黑潮延伸体海洋锋强度和位置变率的对比研究

This study compares the seasonal and interannual-to-decadal variability in the strength and position of the Kuroshio Extension front(KEF) using high-resolution satellite-derived sea surface temperature(SST) and sea surface height(SSH) data. Results show that the KEF strength has an obvious seasonal variation that is similar at different longitudes, with a stronger(weaker) KEF during the cold(warm) season. However, the seasonal variation in the KEF position is relatively weak and varies with longitude. In contrast, the low-frequency variation of the KEF position is more distinct than that of the KEF strength even though they are well correlated. On both seasonal and interannual-to-decadal time scales, the western part of the KEF(142°–144°E) has the greatest variability in strength, while the eastern part of the KEF(149°–155°E) has the greatest variability in position. In addition, the relationships between wind-forced Rossby waves and the low-frequency variability in the KEF strength and position are also discussed by using the statistical analysis methods and a wind-driven hindcast model. A positive(negative) North Pacific Oscillation(NPO)-like atmospheric forcing generates positive(negative) SSH anomalies over the central North Pacific. These oceanic signals then propagate westward as Rossby waves, reaching the KE region about three years later, favoring a strengthened(weakened) and northward(southward)-moving KEF.     

黑潮延伸体海表温度锋位置的变化特征

High spatial resolution sea surface temperature(SST) data from 1993 to 2013 are used to detect the position of the Kuroshio Extension sea surface temperature front(KEF) from 141°E to 158°E,and the seasonal,monthly and interannual-to-decadal variations of the KEF position are investigated.The latitudinal position of the KEF varies with longitudes:the westernmost part of the KEF from 141°E to 144°E is relatively stable,whereas the easternmost part from 153°E to 158°E exhibits the largest amplitude of its north-south displacement.In the light of the magnitudes of the standard deviations at longitudes,then the KEF is divided into three sections:western part of the KEF(KEFw,141°–144°E),central part of the KEF(KEFc,144°–153°E) and eastern part of the KEF(KEFe,153°–158°E).Further analysis reveals that the KEFw position is dominated by the decadal variability,while the KEFc and KEFe positions change significantly both on interannual and decadal time scales.In addition,the KEFw position is well correlated with the KEF path length.The possible mode leading to the decadal oscillation of the KEFw is further discussed.The KEFw position exhibits significant connections with the Pacific decadal oscillation(PDO) index and the north Pacific gyre oscillation(NPGO) index with a time lag of 40 and 33 months,respectively.     

Acoustic propagation analysis with a sound speed feature model in the front area of Kuroshio Extension

This paper aims to analyse acoustic-propagation character in the front area of Kuroshio Extension (KE). By analysing Argo data and the Sea surface height (SSH) data in this KEF area, a two-dimensional (2D) sound-speed feature model (SSPFM) characterising the KEF is proposed. The SSPFM has a transition zone with a width about 100 km and the sound channel changes from 1000 m south of KEF to 300 m north of KEF, resulting in a sharp gradient about 7 m/km. Along with the meandering character of the KEF axis, the sharp gradient results in a rather complicated acoustic environment in the KEF area. With reanalysis data from the hybrid coordinate ocean model, a three-dimensional (3D) sound-speed environment is established. The acoustic propagation character in the KEF area is then analysed with the 2D SSPFM and the 3D acoustic environment. Results show that the KEF affects acoustic propagation mainly by modifying the sound channel depth. Given that acoustic propagation in the KEF area is influenced mainly by the meandering KEF, with the near-real-time SSH data to locate the KEF, the 2D SSPFM is able to provide a near-real-time estimate of the underwater 3D acoustic environment.     

