Submesoscale eddies in the East China Sea detected from SAR images

Seven-year(2005–2011) Synthetic Aperture Radar(SAR) images are applied to study oceanic eddies in the East China Sea. It is found that most of these eddies detected from the SAR images are less than 10 km, which are submesoscale eddies. Seasonal differences are evident in the distribution of eddies, with the highest and the lowest number of eddies noted in summer and winter, respectively. Since slick streaks in SAR images look dark, an eddy identified due to the slicks is referred to as "black eddy". As a result of wave-current interactions in the zones of current shear, it can be seen that an eddy exhibits a bright curve, the eddy is called "white eddy". During the seven years, 95 black eddies and 50 white eddies are identified in the study area. Black eddies are found in the whole study area while white eddies are mainly distributed in the vicinity of the Kuroshio Current. This study suggests that the distribution of the white eddy is denser around the Kuroshio because of the strong shear in the Kuroshio region. In terms of the eddy sizes, white eddies are generally smaller than black eddies.     

苏门答腊岛西北海域中尺度涡源区特征与形成机制

孟加拉湾内和湾口附近有丰富的中尺度现象,本文利用2.0版可分辨低纬地区中尺度涡的Chelton数据集,通过溯源的方法得到中尺度涡的源地分布。苏门答腊岛西北海域(以5°N,94°E为核心的区域)是中尺度涡重要源区之一。通过拉格朗日方法的涡旋追踪表明,1993—2017年该海域(3°N—6°N、92°E—95°E),分别有57个气旋式和40个反气旋式中尺度涡。频谱分析显示海表面高度异常存在180 d和360 d两个显著周期。地形和风场的共同作用是该海域产生中尺度涡的动力机制：沿5°N西传的罗斯贝波在海岭地形的作用下触发了中尺度涡的生成;赤道风场是源区重要的能量来源,局地风场能诱发中尺度涡的极性。本研究也揭示了以往文献虽刻画了苏门答腊岛西北部海域为高涡动能区,却没有识别出较多中尺度涡的原因。     

Statistical characteristics and thermohaline properties of mesoscale eddies in the Bay of Bengal

The statistical characteristics and vertical thermohaline properties of mesoscale eddies in the Bay of Bengal are studied from the view of satellite altimetry data and Argo profiles. Eddy propagation preferences in different lifetimes, eddy evolution process, and geographical distribution of eddy kinetic properties are analyzed in this area. Eddies exist principally in the western Bay of Bengal, and most of them propagate westward. There is a clear southward(equatorward) preference for eddies with long lifetimes, especially for cyclones. Moreover, the eddies in different areas of the bay show different north-southward preferences. Evolution of eddy kinetic properties with lifetime shows that eddies have the significant three-stage feature: the growth period in the former one-fifth lifetime, the stable period in the middle two-fifth to four-fifth lifetime, and the dying period in the last one-fifth lifetime. Large-amplitude and high-intensity eddies occur only in the relatively confined regions of highly unstable currents, such as the East Indian Coastal Current and eastern Sri Lanka. Based on Argo profile data and climatology data, the eddy synthesis method was used to construct three-dimensional temperature and salt structures of eddies in this area. The mean temperature anomaly is negative/positive to the cyclonic/anticyclonic eddies in the upper 300×10~4 Pa, and below this depth, the anomaly becomes weak. The salinity structures of positive anomalies inside cyclonic eddies and negative anomalies inside anticyclonic eddies in the Bay of Bengal are not consistent with other regions. Due to the special characteristics of the water mass in the bay, especially under the control of the low-salinity Bay of Bengal water at the surface and the Indian equatorial water in the deep ocean, the salinity of seawater shows a monotonic increase with depth. For regional varieties of temperature and salinity structures, as the eddies move westward, the temperature anomaly induced by the eddies increases, the effecting depth of the eddies deepens, and the salinity structures are more affected by inflows. In the north-south direction, the salinity structures of the eddies are associated with the local water masses, which comprise lowsalinity water in the northern bay due to the inflow of freshwater from rivers and salty water in the southern bay due to the invasion of Arabian Sea high-salinity water from the north Indian Ocean.     

