Influence of the Oyashio Current and Tsugaru Warm Current on the circulation and water properties of Otsuchi Bay,Japan

Mooring and hydrographic observations were conducted from September 2012 to May 2014 at the mouth of Otsuchi Bay, a ria along the Pacific coast of Japan. Our observations quantitatively demonstrated that the circulation and the water properties of Otsuchi Bay are strongly influenced by the Tsugaru Warm Current (TWC) and Oyashio Current (OY) at seasonal and subseasonal time scales. Two bottom-mounted velocity profilers and temperature and salinity measurements beneath the near-surface halocline showed a counterclockwise lateral circulation pattern related to the TWC, which was enhanced from summer to autumn. From winter to early spring, the lateral circulation patterns related to the TWC weakened and the influence of the OY occasionally increased. When the OY was weak, surface flows became an overturning structure, with outflows in the upper layer and inflows in the lower layer. When the OY was strong and passed close to the Sanriku coast, the circulation became highly variable and intermittent. Intrusions of the markedly low-salinity OY water were observed on two occasions and persisted for periods of several weeks to several months. Salinity was sometimes less than 33.7, the lower limit of the typical TWC from late summer to autumn even when the TWC dominates. We suggested that this is the seasonal fluctuations of the TWC itself, as the upstream current of the Tsushima Warm Current is freshened in summer as a result of the influence of the Changjiang River. The surface water was generally fresher in the south of the bay than in the north, suggesting the Coriolis deflection of the river plume.     

Peculiarities of the currents field in the north-western part of the Black Sea

On the basis of the results derived by theoretical analysis [1] and laboratory physical modelling [2] of the circulation seawater and stratification, induced by buoyancy fluxes, the suggestion has been made that, possibly, the Black Sea currents system is three-layered. It has been supposed that an anticyclonic current flows in the basin. To verify this deduction, the data from MHI's archived dataset have been treated, and results from twoad hoc expeditions are submitted here. Analysis of the vertical structure of currents, based on thein situ velocity measurements, using various means and calculations of the geostrophic circulation, has confirmed that a flow with the theoretically predicted parameters may exist in the Black Sea. Translated by Vladimir A. Puchkin.     

Circulation and transport of water masses in the Lazarev Sea, Antarctica, during summer and winter 2006

The distribution and circulation of water masses in the region between 6°W and 3°E and between the Antarctic continental shelf and 60°S are analyzed using hydrographic and shipboard acoustic Doppler current profiler (ADCP) data taken during austral summer 2005/2006 and austral winter 2006. In both seasons two gateways are apparent where Warm Deep Water (WDW) and other water masses enter the Weddell Gyre through the Lazarev Sea: (a) a probably topographically trapped westward, then southwestward circulation around the northwestern edge of Maud Rise with maximum velocities of about 20 cm s⁻¹ and (b) the Antarctic Coastal Current (AntCC), which is confined to the Antarctic continental shelf slope and is associated with maximum velocities of about 25 cm s⁻¹.Along two meridional sections that run close to the top of Maud Rise along 3°E, geostrophic velocity shears were calculated from CTD measurements and referenced to velocity profiles recorded by an ADCP in the upper 300 m. The mean accuracy of the absolute geostrophic velocity is estimated at ±2 cm s⁻¹. The net baroclinic transport across the 3°E section amounts to 20 and 17 Sv westward for the summer and winter season, respectively. The majority of the baroclinic transport, which accounts for ∼60% of the total baroclinic transport during both surveys, occurs north of Maud Rise between 65° and 60°S.However, the comparison between geostrophic estimates and direct velocity measurements shows that the circulation within the study area has a strong barotropic component, so that calculations based on the dynamic method underestimate the transport considerably. Estimation of the net absolute volume transports across 3°E suggests a westward flow of 23.9±19.9 Sv in austral summer and 93.6±20.1 Sv in austral winter. Part of this large seasonal transport variation can be explained by differences in the gyre-scale forcing through wind stress curl.     

