Interannual variability of transport and bifurcation of the North Equatorial Current in the tropical North Pacific Ocean

The relationship of the interannual variability of the transport and bifurcation latitude of the North Equatorial Current （NEC） to the E1 Nifio-Southern Oscillation （ENSO） is investigated. This is done through composite analysis of sea surface height （SSH） observed by satellite altimeter during October 1992-July 2009, and correspondingly derived sea surface geostrophic currents. During E1 Nifio/La Nifia years, the SSH in the tropical North Pacific Ocean falls/rises, with maximum changes in the region 0-15~N, 130~E-160~E. The decrease/increase in SSH induces a cyclonic/anticyclonic anomaly in the western tropical gyre. The cyclonic/anticyclonic anomaly in the gyre results in an increase/decrease of NEC transport, and a northward/southward shift of the NEC bifurcation latitude near the Philippine coast. The variations are mainly in response to anomalous wind forcing in the west-central tropical North Pacific Ocean, related to ENSO events.

Low-frequency variability of the North Equatorial Current bifurcation in the past 40 years from SODA

The low-frequency variability of the North Equatorial Current （NEC） bifurcation during 1958 to 2001 was investigated with the Simple Ocean Data Assimilation （SODA） 2.0.2 dataset. In agreement with recent observations, the NEC bifurcation latitude （NBL） shifted northward as depth increases, from about 12.7°N near the surface to about 17.1°N at depths around 500 m for the annual average. This study reveals that the interannual variations of NBL, with five years period, mainly focused on the upper 500 m with amplitude increasing as depth increased. The NBL shifted southward in the past 40 years, which was more significant in the subsurface at more than -0.02°/a. The NBL manifests itself in the transports of NMK （NEC-Mindanao Current （MC）-Kuroshio） system in strong relationship with MC （0.7） and Kuroshio （-0.7）. The EOF analysis of meridional velocity off the Philippine coast shows that the first mode, explaining 62% of variance and 5 years period, was highly correlated with the southward shift of NBL with coefficient at about 0.75. The southward shift of NBL consists with the weakening of MC and strengthening of Kuroshio, which exhibited linear trends at -0.24Sv/a and 0.11Sv/a. Both interannual variation and trend of NBL were closely related to the variation of NMK system.

Numerical Study on the Bifurcation of the North Equatorial Current

A 1.5-layer reduced-gravity model forced by wind stress is used to study the bifurcations of the North Equatorial Current(NEC).The authors found that after removing the Ekman drift,the modelled circulations can serve well as a proxy of the SODA circulations on the σθ=25.0 kg m～-3 potential density surface based on available long-term reanalysis wind stress data.The modelled results show that the location of the western boundary bifurcation of the NEC depends on both zonal averaged and local zero wind stress curl latitude.The effects of the anomalous wind stress curl added in different areas are also investigated and it is found that they can change the strength of the Mindanao Eddy(ME),and then influence the interior pathway.

Correlation analysis of the North Equatorial Current bifurcation and the Indonesian Throughflow

Based on monthly mean Simple Ocean Data Assimilation （SODA） products from 1958 to 2007, this study analyzes the seasonal and interannual variability of the North Equatorial Current （NEC） bifurcation latitude and the Indonesian Throughflow （ITF） volume transport. Further, Empirical Mode Decomposition （EMD） method and lag-correlation analysis are employed to reveal the relationships between the NEC bifurcation location, NEC and ITF volume transport and ENSO events. The analysis results of the seasonal variability show that the annual mean location of NEC bifurcation in upper layer occurs at 14.33°N and ITF volume transport has a maximum value in summer, a minimum value in winter and an annual mean transport of 7.75×10^6 m^3/s. The interannual variability analysis indicates that the variability of NEC bifurcation location can be treated as a precursor of El Nino. The correlation coefficient between the two reaches the maximum of 0.53 with a time lag of 2 months. The ITF volume transport is positively related with E1 Nifio events with a maximum coefficient of 0.60 by 3 months. The NEC bifurcation location is positively correlated with the ITF volume transport with a correlation coefficient of 0.43.