Diagnostic calculation of the circulation in the South China Sea during summer 1998

On the basis of hydrographic data obtained from 12 June to 6 July, 1998, the three-dimensional structure of circulation in the South China Sea (SCS) is computed using a three-dimensional diagnostic model. The combination of sea surface height anomaly from altimeter data and numerical results provides a consistent circulation pattern for the SCS, and the main circulation features can be summarized as follows: In the northern SCS there are a cyclonic eddy C1 near Dongsha Islands and an anti-cyclonic eddy W1 west of Luzon Island. In the central SCS a strong anti-cyclonic eddy W3 and a cyclonic eddy C3 compose a quasi-dipole southeast of Vietnam. A coastal northward jet is present at the western boundary near the Vietnam coast above 300 m level. This northward coastal jet flows northward and turns eastward at about 14°N, and then flows southeastward into the area between eddies W3 and C3. In the southern SCS the current is weaker. The most important dynamic mechanism underlying the circulation in the SCS is the joint effect of the baroclinicity and relief (JEBAR), and the second dynamical mechanism is the interaction between the wind stress and relief (IBWSR). Comparison of the characters of circulation in the SCS during summer 2000 with that during summer 1998 reveals no obvious variability of the main characteristics.     

Numerical study on the summer circulation of the upper South China Sea and its establishment

A coupled single-layer/two-layer model is employed to study the South China Sea (SCS) upper circulation and its response before and after the onset of summer monsoon. It is found that, in summer, due to the β effect and the first baroclinic mode of the wind-driven current, a northward western boundary jet current is formed along the Indo-China Peninsula coast, and it leaves the coast at about 13° N and diffuses towards northeast; next to the Indo-China Peninsula, a large anticyclonic     

Oceanic eddy-driven atmospheric secondary circulation in the winter Kuroshio Extension region

In the winter Kuroshio Extension region, the atmospheric response to oceanic eddies is studied using reanalysis and satellite data. The detected eddies in this region are mostly under the force of northwesterly wind, with the sea surface temperature (SST) anomaly located within the eddy. By examining the patterns of surface wind divergence, three types of atmospheric response are identified. The first type, which occupies 60%, is characterized by significant sea surface wind convergence and divergence at the edge and a vertical secondary circulation (SC) aloft, supporting the “vertical momentum mixing mechanism”. The SCs on anticyclonic eddies (AEs) can reach up to 300 hPa, but those on cyclonic eddies (CEs) are limited to 700 hPa. This can be explained by analyzing vertical eddy heat transport: When northwesterly wind passes the warmer center of an AE, it is from the cold to warm sea surface, resulting in stronger evaporation and convection, triggering stronger upward velocity and moist static heat flux. For the cases of CEs, the wind blows from warm to cold, which means less instability and less evaporation, resulting in weaker SCs. The second type, which occupies 10%, is characterized by divergence and a sea level pressure anomaly in the center, supported by the “pressure adjustment mechanism”. The other 30% are mostly weak eddies, and the atmospheric variation aloft is unrelated to the SST anomaly. Our work provides evidence for the different atmospheric responses over oceanic eddies and explains why SCs over AEs are much stronger than those over CEs by vertical heat flux analysis.     

Upwelling of oxygen-depleted water (Sumishio) in Omura Bay,Japan

Wind-induced circulation and the distribution of hypoxia corresponding to the upwelling of oxygen-depleted water (called “Aoshio” in Japan and “Sumishio” locally in Omura Bay) in Omura Bay, Japan, was examined field observations and by three-dimensional modeling. During the calm weather in summer, well-mixed strait water, in rich oxygen at the mouth of the bay intruded into the middle layer of the bay, southward and northward along the west and east coast, respectively, forming basin-scale cyclonic circulation. A stagnant water mass was formed below the center of this cyclonic circulation, and it became hypoxic water. As a result of the prevailing strong southeast (northeast) wind, the bottom hypoxia moved in a southeasterly (northeasterly) direction. This induces the upwelling of hypoxic water, accompanied by mass mortality of marine organisms.     

