Crustal seismic structure beneath the West Philippine Sea, 17°–18° North

Crustal seismic structures beneath the West Philippine Sea are determined by using explosive sources (0.5–108.6 kg) and ocean bottom seismometers to measure refracted compressional waves. Total crustal thicknesses are shown to be thinner in the eastern part of the ocean basin, approaching only 3.5 km. Crustal thinning toward the east is consistent with the Palau Kyushu Ridge being a remnant transform fault connecting the Central Basin Ridge and the Kula Pacific Ridge in the past. A velocity-depth inversion from the westernmost refraction profile indicates the upper transitional crust layer to have strong velocity gradients which gradually decrease with depth; the lower crust is characterized by a nearly constant velocity gradient. The western part of the ocean basin is also shown to have more typical oceanic thicknesses, as is found in deep ocean basins of the Pacific. Spectral energy models using WKBJ synthetic seismograms suggest that there is a sharp seismic discontinuity between the crust and moho in the western part of the basin. Predicted water depths for the West Philippine Basin using an age-depth relation and corrected for an isostatic response to the measured crustal thicknesses, are still 300 meters shallower than observed depths. The depth anomaly can not be fully reconciled by thinner crust in the eastern part of the basin. This observation implies that a deeper seated anomaly is present beneath the West Philippine Basin.     

Currents along the Tsitsikamma coast,South Africa,and potential transport of squid paralarvae and ichthyoplankton

In July 1998, a bottom-mounted Acoustic Doppler Current Profiler was deployed at 36m depth in the centre of the Tsitsikamma National Park on the eastern Agulhas Bank, South Africa. The purpose was to investigate transport of chokka squid Loligo vulgaris reynaudii paralarvae hatched on the inshore spawning grounds (<60m) and ichthyoplankton spawned within the park. Analysis of the first 12 months of data (July 1998–June 1999) shows that surface flow was mainly eastward (alongshore), with a maximum velocity (u-component) of +115cm s^?1 and an average of +24cm s^?1. Generally, velocity decreased with depth, with a maximum bottom velocity (u-component) of +65cm s^?1 and an average of +10cm s^?1. Data from a nearby thermistor array show that the water column was usually isothermal during winter (July–September), with bottom flow in the same direction as the surface layer. In summer (December–March), vertical stratification was most intense, and surface and bottom flows differed in velocity and direction. Potential net monthly displacements calculated for three depths (5m, 23m and 31m) indicate that passive, neutrally buoyant biological material (e.g. squid paralarvae, fish eggs and larvae) would likely be transported eastwards in the surface layer for eight of the 12 months, and would generally exceed distances of 220km month–1. Displacement in the bottom layer was more evenly distributed between east and west, with net monthly (potential) transport typically 70–100km, but reaching a maximum of 200km. Wind-driven coastal upwelling, prevalent during the summer, causes the surface layer of the coastal counter-current to flow offshore for several days, resulting in potential displacement distances of 40km from the coast. These results suggest that squid paralarvae hatched on the inshore spawning grounds are not generally transported towards the 'cold ridge', a prominent semi-permanent oceanographic feature of cold, nutrient-rich upwelled water, where food is abundant, and that fish larvae, whether from the surface or bottom layer, are exported beyond the boundaries of the Tsitsikamma National Park.     

Towards a more detailed seismic picture of the oceanic crust and mantle

A review is presented of current interpretation techniques in marine refraction seismology with particular emphasis on those features which are most useful in structural studies using ocean bottom seismometers. Travel time analysis techniques are discussed for both refraction and variable angle reflection profiles and are compared with systematic travel time inversion methods. Amplitude and waveform analysis techniques allow a more detailed velocity depth profile to be determined, and are illustrated for both surface and bottom receivers. The study of anisotropy and lateral variations in crustal structure may be usefully combined using delay time function methods but a detailed structural model also requires velocity-depth information. Signal processing techniques which are particularly useful for shear wave and long range propagation studies are also metioned. Structural studies using OBS' should lead to detailed knowledge of the oceanic crust and the P wave velocity distribution in the upper mantle.     