珠江磨刀门河口环流结构动力特征分析

河口环流结构关系到物质输运、泥沙沉积和地貌变化等物理过程。根据2019年磨刀门河口原型观测平台洪枯季连续观测分层潮流资料,统计洪枯季、大小潮河口东、西汊的涨落潮流及历时变化特征,利用理论方法解析河口东西汊平面环流和重力环流结构,进一步引入混合参数研究河口纵向环流中的潮汐应变环流。研究发现枯季东、西汊在转潮时刻存在东涨西落的平面环流结构,洪季平面环流特征较不明显;枯季重力环流强度整体略大于洪季,西汊重力环流强于东汊,表层向海环流流速可达0.2～0.25 m/s,而底层向陆环流流速相对较小。洪季大潮期由潮不对称性驱动的潮汐应变环流相对较大,进而增强了纵向环流的强度。河口垂向余流结构同样表现洪枯季、大小潮的变化规律。洪季余流整体较大,西汊在小潮期表层余流流速超过0.6 m/s,而东汊余流则明显呈现表层向海、底层向陆的分布特征,枯季余流整体较小,表明其对物质输运和河口地形塑造作用较弱。     

杭州湾南部主水道内温度、盐度、潮流的变化特征

本文基于船载及锚定ADCP、CTD观测,获得了大、小潮时的温度、盐度、浊度、潮位、流速等观测数据,研究杭州湾南部的一个主要弯曲航道——螺头水道内的潮流动力学特性。螺头水道水深超过100 m、最大潮差大于2.5 m。涨潮时,强潮流速出现在水道北岸,落潮时,强潮流速出现在水道南岸,最大流速值分别为约2 m/s和1.8 m/s。受压强梯度、密度梯度、科氏力和离心力影响,涨落潮过渡时在水道的横断面产生较为明显的环流。夏季存在较弱的层化现象,深水处受环流的影响,盐度、温度的混合较强。锚定观测数据表明,温度、盐度的变化频率与潮流的变化频率相似,但存在高于M2分潮频率的谱峰值。因此,笔者认为潮流与横向环流的相互作用,可能导致更高频率的盐度和温度变化。     

The principal factors contributing to the flux of salt in a narrow, partially stratified estuary

Observations of the velocity and salinity structure of the Tees estuary were made at eight stations along the estuary axis between Victoria Bridge and the sea during the summer of 1975. The measurements were made on ten separate tidal periods covering neap and spring tides.The data were collected over a period of relatively low freshwater flows and the residual current was found to have a strong dependence on the Stokes drift. At the upstream stations, the residuals were more than an order of magnitude greater than the currents anticipated from the freshwater discharge. Although the mean stratification decreased as the tidal range increased, the vertical circulation was stronger on spring tides than on neaps. Vertical variations in the amplitude and phase of the tidal current results in a current which strengthens the vertical circulation. However, this effect only made a relatively small contribution to the observed vertical circulation.The relative contribution of the individual salt flux terms to the net upstream transport of salt varies along the estuary. As the estuary narrows, the contribution by the oscillatory terms dominates that from the shear in the steady state flow. Of these oscillatory terms, the correlation of velocity and salinity fluctuations plays a key rôle in the salt transport. The depth mean values make a greater contribution than deviations from the depth mean and the flux due to phase variations over depth is smaller than either of these. Since the Stokes drift is compensated by a down-stream steady state flow, it does not contribute to the tidal mean transport of salt.At the seaward end of the estuary, the salt fluxes due to the steady state vertical shear and the convariance of the tidal fluctuations act in a complementary way to counter the seaward transport of salt by the freshwater flow. With the possible exceptions of the wide or narrow reaches of the Tees, the longitudinal fluxes of salt due to transverse variations in velocity, salinity and depth and turbulent fluctuations are of secondary importance as contributors to the estuary salt budget.On both neap and spring tides, the computed total salt transports at the Newport and Victoria bridges did not match the values required for a salt balance with the corresponding freshwater flows. These fluxes were probably the cause of the observed downstream displacement of the tidal mean salinity distribution between neap and spring tides.     

2015年6月南海北部陆坡气旋-反气旋涡对陆坡水文和环流影响的观测

利用2015年6月南海北部现场观测的水文数据,结合卫星高度计资料,分析了2015年6月13日—28日南海北部陆坡在气旋涡-反气旋涡的双涡结构影响下的水文和环流特征。结果表明,2015年6月南海北部陆坡调查海区表层50 m以浅盐度存在NE—SW向低盐区,表层盐度最小值低于32,这表明南海北部陆坡存在跨陆架海水输送。在观测期间,南海北部陆坡调查海区受气旋涡和反气旋涡双涡结构影响,使得南海北部陆坡表层100 m以浅存在跨陆坡流,流速最大值出现在两涡交汇区域。此外,通过潜标连续海流资料,发现南海北部陆坡环流呈现了“深入浅出”(100 m以深层为向岸的入侵、以浅层为离岸的出流)的“两层结构”。     

Three-dimensional structure of an observed cyclonic mesoscale eddy in the Northwest Pacific and its assimilation experiment

