Assessment of extreme wind and waves in the Colombian Caribbean Sea for offshore applications

Interest in Colombia’s offshore industry has increased over the past years. Therefore a detailed characterization of extreme wind and waves, in terms of return periods, numbers of events and its duration during the annual cycle, is needed. Two sets of high-resolution data are used in the statistical extreme value analysis (EVA). The significant wave height data (0.125°, 6 h) are from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis available for the past 35 years (1979–2014). Surface winds (0.25°, 6 h) from the Cross-Calibrated Multi-Platform Ocean Surface Wind Vector Analyses (CCMP) of NASA/GSFC/NOAA (NASA/Goddard Space Flight Center/National Oceanic and Atmospheric Administration) are available for the past 24 years (1987–2011). Three well-known methods are applied to the data: the Block Maxima (BM), the Peak-Over-Threshold (POT) and the Method of Independent Storm (MIS). Several probabilistic models (Gumbel, Generalized Extreme Value, Weibull and Pareto) are evaluated for the BM and different threshold values for the POT and MIS. The results show that waves can reach up to 3.8 m and winds can be as strong as 31 m/s when considering the 50–100-year return periods. However, the wave model could underestimate values by up to one meter; hence, there is a probability of higher values in the region. Seasonally, most extreme events occur during the dry season (December–March) and during the Mid-Summer-Drought (MDS) or Veranillo months (June–July). Local conditions, including the reinforcement of the Caribbean Low Level Jet (CLLJ) and the occurrence of cold atmospheric fronts, are important drivers of extreme metoceanic variability. The total number of extreme wind events varied spatially and temporally from 15 to 65 and the mean duration from 15 to 25 h. A total number of extreme wave events ranging from <10 to 80 were computed during the annual cycle in the areas of interest, with a mean duration of less than 40 h.     

Recruitment of Demosponges (Porifera,Demospongiae) in Rocky and Coral Reef Habitats of Santa Marta,Colombian Caribbean*

Abstract. Recruitment of demosponges (Porifera, Demospongiae) was monitored bimonthly for one year (March 1987 – March 1988) on acrylic plates, and compared to cover of adults, in six rocky and coral reef habitats at Santa Marta, Colombian Caribbean, an area subjected to seasonal, upwelling-outwelling regimes. According to suggestive but not significant trends, recruitment rates are more positively influenced by the total sponge cover near groups of plates than by cover at a larger (habitat) scale; recruit mortality is lower where recruitment is higher. These findings are in agreement with a model of higher settlement (probably due to short-distance dispersal) and lower postsettlement mortality within a few meters of adult sponges. Recruitment varied seasonally, being relatively high from May to November-December during the rainy, outwelling season, when seawater temperature was warmer, water turbidity higher, and benthic algal density lower; it was very low from January to April during the dry, upwelling season.     

The assessment of ocean wave energy along the coasts of Taiwan

The wave energy resource around the coasts of Taiwan is investigated with wave buoy data covering a 3-year period(2007~2009).Eleven study sites within the region bounded by the 21.5oN~25.5oN latitudes and 118oE~122oE longitudes are selected for analysis.The monthly moving-average filter is used to obtain the low-frequency trend based on the available hourly data.After quantifying the wave power and annual wave energy,the substantial resource is the result of Penghu buoy station,which is at the northeastern side of Penghu Island in the Taiwan Strait.It is investigated that the Penghu sea area is determined to be the optimal place for wave energy production according to its abundant resource of northeasterly monsoon waves,sheltering of the Taiwan Island,operation and maintenance in terms of seasonal conditions,and constructability of wave power devices.     

