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 Nino events prominently. The strong El Nino 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 interannual signals.

Off-axis sonar beam pattern of free-ranging finless porpoises measured by a stereo pulse event data logger

The off-axis sonar beam patterns of eight free-ranging finless porpoises were measured using attached data logger systems. The transmitted sound pressure level at each beam angle was calculated from the animal's body angle, the water surface echo level, and the swimming depth. The beam pattern of the off-axis signals between 45 and 115 (where 0 corresponds to the on-axis direction) was nearly constant. The sound pressure level of the off-axis signals reached 162 dB re 1 mPa peak-to-peak. The surface echo level received at the animal was over 140 dB, much higher than the auditory threshold level of small odontocetes. Finless porpoises are estimated to be able to receive the surface echoes of off-axis signals even at 50-m depth. Shallow water systems (less than 50-m depth) are the dominant habitat of both oceanic and freshwater populations of this species. Surface echoes may provide porpoises not only with diving depth information but also with information about surface direction and location of obstacles (including prey items) outside the on-axis sector of the sonar beam. 2005 Acoustical Society of America.

The variability of eddy kinetic energy in the South China Sea deduced from satellite altimeter data

We used fifteen years (1993-2007) of altimetric data, combined from different missions (ERS-1/2, TOPEX/Poseidon, Jason-1, and Envisat), to analyze the variability of the eddy kinetic energy (EKE) in the South China Sea (SCS). We found that the EKE ranged from 64 cm(2)/s(2) to 1 390 cm(2)/s(2) with a mean value of 314 cm(2)/s(2). The highest EKE center was observed to the east of Vietnam (with a mean value of 509 cm(2)/s(2)) and the second highest EKE region was located to the southwest of Taiwan Island (with a mean value of 319 cm(2)/s(2)). We also found that the EKE structure is the consequence of the superposition of different variability components. First, interannual variability is important in the SCS. Spectral analysis of the EKE interannual signal (IA-EKE) shows that the main periodicities of the IA-EKE to the east of Vietnam, to the southwest of Taiwan Island, and in the SCS are 3.75, 1.87, and 3.75 years, respectively. It is to the south of Taiwan Island that the IA-EKE signal has the most obvious impact on EKE variability. In addition, the IA-EKE exhibit different trends in different regions. An obvious positive trend is observed along the east coast of Vietnam, while a negative trend is found to the southwest of Taiwan Island and in the east basin of Vietnam. Correlation analysis shows that the IA-EKE has an obvious negative correlation with the SSTA in Nio3 (5A degrees S-5A degrees N, 90A degrees W-150A degrees W). El Nio-Southern Oscillation (ENSO) affects the IA-EKE variability in the SCS through an atmospheric bridge-wind stress curl over the SCS. Second, the seasonal cycle is the most obvious timescale affecting EKE variability. The locations of the most remarkable EKE seasonal variabilities in the SCS are to the east of Vietnam, to the southwest of Taiwan, and to the west of Philippines. To the east of Vietnam, the seasonal cycle is the dominant mechanism controlling EKE variability, which is attributed primarily to the annual cycle there of wind stress curl. In this area, the maximum EKE is observed in autumn. To the southwest of Taiwan Island, the EKE is enlarged by the stronger SCS circulation, which is caused by the intrusion branch from the Kuroshio in winter. Finally, intra-annual and mesoscale variability, although less important than the former, cannot be neglected. The most obvious intra-annual and mesoscale variability, which may be the result of baroclinic instability of the background flow, are observed to the southwest of Taiwan Island. Sporadic events can have an important effect on EKE variability.

A method for individual identification of echolocation signals in free-ranging finless porpoises carrying data loggers

Echolocation click events of a free-ranging juvenile and an adult finless porpoise (Neophocaena phocaenoides) were recorded with an acoustic data logger. Additionally, dive depth and swim speed of the juvenile were recorded with a behavior data logger. Echoes of echolocation signals from the water surface were clearly detected in shallow dives approximately less than 2 m. The delay time between a surface echo and a direct signal corresponded with the two-way transmission time for the animal's depth, indicating that the signals originated from the animal wearing the data loggers. The finless porpoises produced echolocation signals frequently and were thought to be able to detect their depth by listening to echoes from the water surface. (C) 2000 Acoustical Society of America. [S0001-4966(00)01609-X].