Stoneley waves,Lopez Island noise,and deep sea noise from 1 to 5 Hz

The Lopez Island OBS Intercomparison Experiment provided a data set of sufficient spatial density to allow study of the propagation of shot-generated Stoneley waves as well as ambient background noise. The Stoneley waves were observed propagating at velocities of 20 to 50 m s^-1, Phase velocities were determined by fitting peaks in the frequency wave number spectrum. Group velocities were calculated by narrowly filtering the data and determining the arrival time of the peak in the frequency packet. Particle displacement plots illustrate the surface wave character of these waves. The analysis of the ambient background noise failed to produce a clearly defined dispersion curve yet it did allow bounds to be placed on the phase velocities (20 to 50 m s^-1). The data were modeled using eleven layers overlying a half-space. The results indicated that the top 7 m of the sediment column at Lopez Island is best approximated by two zones. In the upper zone there is a fairly rapid change of shear velocity with depth. This zone overlies a region in which the shear velocity gradient is much lower. Deep ocean background noise recorded by University of Washington ocean bottom seismometers was also examined. Although insufficient data precluded any velocity analysis, definite similarities exist between these data and noise data observed at Lopez Island.Hawaii Institute of Geophysics Contribution No. 1174.     

A determination of the seismic velocity structure of sediments using both sources and receiver near the ocean floor

A trial experiment proves the power and practicality of using both sources and receivers near the ocean floor to make precise measurements of deep (∼6000 m) ocean sediment velocity structure. A digitally recording ocean bottom hydrophone receiver operating at a sampling rate of 1800 Hz recorded clear arrivals with bubble pulse frequencies of ∼500 Hz from 41b. explosive charges detonated at depths of 5500m along a 4 km long wide angle reflection profile. It is shown that corrections for changes in source depth may be computed without approximation and without prior knowledge of the velocity structure. The experiment was located at longitude 56° W in the trough of the Kane Fracture Zone. The velocity structure of the 1 km thick sedimentary section reveals a 310 m thickness of 3 km s⁻¹ material overlying igneous basement.     

Crustal structure of the South Florida Platform,eastern Gulf of Mexico: An ocean-bottom seismograph refraction study

Five seismic refraction lines, 70–90 km long, were shot in the South Florida Platform region of the Gulf of Mexico using digital ocean-bottom seismographs. Apparent velocities and depths were calculated from the refracted arrivals using a flat-layer model for the region. The two dominant refractors have apparent compressional-wave velocity ranges of 5.6 to 5.9 km s^–1 and 6.2 to 6.7 km s^–1. On the Sarasota Arch, the depth to the top of a 5.8–5.9km/s layer is 3–4 km below sea level. This depth corresponds to the depth to the crystalline basement. The basement dips to the north and to the south from the arch, with velocity of the upper crust increasing from 5.8–5.9 km s^–1 to a maximum of 6.7 km s^–1 at a depth of 6.3 km. Under the continental slope, the crust has presumably been thinned and extended. The deepest refractor has an apparent velocity of about 7.5 km s^–1 at a depth of 25 km. The thickness of the crustal section and the absence of any mantle arrivals in these long refraction profiles on the platform suggest that thick continental crust underlies the South Florida Platform. A north-south cross-section through the platform suggests the presence of two structural highs separated by a portion of the South Florida Basin, which contains at least 5 km of sediment.     

Response of water column to strong wind forcing,southern Brazilian inner shelf: Implications for sand ridge formation

The result of two sequential oceanographic stations of 36 hours each in the area of sand ridges are presented. One station was located in the trough between two sand ridges and the other was at the crest of a sand ridge. At these stations salinity and temperature of the sea water, currents, winds, waves, and barometric pressure were measured each hour.During the observations, a cold front passed; this generated westerly winds that grew in speed from 24 to 52 km h^?1. The average height of the wind generated waves grew from 1.0 to 1.5 m and their periods increased from 7 to 10 s, and the speed of the northeast directed surface current increased from 40 to 82 cm s^?1. A bottom current (also directed northeast) increased from 26 to 34 cm s^?1.After the cold front had passed, the wind backed to the southeast and decreased in speed from 26 to zero km h^?1. The surface current in a northwest direction decreased from 29 to 8 cm s^?1. A bottom current (also directed northwest) decreased from 22 to 3 cm s^?1. Later, swells from the southeast appeared and their periods increased from 5 to 9 s and their heights grew from 1.0 to 1.5 m. After 3 hours, the speeds of the surface and bottom currents increased from 8 to 72 cm s^?1 and 3 to 62 cm s^?1 respectively.This cold front induced strong winds and storm-wave currents able to erode sediments (assuming a threshold velocity of 20 cm s^?1) and transport them in a north-northeast direction.The origin and the maintenance of these sand ridges is thought to be a function of sediments eroded from troughs and piled up at ridge crests during a storm condition. Some eroded sediments are transported north of Verga lighthouse where they are deposited on a smooth bottom.     