Observed Variability of Thermohaline Fields,Currents and Eddies in the Western Bay of Bengal During BOBMEX-99

Field measurements during the Bay of Bengal Monsoon Experiment (BOBMEX-99), data from a deep sea moored buoy, and satellite altimeter were used to describe variability in the hydrographic and meso-scale features in the Bay of Bengal (BoB) during the summer monsoon of 1999. The thermohaline fields showed two regions of upsloping of isopleths centered at 82°E and 84.75°E, ～110 km and 450 km away from the coast, respectively, followed by downsloping. The upsloping/downsloping of isopleths and the alternating currents was part of cyclonic and anti-cyclonic circulation patterns in the western BoB. In this region, both wind and current were important in the dynamics of coastal upwelling. The observations showed a relationship between the propagating waves and eddy on variability of thermohaline fields. On an annual cycle, four Kelvin waves were observed in the BoB, but only the downwelling Kelvin wave formed during October entered the Arabian Sea. During the monsoon season, four eddies were formed in the western BoB, of which the anticyclonic eddy centered at 15°N, 84°E and the cyclonic eddy centered at 17.5°N, 84.5°E were prominent. The baroclinic instability caused by the opposing currents along the east coast and the wind stress curl favored the formation of eddies. Okhubo-Weiss and Isern-Fontanet parameter confirmed the presence of eddies in the BoB.     

A numerical study of the seasonal circulation in the Seto Inland Sea,Japan

The seasonal variation of water circulation in the Seto Inland Sea is investigated using a high resolution, three-dimensional numerical ocean model. The model results are assessed by comparison with long-term mean surface current and hydrographic data. The simulated model results are consistent with observations, showing a distinct summer and winter circulation patterns. In summer the sea water is highly stratified in basin regions, while it is well mixed near the straits due to strong tidal mixing there. During this period, a cold dome is formed in several basins, setting up stable cyclonic eddies. The cyclonic circulation associated with the cold dome develops from May and disappears in autumn when the surface cooling starts. The experiment without freshwater input shows that a basin-scale estuarine circulation coexists with cyclonic eddy in summer. The former becomes dominant in autumn circulation after the cold dome disappears. In winter the water is vertically well mixed, and the winter winds play a significant role in the circulation. The northwesterly winds induce upwind (downwind) currents over the deep (shallow) water, forming a “double-gyre pattern” in the Suo-Nada, two cyclonic eddies in Hiuchi-Nada, and anticyclonic circulation in Harima-Nada in vertically averaged current fields.     

夏季南海上层环流动力机制的数值研究

通过利用一个分区性的正压-斜压衔接模式来探讨夏季南海的上层环流特征及其动力机制,结果表明:夏季期间,由于风生环流的不稳定性促使在东沙群岛附近的气旋涡的强度及位置发生变化,并间接导致黑潮侵入南海北部的程度变化以及气旋涡南侧的反气旋式环流、西沙群岛西南侧的气旋涡的强度和范围出现波动现象;在南海南部的北向西边界流由于离岸的西南季风所驱动在中南半岛中部沿岸脱离岸线往东北方向的流动,导致沿岸的水体大量流失而在沿岸形成一支南向补偿流并在西沙群岛西南侧诱生一气旋涡,而上述的离岸西边界流则作顺时针方向流动,从而在南海南部形成反气旋式大环流;在南沙海槽附近出现的局地气旋涡和万安滩附近的气旋涡分别受β效应、底形效应的作用而形成.     

