Density Current Descending along Continental Slope and the Associated Deep Water Formation: Two-Dimensional Numerical Experiments with a Nonhydrostatic Model

Numerical experiments with a two-dimensional nonhydrostatic ocean model have been carried out to investigate the dynamical process of descending density current on a continental slope. The associated deep water formation has been also examined by tracking labeled particles. The descending flow along the continental slope occurs in the bottom Ekman layer. The net pressure gradient determining the volume transport consists of not only the pressure gradient due to density deviation but also the surface pressure gradient due to the depth-mean alongshore flow. Since these constituents have the opposite signs and strengthen each other, the oscillation with an alternation of intense up- and downslope flows appears around the shelf break. This temporal variation of the flow field causes the effective mixing on the slope between descending shelf and interior waters and forms the deep water as a mixture of them at a ratio of about 1:3. The present result is applied to the slope current around Antarctica, using velocity and density fields calculated by an ocean general circulation model. The Ekman volume transport is estimated at 0.97 Sv (1 Sv = 10⁶ m³s^–1) in the Weddell Sea, 0.35 Sv in the Ross Sea, and 1.8 Sv in total. About 70% of them is attributed to the depth-mean alongshore flow, such as the East Wind Drift and the Weddell Gyre driven by the wind. This suggests that the pressure gradient due to other factors than density deviation may play an important role in the deep and bottom water formation in the actual oceans.     

Water transport across the subantarctic front and the global ocean conveyer belt

The water transport across the Subantarctic Front involving the Ekman transport, eddy transfer, and transport by the abyssal geostrophic currents is estimated on the basis of different experimental data. This transport accounts for 14 Sv northwards in the upper ocean layer (thermocline) and the same quantity southwards in the lower layer (abyss). This quantity is equal to the magnitude of the water downwelling from the thermocline to the abyssal in the North Atlantic and North-European Basin, which is realized in the field of the Global Ocean Conveyor Belt. This result agrees with the conception according to which the oppositely directed motion of the water from the abyssal into the thermocline takes place in the Antarctic.     

On the structure and the transport of the eastern Weddell Gyre

The circulation pattern and volume transports in the eastern Weddell Gyre are estimated on the basis of hydrographic data collected by R.V. Polarstern between 1989 and 1996. In the northeastern edge of the Weddell Gyre, eastward-flowing water masses from the Antarctic Circumpolar Current and the Weddell Sea converge. Due to the strong effect of topographic constraints on ocean currents in the weakly stratified waters of high latitudes, the wedge-like structure of the Southwest Indian Ridge can cause the convergence. The increased shear leads to instabilities of the current at the eastern end of the ridge, which produce an intense mesoscale eddy field between 15° and 30°E. In the eddies, water from the Weddell cold regime and the Antarctic Circumpolar Current waters mix and form the water masses of the Weddell warm regime. These waters are advected southward and flow towards the westward southern rim current, which is driven by the Antarctic eastwind band. Hence, there is not a continous flow from the northern to the southern rim, but a decay of the mean flow in the northeast and a reformation in the south. Volume transports across the Greenwich Meridian, estimated on the basis of a combined CTD/ADCP data set, result in an eastward flow of 61 Sv in the northern rim current and a westward return flow of 66 Sv in the southern part of the gyre. The transport is about twice as high as previous estimates between Kapp Norvegia and the northern tip of the Antarctic Pensinsula, indicating a significant gyre circulation north of 70°S.     