Mesoscale eddies play an important role in modulating the ocean circulation. Many previous studies on the threedimensional structure of mesoscale eddies were mainly based on composite analysis, and there are few targeted observations for individual eddies. A cyclonic eddy surveyed during an oceanographic cruise in the Northwest Pacific Ocean is investigated in this study. The three-dimensional structure of this cyclonic eddy is revealed by observations and simulated by the four-dimensional variational data assimilation(4 DVAR) system combined with the Regional Ocean Modeling System. The observation and assimilation results together present the characteristics of the cyclonic eddy. The cold eddy has an obvious dual-core structure of temperature anomaly.One core is at 50–150 m and another is at 300–550 m, which both have the average temperature anomaly of approximately-3.5°C. The salinity anomaly core is between 250 m and 500 m, which is approximately-0.3. The horizontal velocity structure is axis-asymmetric and it is enhanced on the eastern side of the cold eddy. In the assimilation experiment, sea level anomaly, sea surface temperature, and in situ measurements are assimilated into the system, and the results of assimilation are close to the observations. Based on the high-resolution assimilation output results, the study also diagnoses the vertical velocity in the mesoscale eddy, which reaches the maximum of approximately 10 m/d. The larger vertical velocity is found to be distributed in the range of 0.5 to 1 time of the normalized radius of the eddy. The validation of the simulation result shows that the 4 DVAR method is effective to reconstruct the three-dimensional structure of mesoscale eddy and the research is an application to study the mesoscale eddy in the Northwest Pacific by combining observation and assimilation methods.     

Numerical modelling of flow around coastal structures and scour prediction

The nearshore circulation and the wave characteristics are important parameters, which control coastline morphology. The interaction of nearshore circulation with coastal structures, modify the wave characteristics and seabed topography, often resulting in scour near the foundation of the structures. This paper deals with the numerical prediction of nearshore circulation induced due to wave setup in the nearshore region with and without the structure [(i) structure resting on seabed (ii) structure raised above the seabed]. It is also helps understand the deficiencies in studying the coastal characteristics by describing the flow field due to the wave velocity potential alone. Comparison of the results of both nearshore circulation and the wave potential model are discussed and the importance of the study and its prototype applications are highlighted.     

海南岛莺歌海近岸的潮汐不对称与潮致余流研究

潮汐不对称与潮致余流在河口海岸区的物质输运中扮演着重要角色。已往的研究表明,在驻波占主导的河口海湾中,涨落潮的历时不对称与流速不对称有较为良好的对应关系。而潮致余流主要由地形与潮波的非线性作用所致。本研究以海南岛莺歌海附近为代表,结合实际观测与数值模型,研究复杂地形的开阔近岸区的潮汐不对称与潮致余流。结果表明,在莺歌海近岸区,涨落潮历时不对称皆表现为涨潮历时短于落潮历时,而流速不对称则出现复杂的空间变化。对流速不对称的机制分解表明,研究区的流速不对称主要由K₁、O₁与M₂的相互作用,以及潮余流与各潮汐分潮的相互作用所控制。其中前者产生涨潮流速大于落潮流速的涨潮优势,而后者则与余流的方向相对应,出现多个涨潮优势与落潮优势的区域。总体而言,研究区的流速不对称由余流与各潮汐分潮的相互作用所决定。这表明,采用涨落潮历时的不对称来确定潮汐不对称的方法在开阔近海区可能并不适用。对潮致余流的研究表明,研究区的欧拉余流远大于斯托克斯余流。欧拉余流表现为多个顺时针与逆时针的涡流。涡流分布与地形具有较好的对应关系,潮流沙脊区多发育顺时针涡流,而深槽区则以发育逆时针涡流为主。摩擦力在涡流的发育中起着重要作用。     

The circulation of the western Mediterranean Sea in spring 2005 as inferred from observations and from model outputs

In situ observations and numerical model simulations have been used to study the circulation of the western Mediterranean Sea during April–May 2005. A hydrological survey and direct current measurements carried out in the western Mediterranean Sea are analyzed with an inverse box model. The model result is a mean circulation of the region during spring 2005 along with simultaneous evaluation of water fluxes through eight transects and associated uncertainties. In order to evaluate the consistency of the results and the weight of currents at shorter temporal and spatial scales, an inter-comparison of differently achieved results is performed. The inverse solution is evaluated against both instantaneous current measurements and simulated velocity fields from a General Circulation Model. The results obtained and the general agreement between the three approaches are encouraging and confirms that the inverse box model is a powerful instrument to investigate flow fields in wide areas of the sea. The picture coming out confirms the previous qualitative knowledge on the mean circulation at all levels, providing, in addition, robust quantitative estimations of the water masses fluxes throughout the western Mediterranean basin.     