基于深度学习的中尺度涡检测技术及其在声场中的应用

针对海洋中尺度涡的检测与参数提取问题,本文使用中尺度涡SAR图像数据集,提出基于深度学习的EddyYolo目标检测模型进行中尺度涡的涡旋中心和涡旋水平尺度的多目标检测,并且提取涡心位置和涡旋水平半径等参数.实验结果表明:本文提出的EddyYolo模型实现了涡旋中心和涡旋水平尺度的多目标检测,检测准确率达到94%.在此基础上,结合二维高斯涡模型和三维中尺度涡模型,本文提出了基于卫星遥感与声学对中尺度涡的联合建模方法.     

Topographic effect of a marine ridge on the spin-down of a cyclonic eddy

The topographic effect of a meridional marine ridge on the spin-down of a cyclonic eddy is discussed by use of a two layer model. It is shown that cyclonic eddies on the eastern side of a marine ridge decay in a shorter time in comparison with those on the western side. The fast decay on the eastern side is mainly due to the planetary effect, which carries the eddy westward to a shallower region on the ridge, where remarkable frictional spin-down is carried out. Conversely, westward shift of the similar cyclonic eddy yields a weak bottom effect in the western side. It is inferred that the resulting remarkable spin-down on the eastern side of a marine ridge is one of the causes of the observed fast decay of the cyclonic cold water mass accompanied by the large meander of the Kuroshio on the eastern side of the Izu Ridge south of Japan.     

基于拉格朗日分析法的中尺度涡精细三维结构研究及其误差估计

在前人的工作中,拉格朗日分析法被用来演示大尺度环流,同时拉格朗日拟序结构可以较好的演示中尺度涡两维结构的发展过程。然而,很少研究关注怎么利用拉格朗日分析法针对中尺度涡三维结构进行演示。与以往利用欧拉方法研究中尺度涡三维结构的工作不同,我们利用拉格朗日分析法,从另一个视角来研究涡旋结构。我们在海山上方模拟出一个理想的气旋涡,涡旋内的下沉流和涡旋旁的上升流形成一个闭合的环流。这种结构很难从欧拉角度来演示。然而,粒子的运动轨迹很好地展示了整个循环:流体在涡旋中旋转下沉,汇聚到底层的上升流区,并通过上升流返回到海表面。我们也将拉格朗日分析法应用于真实的模拟结果中。作为中国南海的一个重要现象,靠近越南中部的海域中的偶极子（反气旋涡/气旋涡）,关于其结构的研究已经比较成熟了,但这些研究主要关注的是海面过程。通过拉格朗日分析,我们很好的演示了偶极子的三维结构:流体在反气旋涡(气旋涡)内部旋转上升(下沉)。更重要的是,粒子的轨迹表明,这两个涡旋之间不存在水团交换,因为强边界急流将它们彼此分开。以上结论均得到了计算误差估计的可信度支持。尽管在强辐散流和强垂直扩散流中,计算误差逐渐增大,但是在一定的时间步长和积分周期内,计算误差始终保持在一个较小的值。     

南海环流的一个约化模式

利用约化数值模式研究了黑潮在巴士海峡的流况及受其影响的南海海盆区的环流,结果为:定常的黑潮入流在巴士海峡不易出现显著的环状流动结构,但在海峡西侧诱生一气旋涡,该涡旋达到一定强度时,β因子和侧边界作用使其向西南移动,因此,模式给出的南海环流呈准半年周期的气旋涡现象。动力分析表明,气旋涡因非线性平流作用将黑潮西侧的气旋性切变涡度向南海北部输送所致。模式同时计算了入流方向和流轴位置呈周期性变化时,巴士海峡和南海的流动结构。     

Inversion of the three-dimensional temperature structure of mesoscale eddies in the Northwest Pacific based on deep learning

