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1.
Since 1987, twice weekly, hydrological variables have been monitored at a fixed station in the R?́a de Vigo (NW Spain), aiming to examine the time scales of variability and the relationships to meteorological conditions. The present paper analyses: (1) the advantage of Box-Jenkins transfer function (TF) models (single output–multiple input), a type of linear stochastic model, to describe the dynamic behaviour of the system; and (2) the coupling between the R?́a and meteorological events at the time scale of autonomy of this coastal inlet affected by the Iberian coastal upwelling, approximately a fortnightly period. In order to achieve these objectives, thermohaline properties have been used to characterize the estuarine ecosystem (output variables), while wind regime, runoff in the drainage basin and incoming solar radiation have been considered as the main forcing variables (input variables). The use of the amplitude time series, derived from principal component analysis (PCA) applied to the deseasonalized meteorological variables, is also explored as a different set of input variables.When compared with standard regression models, all TF models built to describe thermohaline behaviour had reduced residual variance. Similar TF models, as well as percentage of explained variance, were also obtained when meteorological variables or the amplitude time series were used as input variables. The fitted TF models provided an insight into the ‘ inertial ’ behaviour of the system and the time scales of coupling of the system with the forcing variables. The plausible physical mechanisms which link the response of the system with the observed meteorological variability are also discussed. As could be expected, bottom thermohaline properties show a stronger inertial behaviour than the surface ones, which is particularly marked for bottom temperature. Besides, the shelf domain, by means of upwelling-downwelling events, strongly influences surface and bottom temperature, as well as bottom salinity; by contrast, surface salinity is mainly influenced by the effect of wind along the main axis of the R?́a and runoff. In relation to the time scales of coupling between the system and the forcing variables, thermohaline properties show a dependance with the meteorological conditions in, at least, the immediately preceding fortnight period. It was concluded that: (1) TF models that incorporate meteorological information described the dynamic behaviour of the system adequately; and (2) this type of model can be useful as a first approximation to develop more sophisticated (deterministic) models, since, with the purpose of modelling any state variable of the system, both the coupling between different domains and the time scales of the interactions must be taken into account. 相似文献
2.
X. A. lvarez-Salgado J. Gago B. M. Míguez M. Gilcoto F. F. Prez 《Estuarine, Coastal and Shelf Science》2000,51(6):821
A set of hydrographic surveys were carried out in the Ría of Vigo (NW Spain) at 2–4 d intervals during four 2–3 week periods in 1997, covering contrasting seasons. Residual exchange fluxes with the adjacent shelf were estimated with a 2-D, non-steady-state, salinity–temperature weighted box model. Exchange fluxes consist of a steady-state term (dependent on the variability of continental runoff) and a non-steady-state term (dependent on the time changes of density gradients in the embayment). More than 95% of the short-time-scale variability of the exchange fluxes in the middle and outer ría can be explained by the non-steady-state term that, in turns, is correlated (R2>75%) with the offshore Ekman transport. Conversely, 96% of the variability of exchange fluxes in the inner ría rely on the steady-state term. The outer and middle ría are under the direct influence of coastal upwelling, which enhances the positive residual circulation pattern by an order of magnitude: from 102to 103 m3s−1. On the contrary, downwelling provokes a reversal of the circulation in the outer ría. The position of the downwelling front along the embayment depends on the relative importance of Ekman transport (Qx, m3s−1km−1) and continental runoff (R, m3s−1). When Qx/ R>7±2 the reversal of the circulation affects the middle ría. Our results are representative for the ‘Rías Baixas’, four large coastal indentations in NW Spain. During the upwelling season (spring and summer), 60% of shelf surface waters off the ‘Rías Baixas’ consist of fresh Eastern North Atlantic Central Water (ENACW) upwelled in situ. The remaining 40% consists of upwelled ENACW that previously enters the rías and it is subsequently outwelled after thermohaline modification. During the downwelling season (autumn and winter), 40% of the warm and salty oceanic subtropic surface water, which piled on the shelf by the predominant southerly winds, enters the rías. 相似文献
3.