A deep cyclonic gyre in the Australian–Antarctic Basin

The traditional image of ocean circulation between Australia and Antarctica is of a dominant belt of eastward flow, the Antarctic Circumpolar Current, with comparatively weak adjacent westward flows that provide anticyclonic circulation north and cyclonic circulation south of the Antarctic Circumpolar Current. This image mostly follows from geostrophic estimates from hydrography using a bottom level of no motion for the eastward flow regime which typically yield transports near 170 Sv. Net eastward transport of about 145 Sv for this region results from subtracting those westward flows. This estimate is compatible with the canonical 134 Sv through Drake Passage with augmentation from Indonesian Throughflow (around 10 Sv).A new image is developed from World Ocean Circulation Hydrographic Program sections I8S and I9S. These provide two quasi-meridional crossings of the South Australian Basin and the Australian–Antarctic Basin, with full hydrography and two independent direct-velocity measurements (shipboard and lowered acoustic Doppler current profilers). These velocity measurements indicate that the belt of eastward flow is much stronger, 271 ± 49 Sv, than previously estimated because of the presence of eastward barotropic flow. Substantial recirculations exist adjacent to the Antarctic Circumpolar Current: to the north a 38 ± 30 Sv anticyclonic gyre and to the south a 76 ± 26 Sv cyclonic gyre. The net flow between Australia and Antarctica is estimated as 157 ± 58 Sv, which falls within the expected net transport of 145 Sv.The 38 Sv anticyclonic gyre in the South Australian Basin involves the westward Flinders Current along southern Australia and a substantial 33 Sv Subantarctic Zone recirculation to its south. The cyclonic gyre in the Australian–Antarctic Basin has a substantial 76 Sv westward flow over the continental slope of Antarctica, and 48 ± 6 Sv northward-flowing western boundary current along the Kerguelen Plateau near 57°S. The cyclonic gyre only partially closes within the Australian–Antarctic Basin. It is estimated that 45 Sv bridges westward to the Weddell Gyre through the southern Princess Elizabeth Trough and returns through the northern Princess Elizabeth Trough and the Fawn Trough – where a substantial eastward 38 Sv current is hypothesized. There is evidence that the cyclonic gyre also projects eastward past the Balleny Islands to the Ross Gyre in the South Pacific.The western boundary current along Kerguelen Plateau collides with the Antarctic Circumpolar Current that enters the Australian–Antarctic Basin through the Kerguelen–St. Paul Island Passage, forming an energetic Crozet–Kerguelen Confluence. Strongest filaments in the meandering Crozet-Kerguelen Confluence reach 100 Sv. Dense water in the western boundary current intrudes beneath the densest water of the Antarctic Circumpolar Current; they intensely mix diapycnally to produce a high potential vorticity signal that extends eastward along the southern flank of the Southeast Indian Ridge. Dense water penetrates through the Ridge into the South Australian Basin. Two escape pathways are indicated, the Australian–Antarctic Discordance Zone near 125°E and the Geelvinck Fracture Zone near 85°E. Ultimately, the bottom water delivered to the South Australian Basin passes north to the Perth Basin west of Australia and east to the Tasman Basin.     

Volume Transport Variability Southeast of Okinawa Island Estimated from Satellite Altimeter Data

A nine-year-long record of the northeastward volume transport (NVT) in the region southeast of Okinawa Island from 1992 to 2001 was estimated by an empirical relation between the volume transport obtained from the ocean mooring data and the sea surface height anomaly difference across the observation line during 270 days from November 2000. The NVT had large variations ranging from −10.5 Sv (1 Sv ≡ 10⁶ m³s⁻¹) to 30.0 Sv around its mean of 4.5 Sv with a standard deviation of 5.5 Sv. This large variation was accompanied by mesoscale eddies from the east, having a pronounced period from 106 to 160 days. After removal of the eddy, NVT was found to fluctuate from 2 Sv to 12 Sv with a quasi-biennial period.     

