Direct velocity measurements of deep circulation southwest of the Shatsky Rise in the western North Pacific

Direct velocity measurements undertaken using a nine-system mooring array (M1–M9) from 2004 to 2005 and two additional moorings (M7p and M8p) from 2003 to 2004 reveal the spatial and temporal properties of the deep-circulation currents southwest of the Shatsky Rise in the western North Pacific. The western branch of the deep-circulation current flowing northwestward (270–10° T) is detected almost exclusively at M2 (26°15′N), northeast of the Ogasawara Plateau. It has a width less than the 190 km distance between M1 (25°42′N) and M3 (26°48′N). The mean current speed near the bottom at M2 is 3.6±1.3 cm s^?1. The eastern branch of the deep-circulation current is located at the southwestern slope of the Shatsky Rise, flowing northwestward mainly at M8 (30°48′N) on the lower part of the slope of the Shatsky Rise with a mean near-bottom speed of 5.3±1.4 cm s^?1. The eastern branch often expands to M7 (30°19′N) at the foot of the rise with a mean near-bottom speed of 2.8±0.7 cm s^?1 and to M9 (31°13′N) on the middle of the slope of the rise with a speed of 2.5±0.7 cm s^?1 (nearly 4000 m depth); it infrequently expands furthermore to M6 (29°33′N). The width of the eastern branch is 201±70 km on average, exceeding that of the western branch. Temporal variations of the volume transports of the western and eastern branches consist of dominant variations with periods of 3 months and 1 month, varying between almost zero and significant amount; the 3-month-period variations are significantly coherent to each other with a phase lag of about 1 month for the western branch. The almost zero volume transport occurs at intervals of 2–4 months. In the eastern branch, volume transport increases with not only cross-sectional average current velocity but also current width. Because the current meters were too widely spaced to enable accurate estimates of volume transport, mean volume transport is overestimated by a factor of nearly two, yielding values of 4.1±1.2 and 9.8±1.8 Sv (1 Sv=10⁶ m³ s^?1) for the western and eastern branches, respectively. In addition, a northwestward current near the bottom at M4 (27°55′N) shows a marked variation in speed between 0 and 20 cm s^?1 with a period of 45 days. This current may be part of a clockwise eddy around a seamount located immediately east of M4.     

Numerical investigation of the South China Sea deep circulation

Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea（SCS）, the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer（July）, a northeastsouthwest through-flow pattern across the deep basin without a western boundary current in winter（January）,and a transitional pattern in spring and autumn. The sensitivity ...     

Bottom water circulation in Cascadia Basin

A combination of beta spiral and minimum length inverse methods, along with a compilation of historical and recent high-resolution CTD data, are used to produce a quantitative estimate of the subthermocline circulation in Cascadia Basin. Flow in the North Pacific Deep Water, from 900-1900 m, is characterized by a basin-scale anticyclonic gyre. Below 2000 m, two water masses are present within the basin interior, distinguished by different potential temperature-salinity lines. These water masses, referred to as Cascadia Basin Bottom Water (CBBW) and Cascadia Basin Deep Water (CBDW), are separated by a transition zone at about 2400 m depth. Below the depth where it freely communicates with the broader North Pacific, Cascadia Basin is renewed by northward flow through deep gaps in the Blanco Fracture Zone that feeds the lower limb of a vertical circulation cell within the CBBW. Lower CBBW gradually warms and returns to the south at lighter density. Isopycnal layer renewal times, based on combined lateral and diapycnal advective fluxes, increase upwards from the bottom. The densest layer, existing in the southeast quadrant of the basin below 2850 m, has an advective flushing time of 0.6 years. The total volume flushing time for the entire CBBW is 2.4 years, corresponding to an average water parcel residence time of 4.7 years. Geothermal heating at the Cascadia Basin seafloor produces a characteristic bottom-intensified temperature anomaly and plays an important role in the conversion of cold bottom water to lighter density within the CBBW. Although covering only about 0.05% of the global seafloor, the combined effects of bottom heat flux and diapycnal mixing within Cascadia Basin provide about 2-3% of the total required global input to the upward branch of the global thermohaline circulation.     