Making use of a simple two‐layer model, we analyze the impact of freshwater forcing on the thermohaline circulation. We consider the forward‐type circulation dominated by thermal forcing, implying that the freshwater forcing acts to reduce the density contrast associated with the equator‐to‐pole temperature contrast (prescribed in the model). The system is described by two variables: the depth of the upper layer ( H ) and the density contrast between the upper and lower layer (Δρ), which decreases with salinity contrast. The rate of poleward flow of light surface water and the diapycnal flow (i.e., upwelling) driven by widespread small‐scale mixing are both modeled in terms of H and Δρ. Steady states of thermohaline circulation are found when these two flows are equal. The representation of the diapycnal flow ( MD ) is instrumental for the dynamics of the system. We present equally plausible examples of a physically based representation of MD for which the thermohaline circulation either decreases or increases with density contrast. In the latter case, contrary to the traditional wisdom, the freshwater forcing amplifies the circulation and there exists a thermally dominated equilibrium for arbitrary intensity of freshwater forcing. Here, Stommel's famous feedback between circulation and salinity contrast is changed from a positive to a negative feedback. The interaction of such a freshwater boosted thermohaline circulation with the climate system is fundamentally different from what is commonly assumed, an issue which is briefly addressed. 相似文献
4.
Water circulation in the north-west part of the Black Sea during the summer period is calculated by means of a complete non-linear thermohydrodynamic model. We have also studied the contribution of wind forcing, the thermohaline regime, the coastline configuration, and the bottom topography to the generation and transformation of a flow field. It has been shown that the thermohaline regime and wind forcing account for 30 and 70% of the total circulation, respectively. Variation in the nature of circulation at weak winds is due to thermohaline factors. Vertical motion plays a crucial role in theT, S- andH
2
S-field formation.Translated by V. Puchkin. 相似文献
5.
Conceptual box models of the interhemispheric thermohaline circulation are studied with respect to bifurcations. Freshwater fluxes are the main control parameters of the system: they determine the stable states and transitions between stable states of the large-scale thermohaline circulation. In this study of interhemispheric box models both numerical and analytical methods are used to investigate transition mechanisms of the thermohaline circulation. The box model examined first is an interhemispheric four-box model. It is shown that the two bifurcations where the present THC can become unstable, the saddle-node and the Hopf bifurcation, depend in a different way on hemispheric freshwater fluxes. A reduction of the model variables leads to the conclusion that two fixed freshwater fluxes between three surface boxes are the model feature responsible for the bifurcation behaviour found. The significance of the Hopf bifurcation for the stability of the thermohaline circulation is discussed. 相似文献
6.
With all the improvement in wave and hydrodynamics numerical models, the question rises in our mind that how the accuracy of the forcing functions and their input can affect the results. In this paper, a commonly used numerical third-generation wave model, SWAN is applied to predict waves in Lake Michigan. Wind data are analyzed to determine wind variation frequency over Lake Michigan. Wave predictions uncertainty due to wind local effects are compared during a period where wind has a fairly constant speed and direction over the northern and southern basins. The study shows that despite model calibration in Lake Michigan area, the model deficiency arises from ignoring wind effects in small scales. Wave prediction also emphasizes that small scale turbulence in meteorological forces can increase prediction errors by 38%. Wave frequency and coherence analysis show that both models can predict the wave variation time scale with the same accuracy. Insufficient number of meteorological stations can result in neglecting local wind effects and discrepancies in current predictions. The uncertainty of wave numerical models due to input uncertainties and model principals should be taken into account for design risk factors. 相似文献
7.
A. D. Berasategui E. M. Acha N. C. Fernndez Araoz 《Estuarine, Coastal and Shelf Science》2004,60(4):599-610
Two oceanographic surveys were conducted across the salinity gradient of the Río de la Plata Estuary during austral late spring and summer. Ichthyoplankton was dominated by families Sciaenidae, Clupeidae, Engraulidae, Cynoglossidae and Carangidae. The distribution of larval fish assemblages was strongly associated to the salinity structure. Oceanographic conditions were characterized by a large salt wedge (180 km long and a vertical salinity gradient up to 10 units per meter). During both sampling periods each ichthyoplankton assemblage was related to the same region of the salt wedge: the fresh water environment, the bottom salinity front, the mixohaline zone and the outer portion (surface salinity front) of the estuary. Larval assemblages were not related to the thermal structure. Reproductive activity of fishes inside the Río de la Plata Estuary is relatively common (60% of the teleosts sampled use this estuary as a nursery ground). Moreover, the high incidence of pelagic eggs spawners (75% of the species present in the ichthyoplankton) in the Río de la Plata contrasts to the vast majority of estuaries, where this reproductive strategy is poorly represented. Estuarine dynamics plays a dominant role in allowing this reproductive style in this estuary. Spring–summer wind pattern should generate a retentive environment which could minimize advective losses of eggs and larvae to the adjacent ocean. 相似文献
8.