Observations on the Rim Current structure,CIW formation and transport in the western Black Sea

CTD and ADCP measurements together with a sequence of satellite images indicate pronounced current meandering and eddy activity in the western Black Sea during April 1993. The Rim Current is identified as a well-defined meandering jet stream confined over the steepest topographic slope and associated cyclonic–anticyclonic eddy pairs located on both its sides. It has a form of highly energetic and unstable flow system, which, as it propagates cyclonically along the periphery of the basin, is modified in character. It possesses a two-layer vertical structure with uniform upper layer speed in excess of 50 cm/s (maximum value ∼100 cm/s), followed by a relatively sharp change across the pycnocline (between 100 and 200 m) and the uniform sub-pycnocline currents of 20 cm/s (maximum value ∼40 cm/s) observed up to the depth of ∼350 dbar, being the approximate limit of ADCP measurements. The cross-stream velocity structure exhibits a narrow core region (∼30 km), flanked by a narrow zone of anticyclonic shear on its coastal side and a broader region of cyclonic shear on its offshore side. The northwestern shelf circulation is generally decoupled from the influence of the basinwide circulation and is characterized by much weaker currents, less than 10 cm/s. The southward coastal flow associated with the Danube and Dinepr Rivers is weak during the measurement period and is restricted to a very narrow coastal zone.The data suggest the presence of temperature-induced overturning prior to the measurements, and subsequent formation of the Cold Intermediate Water mass (CIW) within the Northwestern Shelf (NWS) and interior of the western basin. The newly formed shelf CIW is transported in part along the shelf by the coastal current system, and in part it flows downslope across the shelf and intrudes into the Rim Current convergence zone. A major part of the cold water mass, however, seems to be trapped within the northwestern shelf. The CIW mass, injected into the Rim Current zone from the shelf and the interior region, is then circulated around the basin.     

Structure,transport and potential vorticity of the Gulf Stream at 68°W: Revisiting older data sets with new techniques

The stream-coordinates mean structure of the Gulf Stream at 68°W is derived using new methods for both defining stream coordinates and interpreting bottom pressure and inverted echo sounder travel times collected during the extensive Synoptic Ocean Prediction experiment. These new analyses provide pictures of the vertical structure of Gulf Stream flows that are demonstrably dynamically consistent with the density field at all depths, in contrast to previous work that relies on simple vertical interpolations to fill gaps between sparse current meter measurements. This new view of the Gulf Stream suggests a slightly higher total mean transport, with the increases coming from both baroclinic and barotropic components, and slightly stronger recirculation cells, particularly on the southern side. The recirculation of the Gulf Stream appears to have a weak baroclinic component, perhaps 10% of the total. A significant advantage of the methodology is the ability to obtain sensible vertical and horizontal gradients of currents and density so that the vertical and cross-stream structures of the components of the mean potential vorticity can be clearly imaged. One new feature from this calculation is that the along-stream gradient of the cross-stream velocity, a term that is often ignored in potential vorticity analyses, is non-negligible (though small) and is asymmetric about the current axis. Both the derived structure and implied dynamics of the circulation can be significantly altered by small changes to the method of calculating daily stream coordinates, e.g., by carefully filtering out observations in rings or not. Arrays of pressure-equipped inverted echo sounders provide the opportunity (at reasonable cost) for properly defining the stream coordinates of energetic jets such as the Gulf Stream.     

Climatic structure of the wind fields in the Black-Sea region

For the Black-Sea region, we perform the dynamic reanalysis of the data on atmospheric circulation for the period 1958–2001 by using the HadRM3P model with a space resolution of 25 km. We estimate the mean climatic atmospheric fields of vorticity and divergence of the wind velocity and study their space structure and seasonal variability. The climatic estimates of the annual course of vorticity of the wind velocity over the sea are presented. The obtained large annual average values of vorticity of the velocity reveal the predominant role played by the wind action in the generation not only of the seasonal variability but also of the mean cyclonic circulation of waters in the Black Sea.     

Features of the currents at a bold coast

Principal regularities of the coastal velocity field formation have been revealed on the basis of processing and analysis of multi-annual measurements of currents at a bold coast. The influence produced by the coastline, the wind force and its spatial-temporal variability, pycnocline, and the general circulation pattern on this field is estimated. The seiche and inertial currents in this region are characterized. Patterns of the transversal upwelling-downwelling circulations are developed for winter and summer.Translated by Mikhail M. Trufanov.     