Numerical investigation of deep water circulation in the Faroese Channels

The overflow of dense water from the Nordic Seas through the Faroese Channels is investigated numerically using the Massachusetts Institute of Technology General Circulation Model. The model is forced by the removal of a barrier that separates different water masses in the bottom layer of the Faroe-Shetland Channel at the north-eastern boundary. An analysis of the output reveals that during its adjustment in the rotating channel the propagating flow is unstable and forms cyclonic and anti-cyclonic eddies in the Faroese Channels. The life-time of the cyclonic eddy is about 10 days, but an anti-cyclonic eddy that is formed upstream of the sill crest of the Faroe Bank Channel has a longer life-time. However, after 50 days it eventually loses its structure below 400 m due to the decay of a counter-rotating current. In the upper 400 m layer this anti-cyclonic eddy remains persistent for longer. Observational evidence of the eddy is confirmed by the tracks of experimental drifters released in the area and by the temperature and salinity fields observed in the Faroese Channels.The pinching of isotherms along the Wyville Thomson Ridge results in the concentration of cold water on the southern side of the Faroese Channels that overflows into the Rockall Trough. The model results demonstrate that the main part of the cold water outflows through the Faroe Bank Channel, rather than across the Wyville Thompson Ridge, due to Earth rotation. The apparent similarity of modelled temperature, salinity and velocity sections to recent measurements in this area adds confidence to these results.     

Set-up of deep circulation in multi-level numerical models

The present study investigates the way an ocean filled with homogeneous warm water is cooled by prescribing cold water formation inside the ocean in the southern part of the southern hemisphere using multi-level numerical models. Cooling of the whole ocean starts with introduction of the cold water from the formation region into the deepest part of the ocean in the equatorial and eastern boundary regions by Kelvin wave-type density currents. The cold water along the eastern boundary extends westward as a Rossby wave-type density current setting up an interior poleward flow, and hits the western boundary to form a northward flowing boundary current in the northern hemisphere. Only then does the western boundary current cross the equator. Cooling of the rest of the ocean basin is accomplished by upwellings in the interior and also along the coasts. During this introduction the cold water is mixed with surrounding warm waters, and the thermocline, rather than forming just below the top level where heating is imposed, tends to spread down to deeper depths. Consequently the circulation at a steady state has a significant vertical structure such that the maximum upwelling in the interior occurs in the mid-depths, and only the deeper part of the deep ocean yields the Stommel and Arons circulation pattern. In the equatorial region higher vertical mode motions dominate, and a set of alternating zonal jets forms along the equator.     

Transport of newly ventilated deep water from the Iceland Basin to the Westeuropean Basin

Increased values of trichlorofluoromethane (CFC-11), tritium and stable tritium in the depth range from 2500 to 3500 m at the eastern flank of the Mid-Atlantic Ridge at 48°N (WHP section A2) indicate an influence of newly ventilated water. Water with similar Θ, S and tracer properties is found on the WHP section A1 (55°N) situated north of the Gibbs Fracture Zone in the Iceland Basin. The high tracer concentrations are due to the influence of Iceland Scotland Overflow Water (ISOW). The ISOW-influenced water found in the Iceland Basin partially passes by the Gibbs Fracture Zone (52°N) and flows southward along the topography of the Mid-Atlantic Ridge. A quantitative analysis of the transport from the Iceland Basin to the Westeuropean Basin is carried out based on the assumption that the water with enhanced tracer values is a two-component mixture of recirculating North East Atlantic Deep Water from the eastern part of the Westeuropean Basin and ISOW-influenced water as found on A1 in the Iceland Basin (NEADW_IB). The composition of the mixture and the transport time for the NEADW_IB are deduced from the temporal evolution of the tracer values. From the distance between the two sections and the area with enhanced tracer values, a transport of NEADW_IB from the Iceland Basin to the Westeuropean Basin of 1.63±0.32 Sv¹ is calculated for the density range 41.37<σ₃<41.475. Transports between 2.4 and 3.5 Sv result if the transport in the former density range is extrapolated to 41.35<σ₃<41.52 (corresponding to σ_Θ>27.8) in different ways.     