Hiroyasu Hasumi 《Journal of Oceanography》2002,58(1):25-33
This paper reviews recent progress and problems in modeling the thermohaline circulation of the world ocean by use of z-coordinate ocean general circulation models. Discussions focus on four issues: sea surface forcing, mixing in the deep ocean
interior, eddy-induced tracer transport, and bottom boundary layer processes. Although some widely used techniques and parameterizations
deal with these issues, some aspects are still overlooked and more sophistication is certainly required.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
9.
10.
This paper presents a simple nonlinear data-based modelling approach for predicting the beach profile volume at Duck, North Carolina, USA. The state-dependent parameter form of the general transfer function (SDP TF) model is used to describe nonlinearity influencing these morphological data in two case examples. Case 1 investigates the nonlinearity associated with the dependency of wave forcing on the preceding beach volume. Case 2 investigates the ability to model the variables within the well-known diffusion equation for beach volume using this data-based approach. The results of this study show that the SDP TF approach can be used successfully to develop statistically robust models for describing nonlinearity in beach morphological systems. Furthermore, these models are shown to predict the beach volumes over both short (1 month ahead) and long (2 years ahead) time periods, and thus show great potential for practical applications in coastal zone management and engineering. 相似文献
11.
Repeated surveys at control sites in the gas accumulation fields from the Ría de Arousa (Galicia, NW Spain) have shown that the top of the acoustic turbidity fluctuates vertically. The vertical movement of gas fronts is more apparent in the gas fields located in the outer ría, where fluctuations of up to 1.5 ms (1.1 m) have been detected. At the inner ría, the depth to the top of the gas horizons varied between 0.5 and 1 ms (40 to 80 cm). Short-term (diurnal) fluctuations associated with the hydrostatic pressure variations of the water column during a tidal cycle have been identified as the most likely factors to explain fluctuations of gas fields in the outer ría. At the inner ría, gas fields are affected by these short-term pressure changes superimposed on long-term temperature and salinity variations of bottom water masses. This combination produces a different pattern of variation in gas fields from the inner and the outer ría. Prominent pockmarks and acoustic plumes have also been monitored throughout the three seismic surveys. Many of these pockmarks exhibit active characteristics, including the observance of plumes of escaping fluids and sediment. 相似文献
12.
V.L. Meccia C.G. Simionato M.E. Fiore E.E. D'Onofrio W.C. Dragani 《Estuarine, Coastal and Shelf Science》2009,85(2):327-343
A long term simulation of the barotropic circulation in the Río de la Plata estuary was performed with the aim of identifying the characteristic patterns of sea surface height (SSH) variability from synoptic to inter-annual time scales and their forcing mechanisms. Hamburg Shelf Ocean Model (HamSOM), forced by tides, monthly mean runoffs and 4-daily 10 m winds and surface atmospheric pressure from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis was run. The solution was analyzed for the period 1965–2004. Inter-annual variability accounts for almost 10% of the variance. The first EOF mode of SSH variability on this time scale is associated with a mean anomaly of approximately 0.25 m at the upper estuary forced by both runoff and winds, which seems to be strongly associated with the ENSO cycles. Other two modes, with periodicities around 2.5 and 10 years were also found. Even though they are linked to weaker SSH anomalies, they are consistent with inter-annual modes of wind variability reported by other authors. Those modes are important, particularly if they act in phase, because they can provide a background for stronger surges. In contrast with the salinity field, SSH variability on seasonal time scales accounts for a very small percentage of variance and it is the combination of an annual and a semi-annual signal forced by winds and runoff, respectively. Approximately 90% of the variance is due to wind driven variability on sub-annual time scales. The most significant SSH anomalies in this band are associated with cyclogenetic events in the atmosphere, occurring either over Uruguay or over the Patagonian Shelf, whereas the strengthening or weakening of the semi-permanent South Atlantic anticyclone displays a relatively smaller influence. In agreement with previous publications, the estuary's spatial patterns in response to short-scale wind variability seem to be determined by wind direction more than by wind speed. 相似文献
13.
