Su-Cheng Pai

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.

Nutrient budgets for the South China Sea basin

Abstract Varying atmospheric forcing and an elaborate geography make for a complex flow in the South China Sea (SCS). Throughout the year, the surface waters of the Kuroshio flow into the SCS, while the surface waters of the SCS flow out through the Bashi Channel. Cumulatively, there is a small (∼1 Sv) net outflow of surface water (0–350-m depth) from the SCS in the wet season, but a net inflow (∼3 Sv) in the dry season through the Bashi Channel. The differences are mainly made up by inflow and outflow of Sunda Shelf Water in the wet and dry seasons, respectively. Seawater, phosphorus, nitrogen and silicate budgets were calculated based on a box model. The results point out an intermediate water outflow (350–1350-m depth) into the West Philippine Sea (WPS) through the Bashi Channel in both the wet and dry seasons, though this, along with the nutrients it carries, is slightly larger in the dry season (2 Sv) than in the wet (1.8 Sv). More importantly, the export of nutrient-laden SCS intermediate water through the Bashi Channel subsequently upwells onto the East China Sea (ECS) shelf. The denitrification rate for shelves in the SCS is 0.11 mol N m−2 year−1, calculated by balancing the nitrogen budget. The oxygen consumption and the nutrient regeneration rates, based on the mass-balance and the one-dimensional advection–diffusion models, stand between those for the Bering Sea and the Sea of Japan.

Determination of dissolved oxygen in seawater by direct spectrophotometry of total iodine

Abstract A modified procedure has been proposed for the colorimetric determination of dissolved oxygen in seawater to improve its precision and accuracy. When a pickled sample is acidified, iodine liberated in the iodometric reaction is measured by direct spectrophotometry at 456 nm. Loss of molecular iodine by volatilization is eliminated by transferring the sample to a flow cuvette without contact with air. The method was calibrated for oxygen by spiking known amounts of potassium iodate. Precision was found at better than 0.2% r.s.d. (full scale). Evaluation of accuracy was made by comparison with calculated oxygen solubilities, which shows a relative bias of no more than 0.5% for oxic waters. The analytical throughput was much faster than that of the standard titration procedure.

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.

Formation kinetics of the pink azo dye in the determination of nitrite in natural waters

Abstract The kinetics of the reaction for the formation of the pink azo dye in the determination of nitrite in both fresh water and sea water was studied at different acidities, temperatures, and concentrations of N -1 naphthylethylenediamine (NED). It was found that the reaction is considerably faster in sea water than in fresh water, and that increase in the acidity slightly increases the molar absorptivity. A concentration of NED (4.29 × 10 −6 M) between the extremes described in the literature and 4.2 × 10 −3 M sulphanilamide are recommended for both manual determination and flow-injection analysis with respect to rapid reaction and a low reagent blank.

Prediction of nitrate concentration from two end member mixing in the southern East China Sea

Abstract Monthly hydrographic surveys were conducted along two transects across the Kuroshio frontal zone in the southern East China Sea from August 1990 to July 1991. The concentration of nitrate in the shelf water was linearly related to temperature. However, the relationship varied with time. This relationship could have resulted from two end member mixing between the nitratedevoid surface water and the nitrate-laden upwelled water. While the temperature of surface water was temporally variable, the temperature-nitrate properties of the upwelled water were almost invariant. The temperature of the surface end member ( T s ) correlated well with the mean air temperature ( T a ) at a weather station in the study area according to the equation: T s = 15.3 + 0.45 × T a The upwelled water, which was brought to the shelf by bottom intrusion of the Kuroshio, had properties similar to those of the Kuroshio subsurface water at σ θ = 25.8 with a mean temperature of 14.8°C and a nitrate concentration of 13.2 μM. With the exception of the data obtained in September 1990, the concentration of nitrate in the shelf water can be predicted from the mean air temperature with a mean error of 0.9 μM from the observed values. The anomaly observed in September 1990 might have been the result of the unsteady condition during the reversal of the direction of the monsoon. There was also a slight deviation from the predicted trend in March 1991 when there was a seaward excursion of the Continent Coastal Water which may have constituted a third end member in the mixing process. In general, this simple mixing model is applicable to the middle and outer shelf north of Taiwan.

Variability of the chemical hydrography at the frontal region between the East China Sea and the Kuroshio north-east of Taiwan

Abstract The hydrography across the frontal region between the East China Sea and the Okinawa Trough north-east of Taiwan, observed during the summer of 1985 and 1988 and the early spring of 1987, was governed mainly by mixing across the front and the topographically induced upwelling of the modified Kuroshio water in the Okinawa Trough during the periodic shelfward migration of the Kuroshio. The location of the front relative to the shelf break seemed to be temporally variable. Topographically induced upwelling was evident during the summer of 1988 and the early spring of 1987 when the front was located close to the shelf break. It might not have occurred in the summer of 1985 when the front was further offshore. The end-member composition of the upwelling water was similar in both seasons. It originated from about 300 m with a temperature and salinity of 13 °C and 34·4 psu. It was rich in nutrients and poor in oxygen with concentrations of nitrate, phosphate, silicate and oxygen of 16, 1, 18 and 160 μM respectively. This upwelling water is potentially a major source of nutrients to the East China Sea. The deep water in the Okinawa Trough at temperatures below 15 C did not participate in cross-shelf mixing. Its chemical characteristics did not change significantly from year to year.

Pre-concentration efficiency of chelex-100 resin for heavy metals in seawater : Part 1. Effects of pH and Salts on the Distribution Ratios of Heavy Metals

Abstract The chelating characteristics of Chelex-100 resin were studied for selected heavy metals in seawater medium. The results of batch equilibrium and breakthrough experiments show that the metal-chelating efficiency of the resin is lower in seawater than in freshwater. These differences are caused by the complicated speciation of heavy metals in seawater medium and by the high concentrations of magnesium and calcium present which act as competitors to heavy metal ions. The optimal pH values for column operation are strongly affected by the salt matrix. Careful choice of experimental conditions is necessary to avoid losses of cadmium and manganese from seawater. For better performance, seawater samples should be adjusted to pH 6–7 before loading onto a Chelex-100 column.

Pre-concentration efficiency of chelex-100 resin for heavy metals in seawater : Part 2. Distribution of Heavy Metals on a Chelex-100 Column and Optimization of the Column Efficiency by a Plate Simulation Method

Abstract Seawater was spiked with heavy metals and passed through a Chelex-100 column string consisting of ten minicolumns. The recoveries of metals from each minicolumn are used to study their distribution on a column. A simplified model was constructed to simulate the chelating efficiency of columns of various sizes, at various pH and flow rates. It is shown that a column containing 2 g of resin in the magnesium form with a flow rate of 4 ml min−1 is suitable for pre-concentration of Cd, Cu, Co, Mn, Ni, Pb, and Zn in seawater after adjustment to pH 6.5.

A low contamination chelex-100 technique for shipboard pre-concentration of heavy metals in seawater

Abstract An easy to handle, on-board procedure is proposed which uses a Chelex-100 ion-exchange column to pre-concentrate Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn from seawater with minimal risk of contamination. Immediately after collection, the seawater sample is poured into a polyethylene terephthalate (PET) bottle containing ammonium maleate buffer at pH 6.5. The bottle is then connected to a pre-packed Chelex-100 column set, and hung upside-down to allow the sample to flow through the column in dust-free conditions. Finally, the whole batch of columns is brought back to the land-based laboratory for further elution and analysis. A concentration factor of 250 is achieved.