The Change Features of the West Boundary Bifurcation Line of the North Equatorial Current in the Pacific

The equatorial Current in the North Pacific(NEC) is an upper layer westward ocean current, which flows to the west boundary of the ocean, east of the Philippines, and bifurcates into the northerly Kuroshio and the main body of the southerly Mindanao current. Thus, NEC is both the south branch of the Subtropical Circulation and the north branch of the Tropical Circulation. The junction of the two branches extends to the west boundary to connect the bifurcation points forming the bifurcation line. The position of the North Pacific Equatorial Current bifurcation line of the surface determines the exchange between and the distribution of subtropical and tropical circulations, thus affecting the local or global climate. A new identification method to track the line and the bifurcation channel was used in this study, focusing on the climatological characteristics of the western boundary of the North Equatorial Current bifurcation line. The long-term average NEC west boundary bifurcation line shifts northwards with depth. In terms of seasonal variation, the average position of the western boundary of the bifurcation line is southernmost in June and northernmost in December, while in terms of interannual variation, from spring to winter in the years when ENSO is developing, the position of the west boundary bifurcation line of NEC is relatively to the north(south) in EI Ni?o(La Ni?a) years as compared to normal years.

BIFURCATION OF THE NORTH EQUATORIAL CURRENT DERIVED FROM ALTIMETRY IN THE PACIFIC OCEAN

The gridded （1/3°＊1/3°） altimetry data from October 1992 through December 2004 were analyzed to study the seasonal and interannual variabilities of the bifurcation of the North Equatorial Current （NEC） at the surface in the western North Pacific Ocean. Calculations show that on annual average the bifurcation occurs at about 13.4°N at the surface. The geostrophic flow derived from Sea Surface Height （SSH） data shows that the southernmost latitude of the NEC bifurcation at the surface is about 12.9°N in June and the northernmost latitude is about 14.1°N in December. Correlation analyses between the bifurcation latitude and the Southern Oscillation Index （SOl） suggest that the bifurcation latitude is highly correlated with the E1 Nino/Southern Oscillation （ENSO） events. During the E1 Nino years the bifurcation of the NEC takes place at higher latitudes and vice versa.

Bifurcation of Pacific North Equatorial Current at the surface

The grid altimetry data between 1993 and 2006 near the Philippines were analyzed by the method of Empirical Orthogonal Function (EOF) to study the variation of bifurcation of the North Equatorial Current at the surface of the Pacific. The relatively short-term signals with periods of about 6 months, 4 months, 3 months and 2 months are found besides seasonal and interannual variations mentioned in previous studies. Local wind stress curl plays an important role in controlling variation of bifurcation latitude except in the interannual timescale. The bifurcation latitude is about 13.3°N in annual mean state and it lies at the northernmost position (14.0°N) in January, at the southernmost position (12.5°N) in July. The amplitude of variation of bifurcation latitude in a year is 1.5°, which can mainly be explained as the contributions of the signals with periods of about 1 year (1.2°) and 0.5 year (0.3°).

Vorticity Budget Study on the Seasonal Upper Circulation in the Northern South China Sea from Altimetry Data and a Numerical Model

