Seasonal characteristics of internal tides and their responses to background currents in the Luzon Strait

The spatial-temporal characteristics of internal tides （ITs） in the southwest Luzon Strait are examined, based on 9-month mooring current records from autumn 2008 to summer 2009. The results of spectral analysis show that the ITs in diurnal and semidiurnal frequencies are prominent at the mooring site, especially for the clockwise rotary component. The diurnal ITs are mostly dominated by the first mode except for that in spring when the second mode is relatively predominant. The semidiurnal ITs display a variable multimodal structure. Moreover, an apparent difference is detected in the kinetic energy of diurnal ITs. The energy is strongest in winter, and followed by that in summer, whereas the value is smallest in spring and autumn. It is suggested that the incoherent motions are responsible for the significant seasonal variations of diurnal ITs, reflecting interaction between diurnal ITs and the varying background conditions. However, the semidiumal ITs are independent of seasonal change, whose energy is smaller and only one-third of the diurnal energy in winter. Nevertheless, the abnormal variations of semidiurnal ITs are also related to the variable background conditions. The incoherent semidiurnal constituent accounts for about 37% of the total semidiurnal tidal kinetic energy, but the diurnal tidal motions contain fewer incoherent component （22.2%）.

A snapshot on spatial and vertical distribution of bacterial communities in the eastern Indian Ocean

Besides being critical components of marine food web,microorganisms play vital roles in biogeochemical cycling of nutrients and elements in the ocean.Currently little is known about microbial population structure and their distributions in the eastern Indian Ocean.In this study,we applied molecular approaches including polymerase chain reaction-denaturant gradient gel electrophoresis（PCR-DGGE） and High-Throughput next generation sequencing to investigate bacterial 16S rRNA genes from the equatorial regions and the adjacent Bay of Bengal in the eastern Indian Ocean.In general,Bacteroidetes,Proteobacteria（mainly Alpha,and Gamma）,Actinobacteria,Cyanobacteria and Planctomycetes dominated the microbial communities.Horizontally distinct spatial distribution of major microbial groups was observed from PCR-DGGE gel image analyses.However,further detailed characterization of community structures by pyrosequencing suggested a more pronounced stratified distribution pattern：Cyanobacteria and Actinobacteria were more predominant at surface water（25m）;Bacteroidetes dominated at 25m and 150m while Proteobacteria（mainly Alphaproteobacteria） occurred more frequently at 75m water depth.With increasing water depth,the bacterial communities from different locations tended to share high similarity,indicating a niche partitioning for minor groups of bacteria recovered with high throughput sequencing approaches.This study provided the first ＂snapshot＂ on biodiversity and spatial distribution of Bacteria in water columns in the eastern Indian Ocean,and the findings further emphasized the potential functional roles of these microbes in energy and resource cycling in the eastern Indian Ocean.

The upper-ocean response to typhoons as measured at a moored acoustic Doppler current profiler

A moored acoustic Doppler current profiler(ADCP) data,satellite-derived sea surface wind data,and the chlorophyll-a concentration were used to examine the influence of typhoon events on the upper ocean in the central Luzon Strait. The data were collected between August 27 and October 6,2011. Large changes in ocean dynamics and marine life were recorded in the upper layers over the short term during the transit of each of the three violent typhoons that passed over the region during the study period. The geostrophic flow during the period of ADCP monitoring was comparable to the Ekman flow,recently shown to be prominent in the upper layer. Based on the influence of the three typhoon events that swept the Luzon Strait or traversed Luzon Island on their way to the South China Sea,we postulated a typhoon-induced upwelling around the ADCP and found that upward isothermal displacements reached 11.8–39.0 m,which was confirmed by the sea-level anomaly data recorded at the same time. This variability in the upper ocean may play an important role in biological activity,especially in offshore deep-sea regions.

Tracking the evolution processes and behaviors of mesoscale eddies in the South China Sea：a global nearest neighbor filter approach

The eddy tracking approach is developed using the global nearest neighbor filter（GNNF） to investigate the evolution processes and behaviors of mesoscale eddies in the South China Sea（SCS）. Combining the Kalman filter and optimal data association technologies, the GNNF algorithm is able to reduce pairing errors to 0.2% in tracking synthetic eddy tracks, outperforming other existing methods. A total of 4 913 eddy tracks that last more than a week are obtained by the GNNF during 1993–2012. The analysis of a growth and a decay based on 3 445 simple eddy tracks show that eddy radius, amplitude, and vorticity smoothly increase during the first half of lifetime and decline during the second half following a parabola opening downwards. The genesis of eddies mainly clusters northwest and southwest of Luzon Island whereas the dissipations concentrate the Xisha Islands where the underwater bay traps and terminates eddies. West of the Luzon Strait, northwest of Luzon Island, and southeast of Vietnam are regions where eddy splits and mergers are frequently observed. Short disappearances mainly distribute in the first two regions. Moreover, eddy splits generally result in a decrease of the radius and the amplitude whereas eddy mergers induce growing up. Eddy intensity and vorticity, on the contrary, are strengthened in the eddy splits and diminished in mergers.