Mesoscale eddies, which are mainly caused by baroclinic effects in the ocean, are common oceanic phenomena in the Northwest Pacific Ocean and play very important roles in ocean circulation, ocean dynamics and material energy transport. The temperature structure of mesoscale eddies will lead to variations in oceanic baroclinity, which can be reflected in the sea level anomaly (SLA). Deep learning can automatically extract different features of data at multiple levels without human intervention, and find the hidden relations of data. Therefore, combining satellite SLA data with deep learning is a good way to invert the temperature structure inside eddies. This paper proposes a deep learning algorithm, eddy convolution neural network (ECN), which can train the relationship between mesoscale eddy temperature anomalies and sea level anomalies (SLAs), relying on the powerful feature extraction and learning abilities of convolutional neural networks. After obtaining the temperature structure model through ECN, according to climatic temperature data, the temperature structure of mesoscale eddies in the Northwest Pacific is retrieved with a spatial resolution of 0.25° at depths of 0–1 000 m. The overall accuracy of the ECN temperature structure is verified using Argo profiles at the locations of cyclonic and anticyclonic eddies during 2015–2016. Taking 10% error as the acceptable threshold of accuracy, 89.64% and 87.25% of the cyclonic and anticyclonic eddy temperature structures obtained by ECN met the threshold, respectively.     

南海东北部中尺度涡对海表叶绿素a浓度的影响规律研究

基于南海东北部1998~2019年的多源卫星遥感数据和风场再分析数据, 较系统地分析了南海东北部涡旋内部叶绿素a浓度的分布特征, 通过量化统计和涡心坐标系参数合成等方法探究了中尺度涡对叶绿素a浓度变化的影响规律及潜在机制。结果表明: (1)南海东北部约有60%的中尺度涡旋内部存在叶绿素a浓度增加和减少的现象。(2)南海东北部中尺度涡内部叶绿素a扰动受到涡旋抽吸和涡致Ekman抽吸机制的共同调控, 其中约有38% (39%)的暖(冷)涡内涡旋抽吸的贡献更大, 21% (24%)的暖(冷)涡内涡致Ekman抽吸的贡献更大。(3)南海东北部中尺度涡生命周期内的海表叶绿素a浓度变化存在显著的阶段性差异, 在冷暖涡的生成期, 涡旋抽吸的作用更为显著, 而在冷暖涡的顶峰和消亡期, 涡致Ekman抽吸的作用更为明显。上述研究结果有助于理解南海东北部初级生产力对中尺度涡的响应过程与机理, 对认识海洋物理-生物耦合过程具有一定的参考价值和研究意义。     

Three dimensional diagnostic study of the circulation in the South China Sea during winter 1998

On the basis of hydrographic data obtained from 28 November to 27 December, 1998, the three-dimensional structure of circulation in the South China Sea (SCS) is computed using a three-dimensional diagnostic model. The combination of sea surface height anomaly from altimeter data and numerical results provides a consistent circulation pattern for the SCS, and main circulation features can be summarized as follows: in the northern SCS there are a cold and cyclonic circulation C1 with two cores C1-1 and C1-2 northwest of Luzon and an anticyclonic eddy (W1) near Dongsha Islands. In the central SCS there is a stronger cyclonic circulation C2 with two cores C2-1 and C2-2 east of Vietnam and a weaker anticyclonic eddy W2 northwest of Palawan Island. A stronger coastal southward jet presents west of the eddy C2 and turns to the southeast in the region southwest of eddy C2-2, and it then turns to flow eastward in the region south of eddy C2-2. In the southern SCS there are a weak cyclonic eddy C3 northwest of Borneo and an anti-cyclonic circulation W3 in the subsurface layer. The net westward volume transport through section CD at 119.125°E from 18.975° to 21.725°N is about 10.3 × 10⁶ m³s⁻¹ in the layer above 400 m level. The most important dynamic mechanism generating the circulation in the SCS is a joint effect of the baroclinicity and relief (JEBAR), and the second dynamical mechanism is an interaction between the wind stress and relief (IBWSR). The strong upwelling occurs off northwest Luzon.     

