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1.
Andreas Macrander Rolf H. Käse Uwe Send Héðinn Valdimarsson Steingrímur Jónsson 《Ocean Dynamics》2007,57(2):75-89
In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability,
no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until
recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic
instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization
of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the
dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared
with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume
can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill. 相似文献
2.
We present a porous medium approach to representing topography, and a new algorithm for the objective interpolation of topography, for use in ocean circulation models of fixed resolution. The representation and algorithm makes use of two concepts; impermeable thin walls and porous barriers. Impermeable thin walls allow the representation of knife-edge sub-grid-scale barriers that block lateral flow between model grid cells. Porous barriers permit the sub-grid scale geometry to modulate lateral transport as a function of elevation. We find that the porous representation and the resulting interpolated topography retains key features, such as overflow sill depths, without compromising other dynamically relevant aspects, such as mean ocean depth for a cell. The accurate representation of the ocean depth is illustrated in a simple model of a tsunami that has a cross-basin travel time very much less dependent on horizontal resolution than when using conventional topographic interpolation and representation. 相似文献
3.
Hydrographic changes in the Labrador Sea, 1960–2005 总被引：1，自引：0，他引：1
The Labrador Sea has exhibited significant temperature and salinity variations over the past five decades. The whole basin was extremely warm and salty between the mid-1960s and early 1970s, and fresh and cold between the late 1980s and mid-1990s. The full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s. The freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the Labrador Sea sustained throughout the subsequent years. It took only a decade for the whole water column to lose most of its excessive freshwater, reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s. If the recent tendencies persist, the basin’s storages of salt and heat will fairly soon, likely by 2008, exceed their historic highs.The main process responsible for the net cooling and freshening of the Labrador Sea between 1987 and 1994 was deep winter convection, which during this period progressively developed to its record depths. It was caused by the recurrence of severe winters during these years and in its turn produced the deepest, densest and most voluminous Labrador Sea Water (LSW1987–1994) ever observed. The estimated annual production of this water during the period of 1987–1994 is equivalent to the average volume flux of about 4.5 Sv with some individual annual rates exceeding 7.0 Sv. Once winter convection had lost its strength in the winter of 1994–1995, the deep LSW1987–1994 layer lost “communication” with the mixed layer above, consequently losing its volume, while gaining heat and salt from the intermediate waters outside the Labrador Sea.While the 1000–2000 m layer was steadily becoming warmer and saltier between 1994 and 2005, the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air-sea heat fluxes in the winter of 1999–2000. This change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent LSW class (LSW2000) penetrating deeper than 1300 m. This layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass, implying that LSW2000 is likely to follow the fate of its deeper precursor, LSW1987–1994. The increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid-depth perimeter of the Labrador Sea.The near-bottom waters, originating from the Denmark Strait overflow, exhibit strong interannual variability featuring distinct short-term basin-scale events or pulses of anomalously cold and fresh water, separated by warm and salty overflow modifications. Regardless of their sign these anomalies pass through the abyss of the Labrador Sea, first appearing at the Greenland side and then, about a year later, at the Labrador side and in the central Labrador Basin.The Northeast Atlantic Deep Water (2500–3200 m), originating from the Iceland–Scotland Overflow Water, reached its historically freshest state in the 2000–2001 period and has been steadily becoming saltier since then. It is argued that LSW1987–1994 significantly contributed to the freshening, density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening LSW, the hydrostatic adjustment to expanding LSW, or both. 相似文献
4.
Emil Jeansson Sara Jutterström Leif G. Anderson E. Peter Jones James H. Swift 《Progress in Oceanography》2008,78(1):12-28
Data from the East Greenland Current in 2002 are evaluated using optimum multiparameter analysis. The current is followed from north of Fram Strait to the Denmark Strait Sill and the contributions of different source waters, in mass fractions, are deduced. From the results it can be concluded that, at least in spring 2002, the East Greenland Current was the main source for the waters found at the Denmark Strait Sill, contributing to the overflow into the North Atlantic. The East Greenland Current carried water masses from different source regions in the Arctic Ocean, the West Spitsbergen Current and the Greenland Sea. The results agree well with the known circulation of the western Nordic Seas but also add knowledge both to the quantification and to the mixing processes, showing the importance of the locally formed Greenland Sea Arctic Intermediate Water for the East Greenland Current and the Denmark Strait. 相似文献
5.
