Temporal and spatial statistics of travelling eddy variability in the South China Sea

Ocean Dynamics - The variability across decades of years of migrating eddy activities in the South China Sea (SCS) has not yet been documented. We employ a daily global eddy-resolving (0.1°)...     

Satellite-observed variability of phytoplankton size classes associated with a cold eddy in the South China Sea

Ocean-color remote sensing has been used as a tool to detect phytoplankton size classes (PSCs). In this study, a three-component model of PSC was reparameterized using seven years of pigment measurements acquired in the South China Sea (SCS). The model was then used to infer PSC in a cyclonic eddy which was observed west of Luzon Island from SeaWiFS chlorophyll-a (chla) and sea-surface height anomaly (SSHA) products. Enhanced productivity and a shift in the PSC were observed, which were likely due to upwelling of nutrient-rich water into the euphotic zone. The supply of nutrients promoted the growth of larger cells (micro- and nanoplankton), and the PSC shifted to greater sizes. However, the picoplankton were still important and contributed ∼48% to total chla concentration. In addition, PSC time series revealed a lag period of about three weeks between maximum eddy intensity and maximum chlorophyll, which may have been related to phytoplankton growth rate and duration of eddy intensity.     

Mesoscale eddy variability in the Caribbean Sea

Ocean Dynamics - The spatial distribution, and the monthly and seasonal variability of mesoscale eddy observations derived from the AVISO eddy atlas are assessed in the Caribbean Sea during...     

Temporal and spatial evolution of a deep-reaching anticyclonic eddy in the South China Sea

The temporal and spatial evolution of a deep-reaching anticyclonic eddy(AE) is studied using a combination of satellite measurements, moored observations and ocean model reanalysis data in the South China Sea(SCS). Three evolutionary stages in eddy's lifecycle are identified from changes in eddy dynamical characteristics estimated from satellite altimetry: birth(22 days), growth(64 days), and decay(47 days). Similar patterns are also distinguished from dynamic signals in HYCOM.Further, flows reversal and upwelling of cold water below 1500 m were captured by the in-situ records when this energetic,highly nonlinear and long-lived(over 19 weeks) AE passed by our mooring position. Its detailed vertical structure is examined through temperature anomalies, vertical shear of horizontal velocities, and horizontal streamlines estimated from ocean model reanalysis data. Results from the model reveal a mesoscale AE with first-mode baroclinic structure: a bowl-shaped anticyclonic flow in the upper ocean connected to a slant-cylinder cyclonic flow at depth, with a transition layer at depths between 400 and 700 m. It is in good agreement with moored observations but showing a shallower transition depth, suggesting a slight deficiency in the model due to limited deep-sea observations. Last, we estimate eddy heat transport at different depths and stages along the AE's path based on the model data. The result reveals that pronounced heat fluxes occur during growth stage(depths 400 m),counting for 73.03% of the total value. In the decay stage, major heat transport occurs at deeper depth(depths 700–1500 m).Dynamical characteristics suggest that the vertical structure and temporal evolution of the eddy play significant roles in basinscale movement and heat transferring. Considering that mesoscale eddies are ubiquitous in the SCS, our results support a recently-proposed mechanism, whereby upper ocean flows produce changes in the deep-sea circulation, potentially influencing boundary layer dynamics. For the first time to track and link an individual AE observed by satellite altimetry and ocean model,comparisons indicate that assimilative HYCOM outputs may be useful for examining the deep ocean properties within the SCS,especially under the impact of such an intensified surface-detected eddy.     