Seasonal variation in the three-dimensional structures of coastal thermal front off western Guangdong

The seasonal structure and dynamic mechanism of oceanic surface thermal fronts(STFs)along the western Guangdong coast over the northern South China Sea shelf were analyzed using in situ observational data,remote sensing data,and numerical simulations.Both in situ and satellite observations show that the coastal thermal front exhibits substantial seasonal variability,being strongest in winter when it has the greatest extent and strongest sea surface temperature gradient.The winter coastal thermal front begins to appear in November and disappears after the following April.Although runoff water is more plentiful in summer,the front is weak in the western part of Guangdong.The frontal intensity has a significant positive correlation with the coastal wind speed,while the change of temperature gradient after September lags somewhat relative to the alongshore wind.The numerical simulation results accurately reflect the seasonal variation and annual cycle characteristics of the frontal structure in the simulated area.Based on vertical cross-section data,the different frontal lifecycles of the two sides of the Zhujiang(Pearl)River Estuary are analyzed.

Effects of Precipitation on Sonic Anemometer Measurements of Turbulent Fluxes in the Atmospheric Surface Layer

Effects caused by precipitation on the measurements of three-dimensional sonic anemometer are analyzed based on a field observational experiment conducted in Maoming, Guangdong Province, China. Obvious fluctuations induced by precipitation are observed for the outputs of sonic anemometer-derived temperature and wind velocity components. A technique of turbulence spectra and cospectra normalized in the framework of similarity theory is utilized to validate the measured variables and calculated fluxes. It is found that the sensitivity of sonic anemometer-derived temperature to precipitation is significant, compared with that of the wind velocity components. The spectra of wind velocity and cospectra of momentum flux resemble the standard universal shape with the slopes of the spectra and cospectra at the inertial subrange, following the-2/3 and-4/3 power law, respectively, even under the condition of heavy rain. Contaminated by precipitation, however, the spectra of temperature and cospectra of sensible heat flux do not exhibit a universal shape and have obvious frequency loss at the inertial subrange. From the physical structure and working principle of sonic anemometer, a possible explanation is proposed to describe this difference, which is found to be related to the variations of precipitation particles. Corrections for errors of sonic anemometer-derived temperature under precipitation is needed, which is still under exploration.

Comparison between MM5 simulations and satellite measurements during Typhoon Chanchu (2006) in the South China Sea

The fifth Pennsylvania State University and National Center for Atmospheric Research mesoscale model （MM5） is utilized to study the precipitation and wind speed during Typhoon Chanchu （2006）. Five model experiments with different physical parameterizations and sea surface temperature （SST） distributions are carried out. It is found that the control experiment configured with the Blakadar boundary scheme, Resiner2 moisture, the Betts-Miller cumulus scheme and daily updated SST has the most reasonable precipitation. The MRF boundary scheme tends to simulate a dryer boundary layer and stronger verticM mixing, which can greatly reduce the intensity of tropical cyclone （TC）, resulting in a smaller maximum wind speed but larger range of medium wind speed （25 30 m/s）. Constant SST through the TC cycle provides more energy from ocean surface, which could cause a significant increase in TC＇s intensity, thus resulting in the largest overestimation on rainfall and maximum wind speed. Longitudinally-uniform SST distribution before the rapid intensification could reduce TC＇s intensity and heat fluxes, which can partially compensate for the overestimation of precipitation in the control experiment.

Spring mesoscale high in the western South China Sea

A recurring spring mesoscale eddy in the western South China Sea （SCS） is studied using remote sensing data and historical in situ observations. The feature first appears east of the central Vietnam coast in Febru- ary as a high sea-level anomaly, grows rapidly to a well-developed anticyclonic eddy by March, matures in April, and decays in May. Besides the warm-core feature, it also has an inherent low-salinity property, so it is named ＂spring mesoscale high （SMH）＂. Though with clear interannual variation in terms of intensity and spatial coverage, the SMH always emerges in the region between ll0~E and l14~E and between 12~N and 16~N. The formation of SMH is ascribed to the combined effects of wind forcing and releasing of po- tential energy set up by winter monsoon. In particular, the wind-stress curl plays an important role in its development, maintenance, and dissipation.

Mesoscale oceanic eddies in the South China Sea from 1992 to 2012:evolution processes and statistical analysis

Automated identification and tracking of mesoscale ocean eddies has recently become one research hotspot in physical oceanography. Several methods have been developed and applied to survey the general kinetic and geometric characteristics of the ocean eddies in the South China Sea（SCS）. However, very few studies attempt to examine eddies＇ internal evolution processes. In this study, we reported a hybrid method to trace eddies＇ propagation in the SCS based on their internal structures, which are characterized by eddy centers, footprint borders, and composite borders. Eddy identification and tracking results were represented by a GIS-based spatiotemporal model. Information on instant states, dynamic evolution processes, and events of disappearance, reappearance, split, and mergence is stored in a GIS database. Results were validated by comparing against the ten Dongsha Cyclonic Eddies（DCEs） and the three long-lived anticyclonic eddies（ACEs） in the northern SCS, which were reported in previous literature. Our study confirmed the development of these eddies. Furthermore, we found more DCE-like and ACE-like eddies in these areas from 2005 to 2012 in our database. Spatial distribution analysis of disappearing, reappearing, splitting, and merging activities shows that eddies in the SCS tend to cluster to the northwest of Luzon Island, southwest of Luzon Strait, and around the marginal sea of Vietnam. Kuroshio intrusions and the complex sea floor topography in these areas are the possible factors that lead to these spatial clusters.