Comparison of overflow simulations on different vertical grids using the Finite Element Ocean circulation Model 总被引：1，自引：0，他引：1
The Finite Element Ocean circulation Model (FEOM) is applied to study the sensitivity of density driven overflows to the vertical discretization and bottom topography representation using the dynamics of overflow mixing and entrainment (DOME) setup. FEOM allows for hybrid grids combining σ, z + σ, full cell, partly shaved cell and fully shaved cell grids within the same numerical kernel thus isolating as far as possible effects of mesh geometry from those of model numerics. The sensitivity of diapycnal mixing, entrainment, plume thickness and plume meridional distribution to vertical discretization and partly to the subgrid process parameterization is explored. It is shown that simulations on pure σ grids or the combination of z + σ resolve the overflow processes best in terms of downslope plume propagation, plume thickness and dilution, and also have the least resolution dependence. Grids using z-levels generate excessive spurious mixing when resolution is insufficient. Applying partial cells improves the plume representation, but still requires higher horizontal and vertical resolution to converge to the σ grid results. It is demonstrated that increasing lateral viscosity causes the plume thickness to reduce whereas increasing lateral diffusivity has opposite effect. When keeping the Prandtl number constant, the increase in diffusivity and viscosity leads to an increase in mixing and plume thickness on z-level grids and also on σ-grids when lateral dissipation is oriented along geopotential surfaces. Using the along σ- diffusion helped to obtain correct plume thickness and entrainment on σ grids. Increasing the vertical mixing coefficients leads to an increase in diapycnal mixing and in downslope penetration as well. 相似文献
6.
北欧海南部沉积物所记录的末次冰盛期以来千年尺度快速气候事件下的溢流水变化特征,对研究历史上热盐环流与气候的相互作用机制有着重要意义.本文基于冰岛东北部陆架IS-1B岩芯(65°36.357′N, 8°59.045′W)上部130 cm的沉积物粒度组成、颜色反射率和高分辨率XRF元素地球化学扫描测试数据,结合有孔虫AMS 14C测年数据和邻近海区站位沉积资料,利用因子分析等方法,重建了研究区3万年(HS 3期)以来的沉积记录,并重点研究了冰岛-苏格兰溢流水(ISOW)的历史变化特征及其对海冰活动的响应.研究结果显示, ISOW在末次冰盛期初期、 HS1末期到 B/A 暖期和全新世时期强度较高,在HS 3期末期、末次冰盛期中后期到HS 1初期和YD时期强度较弱,其中粒度和地球化学证据都指示ISOW在末次冰盛期和YD时期受北冰洋底层水入侵和海平面下降的影响可能发生了多次停滞.总体来说,挪威海南部ISOW强度的变化与表层海水温度有较强的相关性,表现为冰期减弱,间冰期增强的变化趋势.挪威海北部海冰的覆盖范围自HS 3末期至末次冰盛期开始逐渐南移,并在HS 2初期越过冰岛-苏格兰海脊,在冰岛海盆北部形成常年的冰覆盖. HS 1期中后期到B/A暖期,海冰影响范围开始逐渐下降,但在 B/A 暖期末期到YD时期海冰活动便快速恢复,直至进入全新世后,挪威海南部海冰活动强度才逐渐下降,一直保持相对稳定的季节性海冰活动状态. 相似文献
7.
B. Gehrke K. S. Lackschewitz H. -J. Wallrabe-Adams 《International Journal of Earth Sciences》1996,85(3):525-535
Sediment samples from the Mid-Atlantic Reykjanes Ridge (59°N) were taken to get information about sediment genesis and to
identify different sources during the late Quaternary. Samples were investigated by X-ray diffraction and grain-size analyses.
The clay mineral assemblages in sediments of the Reykjanes Ridge reflect paleoceanographic changes during the late Quaternary.
