes in the East China Sea shelf caused by the operation of the Three-Gorge Dam, which is under construction in the middle reach of the Yangtze River.

Seasonal and Interannual Variations in the Velocity Field of the South China Sea

A three-dimensional numerical model is used to simulate sea level and velocity variations in the South China Sea for 1992–1995. The model is driven by daily wind and daily sea surface temperature fields derived from the NCEP/NCAR 40-year reanalysis project. The four-year model outputs are analyzed using time-domain Empirical Orthogonal Functions (EOF). Spatial and temporal variations of the first two modes from the simulation compare favorably with those derived from satellite altimetry. Mode 1, which is associated with a southern gyre, shows symmetric seasonal reversal. Mode 2, which contributes to a northern gyre, is responsible for the asymmetric seasonal and interannual variations. In winter, the southern and northern cyclonic gyres combine into a strong basin-wide cyclonic gyre. In summer, a cyclonic northern gyre and an anticyclonic southern gyre form a dipole with a jet leaving the coast of Vietnam. Interannual variations are particularly noticeable during El Niño. The winter gyre is generally weakened and confined to the southern basin, and the summer dipole structure does not form. Vertical motions weaken accordingly with the basin-wide circulation. Variations of the wind stress curl in the first two EOF modes coincide with those of the model-derived sea level and horizontal velocities. The mode 1 wind stress curl, significant in the southern basin, coincides with the reversal of the southern gyre. The mode 2 curl, large in the central basin, is responsible for the asymmetry in the winter and summer gyres. Lack of the mode 2 contribution during El Niño events weakens the circulation. The agreement indicates that changes in the wind stress curl contribute to the seasonal and interannual variations in the South China Sea.

Assimilating Altimetric Data into a South China Sea Model

Sea surface heights from the TOPEX/Poseidon altimeter are assimilated into a three-dimensional primitive equation model to derive the circulation in the South China Sea. With data assimilation the model resolves not only the basin-wide circulation but also a dipole off Vietnam and a low/high feature near the Luzon Strait. Mesoscale features are missing in the simulation without data assimilation because of poor resolution in the wind field and inadequate knowledge of the transport through the Luzon Strait. Compared to the case without data assimilation, data assimilation reduces the root mean square error between the simulated and observed sea surface heights by a factor of 2–3. Circulation derived from data assimilation under climatological conditions is contrasted with that during El Nino. In the normal winter of 1993–1994, flow at 50 m depth is strong and cyclonic. Flow at 900 m depth is cyclonic as well. The deep cyclone persists into the following summer. During the 1994–1995 El Nino winter, features in the flow field at 50 m depth either weaken or disappear, and circulation at 900 m depth is anticyclonic. In the summer of 1995 the dipole and the eastward jet off Vietnam at 50 m depth are missing, and the anticyclonic circulation at 900 m depth persists. Temperature at 65 m shows significant warming from fall 1994 to summer 1995. A weakened flow field and warming in the upper ocean are consistent with findings from earlier El Nino events.

Wind relaxation as a possible cause of the South China Sea Warm Current

Winter appearance of a northeastward warm current off the southern coast of China against gale force winds is well documented but lacks a plausible explanation. Relaxation of northeasterly winds is envisaged here as a possible cause of the South China Sea Warm Current in winter. A three-dimensional circulation model for the South China Sea is first driven to equilibrium by climatological forcings. Thereafter, wind forcing is relaxed from the 15th day of each month for 9 days. In winterlike months from December to April, the wind relaxation invariably triggers a northeastward current of which the location and alongshore span are comparable to that of the observed warm current. This current is driven by the pressure gradient along the northwestern boundary of the South China Sea, sea level being high to the southwest and low to the northeast. The sea level gradient is built up by the monsoon-driven southwestward coastal current along the northwestern boundary and, after wind relaxes, triggers a return current and a sea level drop that expand southwestward from the southern coast of China to the east coast of Vietnam. The current is initially barotropic, becoming increasingly baroclinic in time as warm waters from the south are advected northeastward. The model also suggests that the sea level gradient is present in most of the months of the year, but is not as dramatic as in winter to trigger fundamental changes in the circulation of the South China Sea.