The seasonal variability of an air-sea heat flux in the northern South China Sea

The seasonal variabilities of a latent-heat flux (LHF), a sensible-heat flux (SHF) and net surface heat flux are examined in the northern South China Sea (NSCS), including their spatial characteristics, using the in situ data collected by ship from 2006 to 2007. The spatial distribution of LHF in the NSCS is mostly controlled by wind in summer and autumn owing to the lower vertical gradient of air humidity, but is influenced by both wind and near-surface air humidity vertical gradient in spring and winter. The largest area-averaged LHF is in autumn, with the value of 197.25 W/m 2 , followed by that in winter; the third and the forth are in summer and spring, respectively. The net heat flux is positive in spring and summer, so the NSCS absorbs heat; and the solar shortwave radiation plays the most important role in the surface heat budget. In autumn and winter, the net heat flux is negative in most of the observation region, so the NSCS loses heat; and the LHF plays the most important role in the surface heat budget. The net heating is mainly a result of the offsetting between heating due to the shortwave radiation and cooling due to the LHF and the upward (outgoing) long wave radiation, since the role of SHF is negligible. The ratio of the magnitudes of the three terms (shortwave radiation to LHF to long-wave radiation) averaged over the entire year is roughly 3:2:1, and the role of SHF is the smallest.     

南海声速跃层分类及其季节变化

南海海底地势复杂,海域内跃层分布有其特殊性,研究声速跃层分布形态对海上军事活动和海洋战场建设有重要影响。利用50 a(1958~2007年)SODA(simple ocean data assimilation)月平均资料,采用垂直梯度法分别求得3种类型声速跃层的特征值。结果表明:冬季,3种类型声速跃层范围全年最小,厚度最薄,强度最弱;夏季,主跃层、双跃层范围全年最大,厚度最厚,深度大都较浅,强度最强。春秋季跃层的示性特征介于冬夏之间,秋季比春季变化明显。     

基于多源融合卫星高度计观测数据的南海海浪有效波高的季节变化研究

The seasonal variability of the significant wave height(SWH) in the South China Sea(SCS) is investigated using the most up-to-date gridded daily altimeter data for the period of September 2009 to August 2015. The results indicate that the SWH shows a uniform seasonal variation in the whole SCS, with its maxima occurring in December/January and minima in May. Throughout the year, the SWH in the SCS is the largest around Luzon Strait(LS) and then gradually decreases southward across the basin. The surface wind speed has a similar seasonal variation, but with different spatial distributions in most months of the year. Further analysis indicates that the observed SWH variations are dominated by swell. The wind sea height, however, is much smaller. It is the the largest in two regions southwest of Taiwan Island and southeast of Vietnam Coast during the northeasterly monsoon, while the largest in the central/southern SCS during the southwesterly monsoon. The extreme wave condition also experiences a significant seasonal variation. In most regions of the northern and central SCS, the maxima of the 99 th percentile SWH that are larger than the SWH theoretically calculated with the wind speed for the fully developed seas mainly appear in August–November, closely related to strong tropical cyclone activities.Compared with previous studies, it is also implied that the wave climate in the Pacific Ocean plays an important role in the wave climate variations in the SCS.     

High-resolution simulation of upper-ocean submesoscale variability in the South China Sea: Spatial and seasonal dynamical regimes

Submesoscale processes in marginal seas usually have complex generating mechanisms, highly dependent on the local background flow and forcing. This numerical study investigates the spatial and seasonal differences of submesoscale activities in the upper ocean of the South China Sea (SCS) and the different dynamical regimes for sub-regions. The spatial and seasonal variations of vertical vorticity, horizontal convergence, lateral buoyancy gradient, and strain rate are analyzed to compare the submesoscale phenomenon within four sub-regions, the northern region near the Luzon Strait (R1), the middle ocean basin (R2), the western SCS (R3), and the southern SCS (R4). The results suggest that the SCS submesoscale processes are highly heterogeneous in space, with different seasonalities in each sub-region. The submesoscale activities in the northern sub-regions (R1, R2) are active in winter but weak in summer, while there appears an almost seasonal anti-phase in the western region (R3) compared to R1 and R2. Interestingly, no clear seasonality of submesoscale features is shown in the southern region (R4). Further analysis of Ertel potential vorticity reveals different generating mechanisms of submesoscale processes in different sub-regions. Correlation analyses also show the vertical extent of vertical velocity and the role of monsoon in generating submesoscale activities in the upper ocean of sub-regions. All these results suggest that the sub-regions have different regimes for submesoscale processes, e.g., Kuroshio intrusion (R1), monsoon modulation (R2), frontal effects (R3), topography wakes (R4).     

