An estimate of water mass transformation in the southern Weddell Sea

Bottom water formation changes the characteristics of water masses entering the southern part of the Weddell Sea through atmosphere-ice-ocean interaction in which both sea and shelf ice play an important role. Modified water, in particular Weddell Sea Bottom Water, recirculates in the west. By comparing the in- and outflowing water masses we have estimated transformation rates on the basis of a data set obtained during the Winter Weddell Gyre Study from September to October 1989. This consisted of a salinity-temperature-depth (CTD) section carried out by R/V “Polarstern” from the northern tip of the Antarctic Peninsula to Kapp Norvegia and data from three current meter moorings maintained from 1989 to 1990 in the eastern boundary current off Kapp Norvegia. Because of the lack of sufficient direct current measurements in the interior and the western boundary current, it was necessary to derive mass transports on the basis of available data combined with physical and geometrical arguments. At the mooring site barotropic currents were measured. They were extrapolated to the interior under the assumption that wind-driven, baroclinic and barotropic current fields are of similar shape. The location of the gyre centre was determined from drifting buoy tracks and geopoten-tial anomaly. A linear current profile from the eastern boundary current to the centre of the gyre was assumed, and the western outflow was determined according to mass conservation. Different assumptions on the transition from the boundary current to the interior and the location of the centre result in a wide range of transports with most likely values between 20 and 56 Sv. The total mass transport was split into individual water masses. Differences between inflow and outflow result in a transformation rate of 3–4 Sv from Winter and Warm Deep Water to Antarctic and Weddell Sea Bottom Water. The net heat and salt transport across the transect implies heat fluxes from the ocean to the atmosphere of 3–10 W m⁻² and ice formation rates of 0.2–0.35 m year⁻¹.     

Effect of vertical viscosity and diffusivity on tropical ocean circulation

Response of the tropical ocean to a uniform zonal wind is studied numerically and analytically. In addition to the Equatorial Undercurrent and surface westward flows on both sides of the equator, an eastward flow at the pycnocline depth is formed at several degrees latitude in both hemispheres. This subsurface eastward flow first appears in the eastern part of the ocean and extends to the west. Then it gradually decreases in speed, and at a steady state the speed is of the order of 1cm sec^–1. The spatial distribution of this subsurface flow is similar to the Subsurface Countercurrent, but the speed is one order smaller than that observed. The obtained thermostad is obscure compared with that observed. Whole of the time evolution produced by a numerical model can be accounted for by linear wave dynamics in a multi-layer model including vertical diffusion and friction. Although diffusion and friction are essential to maintain this subsurface flow, changes in the values of coefficients for vertical viscosity and diffusivity and also in initial density stratification lead only to a minor change in the speed of the subsurface eastward flow. It is concluded that a subsurface eastward flow with speed exceeding 10 cm sec^–1 accompanied by a distinctive thermostad structure cannot be explained by linear wave dynamics including vertical dissipation.     

荔湾3-1气田深水脐带缆竖直铺设技术研究与应用

随着海洋油气开发逐步走向深海,深水脐带缆的应用越来越广泛。我国在脐带缆安装工程中仅具备浅水(300 m以内)水平铺设的能力,深水脐带缆的铺设技术在国内尚属空白。本文以荔湾3-1气田深水脐带缆铺设工程为例,首先介绍铺设方法的选取原则,提出深水脐带缆竖直铺设的技术要求,随后基于Orca Flex软件,对脐带缆铺设进行静力和动力分析,为铺设期间脐带缆长度和张力的监控、作业气候窗的选取提供参考依据。最后制定脐带缆起始抽拉、正常铺设以及末端铺设的作业流程。该工程为我国首次采用脐带缆竖直铺设技术,本文研究成果和计算方法可为国内其它同类工程提供重要借鉴意义。     

Fine-grained sediments in deep water: An overview of processes and facies models

There appears to be a continuum of processes affecting the transport and deposition of fine-grained sediments in the deep sea. This results in a facies continuum within which we can recognize three broadly different facies groups: turbidites, contourites, and pelagites/hemipelagites. Several distinct facies models can be defined for each group on the basis of their chief structural, textural, and compositional attributes.     