Seasonal variability of tides in the deep northern South China Sea

Tides are the major energy source for ocean mixing, regulating the variation of oceanic circulation and sediment transport in the deep sea. Here twenty months of high-resolution current profiles, which were observed via a mooring system at a water depth of 2100 m in the northern South China Sea(SCS), are used to investigate seasonal variability in deep-sea tides.Spectral analysis shows that tides in this region are dominated by diurnal tide, and both diurnal and semidiurnal tide are vertical mode-1 dominant. Baroclinic diurnal tidal current exhibits pronounced seasonal variability, showing its kinetic energy was the strongest in summer, and the maximum depth-averaged value was up to 86.7 cm~2 s~(-2), which was about 1.5 times of that in winter and twice that in spring and autumn. In contrast, baroclinic semidiurnal tide displays no evident seasonal variability. Such seasonal variability in baroclinic tide was mainly modulated by the barotropic forcing from the Luzon Strait. On the other hand,two anticyclonic eddies and one cyclonic eddy, which originated off southwestern Taiwan in winter, crossed the mooring system.The cyclonic eddy had weak impact on current velocity in the deep sea, but the two deep-reaching anticyclonic eddies enhanced the current velocity through the full-water column by inducing strong subinertial flows. Consequently, the kinetic energy of tides was strengthened and the incoherent variance of baroclinic diurnal tide increased in winter, which contributed ～85% of the variability in diurnal tide. Meanwhile, the velocity of baroclinic diurnal tide was reduced in winter, which was attributed to the weakened stratification induced by the passage of anticyclonic eddies in the deep sea. The seasonal variability of tides in the deep northern SCS can provide a dynamic mechanism for interpreting sediment transport processes in the deep sea on different time scales.     

南海北部深海潮汐的季节性变化特征

深海潮汐是深海混合过程的主要能量来源,对深海环流变异和沉积物搬运具有重要的调控作用.文章利用深水锚系观测系统在南海北部2100m水深处开展了近两年的高分辨率海流剖面观测,用于研究深海潮汐的季节性变化特征.通过谱分析显示,南海北部深海潮汐以全日潮为主,且在观测区域的深海中,全日潮和半日潮以垂向第一模态结构为主.斜压全日潮具有显著的季节性变化特征,夏季最强,深度平均的最大动能达86.7cm2s-2,分别是冬季的1.5倍、春秋季的2倍;而斜压半日潮没有明显的季节性变化.通过对比发现斜压全日潮的季节性变化受控于吕宋海峡附近的正压潮.另外,观测期间有三个强中尺度涡经过锚系站位,其中一个气旋式中尺度涡对深海海流影响较小,而另外两个反气旋式中尺度涡经过研究海域时可影响全水深海流,激发强亚惯性流,对深海潮汐产生边缘强化效应,并使深海斜压全日潮冬季的异相成分增强,占全日潮的85%.同时,中尺度涡对深海海水混合的增强弱化了海水层结,导致冬季的斜压全日潮流速减弱,低于夏季.南海北部深海潮汐季节性变化为研究深海盆沉积物在不同时间尺度上的分布及搬运过程提供了重要的动力机制.     

Seasonal variability of the carbon export in the central South China Sea

Ocean Dynamics - The South China Sea (SCS) is strongly influenced by the East Asian monsoon system with seasonal reversal. Measurements from a 7-year continuous sediment trap located in the central...     

The role of remote wind forcing in the subinertial current variability in the central and northern parts of the South Brazil Bight

Data analysis of continental shelf currents and coastal sea level, together with the application of a semi-analytical model, are used to estimate the importance of remote wind forcing on the subinertial variability of the current in the central and northern areas of the South Brazil Bight. Results from both the data analysis and from the semi-analytical model are robust in showing subinertial variability that propagates along-shelf leaving the coast to the left in accordance with theoretical studies of Continental Shelf Waves (CSW). Both the subinertial variability observed in along-shelf currents and sea level oscillations present different propagation speeds for the narrow northern part of the SBB (~?6–7 m/s) and the wide central SBB region (~?11 m/s), those estimates being in agreement with the modeled CSW propagation speed. On the inner and middle shelf, observed along-shelf subinertial currents show higher correlation coefficients with the winds located southward and earlier in time than with the local wind at the current meter mooring position and at the time of measurement. The inclusion of the remote (located southwestward) wind forcing improves the prediction of the subinertial currents when compared to the currents forced only by the local wind, since the along-shelf-modeled currents present correlation coefficients with observed along-shelf currents up to 20% higher on the inner and middle shelf when the remote wind is included. For most of the outer shelf, on the other hand, this is not observed since usually, the correlation between the currents and the synoptic winds is not statistically significant.     