ENSO-induced interannual variability in the southeastern South China Sea

In this study, El Niño Southern Oscillation (ENSO)-induced interannual variability in the South China Sea (SCS) is documented using outputs from an eddy-resolving data-assimilating model. It is suggested that during an El Niño (La Niña) event, off-equatorial upwelling (downwelling) Rossby waves induced by Pacific equatorial wind anomalies impinge on the Philippine Islands and excite upwelling (downwelling) coastal Kelvin waves that propagate northward along the west coast of the Philippines after entering the SCS through the Mindoro Strait. The coastal Kelvin waves may then induce negative (positive) sea level anomalies in the southeastern SCS and larger (smaller) volume transport through the Mindoro and Luzon Straits during an El Niño (La Niña) event.     

Spatial-temporal variability of submesoscale currents in the South China Sea

Spatial and seasonal variabilities of submesoscale currents in the northeastern South China Sea are investigated by employing a numerical simulation with a horizontal resolution of 1km. The results suggest that submesoscale currents are widespread in the surface mixed layer mainly due to the mixed layer instabilities and frontogenesis. In horizontal, submesoscale currents are generally more active in the north than those in the south, since that active eddies, especially cyclonic eddies, mainly occur in the northern area. Specifically, submesoscale currents are highly intensified in the east of Dongsha Island and south of Taiwan Island. In temporal sense, submesoscale currents are more active in winter than those in summer, since the mixed layer is thicker and more unstable in the winter. The parameterization developed by Fox-Kemper et al. is examined in terms of vertical velocity, and the results suggest that it could reproduce the vertical velocity if mixed layer instability dominates there. This study improves our understanding of the submesoscale dynamics in the South China Sea.     

veKdv方程的若干系数在南海北部的地理分布特征及其季节变化分析

应用中国近海及邻近海域海洋再分析资料(简称CORA)研究南海北部第一模态内波场运动学参数的地理分布特征及其季节变化。首先分析了Brunt-Väisälä频率的统计特征;其次,基于弱非线性变系数扩展Kortewed-de Vries (veKdv)方程模型,计算了它的输入系数,即线性长波相速度,平方和立方非线性系数和频散系数,这些参数可用于定性评估内孤立波传播可能的极性,内孤立波的形态,幅度限制以及传播速度等。分析结果表明,南海北部季节性密度跃层从2月开始出现,最大浮力频率约在20 m。它在6—7月达到最强,自8月开始减弱,在10月消退。另一密度跃层出现在8—11月,最大浮力频率约在80 m,冬季大致在120 m。季节性密度跃层在4—9月十分明显,而8—10月双跃层现象显著,冬季仅出现较弱的第二密度跃层。在1—3月和10—12月海盆深水区最大Brunt-Väisälä频率值要大于陆架浅水区;而在5—9月情况则相反。Brunt-Väisälä频率最大值所在深度随季节变化显著,冬季最深,6—7月则最浅。计算的线性内波相速度、频散系数和幅度放大因子的空间特征主要取决于地形变化;平方(立方)非线性系数与地形关系较小,随季节变化明显,它们主要取决于局地海洋环境特征。通过分析veKdv方程的系数特征,解释了为何在夏季南海北部最容易观测到大振幅内孤立波和在吕宋海峡以东海域难以观测到孤立波的原因。     

南海盐度锋的年际变化特征分析

为了探究南海表层5 m盐度锋的年际变化特征,应用50年(1958—2007年)的SODA月平均资料,采用均方差和经验正交函数进行研究。结果表明,盐度锋年际变化显著的区域集中在北部湾、海南岛以东、南海北部、吕宋海峡、东马来西亚西北部和越南最南端;第一模态和第二模态时间系数变化趋势均表明南海盐度锋强度在逐渐减小;前者呈单极子型空间分布,后者空间分布型则为偶极子型。第一模态时间系数功率谱分析表明该模态存在3—5年和7—16年的周期,Morlet小波分析显示1965—2000年该模态主要存在4年、9年和12年左右的周期。第一模态确立的3个负位相中心区域(海南岛以东、吕宋海峡和越南最南端)的均值变化趋势一致,强度都在逐渐增大,具有同位相变化特征;第二模态时间系数功率谱分析表明该模态存在3年、5年和8—12年的周期。Morlet小波分析表明1962—2001年期间该模态存在3年、5年和9—12年的周期。第二模态找出的两个正负位相中心分别在东马来西亚西北部和南海北部,前者盐度锋强度在逐渐减小,而后者强度却在持续增大,强度的年际变化呈现反位相变化特征。     

Multi-scale variability of subsurface temperature in the South China Sea

Using Morlet wavelet transform and harmonic analysis the multi-scale variability of subsurface temperature in the South China Sea is studied by analyzing one-year (from April 1998 to April 1999) ATLAS mooring data. By wavelet transform, annual and semi-annual cycle as well as intrasea-sonal variations are found, with different dominance, in subsurface temperature. For annual harmonic cycle, both the downward net surface heat flux and thermocline vertical movement partially control the subsurface temperature variability. For semi-annual cycle and intraseasonal variability, the subsurface temperature variability is mainly linked to the vertical displacement of thermocline.     