The bimodality of the Luzon Strait deep water

Combined conductivity-temperature-depth(CTD) casts and Argo profiles, 3 086 historical hydrocasts were used to quantify the water column characteristics in the northern South China Sea(SCS) and its adjacent waters. Based on a two-dimensional "gravest empirical mode"(GEM), a gravitational potential(, a vertically integrated variable) was used as proxy for the vertical temperature profiles TG(p,). integrated from 8 MPa to the surface shows a close relationship with the temperature, except in the deep layer greater than 15 MPa, which was caused by the bimodal deep water in the region. The GEM temperature profiles successfully revealed the bimodality of the Luzon Strait deep water, that disparate hydrophic vertical profiles can produce distinct specific volume anomaly() in the SCS and the western Philippine Sea(WPS), but failed in the Luzon Strait, where different temperature profiles may produce a same. A significant temperature divergence between the SCS water and the WPS water confirmed that the bimodal structure is strong. The deepwater bifurcation starts at about 15 MPa, and gets stronger with increasing depth. As the only deep channel connecting the bimodal-structure waters, water column characteristics in the Luzon Strait is in between, but much closer to the SCS water because of its better connectivity with the SCS. A bimodal temperature structure below 15 MPa reveals that there was a persistent baroclinic pressure gradient driving flow through the Luzon Strait. A volume flux predicted through the Bashi Channel with the hydraulic theory yields a value of 5.62×106 m3/s using all available profiles upstream and downstream of the overflow region, and 4.03×106and 2.70×106 m3/s by exclusively using the profiles collected during spring and summer, respectively. No volume flux was calculated during autumn and winter because profiles are only available for the upstream of the Bashi Channel during the corresponding period.     

Biogeochemical evidence of large diapycnal diffusivity associated with the subtropical mode water of the North Pacific

A profiling float equipped with a fluorimeter, a dissolved oxygen (DO) sensor, and temperature and salinity sensors was deployed in the subtropical mode water (STMW) formation region of the North Pacific. It acquired quasi-Lagrangian, 5-day-interval time-series records from March to July 2006. The time-series distribution of chlorophyll showed a sustained and sizable subsurface maximum at 50–100 m, just above the upper boundary of the STMW, throughout early summer (May–July). The DO concentration in this lower euphotic zone (50–100 m) was almost constant and supersaturated in the same period, becoming more supersaturated with time. On the other hand, the DO concentration at 100–150 m near the upper boundary of the STMW decreased much more slowly compared with the main layer of STMW below 150 m, even though oxygen consumption by organisms was expected to be larger in the former depth range. The small temporal variations of DO in the lower euphotic zone and near the upper boundary of the STMW were reasonably explained by downward oxygen transport because of large diapycnal diffusion near the top of the STMW. Assuming that the oxygen consumption rate at 100–150 m was the same as that in the main layer of STMW and compensated by the downward oxygen flux, the diapycnal diffusivity was estimated to be 1.7 × 10⁻⁴ m² s⁻¹. Nitrate transport into the euphotic zone by the same large diffusion was estimated to be 0.8 mmol N m⁻² day⁻¹. All of the transported nitrate could have been used for photosynthesis by the phytoplankton; net community production was estimated to be 5.3 mmol C m⁻² day⁻¹.     

An eddy-driven abyssal circulation in a bowl-shaped basin due to deep water formation

Numerical experiments were performed to investigate the effects of eddies generated in deep water formation processes on an abyssal circulation in a closed bowl-shaped basin. Two sets of experiments were performed. One set was eddy-restricted experiments in which only a volume-driven (upwelling-driven) circulation was simulated and the other set was eddy-permitted experiments in which both a volume-driven circulation and an eddy-driven circulation were simulated. In the two layer experiment where the lower layer water is formed, a mean along-slope current is formed in the lower layer for both the eddy-restricted and eddy-permitted experiments. The direction of the current was not unique in the eddy-restricted experiment, but it was cyclonic in the eddy-permitted experiment. In the three layer experiments where water of the intermediate layer is formed, the mean along-slope current in the lowest layer is negligibly small in the eddy-restricted experiment, while it is large and cyclonic in the eddy-permitted experiment. The driving forcings of the eddy-driven circulation are quantified in terms of eddy fluxes of relative vorticity (Reynolds stress) and layer thickness (bolus velocity). These terms increase as the volume of the newly formed water increases, but they do not change greatly with the slope height. The magnitude of these terms changes with the slope width, but the sum of these terms does not vary greatly. As a result, the intensity of the eddy-driven circulation depends primarily on the volume of newly formed water. These dependences of eddy fluxes were interpreted using downgradient diffusion of potential vorticity.     