Holocene sediments are characterized by high contents of smectite, mainly of less developed crystallinity. In the spatial
distribution of clay minerals high smectite concentrations on the eastern flank and slightly decreasing concentrations on
the western flank of the Reykjanes Ridge indicate the action of bottom-water transport. The smectite originates mainly from
the volcanogenous Icelandic shelf and reflects the influence of Iceland-Scotland Overflow Water (ISOW). Stratigraphic variability
in the clay mineral composition reflects predominantly the influence of different sources, resulting from oceanographic and
glacial transport processes. During glacial time sediment transport is due mainly to input by icebergs. Increasing amounts
of illite, chlorite, and kaolinite characterize ice-rafted sediments of the “Heinrich layers”. In these sediments smectite
crystallinity is well developed. In contrast, several other ice-rafted layers contain smectite with low crystallographic order,
similar to smectites of Holocene age. The icelandic source was proved by distinct amounts of basaltic glass in the coarse-grained
sediment. At approximately 55 ka increasing amounts of chlorite and kaolinite suggest an enhanced influx of warm North Atlantic
surface waters. This hypothesis is supported by a high carbonate shell production at this time. Relative low concentrations
and the well-developed crystallinity of smectite minerals characterize the Last Glacial Maximum (LGM; 18–16 ka), indicating
a reduced supply of fine icelandic material. Shortly after the LGM, at the beginning of termination IA, a distinct increase
in fine-grained quartz (<2μm) and smectite are visible, which are proposed to reflect a supply of fine-grained ice-rafted
material. At 13 ka linear increasing smectite concentrations of lower crystallographic order indicate increasing supply of
fine-grained material from Iceland, linked to reinitiation of bottom currents of the ISOW. Full reinitiation is indicated
at around 10 ka, where a strong increase in smectite of low crystallographic order is detected. 相似文献
8.
The paper discusses the overflow (spillage) and yield rates and the total overflow and total yield over a specified time from a finite discrete stochastic reservoir, in which the yieldY
t
during the working interval (t,t+1) is a function of the storageZ
t
at timet, the inflow sequence {X
t
} being IID.The distribution vector of the spillage rate at timet is a telescoped version of the distribution of a certain Markovian variable whose transition matrix is derived. Formulae are given for the distribution of the total spillageW
h
given suitable initial conditions, forh=1,2,3; and a simple expression derived forE(W
h
). 相似文献
9.
This paper presents the applications of a newly developed free surface flow model to the practical, while challenging overflow problems for weirs. Since the model takes advantage of the strengths of both the level set and volume of fluid methods and solves the Navier-Stokes equations on an unstructured mesh, it is capable of resolving the time evolution of very complex vortical motions, air entrainment and pressure variations due to violent deformations following overflow of the weir crest. In the present study, two different types of vertical weir, namely broad-crested and sharp-crested, are considered for validation purposes. The calculated overflow parameters such as pressure head distributions, velocity distributions, and water surface profiles are compared against experimental data as well as numerical results available in literature. A very good quantitative agreement has been obtained. The numerical model, thus, offers a good alternative to traditional experimental methods in the study of weir problems. 相似文献
10.
Assessment of spill flow emissions on the basis of measured precipitation and waste water data 总被引：1，自引：0，他引：1
Combined sewer overflows (CSOs) are substantial contributors to the total emissions into surface water bodies. The emitted pollution results from dry-weather waste water loads, surface runoff pollution and from the remobilisation of sewer deposits and sewer slime during storm events. One possibility to estimate overflow loads is a calculation with load quantification models. Input data for these models are pollution concentrations, e.g. Total Chemical Oxygen Demand (CODtot), Total Suspended Solids (TSS) or Soluble Chemical Oxygen Demand (CODsol), rainfall series and flow measurements for model calibration and validation. It is important for the result of overflow loads to model with reliable input data, otherwise this inevitably leads to bad results. In this paper the correction of precipitation measurements and the sewer online-measurements are presented to satisfy the load quantification model requirements already described. The main focus is on tipping bucket gauge measurements and their corrections. The results evidence the importance of their corrections due the effects on load quantification modelling and show the difference between corrected and not corrected data of storm events with high rain intensities. 相似文献