On the use of discrete seasonal and directional models for the estimation of extreme wave conditions     

Edward B. L. Mackay  Peter G. Challenor  AbuBakr S. Bahaj 《Ocean Engineering》2010,37(5-6):425-442

Extreme value theory is commonly used in offshore engineering to estimate extreme significant wave height. To justify the use of extreme value models it is of critical importance either to verify that the assumptions made by the models are satisfied by the data or to examine the effect violating model assumptions. An important assumption made in the derivation of extreme value models is that the data come from a stationary distribution. The distribution of significant wave height varies with both the direction of origin of a storm and the season it occurs in, violating the assumption of a stationary distribution. Extreme value models can be applied to analyse the data in discrete seasons or directional sectors over which the distribution can be considered approximately stationary. Previous studies have suggested that models which ignore seasonality or directionality are less accurate and will underestimate extremes. This study shows that in fact the opposite is true. Using realistic case studies, it is shown that estimates of extremes from non-seasonal models have a lower bias and variance than estimates from discrete seasonal models and that estimates from discrete seasonal models tend to be biased high. The results are also applicable to discrete directional models.  相似文献   

Cretan deep water outflow into the Eastern Mediterranean     

M. N. Tsimplis  A. F. Velegrakis  P. Drakopoulos  A. Theocharis  M. B. Collins 《Progress in Oceanography》1999,44(4):2045

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.  相似文献   

Determination of sediment volumes, accumulation rates and turbidite emplacement frequencies on the Madeira Abyssal Plain (NE Atlantic): a correlation between seismic and borehole data     

B. Alib  s  R. G. Rothwell  M. Canals  P. P. E. Weaver  B. Alonso 《Marine Geology》1999,160(3-4):225-250

The sedimentary infill history of the Madeira Abyssal Plain (MAP) is established from correlation of ODP Leg 157 drillsites (Sites 950–952) with an almost regular grid of 7000 km of intermediate-resolution seismic reflection profiles covering the central part of the abyssal plain. The most conspicuous seismic reflectors bounding the seismostratigraphic units have been identified and mapped. Correlation between seismic and borehole data using synthetic seismograms allows the lithological attribution and dating of the reflectors and seismostratigraphic units. Lateral mapping and correlation of seismic units also allows both the volumes and rates of accumulation of sediments within each seismostratigraphic unit and equivalent time periods of deposition to be determined. These calculations have been corrected for the effect of compaction, calculated at around 40% at the base of the drillholes. Three main turbidite types have been identified at the drillsites and their emplacement frequency has been calculated for each site and time period. Our results show that Cretaceous oceanic crust was draped with red pelagic clays, and the fracture-zone valleys were completely infilled and levelled in a geologically rather short time, probably during the latest Oligocene and Early Miocene, by organic-rich turbidites derived from the NW African continental margin. At 16 Ma, the topography was levelled enough to allow large turbidity current flows to cover the entire plain. During the Middle and Late Miocene (16–5.9 Ma), organic-rich turbidites were emplaced on the abyssal plain at a low rate of accumulation (12 m/my). In the uppermost Miocene–Early Pliocene (5.9–3.6 Ma), turbidite emplacement increased markedly in both frequency and accumulation rate (e.g., 26 m/my for organic-rich turbidites). During this time, period emplacement of volcanic-rich turbidites also increased in volume and frequency, a trend that continued into the Pliocene. Increased volcanic-rich turbidite emplacement correlates well with increased volcanic activity on the Canary Islands, and increased organic-rich turbidite emplacement may correlate with periods of erosion on the NW African continental margin. These erosional periods may be related to global cooling and falling sea level, intensification of bottom-water currents, and enhanced upwelling on the margin.  相似文献   

A segment-scale survey of the Broken Spur hydrothermal plume     

《Deep Sea Research Part I: Oceanographic Research Papers》1999,46(4):701-714

We conducted a segment-scale hydrothermal plume survey of the Broken Spur segment, 29°00-20′N, Mid-Atlantic Ridge (MAR). The purpose of the study was to identify the distribution of sources of venting throughout the segment as part of a larger study of hydrothermal fluxes. Evidence from plume particle concentrations (as deduced from in situ nephelometer data) and total dissolvable Mn (TDMn) analyses (from discrete water samples) indicated a restricted source of venting close to the segment centre, coincident with the previously known vent-site. No other pronounced plume signals were observed outside an area bounded by 29°07.5–12.5′N and 43°10–12′W, representing less than 10% of the >300 km² of deep water (>2600 m) within the segment. In addition, however, low-level (<2 nmol l^-1) deepwater TDMn concentrations reveal a pervasive enrichment throughout the segment of ⩾0.15 nmol l^-1. For the 4×1011 m³ of deepwater within the Broken Spur segment, this corresponds to a standing crop of 6×10⁴ mol of hydrothermal Mn. Future studies of long-term current flow will allow the flux of dissolved Mn out of the segment to be established and will investigate the partitioning of its source, between high temperature and axial diffuse flow.  相似文献   