Cretan deep water outflow into the Eastern Mediterranean

A simple hydraulic model is used to estimate the deep water fluxes of Cretan Deep Water (CDW), through the Cretan Arc Straits and into the Eastern Mediterranean Basins. The input to the model consists of the height of the deep water reservoir above sill depth and its density difference from the overlying water masses. Data from four hydrographic cruises, which took place in 1995, 1991 and 1987, are used to estimate the depth of the reservoir above the sill and the density difference. The results show a significant CDW outflow of 0.75×10⁶ m³ s⁻¹ in early 1995. The outflow of CDW through Kassos Strait, in the east, is 0.53×10⁶ m³ s⁻¹, while 0.22×10⁶ m³ s⁻¹ outflows through the Antikithira Strait in the west. The model results agree with fluxes estimated from current meter observations.The CDW outflow has been neither steady nor uniform during the period 1987–95. In the Kassos Strait, the outflow commenced in 1987 and increased rapidly until 1991; since then, it appears to have stabilised. In the Antikithira Strait, in contrast, the outflow has increased steadily since 1987. Such modifications in the CDW outflow are associated with changes in its hydrographic characteristics. The salinity of CDW increased constantly, by approximately 0.1, between 1987 and 1995 while its temperature warmed, between 1987 and 1991, and then cooled.     

Cold deep water in the South China Sea

Two deep channels that cut through the Luzon Strait facilitate deep (>2000 m) water exchange between the western Pacific Ocean and the South China Sea. Our observations rule out the northern channel as a major exchange conduit. Rather, the southern channel funnels deep water from the western Pacific to the South China Sea at the rate of 1.06 ± 0.44 Sv (1 Sv = 10⁶ m³s⁻¹). The residence time estimated from the observed inflow from the southern channel, about 30 to 71 years, is comparable to previous estimates. The observation-based estimate of upwelling velocity at 2000 m depth is (1.10 ± 0.33) × 10⁻⁶ ms⁻¹, which is of the same order as Ekman pumping plus upwelling induced by the geostrophic current. Historical hydrographic observations suggest that the deep inflow is primarily a mixture of the Circumpolar Deep Water and Pacific Subarctic Intermediate Water. The cold inflow through the southern channel offsets about 40% of the net surface heat gain over the South China Sea. Balancing vertical advection with vertical diffusion, the estimated mean vertical eddy diffusivity of heat is about 1.21 × 10⁻³ m²s⁻¹. The cold water inflow from the southern channel maintains the shallow thermocline, which in turn could breed internal wave activities in the South China Sea.     

Global fisheries subsidies: An updated estimate

The aim of this paper is to provide an updated estimate of global fisheries subsidies. It builds on earlier estimates and methodologies to re-estimate and discuss the various types of subsidies provided by governments around the world. The results suggests that total subsidies were about USD 35 billion in 2009 dollars, which is close to the earlier estimate of 2003 subsidies once they are adjusted for inflation. Capacity-enhancing subsidies constituted the highest category at over USD 20 billion. For all regions, the amount of capacity-enhancing subsidies is higher than other categories, except for North America, which has higher beneficial subsidies. The analysis reveals that fuel subsidies constitute the greatest part of the total subsidy (22% of the total), followed by subsidies for management (20% of the total) and ports and harbors (10% of the total). Subsidies provided by developed countries are far greater (65% of the total) than those by developing countries (35% of the total) even though the latter lands well above 50% of total global catch. Asia is by far the greatest subsidizing region (43% of total), followed by Europe (25% of total) and North America (16% of total). Japan provides the highest amount of subsidies (19.7% of total), followed by the United States and China at 19.6% of total.     

赤道太平洋近表层上升流的估计

利用1979-2006年间的卫星跟踪漂流浮标资料计算得到多年平均的赤道太平洋近表层流场,并估计赤道中东太平洋上混合层水平流场的散度和上升流流量.主要结论包括:1)赤道中东太平洋上升流区域主要集中在日界线以东以赤道为中心的±2°纬度带内,并且赤道南北约4°处各有一下沉流区域;2)赤道中东太平洋附近(165°E-85°W,2°S-2°N)散度平均约为2.0×10-7s-1,对应30m深处上升流区域整体体积输送约为43Sv,其中大约一半的上升流水体随热带流圈在赤道外4°N/S附近下沉,其余部分向两极输送;3)赤道中东太平洋辐散在春季达到最大值2.1×10-7s-1,而在秋季最弱;4)在El Ni(n)o期间辐散减弱,而在La Ni(n)a期间辐散增强,其中纬向流所致的辐合辐散也起到一定作用.     