Jake Rice 《Progress in Oceanography》2001,49(1-4)
The conceptual basis for understanding and management of living marine resources is built on three basic ecological principles developed in the first half of the past century: the law of the minimum, competitive exclusion, and succession. This paper highlights aspects of these principles that make them insufficient as a sound foundation for understanding and managing marine ecosystems, points out dangers of continuing to use approaches built on them, and presents alternatives which might be more appropriate and of lower risk.To do this, the paper considers variability of marine ecosystems on annual, medium and long-term time scales, highlighting that these scales correspond to less than, approximately equal to, and much greater than, the generation times of dominant predators in the systems. It also considers how each interval of variability may affect directly ecosystems which are controlled from the bottom up, top down, and middle outward, and how position and duration of forcing affect five types of responses: growth, maturation, recruitment, predation, and competition. Generally these five processes have manifestations at the scale of individuals, populations, and ecosystems, attention is drawn to which manifestations are the most significant for each duration and position of forcing.Effects of some combinations of duration of forcing and position of forcing can be explained reasonably well by conventional ecological theory. For other combinations, particularly forcing at time scales of predator generations on top-down or middle-out ecosystems, theory based on contest competition and equilibria are likely to be misleading. In these systems the major dynamics are transients, when many ecosystems are far from their carrying capacities, so scramble competition dominates, and the carrying capacity is not helpful in explaining the system dynamics. This review clarifies the sorts of questions that we should be asking, in order to begin to understand the transient behaviour of these non-equilibrium ecosystems. The answers to the new classes of questions may lead to great improvements in how ecosystems are managed, as well as how their variation is explained. 相似文献
14.
Existing models of the wave bottom boundary layer have focused on the vertical and temporal dynamics associated with monochromatic forcing. While these models have made significant advances, they do not address the more complicated dynamics of random wave forcing, commonly found in natural environments such as the surf zone. In the closed form solution presented here, the eddy viscosity is assumed to vary temporally with the bed shear velocity and linearly with depth, however, the solution technique is valid for any eddy viscosity which is separable in time and space. A transformation of the cross-shore velocity to a distorted spatial domain leads to time-independent boundary conditions, allowing for the derivation of an analytic expression for the temporal and vertical structure of the cross-shore velocity under an arbitrary wave field. The model is compared with two independent laboratory observations. Model calculations of the bed shear velocity are in good agreement with laboratory measurements made by Jonsson and Carlsen (1976, J. Hydraul. Res., 14, 45–60). A variety of monochromatic, skewed, and asymmetric wave forcing conditions, characteristic of those found in the surf zone, are used to evaluate the relative effects on the bed shear. Because the temporal variation of the eddy viscosity is assumed proportional to the bottom shear, a weakly nonlinear interaction is created, and a fraction of the input monochromatic wave energy is transferred to the odd harmonics. For a monochromatic input wave, the ratio of the third harmonic of velocity at the bed to the first is <10%. However, for a skewed and asymmetric input wave, this ratio can be as large as 30% and is shown to increase with increasing root-mean-square input wave acceleration. The work done by the fluid on the bed is shown to be a maximum under purely skewed waves and is directed onshore. Under purely asymmetric waves, the work done is significantly smaller and directed offshore. 相似文献
15.
巴伦支海-喀拉海是北冰洋最大的边缘海,能够对环境变化做出快速的响应和反馈,是全球气候变化最为敏感的区域之一,其古海洋环境演变及海冰变化研究是全球气候变化研究的重要组成部分。末次盛冰期以来,该区域的古海洋环境受到太阳辐射、海流强度、海平面变化、温盐环流和河流输入等因素影响发生了一系列不同尺度的波动。巴伦支海受到北大西洋暖水和极地冷水两大水团相互作用的影响,在水团交界处 （极锋) 由于不同水团性质的差异,导致其海水温度、盐度及海冰发生剧烈变化。而喀拉海则受到叶尼塞河和鄂毕河大量淡水输入影响,海流系统较巴伦支海相对复杂,沉积物主要来源于河流输入的陆源物质,并可以通过磁化率的分析明确区分两条河流的陆源物质。由于受到冷水和暖水的相互作用,巴伦支海-喀拉海海冰变化迅速,并且在全新世中晚期存在 0.4 ka 和 0.95 ka 的变化周期,但海冰变化的影响因素并不是单一的,而是气候系统内部各因子相互作用的结果。目前古海冰重建研究工作主要为定性研究,定量研究相对较少,所选用的重建指标也相对单一,另外存在年代框架差、分辨率低等不足。本文以巴伦支海和喀拉海为中心,总结了其快速气候突变事件、古温度盐度、海平面及海冰的变化,对影响因素进行了探讨,并通过分析末次盛冰期以来古海洋环境研究的不足,提出了相应的展望。 相似文献
16.