利用地震海洋学方法估算南海中尺度涡的地转流速

中尺度涡是重要的海洋学现象,它在很大程度上影响着海洋内部的能量传递过程.由于传统海洋学观测手段的固有局限性,一直以来对中尺度涡观测和研究的程度都比较低.地震海洋学的诞生和发展为海洋学观测提供了一个全新的手段.对南海的历史地震数据重新处理后,我们首次在本研究海域的地震剖面上看到了透镜状结构.它位于南海西南次海盆(~113.6°E,11.4°N),中心深度约为450 m,中心厚度约为300 m,半径约为55~65 km,具有典型的中尺度涡特征,综合解释为反气旋.我们利用地震海洋学方法估算了地转剪切,结合来自于卫星高度数据的海表面地转流速度进一步得到了绝对流速的垂向剖面.结果显示,流速的最大值约为0.7 m/s,出现在400~450 m处,对应于涡旋的中心深度;西北部分为正,东南部分为负,整体呈现出顺时针的转动方向,说明了它是一个反气旋结构.     

Modelling of waves in the Irish Sea: effects of oceanic wave and wind forcing

This study uses a series of scenarios of wave (boundary) and wind (local) forcing to examine the sensitivity and to quantify the effects associated with nesting ProWAM and POLCOMS models for downscaling predictions of waves in the Irish Sea. The model results show that the response of the modelling system to the wave and wind forcing during the downscaling varies widely depending on wind conditions. Generally, the wave forcing has a greater effect on overall wave prediction in most of the Irish Sea, except for the eastern Irish Sea/Liverpool Bay. The study also suggests detailed look-up tables at specific locations to quantify the impacts of the different forcing scenarios over the Irish Sea, which can be readily extended to the location on any other sites.     

Enhanced turbulent mixing induced by strong wind on the South China Sea shelf

Integrated observations were made on the South China Sea shelf at 19°37’ N, 112°04’ E, under strong wind and heavy raining weather conditions in August 2005. Current data were obtained using a moored 150-kHz Acoustic Doppler Current Profiler, turbulent kinetic energy dissipation rate were measured with TurboMapII, and temperature was recorded by thermistor chains. Both the mixed layer thickness and the corresponding mean dissipation rate increased after the strong wind bursts. Average surface mixed layer thickness was 13.4 m pre-wind and 22.4 m post-wind, and the average turbulent dissipation rate in the mixed layer pre-wind and post-wind were 4.26 × 10^?7 and 1.09 × 10^?6 Wkg^?1, respectively. The post-wind dissipation rate was 2.5 times larger than the pre-wind dissipation rate in the interior layer and four times larger in the intermediate water column. Spectra and vertical mode analysis revealed that near-inertial motion post-wind, especially with high modes, was strengthened and propagated downward toward the intermediate layer. The downward group velocity of near-inertial current was about 8.1 × 10^?5 ms^?1 during the strong wind bursts. The mean percentage of wind work transmitted into the intermediate layer is about 4.2 %. The ratio of post-wind high-mode energy to total horizontal kinetic energy increased below the surface mixed layer, which would have caused instabilities and result in turbulent mixing. Based on these data, we discuss a previous parameterization that relates dissipation rate, stratification, and shear variance calculated from baroclinic currents with high modes (higher than mode 1) which concentrate a large fraction of energy.     

Response of a summertime anticyclonic eddy to wind forcing in Funka Bay,Hokkaido, Japan

Flow fluctuations inside an anticyclonic eddy in summertime Funka Bay were examined using three moorings and hydrographic data. The flow pattern above a sharp pycnocline with a concave isopycnal structure during the mooring period was characterized by high mean kinetic energy and relatively low eddy kinetic energy. The ratios of eddy to mean kinetic energy were equal to or less than one, and the mean flow field and isopycnal structure indicated the existence of a stable anticyclonic eddy above the sharp pycnocline under approximate geostrophic balance. Larger flow fluctuations with periods longer than 7 days were dominant inside the eddy. The low-frequency flow fluctuations are accompanied by north to northeastward movement of the eddy with deepening of the pycnocline and spin-up of the anticyclonic circulation. The wind field over Funka Bay is characterized by bay-scale wind fluctuations. The bay-scale winds are greatly influenced by the land topography around Funka Bay, resulting in non-uniform structure with significant wind stress curl. The bay-scale wind fluctuations are termed “locally modified wind” in the present study. The locally modified wind has a negative (positive) wind stress curl in the central–northeastern (southwestern) region of Funka Bay. The north to northeastward movement of the eddy is caused by horizontal non-uniform supply of vorticity from the locally modified wind forcing by the Ekman pumping process. Through this process, the anticyclonic circulation is enhanced (weakened) in the central–northeastern (southwestern) part of the eddy, resulting in the eddy moving north to northeastward with the pycnocline deepening and spin-up of the anticyclonic circulation. The locally modified wind forcing induces low-frequency flow fluctuations through the movement of the eddy in summertime Funka Bay.     