Effects of the deep ocean circulation on the reservoir characteristics of the ocean

To examine the effects of the deep ocean circulation on the characteristics of the ocean as a reservoir, age distributions of the material whose source and sink are at the ocean surface are calculated using an idealized vertical two-dimensional model of the ocean. The results show that the large-scale vertical circulation of the deep water accelerates the renewal of deep water and reduces the average age of the material. It is also shown that the multi-layered structures of the deep circulation are more realistic than the one-layered structure and promote the renewal of the deep water.     

加拿大海盆深层双扩散对流的观测分析

The Canada Basin(CB) is the largest sub-basin in the Arctic, with the deepest abyssal plain of 3 850 m. The double-diffusive process is the possible passage through which the geothermal energy affects the above isolated deep waters. With the temperature-salinity-pressure observations in 2003, 500-m-thick transition layers and lower1 000-m-thick bottom homogenous layers were found below 2 400 m in the central deep CB. Staircases with downward-increasing temperature and salinity are prominent in the transition layers, suggesting the doublediffusive convection in deep CB. The interface of the stairs is about 10 m thick with 0.001–0.002°C temperature difference, while the thicknesses of the homogenous layers in the steps decrease upward from about 60 to 20 m.The density ratio in the deep central CB is generally smaller than 2, indicating stronger double-diffusive convection than that in the upper ocean of 200–400 m. The heat flux through the deepest staircases in the deep CB varies between 0.014 and 0.031 W/m2, which is one-two orders smaller than the upper double-diffusive heat flux,but comparable to the estimates of geothermal heat flux.     

Direct flows from the Ulleung Basin into the Yamato Basin in the East/Japan Sea

Using the trajectories of ARGO floats, we report direct flows from the Ulleung Basin into the Yamato Basin through a gap between the Oki Spur and the Yamato Rise over the southern part of the East/Japan Sea. The gap is subdivided into two narrow (northern and southern) passages by a seamount located in the middle. The flows, therefore, are narrow and this explains why this flow was not reported earlier. More than half of the 25 ARGO floats, which operated around the gap, drifted through the gap or area near it. The strength of the throughflow estimated using the trajectories of the floats at parking depth is comparable to the mean deep flow found over the southwestern part of the East/Japan Sea. A high resolution regional ocean model whose overall circulation pattern over the Ulleung Basin is consistent with those from previous studies shows that the flow through the gap is supplied mainly by eastward flows crossing the mouth of the basin, and secondarily by the cyclonic circulation following the outer perimeter of the basin. Thus the throughflow is an important component of the deep circulation over the southern East/Japan Sea, and the narrow gap, where the flow is well confined, would be a good place to study the deep circulation.     

Controls on temporal variability of the geochemistry of the deep Cariaco Basin

Studies of the Cariaco Basin on the continental shelf of Venezuela, as a part of the Carbon Retention In A Colored Ocean (CARIACO) program, have revealed that the chemistry of the deeper waters of the system is more variable than previously believed. Small oxygen maxima have been observed on a number of occasions at depths where oxygen was previously absent, suggesting the occurrence of intrusions of oxygenated water into the region of the oxic/anoxic interface (250–300 m). Apparently because of these events, the oxic/anoxic interface deepened by about 100 m during the period of our observations. We also observed a dramatic decrease in H₂S concentrations at all depths below the oxic/anoxic interface during this same period. Bottom waters, for example, had an H₂S concentration of about 75 μM in November 1995, but since November 1997, concentrations in bottom water have not exceeded 55 μM. Water of sufficient density to sink to the bottom of the Basin has been observed on one occasion at sill depth just north of the eastern sill. However, based on a simple box model, the decrease in deep-water sulfide does not appear to be due to intrusion of oxygenated water alone, as concentrations of other measured species, and of hydrographic parameters, have remained constant with time. Instead, we postulate that an earthquake that took place in July 1997 resulted in a turbidity current that transported large quantities of coastal sediment containing oxidized iron into the deep waters of the basin. If the final products of reaction were elemental sulfur and iron sulfide, the sediment associated with the oxidized iron would have produced a turbidite layer about 10 cm thick. Previous earthquakes have produced turbidites of similar thickness.     

