Nutrient and chlorophyll a anomaly in red-tide periods of 2003-2008 in Sishili Bay, China

Sishili Bay is the most important aquiculture and tourism area for the city of Yantai, China; however, red tides occurred frequently and have caused huge economic losses in this bay in recent years. To gain a better understanding of the local ecological environments in the bay, we conducted this research between 2003 and 2008 to analyze variations in nutrients and chlorophyll （chl-a） during high frequency red tide period （May to September）. The results show that the chl-a concentration increased from 2.70 in 2003 to 7.26 mg/m3 in 2008, while the concentration of total inorganic nitrogen （TIN） and silicate （SiO3-Si） increased lineally from 5.18 and 1.45 pmol/L in 2003 to 18.57 and 9.52 pmol/L in 2008, respectively, and the annual phosphate （PO4-P） varied between 0.15 and 0.46 μmol/L. Special attention was given to a red tide in August 2007 occurred when water temperature was high and nutrient concentrations increased sharply because of a heavy rainfall. Overall, the results show the P limitation in Sishili Bay, and reveal that red tides were caused by eutrophication from terrestrial inputs and local warm weather, particularly during rainy periods. Therefore, to control red tide, greater efforts should be made to reduce sewage discharges into Sishili Bay, particularly during rainfall seasons.

Evaluation of an ocean data assimilation system for Chinese marginal seas with a focus on the South China Sea

Data assimilation is a powerful tool to improve ocean forecasting by reducing uncertainties in forecast initial conditions.Recently,an ocean data assimilation system based on the ensemble optimal interpolation(EnOI) scheme and HYbrid Coordinate Ocean Model(HYCOM) for marginal seas around China was developed.This system can assimilate both satellite observations of sea surface temperature(SST) and along-track sea level anomaly(SLA) data.The purpose of this study was to evaluate the performance of the system.Two experiments were performed,which spanned a 3-year period from January 1,2004 to December 30,2006,with and without data assimilation.The data assimilation results were promising,with a positive impact on the modeled fields.The SST and SLA were clearly improved in terms of bias and root mean square error over the whole domain.In addition,the assimilations provided improvements in some regions to the surface field where mesoscale processes are not well simulated by the model.Comparisons with surface drifter trajectories showed that assimilated SST and SLA also better represent surface currents,with drifter trajectories fitting better to the contours of SLA field than that without assimilation.The forecasting capacity of this assimilation system was also evaluated through a case study of a birth-and-death process of an anticyclone eddy in the Northern South China Sea(NSCS),in which the anticyclone eddy was successfully hindcasted by the assimilation system.This study suggests the data assimilation system gives reasonable descriptions of the near-surface ocean state and can be applied to forecast mesoscale ocean processes in the marginal seas around China.

Variability of surface circulation in the South China Sea from satellite altimeter data

11-year satellite altimeter sea surface height (SSH) anomaly data from January 1993 to December 2003 are used to present the dominant spatial patterns and temporal variations of the South China Sea (SCS) surface circulation through Empirical Orthogonal Function (EOF) analysis. The first three EOF modes show the obvious seasonal variations of SSH in the SCS. EOF mode one is generally characterized by a basin-wide circulation. Mode two describes the double-cell basin scale circulation structure. The two cells were located off west of the Luzon Island and southeast of Vietnam, respectively. EOF mode three presents the mesoscale eddy structure in the western SCS, which develops into a strong cyclonic eddy rapidly from July to September. EOF mode one and mode three are also embedded with interannual signals, indicating that the SCS surface circulation variation is influenced by El Ni o events prominently. The strong El Ni o of 1997/98 obviously changed the SCS circulation structure. This study also shows that there existed a series of mesoscale eddies in the western SCS, and their temporal variation indicates intra-seasonal and in-terannual signals.

Spring upper warm water of the Nansha Islands sea area in the South China Sea and the numerical study on its dynamic mechanism

According to the satellite remote sensing monthly mean sea surface temperature data and in situ observational Conductivity-Temperature-Depth data, it is shown that in spring, at the upper layer to the west of Palawan Island, there exists a relatively weak warm water tongue which is distinctly different from the cold water southeast of the Balabac Strait. The relative temperature difference between the warm and cold water reduces gradually from winter to spring. P-vector method is employed to calculate the current field based on the in situ observational data, which shows that the warm water is within an anti-cyclonic meander. Based on the remote sensing wind stress during the observational period, a coupled single-layer/two-layer model is employed to study the dynamic mechanism of this anticyclonic meander current field corresponding to the warm water tongue. According to the numerical results, it is suggested that this anticyclonic meander could be mainly the residue of the winter anticyclonic eddy, rather than formed by the inflow water from the Sulu Sea via the Balabac Strait.

The assimilation experimentin the southwestern South China Sea in summer 2000

We performed an assimilation experiment of the southern South China Sea in summer 2000, and the altimeter data were assimilated into Princeton Ocean Model (POM). The result was evaluated with the in situ data. There were obvious improvements of the currents in the southern South China Sea, and the modeling uncertainty coming from the error of wind stress forcing was reduced. This experiment provides us a new idea about improving the modeling of the ocean currents.

Long-range correlations in remotely sensed chlorophyll in the South China Sea

A 8-year time series of 8-day Seaviewing Wide Field-of-view Sensor (SeaWiFS) data, which spans from Oct 1997 to Oct 2005, was used to study the temporal correlations and scaling behaviour of ocean chlorophyll fluctuations in the South China Sea (SCS) by means of detrended fluctuation analysis (DFA). Long-range correlations in chlorophyll fluctuations were detected in almost all the SCS. The scaling exponents vary over a wide range from 0.5 to 1.14, with an average value of 0.79. High values are found in the upwelling regions, such as the northwest of Luzon and the north of Sunda Shelf. Low values occur in the southwest of Luzon, the east of Hainan Island and a majority of the southern SCS. This spatial pattern is considerably different from that of the scaling exponents of the sea surface temperature (SST) time series. It is also demonstrated that SST exhibits more persistence than chlorophyll in almost all the SCS.