Kinematics of extreme waves in deep water

The velocity profiles under crest of a total of 62 different steep wave events in deep water are measured in laboratory using particle image velocimetry. The waves take place in the leading unsteady part of a wave train, focusing wave fields and random wave series. Complementary fully nonlinear theoretical/numerical wave computations are performed. The experimental velocities have been put on a nondimensional form in the following way: from the wave record (at a fixed point) the (local) trough-to-trough period, T_TT and the maximal elevation above mean water level, η_m of an individual steep wave event are identified. The local wavenumber, k and an estimate of the wave slope, ε are evaluated from ω²/(gk)=1+ε², where ω=2π/T_TT and g denotes the acceleration of gravity. A reference fluid velocity, is then defined. Deep water waves with a fluid velocity up to 75% of the estimated wave speed are measured. The corresponding kη_m is 0.62. A strong collapse of the nondimensional experimental velocity profiles is found. This is also true with the fully nonlinear computations of transient waves. There is excellent agreement between the present measurements and previously published Laser Doppler Anemometry data. A surprising result, obtained by comparison, is that the nondimensional experimental velocities fit with the exponential profile, i.e. e^ky, y the vertical coordinate, with y=0 in the mean water level.     

Nonlinear dynamic analysis of deep water risers

The paper presents a theoretical and numerical approach to the dynamical behaviour of risers in deep water which takes into account two types of nonlinearity; that due to viscous drag forces and that due to the large displacements of the riser when submitted to strong axial loads. As the second nonlinearity may have a significant influence upon the behaviour of risers in deep water, a method for automatically updating the structural geometry during the dynamic analysis is given. A computer programme has been written for this purpose.     

The large deep water transient in the Eastern Mediterranean

The recent changes in the thermohaline circulation of the Eastern Mediteranean caused by a transition from a system with a single source of deep water in the Adriatic to one with an additional source in the Aegean are described and assessed in detail. The name Cretan Sea Overflow Water (CSOW) is proposed for the new deep water mass. CSOW is warmer (θ>13.6°C) and more saline (S>38.80) than the previously dominating Eastern Mediterranean Deep Water (EMDW), causing temperatures and salinities to rise towards the bottom. All major water masses of the Eastern Mediterranean, including the Levantine Intermediate Water (LIW), have been strongly affected by the change. The stronger inflow into the bottom layer caused by the discharge of CSOW into the Ionian and Levantine Basins induced compensatory flows further up in the water column, affecting the circulation at intermediate depth. In the northeastern Ionian Sea the saline intermediate layer consisting of Levantine Intermediate Water and Cretan Intermediate Water (CIW) is found to be less pronounced. The layer thickness has been reduced by factor of about two, concurrently with a reduction of the maximum salinity, reducing advection of saline waters into the Adriatic. As a consequence, a salinity decrease is observed in the Adriatic Deep Water. Outside the Aegean the upwelling of mid-depth waters reaches depths shallow enough so that these waters are advected into the Aegean and form a mid-depth salinity-minimum layer. Notable changes have been found in the nutrient distributions. On the basin-scale the nutrient levels in the upper water column have been elevated by the uplifting of nutrient-rich deeper waters. Nutrient-rich water is now found closer to the euphotic zone than previously, which might induce enhanced biological activity. The observed salinity redistribution, i.e. decreasing values in the upper 500–1400 m and increasing values in the bottom layer, suggests that at least part of the transition is due to an internal redistribution of salt. An initiation of the event by a local enhancement of salinity in the Aegean through a strong change in the fresh water flux is conceivable and is supported by observations.     

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.     

Computations of the almost highest short-crested waves in deep water

The highest short-crested waves have been studied analytically and numerically by several workers, but without a conclusive view. An efficient numerical scheme is proposed in this paper which retains the water-surface elevations in an implicit form in the governing equations, rather than using a series approximation, thus improving the accuracy of the numerical results. Convergence of the numerical scheme is verified. The almost highest short-crested waves in deep water are then evaluated, which are defined for the condition with the largest wave energies. It is found that the critical angle for wave frequency reversal also demarcates the wave characteristics near breaking, for either kinematic or dynamic prominence. The known results available for the limiting two-dimensional cases of standing and progressive waves are compared favourably.