Particulate organic carbon supply to the sea bottom: stable carbon isotope ratio analysis of the sediment trap samples at the mouth of otsuchi bay,Northeastern Japan

An array of sediment traps was deployed for the analysis of the pattern of particulate organic carbon (POC) supply to the sea bottom in April, May and July 1988 at the mouth of Otsuchi Bay (about 80 m depth), Northeastern Japan.On the basis of a simple two-component mixing model using stable carbon isotope ratios, the POC flux was separated into marine planktonic and terrestrial components. Both the planktonic and terrestrial POC fluxes had maximum values at 30 m above the sea bottom throughout the three experiments. The planktonic POC flux showed a significant decrease with depth between 30 m and 10 m or 5 m above the bottom. Vertical supply of the planktonic POC and supply of the resuspended planktonic POC were estimated on the basis of regression lines between water depth and the planktonic POC flux in the depth range where the flux decreases with depth.Vertical supply of the planktonic POC and supply of the resuspended planktonic POC to the sea bottom were largest in May (52.1 mgC m^–2 d^–1 and 19.5 mgC m^–2 d^–1 at 5 m above the sea bottom), and horizontal supplies of the terrestrial POC were almost constant (31.9±3.5 mgC m^–2 d^–1 at 5 m above the bottom) throughout the three experiments.     

Contourite identification along Italian margins: The case of the Portofino drift (Ligurian Sea)

A brief review of the published evidence of current deposits around Italy is the occasion to test the robustness of matching bottom current velocity models and seafloor morphologies to identify contourite drifts not yet documented. We present the result of the regional hydrodynamic model MARS3D in the Northern Tyrrhenian and Ligurian Sea with horizontal resolution of 1.2 km and 60 levels with focus on bottom current: data are integrated over summer and winter 2013 as representative of low and high intensity current conditions.The Eastern Ligurian margin is impacted by the Levantine Intermediate Water (LIW) with modeled mean velocity of bottom current up to 20 cm s⁻¹ in winter 2013 and calculated bottom shear stress exceeding 0.2 N m⁻² in water depth of 400–800 m. By crossing this information with seafloor morphology and geometry of seismic reflections, we identify a sediment drift formerly overlooked at ca 1000 m water depth. The Portofino separated mounded drift has a maximum thickness of at least 150 m and occurs in an area of mean current velocity minimum. Independent evidence to support the interpretation include bottom current modelling, seafloor morphology, seismic reflection geometry and sediment core facies. The adjacent areas impacted by stronger bottom currents present features likely resulted from bottom current erosion such as a marine terrace and elongated pockmarks.Compared to former interpretation of seafloor morphology in the study area, our results have an impact on the assessment of marine geohazards: submarine landslides offshore Portofino are small in size and coexist with sediment erosion and preferential accumulation features (sediment drifts) originated by current-dominated sedimentary processes. Furthermore, our results propel a more general discussion about contourite identification in the Italian seas and possible implications.     

Influence of bathymetric fluctuations on coastal storm surge

Natural events constantly alter nearshore bathymetric properties. Hurricanes particularly affect bathymetry as they pass over a body of water. To compute an accurate forecast or recreate a hurricane's effects through hindcasting techniques, an operational bathymetry data set must be known in advance. However, obtaining and maintaining current and accurate bathymetric data can be costly and difficult to manage. In this paper we examine the extent to which variations in nearshore bathymetry affect the storm surge at the coast. A common question for wave and surge modeling is, “how good is the bathymetric data?” If we can allow for a range of fluctuations in the bathymetry without significantly adjusting the results of the surge predictions, we can potentially save months of field work and millions of dollars. A one-dimensional (1D) analytical solution for waves and water level is developed for initial testing. In the 1D case we find that as long as the amplitudes of the bathymetric fluctuations are less than 60% of the original depth, the surge at the coast is within ± 10% of the surge generated on the initial bottom slope. If the fluctuation produces a hole, a deepening of the local bathymetry, within 80% of the local water depth, the coastal storm surge calculated is still within 10% of the unperturbed value computed for bottom slopes shallower than 1:20. In addition, we find there is an optimum distance offshore for each sloped profile that corresponds to a depth between 25 and 40 m, beyond which the effects of bathymetric fluctuations begin to decrease. A coupled 2D modeling system is implemented to test our hypothesis along a realistic coastline. After selecting three study sites, we vary the bathymetry at the selected locations by ± 20%. Consistent with the 1D tests, the storm surge at the shoreline varies by less than 5%.     