Seismic geomorphological analysis of deepwater gravity-driven deposits on a slope system of the southern Colombian Caribbean margin

Gravity-driven processes are important agents for transporting sediments downslope into deep-marine environments. The Pliocene to Holocene offshore succession of the Colombian Caribbean margin and its stratigraphic distribution, have been affected by faulting and mud diapirism, and have been characterized using 3D seismic data. Nine stratigraphic intervals were characterized within the study, and are interpreted to consist of a range of seismic geomorphologies, including slumps and debrites. Nine gravity-driven deposits were defined within the study area, interpreted to have been transported to the north and northwest. Slumps display high-amplitude, high continuity, elongated, stratified, lobate and confined morphologies, while debrites have a reflection-free pattern or show discontinuous, low-amplitude and chaotic reflections. Mixed slumps-turbidites-debrites deposits are composed by a succession of laterally and vertically interfingered slumps, debrites and turbidites. These deposits are morphologically lobate and broadly scattered. In addition, erosional features such as basal small scours, megascours, linear scours and rafted blocks were used as kinematic indicators within the gravity-driven deposits. There are several candidates triggering mechanism, including over-steepening of slope (related to high sediment supply or slope tectonism). In the study area, confined slumps and debrites with a main transport direction from south to north have been observed, while transport direction of the mixed slumps-turbidites-debrites was toward northwest. Additionally, the fact that slumps and debrites are found in depocenters between periclines suggests a confined environment of deposition. Finally, mixed slumps-turbidites-debrites are unconfined without evident structural control. We suggest that local intraslope sub-basin margin become over-steepened as a result of mud diapirism in the subsurface. In this situation, the paleobathymetry was sufficient to trap the resultant gravity-driven deposits within the sub-basins, suggesting a local origin. Seismic evidence of BSR (Bottom Simulating Reflector) suggests the presence of gas hydrate in the study area, and is taken as an additional potential mechanism to provide instability of slope and generate gravity-driven deposits.     

一种基于声速剖面展开的内波监测新方法

声速剖面时空分布的获取是利用声学方法监测内波的核心问题。在反演算法中,声速剖面通常是采用展开的方式用若干个参数来表示的。这就导致了有时很难从反演结果中直接获得内波的相关信息。本文的目标是找到一种通过展开系数直接获取内波特性的方法。通过推导内波水动力方程,可以从较少的声速剖面样本中提取出水动力简正模态（Hydrodynamic Normal Modes,HNMs）作为声速剖面展开的正交基。较之广泛采用的正交经验函数（Empirical Orthogonal Functions, EOFs）,HNMs直接与内波活动相关,具有更明确的物理含义。然后,基于HNMs对声速剖面的时间序列进行展开,获得展开系数。最后,从前两阶展开系数的时间导数中可以获取内波活动的信息。将方法应用于受内波影响而具有明显时空扰动的南海北陆架区温度链数据,结果表明：只用前两节模态就可以在较好的精度范围内重构声速剖面。前两阶系数的时间导数具有独特的双震荡结构可以用于探测内孤立波。从展开系数也可以获得幅度以及波长信息。理论推导和实验分析证明了本文方法在内波监测中的有效性。HNMs方法使用便利且对样本的依赖性较小,可以在内波活跃海域作为EOFs的有效补充用于声速剖面的展开。     

A Holistic Approach to Upwelling and Downwelling along the South-West Coast of India

An attempt has been made to develop a holistic understanding of upwelling and downwelling along the south-west coast of India. The main objective was to elucidate the roles of different forcings involved in the vertical motion along this coast. The south-west coast of India was characterized by upwelling during the south-west monsoon (May to September) and by downwelling during the north-east monsoon and winter (November to February). The average vertical velocity calculated along the south-west coast from the vertical shift of the 26?°C isotherm is 0.57?m/day during upwelling and 0.698?m/day during downwelling. It was concluded that upwelling along the south-west coast of India is driven by offshore Ekman transport due to the alongshore wind, Ekman pumping, horizontal divergence of currents and by the propagation of coastally trapped waves. Whereas downwelling along the coast is driven only by convergence of currents and the propagation of coastally trapped Kelvin waves. Along the west coast of India, the downwelling-favorable Kelvin waves come from the equator and upwelling-favorable waves come from the Gulf of Mannar region.     

