Effects of a marine ridge to western boundary current in a three-dimensional source-sink flow model

A numerical experiment is made with a three-dimensional source-sink flow model in order to study effects of a marine ridge to the western boundary current. The model represents the Kuroshio current crossing the Izu Ridge which has a submarine narrow passage. It is indicated that most of the surface water of the western boundary current crosses the ridge through the narrow passage. A part of the surface water circulates around a warm eddy formed in an upstream region of the ridge. A water below the thermocline makes a long detour around the ridge and forms a subsurface northward current along the eastern slope of the ridge.     

Shocks in a coastal boundary current

We studied shocks in a coastal boundary current with zero potential vorticity. By coastal boundary current, we mean a semigeostrophic light fluid flow over an infinitely deep dense fluid and along a coast on its right hand side, with its lower interface exposed to the ocean surface at some finite distance from the coast. The shocks are assumed to conserve mass and momentum. It is found that the shocks can be classified into two categories, coastal shocks and frontal shocks, by the signs of the upper layer flux relative to the shocks. Coastal shocks, for which the relative upper layer flux is negative, always propagate downstream. The upper layer at the coast is thicker on the upstream sides of coastal shocks than on the downstream sides. Frontal shocks, for which the relative upper layer flux is positive, propagate upstream as well as downstream. In most cases, the current is wider on the downstream sides of frontal shocks than on the upstream sides. However, under the circumstances that the current is nearly separated from the coast, the current is wider on the upstream sides of frontal shocks. Coastal and frontal shocks both dissipate energy of the current. We also demonstrate that special shocks with no light fluid on the downstream sides cannot exist irrespective of the potential vorticity distribution.     

内孤立波过山脊地形演化实验研究

通过模型实验,研究了下沉型内孤立波通过山脊地形演化特征。实验以三角形障碍物模拟海底山脊地形,采用两种密度的分层水,对上层流体和下层流体的高度比、密度等进行了量化处理。实验研究表明:KdV理论波形可较好模拟本次实验内孤立波波形,但随着内孤立波振幅的增大,误差增加;在内孤立波与障碍物微量作用、中度作用和破波作用三种程度的相互作用中,内孤立波过障碍物具有不同的波形变化和主波能量衰减率。     

The origin and continuation of a western boundary current

The origin of a western boundary current as a jet is accounted for by a transfer of potential energy to kinetic energy, and poleward changes along the western boundary are directly correlated to variation in Coriolis parameter.     

周期性的或伴有贯穿流的西边界流跨隙流动的迟滞变异过程

使用1.5层准地转约化重力模式研究了周期性的或伴有贯穿流的西边界流跨隙流动的迟滞变异过程。当西边界流变化的周期比罗斯贝波在缺口处调整的时间尺度大得多时,在雷诺数增加和减少过程的霍夫分叉点都发生延迟,从而产生新的雷诺数迟滞区间;并且西边界流流态转变的临界值变化显著;且周期强迫越短,雷诺数迟滞区间越大。当西边界流变化的周期与罗斯贝波在缺口处调整的时间尺度相当时,西边界流在缺口的流态呈无迟滞的周期性变化,且西边界流入侵西海盆的程度随周期减少而变小。此外,当贯穿流的流量大于西边界流的一半时,会显著影响西边界流在缺口处的迟滞变异过程;西边界流向西入侵程度和流态转变发生的临界雷诺值均发生变化,且贯穿流流量越大变化越大。     

Volume transport of the western boundary current penetrating into a marginal sea

The amount of penetration of a western boundary current into a marginal sea which is connected to an open ocean by two narrow straits is estimated from a linear, steady and barotropic theoretical model. In this model the western boundary current in the open ocean is driven by a wind stress imposed at the sea surface. The inflow of the water of the open ocean into the marginal sea is caused by the pressure difference between two straits produced by the wind-driven circulation in the open ocean.Main external parameters are combined into two non-dimensional parameters; and (the ratio of the depth of the marginal sea to that of the open ocean), whereb is the distance between north and south boundaries of the ocean,D ₀ is the depth of the open ocean, is the latitudinal variation of the Coriolis parameter andR is the coefficient of friction. The friction is assumed to be proportional to the flow velocity.In the limit of infinite the volume transport into the marginal sea is not affected by the width of two straits and . It is mainly controlled by the wind stress and the positions of two straits. For finite values of , however, the volume transport depends considerably on and the width of the straits.Guided by both this model and physical considerations, we obtained a relation between the volume transport into the marginal sea and the external parameters. This relation predicts that about 2 % of the volume transport of the Kuroshio penetrates into the Japan Sea.     