菲律宾海盆深层温盐结构与环流变化特征研究

基于ECCO2 (Estimating the Circulation and Climate of the Ocean)、 GLORYS12V1 (Global Ocean Reanalysis and Simulations)、ORA-S5 (Ocean ReAnalysis)三种海洋再分析数据,对比研究了菲律宾海盆深层温盐及环流的季节和年际变化特征。结果表明:三种数据显示的海盆深层温盐季节变化特征基本一致,在3 000～4 000 m水深区域,海水呈春夏两季高温低盐而秋冬季低温高盐特性,4 000 m以下海水温盐季节变化很小;沿西边界,温度与内部有明显差异且季节变化幅度相对较大。沿西边界的输运季节变化特征表现为10月至次年4月输运向南,5—9月输运向北,并且在8月份达最大值;表明存在沿西边界的流动,即菲律宾海盆与南端西卡罗林海盆(West Caroline Basin)之间存在季节性水体交换。海盆深层海水温盐年际变化也十分显著,但不同数据显示的变化特征存在较大差异。EOF和相关分析显示,三种再分析数据的深层位温与ENSO均存在一定相关性,ECCO2的深层位温变化与ENSO的相关性最强。由于长期观测数据较少,再分析数据的结果难以验证,因此目前对年际变化特征的研究仍具有很大的不确定性。     

Variability of the deep South China Sea circulation derived from HYCOM reanalysis data

This study aims to investigate variability of the deep South China Sea (SCS) circulation using the Hybrid Coordinate Ocean Model (HYCOM) global reanalysis product. The results reveal that annual cycle is a dominant component in the deep SCS circulation. Meanwhile, the boundary circulation strength is the weakest in January and peaks between June and September. The eastern and southern boundary currents strengthen/weaken one to three months earlier than that in the western and northern boundaries. Vector Empirical Orthogonal Functions (VEOF) analysis results reveal that semiannual and intraseasonal fluctuations are significant components, of which the spatial patterns are mainly confined in the northern and western boundary areas as well as the southwestern sub-basin. Wavelet analysis results show the strength of significant fluctuation varies year to year. Trend analysis results indicate a decadal weakening in the deep SCS circulation. An anomalous anticyclonic circulation, 50–70 km apart from the slope break, tends to weaken the cyclonic boundary circulation in the western and northern boundaries as well as the southwestern sub-basin. This trend is similar to the observed decadal weakening in the North Atlantic deep circulation. Thus, the findings of this study reveal that the variation of the deep SCS circulation has a remarkable response to the climate change. The mechanisms responsible for the variation are worth pursuing if more observations are available.     

Direct measurements of mid-depth circulation in the Shikoku basin by tracking SOFAR floats

Mid-depth circulation of the Shikoku Basin was measured by tracking four SOFAR floats drifting at the 1,500 m layer. Two floats were released on 17 April 1988 at 30°N, 135°59E and tracked for 433 days. Another two were released on 3 November 1988 at 29°52N and 133°25E, and tracked for 234 days. Two floats flowed clockwise around the Shikoku Warm Water Mass with a diameter of 400 km centered at 31°N and 136°E and a mean drift speed of 4.5 cm sec^–1. One of the floats showed about ten counterclockwise rotations with a period of about 8 days and a maximum speed of 80 cm sec^–1 in the sea area west to the Izu Ridge. In the east to Kyushu, a southward flow was observed under the northward flowing Kuroshio. The southward flow of 4 cm sec^–1 drift speed was considered to be a part of the counterclockwise circulation at deep layers along the perimeter of the Shikoku Basin. One float remained for 234 days in a limited area of 100 km by 150 km in the western part of the basin.     