On the circulation of bottom water in the region of the Vema Channel

The circulation and transport of Antarctic Bottom Water (σ₄<45.87) in the region of the Vema Channel are studied along three WOCE hydrographic lines, the geostrophic velocities referenced to previously published direct current measurements. The primary supply of water to the deep Vema Channel is from the Argentine Basin's deep western boundary current, with no indication of an inflow from the southeast. In the northern Argentine Basin, detachment of lower North Atlantic Deep Water from the continental slope is associated with a deep thermohaline front near 34°S. To the north of this front, the upper part of the AABW bound for the Vema Channel (σ₄<46.01) exhibits a significant NADW influence. Further modification of the throughflow water occurs near 30°30′S, where the channel orientation changes by ∼50°. Southward flow of bottom water on the eastern flank of the Vema Channel, amounting to ∼1.5 Sv, represents a significant countercurrent to the deep channel transport. Inclusion of this countercurrent reduces the net flow of AABW through the Vema Channel from 3.2±0.7 to 1.7±1.1 Sv. Water properties imply that the near-zero net flow over the Santos Plateau results from a near-closed cyclonic circulation fed by the deep Vema Channel throughflow. A disruption of the northward boundary current in the upper AABW (lower circumpolar water) is required by this flow pattern. The extension of the cyclonic circulation on the Santos Plateau enters the Brazil Basin as a ∼1 Sv flow distinct from the outflow in the Vema Channel Extension (6.2 Sv). The high magnitude of the latter suggests a southward recirculation of bottom water near the western boundary to the north of the region of study.     

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.     

Uncertainty Reduction of Estimated Geostrophic Volume Transports with Altimeter Observations East of Taiwan

The common geostrophic estimation of ocean current velocity uses only water temperature and conductivity profiles. The geostrophic volume transport of a western boundary current, like the Taiwan Current (Kuroshio east of Taiwan), between the coast and its eastern boundary can be easily estimated based on hydrographic survey data. But the eastern boundary of the Taiwan Current is very uncertain due to extremely variable hydrographic conditions. This uncertainty is strongly correlated with the propagating mesoscale eddies originating from the interior of the western North Pacific Ocean. The uncertainty of estimated transport can be greatly reduced if eddy distribution is considered when determining the integration boundaries with the assistance of satellite altimeter measurements. Eight hydrographic surveys east of Taiwan between November 1992 and June 1996 are demonstrated in this study. The average geostrophic transport of the Taiwan Current with a reference set to 1000 dbar at 22°N between the east coast of Taiwan and 124°E is 22.9 ±14.2 Sv and changes to 22.1 ± 8.3 Sv, the uncertainty of which is nearly halved after taking account of the eddy distribution. The estimation uncertainty is insensitive to vertical displacements of the reference level within the depth range between 800 and 2000 dbar. This revised version was published online in July 2006 with corrections to the Cover Date.     

黑海西北部涡旋与环流对跨陆架水交换的影响

涡旋对海洋中的能量传递和物质交换有重要作用。黑海西北部陆坡边缘是长生命周期涡旋经常发生的海域,但涡旋引起的陆架和海盆之间水交换通量的季节性特征,以及海盆边缘环流对跨陆架水交换的作用等方面的研究还不是很充分。本文对黑海西北部陆架区与深海盆区间的跨陆架水交换进行了研究,利用高分辨率三维原始方程模式模拟的温、盐、流等资料,结合涡旋自动探测方法,统计了黑海西北部海域的涡旋活动,研究了涡致跨陆架水交换的季节性特征,计算对比了2002年到2010年间海盆边缘环流与涡旋对跨陆架水交换通量的各自贡献。结果表明:黑海西北部地区海盆边缘环流强度与跨陆架进、出通量的相关系数分别为0.57和0.67,海盆边缘环流位置与跨陆架交换量的相关系数为0.52;海盆边缘环流强度与位置的季节性变化导致了黑海西北部跨陆架通量的季节性变化。黑海西北部地区表层3月到8月之间涡旋所引起跨陆架交换量约占了跨陆架交换总量的16%~31%;涡致跨陆架通量也具有季节性特征。文中对单个涡旋进行了详细研究:2005年5月5日到2005年7月20日之间在黑海西北部存在一个直径最大值时达到120 km的反气旋式涡旋,涡旋存在期间完成了从陆架区向深海区的水体传输,相当于黑海西北陆架区水体积的30.9%。     