Based on the EOF analyses of Absolute Dynamic Topography satellite data, it is found that, in summer, the northern South China Sea （SCS） is dominated by an anticyclonic gyre whilst by a cyclonic one in winter. A connected single-layer and two-layer model is employed here to investigate the dynamic mechanism of the circulation in the northern SCS. Numerical experi- ments show that the nonlinear term, the pressure torque and the planetary vorticity adveetion play important roles in the circulation of the northern SCS, whilst the contribution by seasonal wind stress curl is local and limited. Only a small part of the Kuroshio water intrudes into the SCS, it then induces a positive vorticity band extending southwestward from the west of the Luzon Strait （LS） and a negative vorticity band along the 200 m isobath of the northern basin. The positive vorticity field induced by the local summer wind stress curl is weaker than that induced in winter in the northern SCS. Besides the Kuroshio intrusion and monsoon, the water trans- ports via the Sunda Shelf and the Sibutu Passage are also important to the circulation in the northern SCS, and the induced vorticity field in summer is almost contrary to that in winter. The strength variations of these three key factors （Kuroshio, monsoon and the water transports via the Sunda Shelf and the Sibutu Passage） determine the seasonal variations of the vorticity and eddy fields in the northern SCS. As for the water exchange via the LS, the Kuroshio intrusion brings about a net inflow into the SCS, and the monsoon has a less effect, whilst the water transports via the Sunda Shelf and the Sibutu Passage are the most important influencing factors, thus, the water exchange of the SCS with the Pacific via the LS changes dramatically from an outflow of the SCS in summer to an inflow into the SCS in winter.

A time-dependent baroclinic model on NEC bifurcation

As it is well-known, the North Equatorial Current (NEC) bifurcates into the Kuroshio flowing northward and the equatorward Mindanao Current, which is well depicted by Munk's theory in 1950 in terms of its climatology. However, Munk's theory is unable to tell the NEC bifurcation variability with time. In the present paper, a time-dependent baroclinic model forced by wind, in which temporal and baroclinic terms are added to Munk's equation, is proposed to examine the seasonal variability of the NEC bifurcation latitude. An analytical solution is obtained, with which the seasonal variability can be well described: NEC bifurcation reaches its northernmost position in December and its southernmost position in June with a range of about 1° in latitude, consistent with previous results with observations. The present solution will degenerate to Munk's one in the case of steady and barotropic state.

黑潮入侵南海的强弱与太平洋年代际变化及厄尔尼诺-南方涛动现象的关系

在黑潮入侵南海强弱的问题上,到底是太平洋年代际变化(Pacific Decadal Oscillation,PDO)还是厄尔尼诺-南方涛动(El Nio-Southern Oscillation,ENSO)现象在起关键作用,目前还存在着较大争议。本文先以高盐水作为黑潮入侵强弱的示踪物,用120°E断面的高盐水数据和北赤道流分叉点(North Equator Current Bifurcation,NEC-Y)的南北变动进行相关分析,接着,进一步用学者所用的黑潮入侵指数(KI指数,Kuroshio intrusion index和NEC指数,North Equatorial Current index)与北赤道流分叉点南北变动进行相关分析。最后,用EMD(Empirical Mode Decomposition)方法和相关关系分析法分别分析了PDO指数、Nio3.4指数与北赤道流分叉点南北变动的关系并用NECP风场数据探讨其影响机制。结果表明:(1)通过对120°E断面的高盐水的KI指数、NEC指数与NEC-Y的相关分析,表明了北赤道流分叉点的南北变动能够很好地指代黑潮入侵南海的强弱;(2)通过PDO指数和Nio3.4指数与北赤道流分叉点的南北变动的相关性分析,发现PDO指数、Nio3.4指数与北赤道流分叉点的南北变动都具有较好的相关性,都在0.5水平。这些良好的相关性表明了PDO和ENSO对黑潮入侵南海的强弱都具有重要的影响;(3)当处于厄尔尼诺年(拉尼娜)时,赤道太平洋发生西(东)风异常,使得北赤道流分叉点偏北(南),使吕宋岛东侧的黑潮流速减弱(加强),黑潮入侵南海增强(减弱);当PDO处于暖(冷)阶段时,会加强热带太平洋的西(东)风异常,使得黑潮入侵南海增强(减弱)。