Numerical study on the interactions between a mesoscale eddy and a western boundary current

A reduced-gravity primitive equation eddy resolving model has been used to study the decay of a mesoscale eddy as it migrates toward a western boundary current (WBC) region. The model results indicated that the gradient of the relative vorticity to the east of the WBC is an important factor in the interaction between an eddy and a WBC. A circular eddy is deformed into an elliptical form during the eddy–WBC interaction with the major axis of a cyclonic/anticyclonic eddy aligning in the NW/NNE direction, respectively. Because of the difference in the major axes orientations for the cyclonic and anticyclonic eddies, the kinetic energy transfer between a WBC and a particular eddy has very different behavior. A cyclonic eddy loses its energy to the mean field, whereas an anticyclonic eddy can obtain energy from the mean flow during the WBC–eddy interaction. An anticyclonic eddy, however, still decayed from losing its water and friction dissipation during the interaction period.     

琉球群岛以东的西边界流与东海黑潮流量时空特征的研究

通过最新的高分辨率再分析海洋数据资料,对于东海黑潮以及琉球群岛以东海域的海流进行了研究。结果表明琉球群岛以东西边界流最大流速出现在600~1200 m深度的地形坡度最大处,大小约为0.2 m/s。由于冲绳岛以南庆良间水道的水交换对于东海黑潮流量有重要的影响,东海黑潮的平均流量从南向北逐渐递增,平均流量为28×106~35×106m3/s;琉球群岛以东的西边界流流量则比东海黑潮小一个量级,平均值小于其变化的方差;由于受庆良间水道海流的影响,冲绳岛东侧的流量要远小于奄美大岛东侧的流量。同一纬度大洋中西传的Rossby波对琉球群岛以东的西边界流有较大影响,因此琉球群岛以东西边界流的流量有大约100 d的显著变化周期。庆良间水道以南的东海黑潮由于主要受台湾以东黑潮流量的控制,也有大约100 d的显著变化周期,庆良间水道以北的东海黑潮则没有该特征。     

Downscaling biogeochemistry in the Benguela eastern boundary current

Dynamical downscaling is developed to better predict the regional impact of global changes in the framework of scenarios. As an intermediary step towards this objective we used the Regional Ocean Modeling System (ROMS) to downscale a low resolution coupled atmosphere–ocean global circulation model (AOGCM; IPSL-CM4) for simulating the recent-past dynamics and biogeochemistry of the Benguela eastern boundary current. Both physical and biogeochemical improvements are discussed over the present climate scenario (1980–1999) under the light of downscaling.Despite biases introduced through boundary conditions (atmospheric and oceanic), the physical and biogeochemical processes in the Benguela Upwelling System (BUS) have been improved by the ROMS model, relative to the IPSL-CM4 simulation. Nevertheless, using coarse-resolution AOGCM daily atmospheric forcing interpolated on ROMS grids resulted in a shifted SST seasonality in the southern BUS, a deterioration of the northern Benguela region and a very shallow mixed layer depth over the whole regional domain. We then investigated the effect of wind downscaling on ROMS solution. Together with a finer resolution of dynamical processes and of bathymetric features (continental shelf and Walvis Ridge), wind downscaling allowed correction of the seasonality, the mixed layer depth, and provided a better circulation over the domain and substantial modifications of subsurface biogeochemical properties. It has also changed the structure of the lower trophic levels by shifting large offshore areas from autotrophic to heterotrophic regimes with potential important consequences on ecosystem functioning. The regional downscaling also improved the phytoplankton distribution and the southward extension of low oxygen waters in the Northern Benguela. It allowed simulating low oxygen events in the northern BUS and highlighted a potential upscaling effect related to the nitrogen irrigation from the productive BUS towards the tropical/subtropical South Atlantic basin. This study shows that forcing a downscaled ocean model with higher resolution winds than those issued from an AOGCM, results in improved representation of physical and biogeochemical processes.     