Direct observation of subtropical mode water circulation in the western North Atlantic Ocean

Eighteen Degree Water (EDW) is the dominant subtropical mode water of the North Atlantic subtropical gyre and is hypothesized as an interannual reservoir of anomalous heat, nutrients and CO₂. Although isolated beneath the stratified upper-ocean at the end of each winter, EDW may re-emerge in subsequent years to influence mixed layer properties and consequently air–sea interaction and primary productivity. Here we report on recent quasi-Lagrangian measurements of EDW circulation and stratification in the western subtropical gyre using an array of acoustically-tracked, isotherm-following, bobbing profiling floats programmed to track and intensively sample the vertically homogenized EDW layer and directly measure velocity on the 18.5 °C isothermal surface.The majority of the CLIVAR Mode Water Dynamics Experiment (CLIMODE) bobbers drifted within the subtropical gyre for 2.5–3.5 years, many exhibiting complex looping patterns indicative of an energetic eddy field. Bobber-derived Lagrangian integral time and length scales (3 days, 68 km) associated with motion on 18.5 °C were consistent with previous measurements in the Gulf Stream extension region and fall between previous estimates at the ocean surface and thermocline depth. Several bobbers provided evidence of long-lived submesoscale coherent vortices associated with substantial EDW thickness. While the relative importance of such vortices remains to be determined, our observations indicate that these features can have a profound effect on EDW distribution. EDW thickness (defined using a vertical temperature gradient criterion) exhibits seasonal changes in opposition to a layer bounded by the 17 °C and 19 °C isotherms. In particular, EDW thickness is generally greatest in winter (as a result of buoyancy-forced convection), while the 17°–19 °C layer is thickest in summer consistent with seasonal Ekman pumping. Contrary to previous hypotheses, the bobber data suggest that a substantial fraction of subducted EDW is isolated from the atmosphere for periods of less than 24 months. Seasonal-to-biennial re-emergence (principally within the recirculation region south of the Gulf Stream) appears to be a common scenario which should be considered when assessing the climatic and biogeochemical consequences of EDW.     

海底大型多金属硫化物矿体内的流体过程

海底多金属硫化物矿体内热场和流场分布控制着多金属硫化物矿体的形成过程和成矿机制.在大洋钻探计划(ODP)和已有模拟实验研究的基础上,构建了一个具有三层结构的非均质海底多金属硫化物矿体模型,并利用地下热水系统体系模拟软件(Hydrothermal)模拟了不同渗透率情况下多金属硫化物矿体内部热场和流场的形态特征.模拟结果表...     

WOA18模型在远海地形测量中的运用研究

随着我国远海地形测量的日益频繁,进一步提高测量的效率和精度成为研究的热点。利用WOA18数据,对远海地形测量所涉及海域温盐等海洋要素时空分布规律展开预先分析,由此得到该海域声速垂直和水平分布规律,再利用层内常梯度的声线跟踪方法,对相关海域声速剖面获取频次和线性变化开始深度展开定量研究。结果表明,WOA18数据不但能较好优化远海多波束地形测量声速剖面,还能对声速剖面获取的频次和线性变化开始深度做出较好预测,研究结果对提高远海地形测量的精度和效率具有较高价值。     

Surface circulation of the Levantine Basin: Comparison of model results with observations

The eastern Mediterranean (Levantine Basin) hydrography and circulation are investigated by comparing the results of a high-resolution primitive equation model with observations. After a 10-year integration, the model is able to reproduce the major water masses and the circulation patterns of the eastern Mediterranean. Comparisons with the POEM hydrographical observations show good agreement. The vertical distribution of the water masses matches that of the observations quite well in terms of monthly mean. The model surface circulation is in agreement with circulation schemes derived from recent observations. Some well-known mesoscale features of the upper thermocline circulation are also realistically reproduced. In agreement with satellite observations, the model shows that high-energy mesoscale eddies dominate the upper thermocline circulation in the southern and the central parts of the Levantine Basin. Most of the Atlantic Water follows the north African coast and forms a strong coastal jet near the Libyan coast rather than forming the Mid-Mediterranean Jet described by several authors. The sub-basin circulation shows a strong seasonal signal. A strong and stable current flows along the isobaths in winter, becoming weaker and with more meanders in summer. The mesoscale eddies throughout the whole basin are more energetic in summer than in winter.     