ENSO循环相联系的北太平洋低纬度异常西边界流

用SODA海洋同化和NCEP大气再分析资料,分析了热带太平洋次表层海温异常主要模态与北太平洋低纬度西边界流海域上层海洋环流和亚洲-北太平洋地区大气垂直和水平流场变化之间的关系,得到以下结果:(1)在热带太平洋海洋次表层ENSO事件具有两种模态,二者组合构成ENSO循环。第一模态为ENSO成熟期,主要出现在冬季,第二模态为ENSO过渡期,主要出现夏季。(2)ENSO循环对北太平洋低纬度西边界流区上层海洋环流有重要影响。在El Nio发展期或La Nia衰退期,该区出现气旋性异常环流,北赤道流(NEC)加强,NEC分叉位置北移,棉兰老海流(MC)加大,菲律宾以东黑潮(KC)减小,北赤道逆流(NECC)最强。在El Nio(La Nia)成熟期,该区气旋性(反气旋性)异常环流达最强,NEC最强(最弱),NEC分叉位置最北(最南),MC最大(最小),KC最小(最大),NECC减弱(加强)。在El Nio衰退期或La Nia发展期与El Nio发展期相反,该区出现反气旋性异常环流,由此导致相应流系异常发生反位相变化。(3)ENSO循环对北太平洋低纬度西边界流海域上层海洋环流的影响是通过ENSO事件期间热带太平洋热力状况异常改变上空大气环流来实现的。ENSO事件首先造成热带太平洋海洋热力状况异常,导致其上空对流活动异常,后者直接或间接通过"大气桥"能量传输引起相关地区大气环流场的变化,致使海面风应力场异常,进而强迫上层海洋环流场的相应变化。文章最后还分析了ENSO事件期间菲律宾附近异常反气旋或异常气旋性风场的产生和持续原因,讨论了北太平洋低纬度西边界流海域海气相互作用在ENSO循环中的贡献。

北赤道流分叉点及南海北部环流的研究进展

介绍了北赤道流分叉点、南海北部环流的一些研究成果,并就黑潮对南海的影响所作的研究进行了回顾。北赤道流分叉点的位置对于北赤道流系水体疏运变化及在黑潮和MC之间水体、热量、盐度输运的分配中起着重要的作用。北赤道流分叉点位置约在14.6°N上,分叉点位置随深度增加而北移。分叉点有明显的季节变化和年际变化,在春、夏季向南移动,而在秋、冬季则向北移动。年际变化与ENSO现象相关紧密,在El Ni^no事件NEC分叉纬度处于最北端,在La Ni^no事件处于最南端。对于分叉点位置的定量化研究,仍然需要更多的观测结果进行研究。季风和黑潮是影响南海北部环流的两种主要因素。南海北部上层流场主要由广东沿岸流、黑潮入侵流套、东沙海流、南海暖流和吕宋海流组成。除海盆、次海盆尺度环流外,受季风、黑潮和地形等因素的影响,南海表现出多涡结构。通过近些年的卫星观测和数值模拟的结果,人们对南海中尺度涡的认识大大加深,但要想模拟出风应力形成涡的机制,还需要提高风场和模式的分辨率。由于观测资料的限制,对南海流场的垂直结构、以及春-秋季季风转相时期的流场结构等还研究得较少。吕宋海峡水交换是西太平洋对南海影响的主要途径。黑潮在吕宋海峡附近的形变一直是有争议的热点问题,目前对于黑潮入侵有3种观点:(1)认为黑潮经过吕宋海峡形成流套结构,并分离出中尺度涡影响南海流场、水团结构;(2)认为黑潮有一直接分支分离出来进入南海形成黑潮分支;(3)认为吕宋海峡水交换不属于以上两种情况,西太平洋对南海的水团输送另有机制。北赤道流分叉点在对黑潮的水体、热量、盐度输运的分配中起着重要的作用,黑潮对南海北部环流的影响可能与NEC的分叉点位置有关,但目前对NEC的分叉点位置与南海北部环流相关性的研究甚少。最后提出了对未来加强该方面研究的一些展望。