On the choice of data transformation for modelling time series of significant wave height

Autoregressive models have been shown to adequately model the time series of significant wave height. However, since this series exhibits a seasonal component and has a non-gaussian nature, it is necessary to transform the series before a model can be fit to the data. Two different transformations that have been used in earlier work are shown not to be appropriate for all types of applications. A third transformation is proposed here, which combines the better features of the two earlier ones and which is appropriate for simulation work. This is demonstrated with an example of a series from Figueira da Foz, a location of the Portuguese Coast.     

Prediction of the dynamic response of a slender marine structure under an irregular ocean wave using the NARX-based quadratic Volterra series

The time series of the dynamic response of a slender marine structure was predicted in approximate sense using a truncated quadratic Volterra series. The wave-structure interaction system was identified using the NARX (Nonlinear Autoregressive with Exogenous Input) technique, and the network parameters were determined through supervised training using prepared datasets. The dataset used for network training was obtained by nonlinear finite element analysis of the slender marine structure under random ocean waves of white noise. The nonlinearities involved in the analysis were both large deformation of the structure under consideration and the quadratic term of the relative velocity between the water particle and structure in the Morison formula. The linear and quadratic frequency response functions of the given system were extracted using the multi-tone harmonic probing method and the time series of the response of the structure was predicted using the quadratic Volterra series. To check the applicability of the method, the response of a slender marine structure under a realistic ocean wave environment with a given significant wave height and modal period was predicted and compared with the nonlinear time domain simulation results. The predicted time series of the response of structure with quadratic Volterra series successfully captured the slowly varying response with reasonably good accuracy. This method can be used to predict the response of the slender offshore structure exposed to a Morison type load without relying on the computationally expensive time domain analysis, especially for screening purposes.     

A global wave parameter database for geophysical applications. Part 1: Wave-current–turbulence interaction parameters for the open ocean based on traditional parameterizations

Ocean surface mixing and drift are influenced by the mixed layer depth, buoyancy fluxes and currents below the mixed layer. Drift and mixing are also functions of the surface Stokes drift U_ss, volume Stokes transport T_S, a wave breaking height scale H_swg, and the flux of energy from waves to ocean turbulence Φ_oc. Here we describe a global database of these parameters, estimated from a well-validated numerical wave model, that uses traditional forms of the wave generation and dissipation parameterizations, and covers the years 2003–2007. Compared to previous studies, the present work has the advantage of being consistent with the known physical processes that regulate the wave field and the air–sea fluxes, and also consistent with a very large number of in situ and satellite observations of wave parameters. Consequently, some of our estimates differ significantly from previous estimates. In particular, we find that the mean global integral of Φ_oc is 68 TW, and the yearly mean value of T_S is typically 10–30% of the Ekman transport, except in well-defined regions where it can reach 60%. We also have refined our previous estimates of U_ss by using a better treatment of the high frequency part of the wave spectrum. In the open ocean, U_ss  0.013U₁₀, where U₁₀ is the wind speed at 10 m height.     

Effect of Salinity and Currents on Sea Level Variation Along the Indian Coast: A Numerical Study

The process of upwelling/sinking and associated sea level variations are seen as a response of coastal ocean to pure wind stress forcing. Further,precipitation and monsoonal floods, apart from the marine meteorological parameters, are expected to influence the sea level fluctuations along the coast. This study comprises determining the sea level from the various parameters together with the pure wind stress forcing, which is compared with the observed cycle. However, it is found that there is considerable difference between the computations and observations. This suggests that the sea level is dependent not just on the local forcing alone, but also on the induced background circulation as well. For example, the sea level changes along the east coast of India, particularly the northern region, are more sensitive to freshwater discharge from various rivers joining the Bay of Bengal. This is due to more frequently occurring pre- and postmonsoon cyclonic storms and the associated surges in the Bay of Bengal as compared to the Arabian Sea. Hence the salinity effects are particularly important in the coastal waters off the east coast of India during monsoon months (June-September). For the west coast of India, however, it is expected that the large-scale coastal circulation may play a role in determining sea level changes in addition to other forcings. The salinity effects are negligible along the west coast in the absence of any major river systems that join the Arabian Sea. The local advection currents caused by the offshore directed freshwater discharge from various estuaries joining the coastal bay also seemed to influence the sea level. In order to elucidate the essential dynamics involved and to study the effect of the remote forcing, a three-dimensional baroclinic, nonlinear numerical model is used with appropriate open boundary conditions. The local effect of the current has been incorporated in the west coast model by means of opening a channel at Cochin through which the rainwater is carried away to the model ocean. The low saline plume, cascading from north along the east cost of India, has been incorporated in the east coast model through a proper forcing applied at the northern boundary of the model. With the inclusion of these remote forcings in the models, the disagreement between the simulations and the observations is minimized.     