Internal waves and temperature fronts on slopes     

S. A. Thorpe  U. Lemmin 《Annales Geophysicae》1999,17(9):1227-1234

Time series measurements from an array of temperature miniloggers in a line at constant depth along the sloping boundary of a lake are used to describe the ‘internal surf zone” where internal waves interact with the sloping boundary. More small positive temperature time derivatives are recorded than negative, but there are more large negative values than positive, giving the overall distribution of temperature time derivatives a small negative skewness. This is consistent with the internal wave dynamics; fronts form during the up-slope phase of the motion, bringing cold water up the slope, and the return flow may become unstable, leading to small advecting billows and weak warm fronts. The data are analysed to detect ‘events’, periods in which the temperature derivatives exceed a set threshold. The speed and distance travelled by ‘events’ are described. The motion along the slope may be a consequence of (a) instabilities advected by the flow (b) internal waves propagating along-slope or (c) internal waves approaching the slope from oblique directions. The propagation of several of the observed ‘events’ can only be explained by (c), evidence that the internal surf zone has some, but possibly not all, the characteristics of the conventional ‘surface wave’ surf zone, with waves steepening as they approach the slope at oblique angles.  相似文献   

On Probing the Inertial Subrange     

Katja Henjes 《Boundary-Layer Meteorology》1999,91(3):367-384

For an extensive turbulent wind speed data set collected on the open ocean, the optimum sampling time is determined to calculate inertial-range spectra. Such spectra are widely used to estimate surface fluxes. The optimum sampling time corresponds to the turbulent memory time, the minimum period after which a given time series can be interpreted as a new independent measurement. In our case, this interval is found to be only about six seconds.Separate spectral levels are calculated from the three measured vector components of the wind. Their ratios are compatible with the assumption of an isotropic inertial subrange, but scatter considerably. Alignment factors are developed based on Kolmogorov's theory, and the aligned spectral levels coincide within a few percent. It is proposed that alignment might be a more reliable test than the ratio of the spectral levels to determine whether observations have taken place in a universal inertial subrange. It is recommended that all three wind components are used to estimate the dissipation rate.  相似文献   

Reflection Coefficient Calculation from Marine High Resolution Seismic Reflection (Chirp) Data and Application to an Archaeological Case Study   总被引：2，自引：0，他引：2  

Bull  Jonathan M.  Quinn  Rory  Dix  Justin K. 《Marine Geophysical Researches》1998,20(1):1-11

Chirp sub-bottom profilers produce high-resolution images of the near-surface. An attribute of the sea-bed reflection in chirp data are fluctuations in polarity between adjacent traces. Two models are proposed and presented to explain this: the first incorporates changes in an acoustic impedance gradient at the sea bed; the second uses changes in the thickness of the uppermost sediment layer. Mixing of adjacent traces produces a consistent polarity for the sea-bed reflector. Reflection coefficients are calculated, using amplitude information derived from single-traces, and polarity information from trace mixing, with application to a marine archaeological case study. The reflection coefficient calculated for the top of a buried 18th century wooden wreck is -0.26.  相似文献   

Introduction: marine geoscience research by the UNESCO-IOC Training-through-Research program in the Mediterranean and Black seas     

N. H. Kenyon 《Geo-Marine Letters》1998,18(2):79-80

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OBSERVATION OF A SHALLOW WATER INTERNAL OSCILLATION EVENT     

高抒 《中国海洋湖沼学报》2002,20(1):86-90

At a shallow water station (6 m in depth), an internal oscillation event which consisted of one or two wave-like features, with a period of 3 h and a height of 1.5 m, was observed. The velocities within the water column were modified by the event during the flood phase of the tide ; a multilayered velocity structure and intense shear were generated. Further investigations are required to understand fully the mechanism for the formation of such an event.  相似文献   

Development of a 2D horizontal biogeochemical model for the Irish Sea DYMONIS   总被引：2，自引：0，他引：2  

Le Gall  A. C.; Hydes  D. J.; Kelly-Gerreyn  B. A.; Slinn  D. J. 《ICES Journal of Marine Science》2000,57(4):1050-1059

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