The advection effect of planetary vorticity on sea level slope in a western boundary current

Western boundary currents, such as the Gulf Stream, are often modeled as flows in near cross‐stream geostrophic balance with paths that are straight or nearly straight. The effect of planetary vorticity advection on the downstream sea‐level slope in these rectilinear flows is re‐examined and found negligible. Instead, the re‐examination reveals a much greater effect of the ageostrophic component of the horizontal divergence in determining the downstream slope. Overlooked in previous studies is the geostrophic component of the horizontal divergence because of a hidden assumption of parallel flow. To avoid this pitfall, we employ a natural coordinate system to follow precisely the downstream direction. Further, we differentiate between weak geostrophic flows with small accelerations and flows in cross‐stream geostrophic balance where downstream acceleration might be appreciable. Two Rossby numbers are employed: a small cross‐stream number (≤0.01) to describe the near cross‐stream balance, and a large downstream number (≤0.1) to describe the large downstream accelerations that are found in western boundary flows. Finally, by means of a scale analysis we show that over the whole range of possible Rossby numbers, the advection effect of planetary vorticity on downstream sea level slope is negligible compared to the effect of the ageostrophic component of the horizontal divergence. Some new data on the nearshore gradient of the Reynolds stress are also included.     

A review on the South China Sea western boundary current

The advances in understanding the South China Sea (SCS) western boundary current (SCSwbc) have been reviewed since the works of Dale (1956) and Wyrtki (1961) in the middle of the 20th century. The features of the pattern of SCSwbc and the oceanic phenomena associated with it are focused on. The current is driven mainly by monsoon over the SCS and partially by winds over the tropical Pacific governed by the island rule. The SCSwbc exhibits strong seasonal variation in its direction and patterns. In winter, the current is strong and flows southwestward along the South China shelf and slope from the east of Dongsha Islands to the northern central Vietnamese coast, then turns to the south along the central and southern Vietnamese coast, and finally partially exits the SCS through the Karimata Strait. In summer and early fall, the SCSwbc can be divided into three segments based on their characteristics. The southern segment is stable, flowing northward from the Karimata Strait up to about 11 N, where it separates from the coast forming an eastward offshore current. The separation of the current from Vietnamese coast induces some striking features, such as upwelling and cold sea-surface temperature. The middle segment off the central Vietnamese coast may have a bimodal behavior: northward coastal current and meandering current in early summer (June-July), and cyclonic gyre in later summer and early fall (August-September). The northern segment is featured by the summer SCS Warm Current on the South China shelf and a southwestward subsurface current along the continental slope.     

Wave-adjusted boundary condition for longshore current in finite-volume circulation models

Numerical modeling of coastal circulation encompassing the nearshore requires forcing by tide, surface gravity waves, and possibly other factors. In the nearshore, the wave-induced longshore current and setup are dominant hydrodynamic processes, and lateral boundary conditions representing tide and oceanic forcing typically do not include surface-wave contributions. Without proper boundary conditions, significant gradients in current and water level can occur that contaminate the solution in the internal domain. A standard strategy is to place the boundaries far from the site of interest, but this strategy greatly increases computational demands, and it may not be appropriate for long-term simulations. This paper describes a wave-adjusted boundary condition that accounts for wave-induced water level and current acting in combination with tidal forcing. The wave-adjusted boundary condition is demonstrated for an idealized case of a parallel-contour beach and for an engineering application at Ocean City, MD.     