南海等密度面环流季节变化的数值模拟研究

In this study, we develop a variable-grid global ocean general circulation model(OGCM) with a fine grid(1/6)°covering the area from 20°S–50°N and from 99°–150°E, and use the model to investigate the isopycnal surface circulation in the South China Sea(SCS). The simulated results show four layer structures in vertical: the surface and subsurface circulation of the SCS are characterized by the monsoon driven circulation, with basin-scaled cyclonic gyre in winter and anti-cyclonic gyre in summer. The intermediate layer circulation is opposite to the upper layer, showing anti-cyclonic gyre in winter but cyclonic gyre in summer. The circulation in the deep layer is much weaker in spring and summer, with the maximum velocity speed below 0.6 cm/s. In fall and winter, the SCS deep layer circulation shows strong east boundary current along the west coast of Philippine with the velocity speed at 1.5 m/s, which flows southward in fall and northward in winter. The results have also revealed a fourlayer vertical structure of water exchange through the Luzon Strait. The dynamics of the intermediate and deep circulation are attributed to the monsoon driving and the Luzon Strait transport forcing.     

吕宋岛西侧中、深层环流的诊断分析

利用P矢量方法和第3版本的美国海军全球数字环境模式(GDEM)气候态月平均温、盐数据诊断分析了南海吕宋岛西侧中层与深层环流结构。结果显示在南海吕宋岛西侧中层存在一个反气旋涡,与前人在此处深层发现的气旋涡相反,且2个涡旋均与低盐中心位置相对应。诊断结果还表明中层反气旋涡从上到下流速逐渐减弱,深层气旋涡则随深度逐渐加强,在两涡旋之间的过渡层流速非常弱。中层反气旋涡的平均流速大小和低盐中心的平均盐度存在明显的季节变化,5—7月最强,这可能是中层环流在一定程度上受上层环流影响的表现。     

Physical processes in the transition zone between North Sea and Baltic Sea. Numerical simulations and observations

The dynamics in the transition zone between the North Sea and Baltic Sea are analyzed here using data from a 22-year-long climatic simulation with a focus on the periods 1992–1994 and 2001–2003 when two recent major inflow events occurred. Observations from gauges and in situ measurements are used to validate the model. Parameters, which cannot be easily measured, such as water and salt transports through straits, have been compared against similar previous estimates. The good performance of simulations is attributed to the finer resolution of the model compared to earlier set ups. The outflow in the Kattegat, which is an analogue of the tidal outflows, tends to propagate to the North over the shallows without showing a substantial deflection to the right due to the Earth's rotation. The inflow follows the topography. The different inflow and outflow pathways are explained as a consequence of the specific combination of bathymetry, axial and lateral processes. The circulation in Kattegat is persistently clockwise with an eastern intensification during inflow and a western one during outflow regimes. The tidal wave there propagates as Kelvin wave, keeping the coast on its right. The flows in the two main straits reveal very different responses to tides, which are also highly asymmetric during inflow and outflow conditions. The circulation has a typical two-layer structure, the correlation between salinity and velocity tends to increase the salt transport in the salinity conveyor belt. The transversal circulation in the entrance of the Sound enhances the vertical mixing of the saltier North Sea water. The long-term averaged ratio of the water transports through the Great Belt and the Sound is ∼2.6-2.7 but this number changes reaching lower values during the major inflow in 1993. The transports in the straits are asymmetric. During inflow events the repartition of water penetrating the Baltic Sea is strongly in favor of the pathway through the Sound, which provides a shorter connection between the Kattegat and Baltic proper. The wider Great Belt has a relatively larger role in exporting water from the Baltic into the North Sea. A demonstration is given that the ventilation of the Baltic Sea deep water is not only governed by the dynamics in the straits and the strong westerly winds enhancing the eastward propagation of North Sea water (a case in 1993), but also by the clockwise circulation in the Kattegat acting as a preconditioning factor for the flow-partitioning.     

基于Argo资料的北太平洋绝对地转流计算

针对海洋实测流速资料极其匮乏的事实, 利用2004 年1 月~2009 年12 月月平均的Argo 温盐格点资料, 结合改进的P-vector 方法重构北太平洋绝对地转流流场。与卫星高度计和实测流速的比较以及相关性分析表明, 重构的绝对地转流是可信的, 可以用来研究不同大尺度环流特征及其动力结构。可以为研究海洋动力过程和气候变化提供一套有用的流场数据。