冬季黑潮延伸体海表温度对阿留申低压活动的双周期响应

Based on our previous work, the winter sea surface temperature(SST) in the Kuroshio Extension(KE) region showed significant variability over the past century with periods of ~6 a between 1930 and 1950 and ~10 a between1980 and 2009. How the activity of the Aleutian Low(AL) induces this dual-period variability over the two different timespans is further investigated here. For the ~6 a periodicity during 1930–1950, negative wind stress curl(WSC)anomalies in the central subtropical Pacific associated with an intensified AL generate positive sea surface height(SSH) anomalies. When these wind-induced SSH anomalies propagate westwards to the east of Taiwan, China two years later, positive velocity anomalies appear around the Kuroshio to the east of Taiwan and then the mean advection via this current of velocity anomalies leads to a strengthened KE jet and thus an increase in the KE SST one year later. For the ~10 a periodicity during 1980–2009, a negative North Pacific Oscillation-like dipole takes2–3 a to develop into a significant positive North Pacific Oscillation-like dipole, and this process corresponds to the northward shift of the AL. Negative WSC anomalies associated with this AL activity in the central North Pacific are able to induce the positive SSH anomalies. These oceanic signals then propagate westward into the KE region after 2–3 a, favoring a northward shift of the KE jet, thus leading to the warming of the KE SST. The feedbacks of the KE SST anomaly on the AL forcing are both negative for these two periodicities. These results suggest that the dual-period KE SST variability can be generated by the two-way KE-SST-AL coupling.     

黑潮延伸体区域脱落涡旋的时空特征分析

本文利用1993-2015年AVISO卫星高度计融合数据，统计分析了从黑潮延伸体流轴脱落涡旋的空间分布特征、运动属性以及季节、年际和类年代际变化。研究结果表明，23年间共追踪到242个气旋涡，276个反气旋涡，脱落的涡旋主要分布在沙茨基海隆以西区域。从脱落涡旋的源地空间分布来看，气旋涡的形成区域有两个高值区，一个位于黑潮延伸体流轴稳定弯曲处，即144°~146°E之间的上游区域；另一个位于沙茨基海隆西侧156°E处。而反气旋涡的形成区域也有两个高值区，一个位于沙茨基海隆以西的下游区域，另一个位于148°E处。这些在上游和下游脱落的涡旋大多向西移动，其中有88%的涡旋再次被流轴吸收。脱落涡旋的数量显示出了明显的年际和类年代际变化。在流轴的上下游区域，类年代际和年际变化分别占主导地位。并且在上游区域，脱落涡旋的类年代际变化与黑潮延伸体的强度呈负相关。在季节变化上，夏季脱落形成的涡旋最多，冬季最少。     

Formation Process of North Pacific Intermediate Water Revealed by Profiling Floats Set to Drift on 26.7 σθ Isopycnal Surface

Six newly developed floats, which were set to drift on the 26.7 σ_θ isopycnal surface and to profile temperature, salinity and pressure above 1000 dbar once a week, were deployed in the Oyashio and Kuroshio Extension (KE) in order to examine the circulation, formation site and time scale of newly formed North Pacific Intermediate Water (NPIW). The floats were deployed in February or May 2001, and the data from their deployments to December 2002 are analyzed here. Four of the six floats were deployed near the KE axis at around the first meander crest, and they moved eastward to 157°E–176°W at latitudes of 30°N–45°N. The other two floats deployed in the Oyashio water with low-potential vorticity near the south coast of Hokkaido moved southward to reach the KE front and then moved eastward to the same region as the first four floats. The temperature and salinity at 26.7 σ_θ measured by the profiling floats indicate that the source waters of NPIW, Oyashio and Kuroshio waters are drastically mixed and modified in the mixed water region west of 160°E. The floats were separated into the three paths east of 160°E between the Kuroshio Extension front and the north of Water-Mass front (nearly subarctic front). New NPIW is judged to be formed along these three paths since the vertical profiles of temperature and salinity are quite smooth, having a salinity minimum at about 26.7σ_θ along each path. Kuroshio-Oyashio isopycnal mixing ratios of the new NPIW are 7:3, 6:4 and 5:5 at 26.7σ_θ along the southern, middle and northern paths, respectively. Potential vorticity converges to about 14–15 × 10⁻¹¹ m⁻¹s⁻¹ along these paths. The time scale of new NPIW formation is estimated to be 1–1.5 years from the merger of Oyashio and Kuroshio waters to the formation of the new NPIW. This revised version was published online in July 2006 with corrections to the Cover Date.     

日本南部黑潮与黑潮延伸体路径状态关联性的定量分析

日本南部黑潮存在多种路径模态:近岸非大弯曲路径、离岸非大弯曲路径和大弯曲路径。黑潮延伸体的路径存在两种典型模态:收缩态和伸展态。从地理位置看, 日本南部黑潮和黑潮延伸体是相邻的, 但它们的路径状态是否存在关联一直存在争议。本文基于卫星观测的海表高度资料和长期的海洋高分辨率再分析资料, 对日本南部黑潮和黑潮延伸体路径状态之间的关联性进行定量分析, 结果表明:日本南部黑潮和黑潮延伸体的路径状态存在一定的关联。当日本南部黑潮处于近岸非大弯曲和大弯曲路径时, 黑潮延伸体主要处于伸展态; 当日本南部黑潮处于离岸非大弯曲时, 黑潮延伸体处于伸展态和收缩态的比例相当。进一步分析表明, 黑潮流轴处于伊豆海脊的位置部分决定了上述关系, 可能存在其他因素调制了两者的关联性。     

