Shenn-Yu Chao

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.

Seasonal variation of the circulation in the Taiwan Strait

Abstract The Taiwan Strait is an essentially meridional channel connecting the East and South China Seas. There is often a northward current on the east side and a southward current on the west side. The source water feeding the eastern boundary current is South China Sea Water in summer and Kuroshio Branch Water in other seasons. The current on the west side carries colder and fresher China Coastal Water southward. Both currents are modulated by the annual cycle of monsoon wind forcing, which reinforces the northward current in summer but southward current in other seasons. Further, both currents are partially impeded by a bottom ridge (Changyun Rise) in the middle reaches of the strait. The combination of monsoon and topography forcing leads to the winter blocking of northward current, spring renewal of northward intrusion, minimal blocking of northward intrusion in summer, and fall emergence of China Coastal Current. A recent hydrographic data set, satellite images and a numerical model lend support to these findings.

Onset of Estuarine Plumes

Abstract The onset of estuarine plumes is numerically studied here, using a three-dimensional, primitive-equation model. The model ocean consists of a narrow estuary that is connected to an otherwise enclosed ocean basin. The basin is initially filled with saline water. Subsequently, freshwater is pumped in near the surface and the saline water is withdrawn from below at the head of the estuary. To maximize the chance of development for a baroclinic flow field, a rigid-lid and a flat bottom are assumed, and the inflow–outflow profile has no barotropic component. The plume expands in the direction of propagation of the coastally trapped waves after the freshwater release. The intrusion speed inside the estuary is consistently higher than that along the shelf. Energy is therefore accumulated near the estuary mouth, forming a bulge of anticyclonic surface flow. The far-field flow consists of a bore intrusion along the shelf. The transitional zone between the near-field and far-field flows is characterized by...

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.

A climatological description of circulation in and around the East China Sea

Abstract To provide baseline information for the biogeochemical studies, the circulation in the three-sea (Bohai, Yellow Sea and East China Sea) system is derived from climatological forcing, using a three-dimensional general circulation model. In the light of extensive literature dealing with similar subjects, the discussion is focused mostly on previously under-represented parts of the circulation, namely, seasonal variations of circulation in the surface layer, Changjiang plume dispersal, sea-level variations, and upwelling–downwelling locations and intensities. In terms of circulation, notable features include the development of China Coastal Current in fall and winter, appearance of a southward coastal jet off the west coast of Korean Peninsula in winter, and northward expansion of Taiwan Warm Current in summer. In response to the seasonally changing circulation, the Changjiang plume dispersal is primarily southward in winter and northeastward in summer. Two upwelling centers near the shelfbreak are worth noting. One is to the northeast of Taiwan and the other is to the southwest of Kyushu. Both are associated with anticyclonic meanders of the Kuroshio Current. Along the Kuroshio axis, a series of upwelling and downwelling centers develops in response to topographic forcing. East of Ryukyu Islands, the model also predicts a persistent upwelling location east of Okinawa Island.

River-forced estuarine plumes

Abstract The development, maintenance, and dissipation of river-forced estuarine plumes with and without seaward sloping bottom are studied by use of a three-dimensional, primitive-equation model. Inside the estuary, discussion is focused on how the Coriolis force induces lateral asymmetries in the circulation. Four physical processes dictate the transient as well as the quasi-steady circulation in moderately stratified estuaries: 1) upper-layer convergence during the transient spinup phase, 2) upward entrainment leading to the two-layer steady circulation pattern, 3) upward stretching of the lower-layer vortex during the spindown period, and 4) in the presence of a seaward bottom slope, the left-bounded tendency of the landward undercurrent for all phases of development. Deviations from the laterally averaged circulation pattern caused by these processes are discussed. Over the shelf, various types of plumes are defined according to the visual appearance of the surface salinity field. The four types of p...

The formation and fate of internal waves in the South China Sea

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

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.

Response of Kuroshio upwelling to the onset of the northeast monsoon in the sea north of Taiwan: Observations and a numerical simulation

A cold water anomaly, which manifests upwelling of the subsurface Kuroshio Water, has been frequently observed at the shelf break of the East China Sea to the north of Taiwan. Its response to the onset of northeast monsoon was observed during August–October 1990. The wind direction reversed in mid-September, indicating the onset of northeast monsoon. Shortly thereafter, the sea surface temperature at the center of the cold eddy showed a pulselike sudden drop, and a significant concentration of nitrate (up to 5 μM) appeared in the surface water, suggesting intensification of upwelling. Subsequently, the upper layer of the Kuroshio Water intruded onto the shelf. The general circulation model of the East China Sea previously developed by Chao was used to simulate the overall response of the East China Sea. The numerical simulation reproduced the intensification of upwelling. It also predicted extensive Kuroshio intrusion along the shelf break farther north of Taiwan as well as the temporary intensification of the northeast branch of the Yangtze River outflow. The energy source of this sudden intensification comes from the potential energy released by the Kuroshio as the isopycnals maintaining the Kuroshio rise in response to the wind change.