Interannual variability of the Kuroshio intrusion in the South China Sea

The interannual variability of intrusions of the Kuroshio into the South China Sea (SCS) is investigated using satellite remote sensing data supported by in-situ measurements. The mesoscale circulation of the SCS is predominantly wind-forced by the northeast winter and southwest summer monsoons. Although the region has been studied extensively, considerable uncertainty remains about the annual and interannual mesoscale nature of the circulation. The frequency and characteristics of Kuroshio intrusions and their effect on circulation patterns in the northeast SCS are also not well understood. Satellite observations of Sea Surface Temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM) and the Advanced Very High Resolution Radiometer (AVHRR) and Sea Surface Height Anomalies (SSHA) from TOPEX/ Poseidon for the period 1997–2005 are used here to analyze the annual and interannual variability in Kuroshio intrusions and their effects on the region. Analysis of SST and SSHA shows the formation and characteristics of intrusions vary considerably each year. Typically, the intrusion occurs in the central region of Luzon Strait and results in an anticyclonic circulation in the northeastern SCS. However, in some years, the intrusion is located in the northern portion of Luzon Strait and a cyclonic intrusion results. Wind stress and wind stress curl derived from the National Aeronautics and Space Administration (NASA) QuikSCAT satellite scatterometer are used to evaluate the relationship between wind stress or wind stress curl and the presence of winter Kuroshio intrusions into the SCS.     

Modelling of the Black Sea seasonal variability

The paper discusses the results of numerical experiments on response of the Black Sea to the ‘real’ (mean monthly) atmospheric forcing. A new version of the multi-layer quasi-isopycnic model is applied which does not use the rigid-lid approximation and allows for a salt flux through the sea surface. Ways of obtaining quantitative agreement between the calculated data due only to the external forcing, without invokingin situ temperature/salinity observations are suggested. Translated by Vladimir A. Puchkin.     

The variability of internal tides in the Northern South China Sea

An array of three bottom-mounted ADCP moorings was deployed on the prevailing propagation path of strong internal tides for nearly 1 year across the continental slope in the northern South China Sea. These velocity measurements are used to study the intra-annual variability of diurnal and semidiurnal internal tidal energy in the region. A numerical model, the Luzon Strait Ocean Nowcast/Forecast System developed at the U.S. Naval Research Laboratory that covers the northern South China Sea and the Kuroshio, is used to interpret the observed variation of internal tidal energy on the Dongsha slope. Internal tides are generated primarily at the two submarine ridges in the Luzon Strait. At the western ridge generation site, the westward energy flux of the diurnal internal tide is sensitive to the stratification and isopycnal slope associated with the Kuroshio. The horizontal shear at the Kuroshio front does not modify the propagation path of either diurnal or semidiurnal tides because the relative vorticity of the Kuroshio in Luzon Strait is not strong enough to increase the effective inertial frequency to the intrinsic frequency of the internal tides. The variation of internal tidal energy on the continental slope and Dongsha plateau can be attributed to the variation in tidal beam propagation in the northern South China Sea.     

南海海面高度年际变化的S-EOF分析

应用依赖于季节的经验正交甬数(S-EOF)分析,探讨了最近15a南海海面高度随季节演变的年际变化.S-EOF分析得出南海海面高度异常各模态不同季节的空间结构以及时间演变过程,证实了季风强盛期冬季和夏季基本模态的结构,还分离出了季风转换期(春季和秋季)海面高度的分布格局.结果表明,南海海面高度随季节演变的年际变化与厄尔尼诺和拉尼娜事件密切相关.S-EOF1的结果表明,南海海面高度的变化具有明显而稳定的季节振荡,但在ENSO年海面高度的季节振荡相对减弱;S-EOF2模态显示了1998-2001年间冬季吕宋岛西侧存在一个较强的正异常,且能一直持续到春季;S-EOF3模态主要体现了南海西部一系列中尺度涡状结构的年际差异,包含1997/1998年厄尔尼诺对南海环流的影响.     