线边界法在潮流模拟中的应用

本文针对有限差分法在模拟潮流运动中遇到的边界问题,如岸边界问题,动边界问题和工程边界问题等,引入线边界的概念,并以此为研究对象,提出了较普遍适用的线边界法,实例应有分析表明,用线边界法处理各类边界问题,实用有效,简便易行。     

The western boundary current in the Bransfield Strait,Antarctica

The Bransfield Strait west of the Antarctic Peninsula has been considered as a highly productive region for all trophic levels from primary production, to zooplankton aggregations, especially krill, to birds and mammals. The western boundary current, referred to as the Bransfield Current, plays an important role in determining the transport and retention of biota in the Bransfield Strait. Following the study of surface current characteristics in the strait using 39 tracks of mixed-layer drifters deployed between 1988 and 1990, a high-resolution transect of temperature, salinity and current measurements crossing the Bransfield Current was conducted between 13 and 14 March 2004, for understanding its horizontal and vertical structure and dynamics. The results from current, temperature and salinity measurements using a vessel mounted narrow band acoustic doppler current profiler and conductivity–temperature–depth (CTD) sensors revealed the magnitude of this current of approximately 50 cm/s within a horizontal distance of 15 km associated with a narrow and deep density front 4–6 km wide and 500 m deep. The comparison between the direct current measurements and the geostrophic current estimates from the density field implies that the Bransfield Current is geostrophically balanced. The mechanism forming this current is explored with Sverdrup dynamics. Results indicate that the negative wind stress curl and β-effect lead to a southwestward transport in the Bransfield Strait. When this transport is restricted by land and shelves, a narrow western boundary current is formed.     

The effect of a current upon the generation of internal waves by a semi-diurnal tide in the vicinity of a ridge with a continuously changing height

In the framework of the linear theory for long waves, the paper studies internal waves generated by a semi-diurnal barotropic tide impinging on a bottom ridge at an arbitrary angle. The ocean is assumed to be double-layered. In the vicinity of the ridge, whose height is continuously changing, geostrophic flows occurring due to the inclination of the free surface and interface are considered. The dependencies of the generated wave's amplitudes on the angle of incidence of the tide and on the magnitude and direction of the geostrophic current velocity are determined, allowing a conclusion that the current contributes to the generation of internal waves. Translated by Vladimir A. Puchkin.     

Nutrients in an oligotrophic boundary current: Evidence of a new role for the Leeuwin Current

New observations along the continental shelf of Western Australia provide a novel explanation for the established ∼60 years relationship between Leeuwin Current (LC) strength and greater winter nitrate concentrations at 32°S plus the inter-annual variation in the magnitude of the annual, shelf-scale, phytoplankton bloom. The potential source of dissolved nitrogen to support the annual shelf scale phytoplankton bloom was identified as thin layers of an unprecedented areal extent, nitrate concentration and shallow nature that were observed off the northwest of Australia. We propose that the dissolved inorganic nitrogen (DIN) in these layers enters the LC at depth and then enters the euphotic zone via by three mechanisms: instability that results in a warm core eddy, cooling that deepens the surface mixed layer and shallowing of the thin layer. During the onset of the annual phytoplankton bloom along the west coast of Australia from 22°S to 34°S the poleward flowing LC was clearly evident as a surface intensified ocean boundary current transporting warmer, lower-salinity, greater-silicate waters in a shallow mixed layer rapidly southward. Between 24 and 26°S the core of the LC was present as a 50–100 m deep layer over one or more thin layers, 15–50 m thick, with high nitrate and low dissolved oxygen (DO). These layers were of lower salinity, cooler water with markedly reduced DO, high nitrate concentrations and distinct nitrate:silicate (NO₃:Si(OH)₄) nutrient ratios. As the LC flowed south it cooled and deepened thereby entraining the thin layers of high nitrate water into the euphotic zone. The LC also formed large (greater than 100 km diameter) warm core eddies with a deep surface mixed layer that also entrained nitrate from these thin layers. In some locations as far south as 32°S the LC was still present with the thin layer of high nitrate intact but now within the euphotic zone. Thus, the available evidence suggests the LC arises under conditions that favour rapid and shallow nitrification. This nitrification fuels a shelf-scale bloom on a downwelling favourable coast. Depending upon the rate of nitrification the source of the particular organic matter may be local or delivered from the tropics via horizontal advection in a subsurface layer of the LC.     