Structure of Subsurface Intrusion of the Oyashio Water into the Kuroshio Extension and Formation Process of the North Pacific Intermediate Water

In order to understand the actual formation process of the North Pacific Intermediate Water (NPIW), structure of subsurface intrusions of the Oyashio water and the mixing of the Oyashio and the Kuroshio waters in and around the Kuroshio Extension (KE) were examined on the basis of a synoptic CTD observation carried out in May-June 1992. The fresh Oyashio water in the south of Hokkaido was transported into KE region through the Mixed Water Region (MWR) in the form of subsurface intrusions along two main paths. The one was along the east coast of northern Japan through the First Branch of the Oyashio (FBO) and the other along the eastern face of a warm streamer which connected KE with a warm core ring through the Second Branch of the Oyashio (SBO). The fresh Oyashio water extended southward through FBO strongly mixed with the saline NPIW transported by the Kuroshio in the south of Japan (old NPIW) in and around the warm streamer. On the other hand, the one through SBO well preserved its original properties and extended eastward beyond 150°E along KE with a form of rather narrow band. The intrusion ejected Oyashio water lens with a diameter of 50–60 km southward across KE axis and split northward into the MWR involved in the interaction of KE and a warm core ring, which were supposed to be primary processes of new NPIW formation.     

Analysis of sea surface height variabilities in the Kuroshio Current region by using Geosat altimeter data

INTRODUCTIONBeing a current of high temperature and high salinity, the Kuroshio carries a large amount ofheat from low latitude tropical ocean to high latitude ocean, and plays an imPOrtant role in theheat balance in East Asia. The variability of the Kurosl,io can affect the climate of East Asia, aswell as the ocean environment and the fishery resources. A lot of studies showed that the variabilitiies of the Kuroshio were related to the global changes especially to the onset of ENSO.…     

黑潮延伸体区域50~100 km涡旋分布特征

本文使用基于热成风速度的涡旋识别拓展方法,通过海表面温度数据对黑潮延伸体区域50~100 km涡旋进行研究,发现50~100 km涡旋主要分布在黑潮延伸体流轴两侧,气旋涡和反气旋涡的寿命、半径分布具有一致性。气旋涡多出现在35°N以北,反气旋涡在35°N以南比较集中,与尺度较小的中尺度涡旋分布特征较为相似。冬夏两季涡旋地理分布存在一定差异,主要与不同季节该区域海表温度梯度及风应力旋度的变化有关。35°N以南50~100 km涡旋数量的季节性变化与风速大小的季节性变化存在明显的正相关性。35°N以南50~100 km涡旋三倍半径内风速异常和风应力旋度归一化表明,气旋涡对应风速负异常而反气旋涡对应风速正异常,反气旋涡的产生依赖于风应力负旋度,气旋涡的生成与风应力正旋度有关。     

Quasiperiodic small meanders of the Kuroshio off Cape Ashizuri and their inter-annual modulation caused by quasiperiodic arrivals of mesoscale eddies

A quasiperiodic variation of 100–110 days in the Kuroshio path off Cape Ashizuri, resulting from the passage of small meanders, was detected by observation with moored current meters during 1993–1995. TOPEX/POSEIDON altimeter data covering 9 years showed that the quasiperiodic variation period was not persistent and modulated twice, with a ∼110-day period from mid-1993 to late 1996, a ∼150-day period from late 1996 to mid-1999, and a ∼110-day period from mid-1999 to late 2001. The quasiperiodic variations of the Kuroshio path migration were contemporaneous with the quasiperiodic arrivals of mesoscale eddies from the east along 27–32°N over the same ∼110- and ∼150-day period quasiperiodic variations. The periodic arrivals of the eddies configure the periodic variations of the Kuroshio path and its inter-annual modulation.     