卫星高度计资料在海洋潮汐研究中的应用

本文依据收集到的392个地面验潮站8个主要分潮（M2、S2、K1、O1、N2、K2、P1及Q1）的调和常数，对现有7个全球大洋潮汐模式的准确度进行了检验，结果显示各模式在深海区域均达到了比较高的准确度，相互之间差别也不大。经验模式GOT00和CSR4.0、同化模式NAO99、反演同化模式TPXO7.0、数值同化模式FES2002和FES2004的M2分潮均方根偏差在3 cm左右，其它分潮（S2、K1、O1、N2、K2、P1及Q1）大约在1～2 cm。本文还依据中国近海18个岛屿的调和常数对其中的5个大洋潮汐模式的准确度进行了检验，结果表明，M2分潮均方根偏差在6～14 cm，明显高于大洋部分的偏差，其中日本国家天文台的潮汐模式NAO99在中国近海的结果相对较准确。 我们利用1992年8月至2008年8月的TOPEX/POSEIDON和JASON-1(T/P-J)卫星高度计资料，对沿卫星轨道的302816个站点进行了14个分潮的潮汐调和分析，得到了全球大洋潮汐的8个主要分潮以及2个气象分潮Sa、Ssa的经验同潮图。主要结果有：（1）各分潮在卫星上升轨道与下降轨道的交叉点(约7000个)相关性分析表明：M2分潮的振幅和迟角的相关系数很高（分别为0.9965和0.9961）；S2，K1，O1和Sa分潮也有较好的相关性（相关系数为0.94～0.99）；（2）该结果与392地面个验潮站吻合较好，其中M2分潮的振幅、迟角和向量的均方根偏差分别为：1.73 cm，2.340和2.93 cm；S2，K1和O1分潮的振幅、迟角和向量的均方根偏差为1 cm左右，5.250～7.270和1.5～2.1 cm，该精度与最近几年国际上的主要大洋潮汐模式的准确度相近；（3）首次通过卫星资料获得了Sa、Ssa分潮的同潮图。周期为1年的Sa分潮与大洋105个地面站相比，振幅、迟角和向量的均方根偏差分别为1.50 cm、18.360和2.16 cm。在此基础上，进一步分析了构成Sa、Ssa气象分潮的两个主要因素（海水密度以及海面气压）在全球的分布。 在T/P-J等卫星资料无法覆盖到南大洋和北冰洋，本文利用Princeton Ocean Model（POM）进行了数值模拟，模拟结果与162个地面实测站（其中南大洋30个，北冰洋132个）的观测比较一致。基于卫星资料分析的结果和数值模拟结果合并得到了全球大洋的8个主要分潮同潮图。在此基础上通过全球潮汐能量耗散的计算得到潮能通量的分布，并得到全球M2、S2、K1和O1分潮的潮汐能量耗散率为2.431TW、0.401TW、0.336TW和0.176TW。 本文还利用卫星资料对南海潮汐进行了研究，在中国南海，获得了主要的半日潮、全日潮、四分日分潮和长周期分潮（M2，S2，N2，K2，K1，O1，P1，Q1，M4, MS4，Sa, Ssa）的经验同潮图。与南海沿岸94个地面验潮站的数据符合得比较好，M2，S2，K1及O1等4个主要分潮的平均振幅差为2～4 cm，均方根偏差分别是9～11 cm.其它4个主要分潮N2，K2，P1，Q1的平均振幅差为1～2 cm，均方根偏差为2～4 cm。此外，本文还利用卫星高度计资料潮汐分析结果沿卫星轨道进行高通滤波，分离得出中国近海的M2，S2，K1及O1分潮的内潮信息。