Travel-time curves for a simple sea floor model

This paper reviews a simple technique for interpreting the velocity structure of upper oceanic crust from travel-time data of sonobuoy and ocean bottom receiver refraction experiments. The technique does not involve sophisticated digital processing or synthetic seismogram analysis. Interpretations can be carried out with a pencil, paper and slide rule.Travel-time inversion procedures based on the -p transformation require the assumption of the shallowmost velocity. In some cases, however, such as oceanic crustal studies, the shallowmost velocity is one tf the critical parameters for which one wishes to invert. An inversion method for the shallowmost velocity is discussed which assumes a constant velocity gradient. The time, range and ray parameter of a point on the travel-time curve are sufficient to obtain the velocity at the top of the gradient zone and the gradient. The method can be used to interpolate the velocity-depth function into regions from which no seismic energy is returned as a first arrival. Once an estimate of the upper crustal velocity is obtained the traditional -p procedures can be applied.The model considered consists of a homogeneous layer over a layer in which velocity increases linearly with depth. For such a geometry there are three classes of behaviour of the travel-time curve based on the number of cusps: zero, one or two. The number of cusps depends on the uppermost velocity in the crust, the velocity gradient of the upper crust and the depth of the sources and receivers. It has not been previously recognized that two cusps in the travel time curve may be observed for this simple model. Since estimating the ray parameter from first arrival times is less ambiguous when there are no cusps, understanding the relations involved with the three classes aids in the design of experiments. It is reasonable to apply the model to shallow sea floor structure because of the high quality of marine refraction data which has recently been obtained.     

Hydrography and circulation of the bay of Bengal during withdrawal phase of the southwest Monsoon

Hydrographic data were collected from 3 to 10 September 1996 along two transects; one at 18° N and the other at 90° E. The data were used to examine the thermohaline, circulation and chemical properties of the Bay of Bengal during the withdrawal phase of the southwest monsoon. The surface salinity exhibited wide spatial variability with values as low as 25.78 at 18° N / 87° E and as high as 34.79 at 8° N / 90° E. Two high salinity cells (S > 35.2) were noticed around 100 m depth along the 90° E transect. The wide scatter in T-S values between 100 and 200 m depth was attributed to the presence of the Arabian Sea High Salinity (ASHS) water mass. Though the warm and low salinity conditions at the sea surface were conducive to a rise in the sea surface topography at 18° N / 87° E, the dynamic height showed a reduction of 0.2 dyn.m. This fall was attributed to thermocline upwelling at this location. The geostrophic currents showed alternating flows across both the transects. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18° N transect with velocity slightly in excess of 30 cm s⁻¹. Similar high velocity (> 40 cm s⁻¹) pockets were also noticed to extend up to 30 m depths in the southern region of the 90° E transect. However, the currents below 250 m were weak and in general < 5 cm s⁻¹. The net geostrophic volume transports were found to be of the order of 1.5 × 10⁶ m³ s⁻¹ towards the north and of 6 × 10⁶ m³ s⁻¹ towards west across the 18° N and 90° E transects respectively. The surface circulation patterns were also investigated using the trajectories of drifting buoys deployed in the eastern Indian Ocean around the same observation period. Poleward movement of the drifting buoy with the arrival of the Indian Monsoon Current (IMC) at about 12° N along the eastern rim of the Bay of Bengal has been noticed to occur around the beginning of October. The presence of an eddy off the southeast coast of India and the IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data.     