17.
K. Hasselmann 《Progress in Oceanography》1982,11(2):69-92
Models of the time dependent ocean circulation can be simplified considerably by filtering out all short term, small scale motions which are unimportant for climatic processes. For time scales large compared with a day and space scales large compared with the internal Rossby radius of deformation (～50 km), the currents in most of the interior ocean can be determined diagnostically as quasi-equilibrium fields, so that only the salinity and temperature fields need be treated prognostically.Regions of closed f/h contours, however, represent exceptions. Here trapped vorticity gyres exist as free flow solutions without external forcing, and in the presence of forcing the barotropic velocity field must therefore be determined prognostically through a potential vorticity equation for the gyres.Lateral boundary layers and the equatorial regions also require separate treatment. These were not considered specifically, but it is suggested that integrated (parametrical) models analogous in structure to mixed-layer models or the integrated boundary layer models of aerodynamics may be the most appropriate technique for coupling these regions to the interior ocean in a comprehensive ocean model suitable for climate studies.A coupled multi-region model of the global ocean circulation based on these scale considerations could be sufficiently cost-effective to permit systematic investigation of the role of the oceanic heat storage and transport in climate variability studies over a wide spectrum of space and time scales.The analysis of the seasonal variations of the interior ocean circulation represents a simple example in which the filtered model yields considerably simpler and more readily interpretable results than a fully three-dimensional, unfiltered model. 相似文献
18.
The Hadley Centre climate model HadCM3 simulates a stable thermohaline circulation driven by deep water formation in the Norwegian and Labrador Seas without the need for flux adjustments. It has however been suggested that this result is the fortuitous consequence of the local use of the Roussenov convection scheme in this region, and that the model simulation may depend sensitively on this parameterisation. Here we investigate the sensitivity of the thermohaline circulation (THC) to the model’s treatment of the overflows from the Nordic Seas for both pre-industrial and increasing greenhouse gas forcings. We find that although the density structure in the Labrador Sea does depend upon the specifics of how the overflows are modelled, the global thermohaline circulation and climate responses are not sensitive to these details. This result gives credibility to previously published modelling studies on the response of the thermohaline circulation to anthropogenic greenhouse gas forcing, and implies that research may profitably be focussed on the large scale transports, where models are known to disagree. 相似文献
19.
《Ocean Modelling》2001,3(1-2):67-94
The effect of variable vertical diffusivity is investigated in dynamically reduced models of the thermohaline circulation (THC) in a rectangular basin. In a simple box model, sufficiently strong variation of the diffusivity κv with stability G can lead to the existence of two stable equilibria. Related behaviour is found in well-resolved frictional geostrophic (FG) models. A hierarchy of under-resolved FG models is constructed, the simplest of which is an 8-cell cube, to connect the two extremes of resolution. Multiple solutions in low-order models are found to correspond to the formation of high-gradient layers which are unlikely to be resolved by current ocean models. Physical arguments show that layering and multiple solutions require κv to decrease more rapidly than 1/G and sensitivity experiments suggest that, in addition, κv must vary by a factor of 10–100. In two-hemisphere runs with salinity forcing included, the dependence of diffusivity on stratification is found to marginally favour equatorially symmetric states. Finally, such variation is shown to have a profound effect on the periodic, flush-collapse cycle under strong saline forcing; specifically, if diffusivity is taken to be a function of stratification rather than depth, regime transitions can occur much more easily. It will therefore be important for climate modelling to determine which is more realistic. 相似文献
20.
潮致混合对海洋环流的调整起着重要作用。陆架环流的数值模拟中如果不考虑潮汐作用,往往不能得到与观测相符的垂向温盐结构。本文基于调和分析方法,建立了一套潮致混合参数化方案。该方案通过对垂向混合系数进行调和分析,从而得到随时间变化的潮致混合系数。将该方案用于黄海冷水团数值模拟的结果显示,其能够得到与在数值模式开边界直接加入潮汐强迫相当的冷水团温盐结构。和直接引入潮汐强迫相比,这一潮致混合参数化方案的优势在于,它能够大大节省数值模拟计算机时,因此有望显著提高大规模高分辨率的海洋环流及气候模式的模拟能力和计算效率。 相似文献