Effect of Salinity and Currents on Sea Level Variation Along the Indian Coast: A Numerical Study

The process of upwelling/sinking and associated sea level variations are seen as a response of coastal ocean to pure wind stress forcing. Further,precipitation and monsoonal floods, apart from the marine meteorological parameters, are expected to influence the sea level fluctuations along the coast. This study comprises determining the sea level from the various parameters together with the pure wind stress forcing, which is compared with the observed cycle. However, it is found that there is considerable difference between the computations and observations. This suggests that the sea level is dependent not just on the local forcing alone, but also on the induced background circulation as well. For example, the sea level changes along the east coast of India, particularly the northern region, are more sensitive to freshwater discharge from various rivers joining the Bay of Bengal. This is due to more frequently occurring pre- and postmonsoon cyclonic storms and the associated surges in the Bay of Bengal as compared to the Arabian Sea. Hence the salinity effects are particularly important in the coastal waters off the east coast of India during monsoon months (June-September). For the west coast of India, however, it is expected that the large-scale coastal circulation may play a role in determining sea level changes in addition to other forcings. The salinity effects are negligible along the west coast in the absence of any major river systems that join the Arabian Sea. The local advection currents caused by the offshore directed freshwater discharge from various estuaries joining the coastal bay also seemed to influence the sea level. In order to elucidate the essential dynamics involved and to study the effect of the remote forcing, a three-dimensional baroclinic, nonlinear numerical model is used with appropriate open boundary conditions. The local effect of the current has been incorporated in the west coast model by means of opening a channel at Cochin through which the rainwater is carried away to the model ocean. The low saline plume, cascading from north along the east cost of India, has been incorporated in the east coast model through a proper forcing applied at the northern boundary of the model. With the inclusion of these remote forcings in the models, the disagreement between the simulations and the observations is minimized.     

基于复杂网络的海洋涡旋移动特征研究——以南海为例

海洋涡旋作为一种快速连续变化的海洋现象,如何分析和挖掘其移动特征成为当前海洋涡旋定量研究的重点。本文引入空间数据挖掘的社区网络划分方法,将涡旋过程看作复杂的移动网络,对涡旋移动的聚集性特征进行探索和分析。首先,以网格为统计单元对1992-2011年近20年南海海洋涡旋移动数据进行组织,基于图论模型构建了涡旋瞬时移动（TP）,涡旋移动起止点（OD）,涡旋最小描述距离的特征点移动网（MDL）和涡旋过程移动再生数据（RSP）4种状态的海洋涡旋的移动网络图;其次,采用基于快速模块度优化的区域划分方法分别得到4种状态下涡旋移动的聚集性区域;最后,利用弦图对区域内和区域间涡旋移动规律进行了可视化分析,发现海洋涡旋的RSP数据能够弥补原始涡旋移动数据在区域划分方法中呈现的数量不足的问题,能够在足够数据量的情况下,有效地发现从起点到终点的主要移动通道和涡旋移动的聚集性区域,这些区域反映了南海涡旋从其产生、发展到结束整个演化过程的聚集性特征。     

A comparison of three marine ecosystems surrounding the Korean peninsula: Responses to climate change