冬季黑潮延伸体海表温度变率的年代际变化及其与阿留申大气低压系统的关系

By utilizing multiple datasets from various sources available for the last 100 years, the existence for the interdecadal change of the winter sea surface temperature(SST) variability in the Kuroshio Extension(KE) region is investigated. And its linkage with the Aleutian Low(AL) activity changes is also discussed. The results find that the KE SST variability exhibits the significant ~6 a and ~10 a oscillations with obvious interdecadal change. The ~6 a oscillation is mainly detected during 1930–1950, which is largely impacted by the anomalous surface heat flux forcing and Ekman heat transport associated with the AL intensity variation. The ~10 a oscillation is most evident after the 1980s, which is predominantly triggered by the AL north-south shift through the bridge of oceanic Rossby waves.     

黑潮延伸体区域脱落涡旋的三维合成结构研究

Using AVISO satellite altimeter observations during 1993–2015 and a manual eddy detection method, a total of 276 anticyclonic rings and 242 cyclonic rings shed from the Kuroshio Extension(KE) were identified, and their three-dimensional(3D) anomaly structures were further reconstructd based on the Argo float data and the Japan Agency for Marine-Earth Science and Technology(JAMSTEC) cruise and buoy data through an interpolation method. It is found that the cyclonic(anticyclonic) rings presented consistent negative(positive) anomalies of potential temperature;meanwhile the relevant maximum anomaly center became increasingly shallow for the cyclonic rings whereas it went deeper for the anticyclonic rings as the potential temperature anomaly decreased from the west to the east. The above deepening or shoaling trend is associated with the zonal change of the depth of the main thermocline. Moreover, the composite cold ring between 140° and 150°E was found to exhibit a double-core vertical structure due to the existence of mode water with low potential vorticity. Specifically, a relatively large negative(positive) salinity anomaly and a small positive(negative) one appeared for the composite cyclonic(anticyclonic) ring at the depth above and below 600 m, respectively. The underlying driving force for the temperature and salinity anomaly of the composite rings was also attempted, which varies depending on the intensity of the background current and the temperature and salinity fields in different areas of the KE region, and the rings’ influences on the temperature and salinity could reach deeper than 1 000 m on average.     

黑潮延续体温盐锋的时空变化特征及其声场的初步分析

通过对WOA13多年(1955—2012年)季节平均数据的分析,利用绝对梯度最大值连线的方法,得到日本东海岸黑潮延续体温盐锋轴的空间信息,对锋轴线上的锋强度的季节变化特征进行了分析,并利用BELLHOP模型初步讨论了锋区内的声场特点。认为黑潮延续体锋轴线的位置随深度增加逐渐南移,其温盐强度随季节随深度都有明显变化,1—3月份混合层现象明显,此时水下声道受到声源深度的影响明显。     

Unusual behavior of the Kuroshio Current System from winter 1996 to summer 1997 revealed by ADEOS-OCTS and other data— Suggestion of topographically forced alternating-jet instability

While the Advanced Earth Observing Satellite (ADEOS) was operating, the Kuroshio and the Kuroshio Extension, or the Kuroshio Current System, exhibited unusual behavior from the winter of 1996 to the summer of 1997. This behavior of the Kuroshio Current System has been closely studied using a time series of satellite observation images of SST and ocean color obtained by ADEOS-OCTS, reinforced by SST images obtained by NOAA-AVHRR. Our findings include (i) a long lasting, very southerly path of the Kuroshio Extension; (ii) a Kuroshio path very distant from Japan with the following alternating-jet-like north-south flow pattern of the Kuroshio Extension, which occurred twice, once in February and once in April 1997, as independent events and which was observed to be affected by the bottom topography of the Izu-Ogasawara Ridge and Trench, and of the Japan Trench; (iii) cutting off of a cold water mass after the February event; and (iv) the formation of a vortex pair after the April event. A new mechanism is suggested for the formation of the alternating-jet flow pattern: a topographically forced alternating-jet instability (AJI). An SST-Chlorophyll Diagram (T-Chl Diagram) generated using simultaneous data from a single satellite is useful for analyzing the water mass structure of this region, including biological processes.     

Kuroshio extension variability explored through assimilation of TOPEX/POSEIDON altimeter data into a quasi-geostrophic model

Mesoscale features in the eastward extension of the Kuroshio were investigated using assimilation of TOPEX/POSEIDON (T/P) data into a three-layer quasi-geostrophic model. The T/P data exhibited an elongated state of the southern recirculation gyre in 1993–95 and 1997, between whose two periods the gyre had a contracted state in 1995–96. A few stationary eddies were located in the southern gyre during the contracted state. The baroclinic instability, which was indicated by the phase shift from the uppermost-to the lowest-layer anomalies toward the downstream side, was evident near the Kuroshio Extension (KE) path. Since the instability never appeared in the artificial model without bottom topography, the topographic barrier for the eastward flow in the lowest layer was a necessary condition for the instability. The instability synchronized with the transition in the western region of the KE axis from the elongated to the contracted states. This evolution was interpreted as if the baroclinic instability played some part in the KE states and was a trigger for the transition from the elongated to the contracted states.     