一次台风过程对西北太平洋西边界流系源区影响的数值模拟研究

利用POM及其与WRF的耦合模式对"格美"台风影响下的该海区进行了5组数值模拟试验,在对结果分析的基础上,得到了西北太平洋西边界流系源区对此次台风过程的响应。研究表明:在台风影响下,最大风速区及热通量输送决定了海表温度(SST)降温中心范围,热通量的输送对SST的降低贡献超过16.7%;受此次台风影响的混合层(OML)加深、维持的时间为42 h,热通量对OML的加深有正作用,但不如风应力的贡献明显。台风移动方向右侧,OML加深范围更大,且SST最大降低区并不是OML最大加深区。此次台风过程对黑潮南向流的影响较弱,主要增加了海洋混合层的北向流流量。利用耦合模式,考虑了海气间的相互作用,在台风中心附近模拟出由于低压引起的海面升高现象。     

Calm-weather spring and neap tidal current characteristics on a shoreface-connected ridge complex in the German Bight (southern north Sea)

While shoreface-connected sand ridges may be molded by storm-generated waves and currents, calmweather counterparts may determine their longevity in the German Bight. Fair-weather flow measurements on shoreface ridges off Spiekeroog Island show that: (1) peak velocities (U₁₀₀ max) mostly range from 30 to 60 cm s^–1 and are flood asymmetric, except at neap tide when ebb flows are dominant in ridge troughs; (2) velocity contrast between accelerating and decelerating flow phases is higher for ebb than flood currents, suggesting intense interaction between inlet and shoreface ebb currents; and (3) tidal currents play a primary role in ridge maintenance.     

海底边界效应对海流发电水轮机水动力性能影响研究

对30 W海流能水平轴水轮机进行叶片设计,应用FLUENT软件对水轮机的水动力性能进行数值模拟,研究了边界效应对叶片表面压力、流场、湍流强度、获能和轴向力的影响。受海底边界效应影响,海流速度沿深度呈现梯度变化,底层流速较小,中上层流速较大。边界效应导致水轮机的水动力性能呈现周期性变化,降低了水轮机的获能和轴向力。机组布置时,宜选择水流稳定且流速较大的中上层区域。     

A western boundary current east of New Caledonia: Observed characteristics

Waters from the South Equatorial Current (SEC), the northern branch of the South Pacific subtropical gyre, are a major supply of heat to the equatorial warm pool, and have an important contribution to climate variability and ENSO which motivated the Southwest Pacific Ocean and Climate Experiment (SPICE, CLIVAR/WCRP). Initially a broad westward current extending from the equator to 30°S, the SEC splits upon arriving at the major islands and archipelagoes of Fiji (18°S, 180°E), Vanuatu (16°S, 168°E), and New Caledonia (22°S, 165°E), resulting in a complex system of western boundary currents and zonal jets that feed the Coral and Solomon Seas. We focus here on the formation of one specific jet feeding the Coral Sea, the North Caledonian Jet (NCJ). Using a combination of recent oceanographic cruises, we describe the ocean circulation to the northeast of New Caledonia, where the SEC forms a western boundary current that ultimately becomes the NCJ. This current, which we document for the first time and propose to refer to as the East Caledonian Current (ECC), has its core located 10-100 km off the east coast of New Caledonia, and extends vertically to at least 1000 m depth. Water mass properties show continuous westward transports through the ECC, from the SEC to the NCJ in both the South Pacific Tropical Waters in the thermocline and Antarctic Intermediate Waters near 700 m depth. The ECC extends about 100 km horizontally; its average 0-1000 m transport was estimated at 14.5±3 Sv off the north tip of the New Caledonian reef, with a maximum of 20 Sv in May 2010. South of that the upstream branch of the ECC east of the Loyalty is close to 8 Sv suggesting an important additional contribution from central Pacific waters carried by the SEC at 16°S and diverted to our region through the western boundary current system east of Vanuatu.