The sources of Antarctic bottom water in a global ice–ocean model

Two mechanisms contribute to the formation of Antarctic bottom water (AABW). The first, and probably the most important, is initiated by the brine released on the Antarctic continental shelf during ice formation which is responsible for an increase in salinity. After mixing with ambient water at the shelf break, this salty and dense water sinks along the shelf slope and invades the deepest part of the global ocean. For the second one, the increase of surface water density is due to strong cooling at the ocean–atmosphere interface, together with a contribution from brine release. This induces deep convection and the renewal of deep waters. The relative importance of these two mechanisms is investigated in a global coupled ice–ocean model. Chlorofluorocarbon (CFC) concentrations simulated by the model compare favourably with observations, suggesting a reasonable deep water ventilation in the Southern Ocean, except close to Antarctica where concentrations are too high. Two artificial passive tracers released at surface on the Antarctic continental shelf and in the open-ocean allow to show clearly that the two mechanisms contribute significantly to the renewal of AABW in the model. This indicates that open-ocean convection is overestimated in our simulation. Additional experiments show that the amount of AABW production due to the export of dense shelf waters is quite sensitive to the parameterisation of the effect of downsloping and meso-scale eddies. Nevertheless, shelf waters always contribute significantly to deep water renewal. Besides, increasing the P.R. Gent, J.C. McWilliams [Journal of Physical Oceanography 20 (1990) 150–155] thickness diffusion can nearly suppress the AABW formation by open-ocean convection.     

Nonlinear inertial oscillations of a multilayer eddy: An analytical solution

Nonlinear axisymmetric oscillations of a warm baroclinic eddy are considered within the framework of an reduced-gravity model of the dynamics of a multilayer ocean. A class of exact analytical solutions describing pure inertial oscillations of an eddy formation is found. The thicknesses of layers in the eddy vary according to a quadratic law, and the horizontal projections of the velocity in the layers depend linearly on the radial coordinate. Owing to a complicated structure of the eddy, weak limitations on the vertical distribution of density, and an explicit form of the solution, the latter can be treated as a generalization of the exact analytical solutions of this form that were previously obtained for homogeneous and baroclinic eddies in the ocean.     

The Canary Eddy Corridor: A major pathway for long-lived eddies in the subtropical North Atlantic

We report, from remote sensing and in situ observations, a new type of permanent structure in the eastern subtropical Atlantic Ocean, that we call the “Canary Eddy Corridor”. The phenomenon, is a zonal long-lived (>3 months) mesoscale eddy corridor, whose source is the flow perturbation of the Canary Current and the Trade Winds at the Canary Islands. The latitudinal range of the corridor spans 22°N–29°N and extends from the Canaries to at least 32ºW, near the mid-Atlantic. This is the main region of long-lived westward-propagating eddies in the subtropical northeast Atlantic. From a age-distribution study we observe that at least 10% of mesoscale eddies in this region are long-lived, with a dominance of anticyclones over cyclones. Another four westward-propagating eddy corridors were also detected: two small corridors north and south of the Azores Front; a small zonal corridor located near 31ºN, south of the island of Madeira; and a small corridor located near the Cape Blanc giant filament. The existence of these corridors may change, at least for the northeastern subtropical Atlantic, the general idea that mesoscale eddies are disorganized, ubiquitous structures in the ocean. The Canary Eddy Corridor constitutes a direct zonal pathway that conveys water mass- and biogeochemical properties offshore from the Canary Island/Northwest Africa upwelling system, and may be seen as a recurrent offshore pump of organic matter and carbon to the oligotrophic ocean interior. Estimates of volume and mass transport indicate that Canary Eddy Corridor westward transport is more than one-fourth of the southward transport of the Canary Current. The westward transport of kinetic energy by the eddies of the Canary Corridor is as important as the southward transport by the Canary Current. The total primary production related to the Corridor may be as high as the total primary production of the northwest Africa upwelling system for the same latitude range.     