太平洋北赤道流表层流速及分叉点位置

确定太平洋北赤道流表层流速及分叉点位置的变化是海洋环境研究中的1个重要问题。使用1987～1998年的WOCE浮标资料,通过估算得到了季节平均和年平均意义下北赤道流表层流速(1989～1998)。计算结果显示:北赤道流表层流的流速冬季最大,夏季最小,春秋两季相仿。在厄尔尼诺发生年的第2年,夏季平均流速往往较大。对浮标轨迹的逐年分析表明表层北赤道流分叉点的位置在11°N～14 .7°N之间,从轨迹较为密集的4年的分析可以看出,表层流分叉点的位置具有年际变化,其中,2个ElNino年分叉点偏北。

南海北部次表层盐度的变化及其与北赤道流分叉点的关系

吕宋海峡是南海与外界水交换的重要通道,黑潮作为北太平洋最强的1支西边界流,在经过吕宋海峡时会对南海北部的环流和环境产生重要影响.本文用1991-2011年期间CTD断面实测资料和高度计资料,提取23.0～25.5 kg/m3等密度面之间的盐度极大值,研究了南海北部不同年月盐度极大值变化、黑潮入侵方式与强弱,以及盐度极大值变化与北赤道流分叉点南北移动的关系,结果表明：（1）黑潮入侵南海方式多样,既有分支形式,也有弯曲、流套形式.（2）不同年月间,黑潮入侵南海的强弱存在较大差别,120°E 断面的次表层盐度极大值的变动可超过0.3.（3）北赤道流分叉点位置的南北变动对黑潮入侵南海的强弱具有重要影响：北赤道流分叉点位置偏北,黑潮入侵南海较强；北赤道流分叉点位置偏南,则黑潮入侵相对较弱.

对北赤道流分叉纬度变化规律的研究分析

关于北赤道流分叉研究的两类方法,即水文实测数据分析和数值模拟,其本身都因存在突出的问题而引起估算结果的偏差,致使相互结果差别很大。所以,如何找到一种高时空分辨率的数据或提出一个比较符合物理实际并得到实测数据验证的数值模式是研究赤道流分叉的关键。本文将利用从1992年10月到2004年12月高时空分辨率(时间分辨率:7 d;空间分辨率:1/3°×1/3°)的卫星高度计数据,对北赤道流在表层的分叉进行研究,包括确定分叉点,求得其季节和年际变化的规律。

南海北部次表层高盐水的季节变化及其与西北太平洋环流的关系

次表层高盐水（〉34.68‰）作为北太平洋热带水（NPTW）的示踪, 可用来研究黑潮入侵, 了解南海与太平洋的水体交换.文章利用基于高分辨率混合坐标海洋模式（HYCOM）的海洋再分析资料, 研究了南海北部次表层高盐水的季节变化及其影响因子.模拟结果显示, 南海北部高盐水位于100-200m 水深, 最西可达111°E, 其体积存在明显的季节变化,12 月达到极大值, 6 月达到极小值.进一步的分析表明, 其季节变化主要受低纬度西北太平洋大尺度环流的调制, 与北赤道流分叉点位置的季节变化（1 月到达最北端、6 月到达最南端）呈现很好的相关性.受太平洋大尺度风场的影响, 北赤道流分叉点上半年（下半年）向南（北）移动, 导致黑潮输运增强（减弱）, 通过吕宋海峡进入南海的盐通量减少（增加）, 从而使南海次表层高盐水的盐度降低（升高）.吕宋海峡断面的流速和盐通量分布特征显示, 西太平洋高盐水主要通过吕宋海峡中部（20？~21？18？N）进入南海北部.