Responses of foraminiferal isotopic variations at ODP Site 1143 in the southern South China Sea to orbital forcing

1 Astronomical theory One of the greatest achievements of the EarthSciences in the 20th century is the establishment of theMilankovitch theory or the astronomical theory[1]. AsImbrie pointed out[2], the astronomical theory can bedescribed as a simple system model. The input to thesystem can be defined as changes in the geometry ofthe Earth’s orbit or the seasonal and latitudinal distri-bution of incoming solar radiation, whereas the outputto the system can be defined as one or more calc…     

Responses of foraminiferal isotopic variations at ODP Site 1143 in the southern South China Sea to orbital forcing

The foraminiferal δ ¹⁸O and δ ¹³C records for the past 5 Ma at ODP Site 1143 reveal the linear responses of the Plio-Pleistocene climatic changes in the southern South China Sea to orbital forcing at the obliquity and precession bands. The phase of the δ ¹⁸O variations with the orbital forcing is opposite to that of the δ ¹³C, which may be caused by the frequent El Niño events from the equatorial Pacific. The amplification of the Northern Hemisphere Ice Sheet at ñ3.3 Ma probably affected the development of the 100-ka climatic cycles. Its further spreading may spur the 100-ka climatic cycle to become the dominant cycle in the late Pleistocene. The “Mid- Pleistocene Transition” event has localized influence on the isotopic variations in the southern South China Sea. The foraminiferal δ ¹³C records for the past 5 Ma at Site 1143 are highly coherent with the orbital forcing at the long eccentricity band, and lead the δ ¹⁸O records at the shorter eccentricity band, highlighting the importance of the carbon cycle in the global climate change.     

Impact of multiple tidal forcing on the simulation of the M2 internal tides in the northern South China Sea

Ocean Dynamics - Previous studies have indicated that internal tides (ITs) in the South China Sea (SCS) are dominated by the M2, K1, and O1, among which intense nonlinear interaction occurs. In...     

Geographical variation and controlling mechanism of eddy-induced vertical temperature anomalies and eddy available potential energy in the South China Sea

Ocean Dynamics - Using observational temperature profiles, this study reveals that eddy-induced oceanic vertical temperature anomalies are geographically varying in the South China Sea (SCS). The...     

Numerical study of seasonal circulation and variability over the inner shelf of the northern South China Sea

This study examines seasonal circulation, hydrography, and associated spatial variability over the inner shelf of the northern South China Sea (NSCS) using a nested-grid coastal ocean circulation model. The model external forcing consists of tides, atmospheric forcing, and open boundary conditions based on the global ocean circulation and hydrography reanalysis produced by the Hybrid Coordinate Ocean model. Five numerical experiments are conducted with different combinations of external forcing functions to examine main physical processes affecting the seasonal circulation in the study region. Model results demonstrate that the monthly mean circulation in the study region features the Guangdong Coastal Current (GCC) over coastal waters and the South China Sea Warm Current (SCSWC) in the offshore deep waters. The GCC produced by the model flows nearly southwestward in winter months and northwestward in summer months, which agrees with previous studies. The SCSWC flows roughly northeastward and is well defined in summer months. In winter months, by comparison, the SCSWC is superseded by the southwestward strong wind-driven currents. Analysis of model results in five different experiments demonstrates that the monthly mean circulation over coastal and inner shelf waters of the NSCS can be approximated by barotropic currents forced by the southwestward monsoon winds in winter months. In summer months, by comparison, the monthly mean circulation in the study region is affected significantly by baroclinic dynamics associated with freshwater runoff from the Pearl River and advection of warm and saline waters carried by the SCSWC over the NSCS.