Variability in the Canary Islands area of filament-eddy exchanges

The physical background to a suite of biological studies carried out in the Canary Islands upwelling region is presented. The area is unique in that the coastal transition zone is spanned by an archipelago of islands that shed mesoscale eddies of diameter 50–100 km into the alongshore flow. A recurrent filament and eddy system was sampled intensively to study the changing properties of waters as they are advected towards the open ocean in the filament and to investigate the exchanges between filament and eddies. The system was more complex than previously revealed. In early August, a single filament extended offshore from near Cape Juby. Two weeks later, a second filament had developed slightly farther north and extended offshore to merge with the first at 100 km offshore. The merged filament was entrained around a recurrent, topographically trapped cyclonic eddy and interacted with transient cyclonic and anticyclonic eddies shed from the island of Gran Canaria. Between the two filaments and the coast, a pair of counter-rotating eddies re-circulated water parcels for several weeks. Surface layer drifters cycled around this near-shore re-circulation several times before following convoluted paths that demonstrate significant exchange between continental shelf and open ocean waters.     

Taiwan Current (Kuroshio) and Impinging Eddies

Considerable westward or nothwestward propagating eddies were found east of Taiwan that cross-explains the anomalies in the repeated hydrography, trajectory of drifting buoys and altimetric analyses. The sea level differences (SLD) across the Taiwan Current (Kuroshio) in the East Taiwan Channel (ETC) are utilized in order to examine the possible implication of eddies in the Taiwan Current transport. It is concluded that Taiwan is impinged by both cyclonic and anticyclonic mesoscale eddies at an interval of about 100 days. An approaching anticyclonic eddy will result in a higher SLD across the ETC and a larger mass transport of Taiwan Current, and, vice versa, a reduction of both SLD and the mass transport in the ETC as a cyclonic eddy arrives. The SLD-inferred northward transport in the ETC is highly coherent at the 100-day band with westward propagating eddies that originated in the interior ocean. The generation mechanism of these eddies are, however, still unclear. Leakage of the Kuroshio water to the east of the Ryukyu Islands is suggested due to the presence of cyclonic eddies. This 100-day rate of eddy-impingement invalidates any observation of 4 months or less, whether with direct or indirect measurements, because any conclusions depend on the presence or absence of eddies. To minimize the contamination from eddies, either long-term observations or eddy-removal procedures are required.     

Southern Ocean transformation in a coupled model with and without eddy mass fluxes

A coupled air–sea general circulation model is used to simulate the global circulation. Different parameterizations of lateral mixing in the ocean by eddies, horizontal, isopycnal, and isopycnal plus eddy advective flux, are compared from the perspective of water mass transformation in the Southern Ocean. The different mixing physics imply different buoyancy equilibria in the surface mixed layer, different transformations, and therefore a variety of meridional overturning streamfunctions. The coupled‐model approach avoids strong artificial water mass transformation associated with relaxation to prescribed mixed layer conditions. Instead, transformation results from the more physical non‐local, nonlinear interdependence of sea‐surface temperature, air–sea fluxes, and circulation in the model's atmosphere and ocean. The development of a stronger mid‐depth circulation cell and associated upwelling when eddy fluxes are present, is examined. The strength of overturning is diagnosed in density coordinates using the transformation framework.     

Dynamics of the current system in the southern Drake Passage

It has long been seen from satellite ocean color data that strong zonal gradients of phytoplankton biomass persistently occur in the southern Drake Passage during austral summer and fall, where the low productivity Antarctic Surface Water (ASW) within the Antarctic Circumpolar Current (ACC) region transforms to the high productivity water. An interdisciplinary cruise was conducted in February and March 2004 to investigate potential physical and biogeochemical processes, which are responsible for transporting nutrients and metals and for enhancing primary production. To explore physical processes at both the meso- and large-scales, surface drifters, a shipboard Acoustic Doppler Current Profiler and conductivity–temperature–depth sensors were used. Analyzing meso- and large-scale hydrography, circulation and eddy activities, it is shown that the topographic rise of the Shackleton Transverse Ridge plays the key role in steering an ACC branch southward west of the ridge, forming an eastward ACC jet through the gap between the ridge and Elephant Island and causing the offshelf transport of shelf waters approximately 1.2 Sv from the shelf near Elephant Island. High mesoscale eddy activities associated with this ACC southern branch and shelf waters transported off the shelf were found. The mixing between the iron-poor warmer ASW of the ACC and iron-rich waters on the shelf through horizontal transport and vertical upwelling processes provides a physical process which could be responsible for the enhanced primary productivity in this region and the southern Scotia Sea.     