Three-dimensional properties of mesoscale cyclonic warm-core and anticyclonic cold-core eddies in the South China Sea

In general, a mesoscale cyclonic (anticyclonic) eddy has a colder (warmer) core, and it is considered as a cold (warm) eddy. However, recently research found that there are a number of “abnormal” mesoscale cyclonic (anticyclonic) eddies associated with warm (cold) cores in the South China Sea (SCS). These “abnormal” eddies pose a challenge to previous works on eddy detection, characteristic analysis, eddy-induced heat and salt transports, and even on mesoscale eddy dynamics. Based on a 9-year (2000–2008) numerical modelling data, the cyclonic warm-core eddies (CWEs) and anticyclonic cold-core eddies (ACEs) in the SCS are analyzed. This study found that the highest incidence area of the “abnormal” eddies is the northwest of Luzon Strait. In terms of the eddy snapshot counting method, 8 620 CWEs and 9 879 ACEs are detected, accounting for 14.6% and 15.8% of the total eddy number, respectively. The size of the “abnormal” eddies is usually smaller than that of the “normal” eddies, with the radius only around 50 km. In the generation time aspect, they usually appear within the 0.1–0.3 interval in the normalized eddy lifespan. The survival time of CWEs (ACEs) occupies 16.3% (17.1%) of the total eddy lifespan. Based on two case studies, the intrusion of Kuroshio warm water is considered as a key mechanism for the generation of these “abnormal” eddies near the northeastern SCS.     

Numerical study on tidal currents and seawater exchange in the Benoa Bay, Bali, Indonesia

A three-dimensional （3-D） finite volume coastal ocean model （FVCOM） was used for the study of water cir culation and seawater exchange in the Benoa Bay, Bali Island. The M2 tidal component was forced in open boundary and discharge from six rivers was included in the numerical calculation. The M2 tidal elevation produced by the FVCOM has a good agreement with the observation data. The M2 tidal current is also suc cessfully calculated under the ebb tide and flood tide conditions. The non-linear M2 tidal residual current was produced by the coastline geometry, especially surrounding the narrow strait between the Serangan Is- land and the Benoa Peninsula. The tidal residual current also generated two small eddies within the bay and one small eddy in the bay mouth. The salinity distribution influenced by river discharge could be success- fully calculated, where the numerical calculation and the observation results have a good correlation （r2） of 0.75. Finally in order to examine the seawater exchange in the Benoa Bay, the Lagrangian particle tracking method and calculation of residence time are applied. The mechanism of particle transport to the flushing of seawater is depicted clearly by both methods.     

Wind- and density-driven circulation in a bay on the Sanriku ria coast,Japan: study of Shizugawa Bay facing the Pacific Ocean

Flow fields in Shizugawa Bay on the Sanriku ria coast, which faces the Pacific Ocean, were investigated using hydrographic observations for the purpose of understanding oceanographic conditions and the process of water exchanges in the bay after the 2011 earthquake off the Pacific coast of Tohoku. In spring to summer, density-driven surface outflow is part of estuarine circulation and is induced by a pressure gradient force under larger longitudinal gradients in density along with lower salinity water in the innermost part of the bay, regardless of wind forcing. In winter to summer, another density-driven current with a thermal structure is induced by a pressure gradient force under the smaller longitudinal density gradients in calm wind conditions. Particularly in winter, Tsugaru Warm Current water can be transported in the surface layer inside the bay. Wind-driven bay-scale circulation with downwind and upwind currents in the surface and deeper layers, respectively, is induced by strong longitudinal wind forcing under the smaller longitudinal density gradients, irrespective of season. Particularly in fall to spring, this circulation can cause the intrusions of oceanic water associated with Oyashio water and Tsugaru Warm Current water in the deeper layer. These results suggest that wind- and density-driven currents can produce the active exchange of water from inside and outside the bay throughout the year.