一个高分辨率太平洋-印度洋海盆环流模式的初步结果

利用LASG/IAP发展的一个0.25°×0.25°高分辨率太平洋-印度洋海盆环流模式初步分析了模式在太平洋区域的模拟结果,并与海洋同化资料的数据以及前人的研究结果作比较,检验此模式对该区域平均气候态、年际变化的模拟能力。分析表明,模式较好地再现了海表温度(SST)分布、赤道温跃层和纬向流结构、赤道流系分布形态、海表高度以及正压流函数空间分布特征;同时,对显著的El Nio和La Nia事件的模拟等方面与Simple Ocean Data Assimilation(SODA)2.0.2版本结果相近。此外,模式模拟北赤道流(NEC)分叉点位置的季节和年际变化以及吕宋海峡流量的年际变化与已有研究结果基本一致。进一步分析还发现,在年际尺度上,NEC分叉点位置和吕宋海峡流量与ENSO密切相关。

基于OFES模式数据的北赤道流分叉研究

利用1957—2006共50年高分辨率的长时间序列海洋模式OFES(OGCM for the Earth Simulator)数据,对北赤道流(NEC)的分叉规律及其与ENSO循环的关系进行了分析。结果表明:(1)NEC分叉纬度具有明显的季节和年际变化,周期主要呈现为3—6个月的季节内振荡和2年、2—7年左右ENSO尺度周期振荡以及10年以上的年代际变化。在季节尺度上,分叉位置春季偏南,秋季偏北,并且分叉纬度随深度的增加向北移动,其北移幅度冬季最大,夏季最小。而在年际变化尺度上,NEC分叉纬度具有较强的年际变化信号,与ENSO循环密切相关,El Ni?o年分叉位置偏北,La Ni?a年分叉位置偏南。(2)NEC分叉纬度的变化与北太平洋0—30°N之间的纬向风应力旋度积分零线位置密切相关,零线的南北偏移导致了分叉位置的改变,在不同深度上,零线位置对分叉纬度改变的影响时间不同,表层需1个月,而500 m深度则需4个月左右。(3)NEC分叉影响着黑潮(KC)与(棉兰老流)MC的流量分配率,其年际异常变化与冷、暖ENSO事件发生密切相关,当El Ni?o发生时,KC流量分配率减少,MC流量分配率增加;La Ni?a年情况则相反。

Low-frequency variability of the shallow meridional overturning circulation in the South China Sea

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.

南海贯穿流季节变化数值模拟研究

本文采用美国伍兹霍尔研究所研发的海洋-大气-波浪-泥沙输运耦合模式COAWST(Coupled Dcean-Atmosphere-Wave-Sediment Transport)对南海及邻近海域进行了9 km分辨率的数值模拟研究。结果表明,南海贯穿流的季节变化再现了冬强夏弱的特征,在南海内部冬季呈现气旋环流结构,夏季呈现反气旋环流结构,尤其在冬季其流轴结构更为清晰和稳定,海水从吕宋海峡进入南海,从民都洛海峡、卡里马塔海峡、台湾海峡和巴拉巴克海峡流出,吕宋海峡断面流量与其他4个海峡流量合计在数量级上相当,保持南海海水总量不变。吕宋海峡、卡里马塔海峡、民都洛海峡的流量呈现明显相关性,吕宋海峡流量增大时,民都洛海峡和卡里马塔海峡的流量也相应增大,相关系数分别达到0.78和0.9。通过更适于分析中短期变化的简化绕岛环流理论,定量计算2019年吕宋海峡、黑潮和棉兰老流流量与北赤道流分叉点位置的关系,发现夏季北赤道流分叉点NECBL(North Equatorial Current Bifurcation Latitude)偏南,在13.6°N附近;冬季NECBL偏北,在15.6°N左右,同期黑潮流量减少,棉兰老流流量增加,作为南海贯穿流入流的吕宋海峡流量可达13.4 Sv。吕宋海峡输运补偿了北赤道流到达菲律宾海岸后的北向分支的流量,与棉兰老流的流量呈正相关,相关系数达到0.5361。