南大洋太平洋扇区中尺度涡旋的统计特性及其变化

中尺度涡旋在南大洋海洋动力学中具有重要地位,其对气候变化的响应表现也引起了海洋学家与气候学家的广泛关注。本文利用涡动动能与涡旋自动探测技术两种方法对南大洋太平洋扇区的涡旋特性及其变化进行了分析。与前人结果相一致的是,高值的涡动动能主要集中在南极极锋海区,并且自西向东逐渐减弱。在过去的20年里,涡动动能在太平洋扇区的显著增强也集中在中西部海域,这里也是南极绕极流斜压性较强的海域。涡旋统计特性揭示了涡动动能的空间分布及其年际变化主要归因于涡旋振幅与旋转速度,而并非涡旋个数或者涡旋半径。这些结果进一步确认了对应于南半球环状模正位相的绕极西风异常改变了南大洋的涡旋特性,从而表现出涡旋活跃性增强。     

Synoptic eddies of the Subantarctic and Agulhas fronts and generation of the Antarctic Intermediate Water in the Atlantic Ocean

The concept developed in [9] and [10], according to which the Antarctic Intermediate Water is generated owing to the formation and subsequent decay of cyclonic eddies of the Subantarctic Front, is tested for the Atlantic Ocean. The cross frontal water and salt transport by the Subantarctic Front eddies and the Agulhas Front anticyclones is estimated by developing cinematic eddy models and analyzing satellite altimetry maps. Calculations of the water and salt balance for a box in the South Atlantic based on these results and various experimental data confirmed the above-mentioned concept.     

A late winter hydrographic section from Tasmania to Antarctica

A hydrographic section between Tasmania and Antarctica was occupied in late winter 1991 as part of the World Ocean Circulation Experiment (WOCE). The primary purpose of the WOCE repeat section SR3 is to measure the exchange between the Indian and Pacific Oceans south of Australia. This paper describes the fronts, water masses and transport observed on the first occupation of the repeat section. The Subantarctic Front (SAF) is located between 50°S and 51°S and is the most striking feature of the vertical sections. Two additional fronts at 53°S and 59°S are associated with the Polar Front (PF), part of which turns northward to flow along the section before turning back to the east near 53°S. Very deep (>500 m) mixed layers are found north of the SAF, confirming that Subantarctic Mode Water (SAMW) is formed in this region by deep convection in winter. Chlorofluorocarbons (CFCs) are significantly undersaturated (≈90–92% of equilibrium values) in these deep mixed layers, indicating that gas exchange rates are not rapid enough to bring these deep mixed layers to equilibrium by the end of the winter period of deep convective mixing. Northward Ekman drift of cold, fresh water across the SAF is likely to be responsible for the cooler, fresher mixed layers observed immediately north of the SAF. The Antarctic Intermediate Water (AAIW) on the SR3 section is relatively low in oxygen and CFCs (≈60–70% and 10–20% of saturation values, respectively), high in potential vorticity, and high in nutrients. These characteristics suggest that the AAIW on this section is not renewed by direct and rapid ventilation near this location. Water mass properties suggest that water from the Tasman Sea spreads south and west across the northern portion of the SR3 section between 800 and 3000 m depth. A cold, fresh, CFC-rich variety of Antarctic Bottom Water is formed along the Wilkes-Adelie coast of Antarctica. The net transport across the section relative to the deepest common depth is 160 Sv. The band of eastward flow between 50°S and 53°S including the SAF carries 137 Sv to the east and dominates the net transport. Weaker flow south of 58°S contributes an additional 70 Sv. The eastward flow is compensated in part by 37 Sv of westward flow between Tasmania and 48.5°S and 8 Sv of flow to the west over the southern flank of the mid-ocean ridge. The trajectories of six ALACE floats deployed at about 950 m confirm the sense of flow inferred from the choice of a deep reference level.