南海上层海洋次中尺度过程空间差异和季节变化特征

南海是西太平洋最大的边缘海, 由于受季风影响显著以及北部海域的黑潮入侵, 其动力环境复杂多变, 次中尺度过程丰富, 且在空间上和时间上存在多变性。文章基于高分辨率数值模式的结果, 通过对次中尺度动力参数的分析, 对比讨论了南海北部、中部、西部和南部海域4个典型子区域上层海洋次中尺度过程的空间差异、季节变化、影响深度、影响因素等问题。研究发现各区域季节性变化特征和机制有所不同: 北部海域受冬季风和黑潮入侵影响, 冬季次中尺度的混合层不稳定较强; 中部海域同样表现为“冬强夏弱”; 西部海域受夏季风影响显著, 夏季次中尺度过程更为活跃; 而南部海域主要受岛屿地形影响较大, 容易产生地形尾涡, 季节性特征不明显。统计分析表明, 次中尺度过程往往表现出强正相对涡度与高应变特征, 在表层更容易出现负位涡, 流体稳定性较差。此外, 文章从能量学角度对次中尺度过程的主要能量来源、控制因素等进行了讨论。     

Seasonal variability of tropical cyclones generated over the South China Sea

The seasonal variability of tropical cyclones (CTCs) generated over the South China Sea (SCS) from 1948 to 2003 is analyzed. It peaks in occurrence in August and few generate in late winter (from January to March). The seasonal activity is attributed to the variability of atmosphere and ocean environments associated with the monsoon system. It is found that the monsoonal characteristics of the SCS basically determine the region of tropical cyclone (TC) genesis in each month.     

南海QuikSCAT海面风场变化特征分析

基于QuikSCAT海面风场产品,对海面风场资料进行了EOF分析和随机动态分析,以此分析南海海面风场的变化特征。研究发现:海面原始风场风速季节变化最为明显,其变化占总变化方差的59.1%,黑潮的季节变化通过海气相互作用对南海局地风场有较明显的影响;原始风场第三模态及异常风场第二模态时间变化函数与SOI和PDO弱相关,且异常风场第二模态时间变化函数谱分析结果主要呈现5年的周期变化,南海海面风场变化与年际振荡有关;南海大部分海区风速呈现增长的趋势,但增长速率较小;风速增大最快的区域是台湾海峡以南海域和北部湾,增长速度达到0.05 ms-1a-1。     

南海上层环流季节变化的诊断计算

通过一个全球的二维诊断模型,采用Levitus温盐资料和COADS风应力资料,并结合动力计算来研究南海上层环流的季节变化。计算结果与其它模式结果和观测结果非常相似。南海北部(南部)全年存在一气旋式(反气旋式)环流。在冬季气旋式环流几乎占据了整个南海,夏季则以反气旋式环流为主。泰国湾的环流在冬季(夏季)是气旋式的(反气旋的)。南海的西边界流有明显的季节变化,其在冬季从卡里马塔海峡流出南海,夏季部分西边界流从台湾海峡流出南海。越南离岸流在春季就开始出现,其位置比夏季的越南离岸流的位置偏北。     

南海风生近惯性能通量的敏感性分析

After validated by the in-situ observation, the slab model is used to study the wind-generated near-inertial energy flux(NIEF) in the South China Sea(SCS) based on satellite-observed wind data, and its dependence on calculation methods and threshold criteria of the mixed layer depth(MLD) is investigated. Results illustrate that the total amount of NIEF in the SCS could be doubled if different threshold criteria of MLD are adopted. The NIEF calculated by the iteration and spectral solutions can lead to a discrepancy of 2.5 GW(1 GW=1×109 W). Results also indicate that the NIEF exhibits spatial and temporal variations, which are significant in the boreal autumn,and in the southern part of the SCS. Typhoons are an important generator of NIEF in the SCS, which could account for approximately 30% of the annual mean NIEF. In addition, deepening of the MLD due to strong winds could lead to a decrease of NIEF by approximately by 10%. We re-estimate the annual mean NIEF in the SCS,which is(10±4) GW and much larger than those reported in previous studies.     

南海上层经向翻转环流的低频变化

The low-frequency variability of the shallow meridional overturning circulation(MOC) in the South China Sea(SCS) is investigated using a Simple Ocean Data Assimilation(SODA) product for the period of 1900–2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells: a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport(LST) that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current(NEC) bifurcation and thus a stronger transport of the Kuroshio east of Luzon.     

基于数值模式的南海北部深层内潮季节变化特征分析

本文基于MITgcm非静力数值模式,采用实际地形、层结和潮流强迫,开展南海北部内潮数值模拟敏感性试验,分析夏冬两个季节南海北部深层内潮的差异。结果显示在南海北部深层,冬季K₁和M₂内潮流速振幅比夏季强10.1%和44.7%。垂向模态分析结果进一步表明,尽管南海北部深层冬季第一模态内潮动能密度比夏季低15.5%,但第二和第三模态内潮则是冬季比夏季高约25.1%和33.2%,导致冬季深层流速的垂向剪切大于夏季,表明冬季较强的高模态内潮可能是冬季南海深层强混合的一个原因。