Decadal seesaw of the Central and Subtropical Mode Water formation associated with the Kuroshio Extension variability

Available Argo profiling float data from 2002 to 2011 were analyzed to examine the effect of the Kuroshio Extension (KE) current system variability on the formation of the Central Mode Water. Just north of the upstream portion of the KE at 140–152°E, formation of a lighter variety of the Central Mode Water in winter was active during the unstable period of the upstream KE in 2006–2009 and was reduced when the upstream KE was in the stable period of 2002–2005 and 2010–2011. This decadal formation variability is out of phase with that of the Subtropical Mode Water just south of the KE.     

Assimilation of sea surface temperature in the Northwest Pacific Ocean and its marginal seas using the ensemble Kalman filter

Satellite-borne sea surface temperature (SST) data were assimilated with the ensemble Kalman filter (EnKF) in a Northwest Pacific Ocean circulation model to examine the effect of data assimilation. The model domain included the northwestern part of the Pacific Ocean and its marginal seas, such as the Yellow Sea and East/Japan Sea. The performance of the data assimilation was evaluated by comparing the simulated ocean state with that observed. Spatially averaged root-mean-squared errors in the SST and sea surface height (SSH) decreased by 0.44 °C and 4 cm, respectively, by the assimilation. The results of the numerical experiments substantiated the effectiveness of the SST assimilation via the EnKF for all marginal seas, as well as the Kuroshio region. The benefit of the data assimilation depended on the characteristics of each marginal sea. The variation of the SST in the East/Japan Sea and the Kuroshio extension (KE) region were improved 34% and those in the Yellow Sea 12.5%. The variation of the SSH was improved approximately 36% in the KE region. This large improvement was achieved in the deep-water regions because assimilation of SST data corrected the separation point of the western boundary currents, such as the Kuroshio and the East Korea Warm Current, and the associated horizontal surface currents. The SST assimilation via the EnKF also improved the subsurface temperature profiles. The effectiveness of SST assimilation was seasonally dependent, with the improvement being relatively larger in winter than in summer, which was related to the seasonal variation of the vertical mixing and stratification in the ocean surface layer.     

黑潮延伸体区域50-100公里涡旋分布特征

本文使用基于热成风速度的涡旋识别拓展方法,通过海表面温度数据对黑潮延伸体区域50-100公里涡旋进行研究,发现50-100公里涡旋主要分布在黑潮延伸体流轴两侧,气旋涡和反气旋涡的寿命、半径分布具有一致性。气旋涡多出现在35°N以北,反气旋涡在35°N以南比较集中,与尺度较小的中尺度涡旋分布特征较为相似。冬夏两季涡旋地理分布存在一定差异,主要与不同季节该区域海表温度梯度及风应力旋度的变化有关。35°N以南50-100公里涡旋数量的季节性变化与风速大小的季节性变化存在明显的正相关性。35°N以南50-100公里涡旋三倍半径内风速异常和风应力旋度归一化表明,气旋涡对应风速负异常而反气旋涡对应风速正异常,反气旋涡的产生依赖于风应力负旋度,气旋涡的生成与风应力正旋度有关。     

Time-Dependent Spatial Wavenumber Spectra Derived from Merged Sea Surface Temperature Data

Monthly wavenumber spectra of sea surface temperatures (SST) have been estimated in two regions near the Kuroshio, in the recirculation and the Kuroshio Extension regions, using the merged SST product to determine the statistical parameter (spatial decorrelation scale) required for optimal interpolation of a high-resolution SST dataset. The two-dimensional wavelet transform was used for analysis. Estimates were made of daily mean and daily minimum SSTs. These do not significantly differ, which suggests that the same covariance matrix can be used for the daily mean and minimum in the merging procedure. The seasonality of wavenumber spectra is significant. There are also large differences between those in the recirculation region and in the Kuroshio Extension region. Therefore, it is recommended that the covariance matrix in the merging process for high-resolution SST dataset be defined as a function of time and space. Improvements of the merging methodology are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.