This study uses a comparative approach to examine responses of marine ecosystems to climatic regime shifts. The three seas surrounding the Korean peninsula, the Japan/East Sea, the East China Sea and the Yellow Sea represent three contiguous but distinct ecosystems. Sampling has been carried out by the National Fisheries Research and Development Institute of South Korea since 1965, using the same methods in all three seas. Sampling was generally synoptic. Amplitude time series of 1st EOF modes for temperature, salinity, zooplankton biomass and concentrations of four major zooplankton taxa were used to determine whether the three marine ecosystems respond in a similar manner to climate variations. Temporal patterns of the variables were strongly similar among the three seas at decadal time scales, but very weakly similar at interannual scales. All three seas responded to a climatic regime shift that occurred in 1989. Temperature, zooplankton biomass and copepod concentrations increased in the late 1980s or early 1990s in all three seas. Concentrations of amphipods, chaetognaths and euphausiids also increased in the Japan/East Sea and the East China Sea, but not the Yellow Sea. The Yellow Sea ecosystem differs strongly from the other two seas, and water exchange between the Yellow Sea and the East China Sea is much weaker than that between the East China Sea and Japan/East Sea. Spatial patterns of zooplankton determined by the EOF analysis were closely related to currents and fronts in each of the three seas.     

Seasonal variability of the sea surface height in the East China Sea along the China coast: A model study

The phase of the sea surface height annual variation in the East China Sea along China’s continental coast is delayed from that in the open ocean area, most probably because of seasonal strong monsoon winds. To elucidate this mechanism, we conducted an idealized model experiment using a rectangular shallow ocean with a sloped seafloor forced by southward blowing winds. We obtain a locally confined high SSH near the western boundary found in the East China Sea. The delay of the phase of the sea surface height (SSH) along the China coast can be interpreted as follows. The SSH of the East China Sea is high over large areas in September and low in March due to the expansion/contraction of seawater, which is attributable to the sea surface heat flux. However, near the continental boundary SSH becomes high in January and low in July under the influence of a monsoon winds. The phase delay along the continental boundary should appear by superposing these two time series with a phase difference near the boundary.     

The impact of bio-optical heating on the properties of the upper ocean: A sensitivity study using a 3-D circulation model for the Labrador Sea

The impact of bio-optical heating on the properties of the upper Labrador Sea water was investigated by considering changes in light attenuation in water associated with the seasonal change of chlorophyll distribution. The time- and depth-dependent attenuation coefficients were obtained from remotely sensed SeaWiFS ocean-colour data. Sea-surface temperature (SST) and mixed-layer depth (MLD) were computed from a three-dimensional ocean circulation model. The model was integrated from 1999 to 2003 with 6-hourly atmospheric forcing. The changes in SST and MLD attributable to bio-optical heating were determined by comparing the model results using the observed attenuation coefficients (chlorophyll) to those using a weak and constant attenuation (clear water). The model results show that bio-optical heating is controlled mainly by chlorophyll concentration and MLD. The increase in SST is around 1 °C in most parts of the Labrador Sea and the shelves, and up to 2.7 °C in areas of shallow MLD and high chlorophyll concentrations (the Grand Banks and Northeastern Newfoundland Shelf). The increase is much higher than that found in previous studies, which was typically a fraction of a degree. Bio-optical heating also can enhance the stratification of the upper ocean and reduce the MLD by 20–50%.     

Storminess and vulnerability along the Atlantic coastlines of Europe: analysis of storm records and of a greenhouse gases induced climate scenario

Identification of the distinctive circulation patterns of storminess on the Atlantic margin of Europe forms the main objective of this study; dealing with storm frequency, intensity and tracking. The climatology of the extratropical cyclones that affect this region has been examined for the period 1940–1998. Coastal meteorological data from Ireland to Spain have been linked to the cyclone history for the North Atlantic in the analysis of storm records for European coasts. The study examines the evolution in the occurrence of storms since the 1940s and also their relationship with the North Atlantic Oscillation (NAO). Results indicate a seasonal shift in the wind climate, with regionally more severe winters and calmer summers established. This pattern appears to be linked to a northward displacement in the main North Atlantic cyclone track.