Gas hydrate within the Winona Basin, offshore western Canada

In western Canada gas hydrates have been thought to exist primarily in the Cascadia accretionary prism off southern Vancouver Island, British Columbia (BC). We present evidence for the existence of gas hydrate in folds and ridges of the Winona Basin up to 40 km seaward from the foot of the continental slope off northern Vancouver Island. The occurrence of a bottom-simulating reflector (BSR) observed in a number of vintage seismic reflection profiles is strongly correlated to faulted, and folded sedimentary ridges and buried folds. The observed tectonic structures of the Winona Basin are within the rapidly evolving Juan de Fuca - Cascadia - Queen Charlotte triple junction off BC. Re-processing of multi-channel data imaged mildly to strongly deformed sediments; the BSR is confined to sediments with stronger deformation. Changes in the amplitude character of sediment-reflections above and below the depth of the base of gas hydrate stability zone were also used as an indicator for the presence of gas hydrate. Additionally, regional amplitude and frequency reduction below some strong BSR occurrences may indicate free gas accumulations. Gas hydrate formation in the Winona Basin appears strongly constrained to folds and ridges and thus correlated to deeper-routed fluid-advection regimes. Methane production from in situ microbial activities as a source of gas to form gas hydrates, as proposed to be a major contributor for gas hydrates within the accretionary prism to the south, appears to be insufficient to produce the widespread gas hydrate occurrences in the Winona Basin. Potential reasons for the lack of sufficient in situ gas production may be that sedimentation rates are 5-100 times higher than those in the accretionary prism so that available organic carbon moves too quickly through the gas hydrate stability field. The confinement of BSRs to ridges and folds within the Winona Basin results in an areal extent of gas hydrate occurrences that is a factor of five less than what is expected from regional gas hydrate stability field mapping using water-depth (pressure) as the only controlling factor only.     

Seismic stratigraphic framework and depositional sequences in the Santos Basin,Brazil

The Santos Basin, situated offshore southern Brazil, is one of nine marginal rift basins in the equatorial South Atlantic. It formed by the collapse of a thermal dome in the late Jurassic and by subsequent rifting and opening of the South Atlantic in the early Cretaceous. Rifting was accompanied by immense volcanic outpouring seen at the surface today throughout the onshore Paraná Basin and thought to underlie the entire Santos Basin, and the adjacent São Paulo Plateau. Vulcanism was followed by subsidence of up to 10 km from Aptian to Recent time, and a coastal hingeline coincides with the Serra do Mar uplift. The basin depocentre, which is 700 km long, is bounded to the north and south by basement and volcanic highs, respectively. A restricted water circulation in the ocean basin, which prevailed up to the Santonian stage, has important repercussions for the hydrocarbon potential of the area. The nine genetically related basins have collective reserves of ≈ 5 billion barrels of oil and associated gas. A stratigraphic framework, based largely on seismic data, has been erected for the Santos Basin. Seven regional unconformities, or ‘R’ reflections, can be traced throughout the basin and form the boundaries for seismic sequences. Isopaching the seismic sequences defines the principal depositional units in the basin and also shows how the basin depocentre shifted with time. Limited well control has enabled the seismic sequences to be correlated with litho-environmental sequences which more fully reflect the geological evolution and provide a working exploration model. Finally, an attempt has been made to recognize and map seismic facies within the seismic sequences and to predict the lithofacies in areas away from well control