An experiment with the ECHAM4 A-GCM at high resolution (T106) has also been used to model the effect of a greenhouse gases induced warming climate on the climatology of coastal storms in the region. The experiment consists of (1), a 30-year control time-slice representing present-day equivalent CO₂ concentrations and (2), a 30-year perturbed period corresponding to a time when the radiative forcing has doubled in terms of equivalent CO₂ concentrations. The boundary conditions have been obtained from an atmosphere-ocean coupled OA-GCM simulation at low horizontal resolution. An algorithm was developed to allow the identification of individual cyclone movements in selected coastal zones. For most of the northern part of the study region, covering Ireland and Scotland, results describe the establishment by ca. 2060 of a tendency for fewer but more intense storms.

The impacts of these changes in storminess for the vulnerability of European Atlantic coasts are considered. For low-lying, exposed and ‘soft’ sedimentary coasts, as in Ireland, these changes in storminess are likely to result in significant localised increases in coastal erosion.     

Tracing the inputs and fate of marine and terrigenous organic matter in Arctic Ocean sediments: A multivariate analysis of lipid biomarkers

An understanding of the carbon cycle within arctic sediments requires discrimination between the terrigenous and marine components of organic carbon, insight into the removal mechanisms for labile carbon during burial and appreciation of shelf-to-basin processes. Using a large data set of multiple molecular organic markers (alkanes, alkanols, sterols, saturated and unsaturated fatty acids, dicarboxylic acids), we apply (1) principal components analysis (PCA) to obtain a robust comparison of biomarker compositions in Arctic Ocean sediments, (2) geometric mean (GM) linear regression of the PCA variables to estimate the relative contributions of labile/marine and stable/terrigenous sources to each biomarker and (3) the slope of the GM regression of each biomarker with TOC to provide a novel measure of the removal rate of each biomarker relative to phytol. The PCA- and TOC-based indices generally increase together: biomarkers with very high TOC-based removal rates such as the saturated and unsaturated n-alkanoic acids generally have a high labile/marine content from PCA, while the sterols have low removal rates, but exhibit a range of labile/marine content values and the n-alkanes and n-alkanols have low values for both. A dominant feature of all PCA models examined is a progressive decrease in the autochthonous/marine biomarkers with each increase in sediment core depth, which points to a universal diagenetic alteration of organic carbon with depth in the cores. The PCA model also displays a shelf to basin trend that is non-diagenetic and implies the ongoing (centuries or more) delivery of long-chain n-alkanes, n-alcohols and n-alkanoic acids in a matrix that is pre-formed and well-preserved within the sediments. Terrigenous biomarker distributions within the PCA model suggest that atmospheric transport of plant waxes in aerosols and the water borne transport of very fine plant macerals likely have significant roles in the export of these vascular plant biomarkers to the basins. Biomarker ratios and profiles of the PCA-based labile/marine content with core depth indicate that the PCA model is more strongly influenced by the biomarker lability than the marine content, while increases in the marine content are largely responsible for the shifts in composition for near-surface core sections.     

A critical analysis on the response of macroinvertebrate communities along disturbance gradients: description of MEDOCC (MEDiterranean OCCidental) index

Macroinvertebrate communities inhabiting shallow sandy bottoms from Catalonia and Balearic Islands coastal waters (Western Mediterranean) were used to develop the Mediterranean Occidental (MEDOCC) index, a new methodology to assess the ecological status (ES) according to Water Framework Directive (WFD) requirements. The dominance pattern of four ecological groups (EG; sensitive, indifferent, tolerant and opportunistic organisms) was studied along a disturbance gradient. Sensitive taxa were present in disturbed situations and tolerant species were related to opportunistic taxa. The thresholds between ES were scaled according to the shifts in the dominance pattern of the EGs. The distribution of the EGs along the gradient of disturbance may vary depending on the studied communities, thus influencing the boundaries defining the ES. With the development of the MEDOCC index it is shown that analysing the response of the communities to the increasing gradient of disturbance is critical to enable an accurate proposal for boundary setting to be made, and to improve the applicability of the indices. AMBI (AZTI Marine Biotic Index), Bentix (Biotic Index), and MEDOCC indices were applied to the dataset to compare the ES obtained.