XGBoost算法在多光谱遥感浅海水深反演中的应用

在多光谱遥感浅海水深反演过程中,考虑到水体和底质影响,水深值和海水表面辐射亮度之间的线性关系不成立。本文以甘泉岛南部0～25m范围的沙质区域为研究区域,利用GeoEye-1多光谱遥感影像和多波束实测水深数据构建XGBoost非线性水深反演模型,研究了XGBoost算法用于水深反演的性能。以决定系数(R~2),均方误差(MSE)和平均绝对误差(MAE)作为评价指标,并与3种传统线性回归模型进行了对比分析。结果表明, XGBoost非线性水深反演模型的R~2、MSE和MAE分别为0.991、0.33m和0.44m,拟合程度最好,精度优于线性回归模型。为进一步探究各模型在不同水深的反演精度,将水深范围分成3段(0～8 m, 8～15 m, 15～25 m)分别进行精度验证和误差分析。结果表明, XGBoost模型在各分段的精度均优于线性回归模型, MSE依次为0.56 m, 0.14 m和0.43 m。可见,在单一底质区域下XGBoost模型的水深反演精度更高,且反演效果更稳定。     

Measurements of the water-following capability of holey-sock and TRISTAR drifters

Since 1985, a number of measurements have been made in deep water to determine the water-following characteristics of mixed layer drifters with both holey-sock and TRISTAR drogues at 15 m depth. The measurements were done by attaching two neutrally buoyant vector measuring current meters (VMCMs) to the top and the bottom of the drogues and deploying the drifters in different wind and upper ocean shear conditions for periods of 2–4 h. The average velocity of the VMCM records was taken to be a quantitative measure of the slip of the drogue through the water, observed to be 0.5-3.5 cm s⁻¹. The most important hydrodynamic design parameter which influenced the slip of the drogue was the ratio of the drag area of the drogue to the sum of the drag areas of the tether and surface floats: the drag area ratio R. The most important environmental parameters which affected the slip were the wind and the measured velocity difference across the vertical extent of the drogue. A model of the vector slip as a function of R, vector wind and velocity difference across the drogue was developed and a least squares fit accounts for 85% of the variance of the slip measurements. These measurements indicated that to reduce the wind produced slip below 1 cm s⁻¹ in 10 m s⁻¹ wind speed, R > 40. Conversely, if the daily average wind is known to 5 m s⁻¹ accuracy, the displacement of the R = 40 drifter can be corrected to an accuracy of 0.5 km day⁻¹.     

Detailed observation of the wind-exerted surface flow by use of flow visualization methods

Detailed observations were performed of the wind-exerted surface flow, before and after the generation of wind waves. As flow visualization techniques, 6 classes of polystyrene beads of from 0.33 mm to 1.93 mm in diameter, with a specific gravity of 0.99, and also, hydrogen bubble lines, were used. Experiments were carried out at three ranges of the wind speed: 4.0, 6.2 and 8.6ms^–1 in the mean in the wind-wave tunnel section, and the observations were made at 2.85 m in fetch. In the case of 6.2 m s^–1, when the initial surface skin flow attains 0.22 cm in the scale thickness and 16 cm s^–1 in the surface velocity in about 3 second from the onset of the wind, regular waves of about 1.7 cm in wave length appear on the water surface. In one second after that, the downward thrust of the surface flow and the consequent forced convection commences, and the transition of the surface layer to a turbulent state occurs. Ordinary wind waves begin to develop from this state. In developed wind waves the viscous skin flow grows on the windward side of the crests, frequently producing macroscopic skin flows, and these skin flows converge to make a downward thrust at the lee side, and the viscous skin layer disappears there. The velocity of the downward flow has a maximum at the phase of about 30, and the value is of the order of 10 cm s^–1 at 4-mm depth after the orbital velocity of the sinusoidal wave is subtracted. As the process through which the wind stress acts on the water surface, it is considered that the following particular one may be real: the skin friction concentrated at the windward side of the crest produces skin flows, which thrust into the inner region to make the forced convection, carrying the acquired momentum. The viscous shearing stress just before the generation of the surface undurations was about 1/4 of the total shearing stress under the existence of wind waves. It is considered that the increase of the wind stress by wind waves is caused by this mechanism.     

Uncertainties in gas exchange parameterization during the SAGE dual-tracer experiment

A dual tracer experiment was carried out during the SAGE experiment using the inert tracers SF₆ and ³He, in order to determine the gas transfer velocity, k, at high wind speeds in the Southern Ocean. Wind speed/gas exchange parameterization is characterised by significant variability and we examine the major measurement uncertainties that contribute to that scatter. Correction for the airflow distortion over the research vessel, as determined by computational fluid dynamics (CFD) modelling, had the effect of increasing the calculated value of k by 30%. On the short time scales of such experiments, the spatial variability of the wind field resulted in differences between ship and satellite QuikSCAT winds, which produced significant differences in transfer velocity. With such variability between wind estimates, the comparison between gas exchange parameterizations from diverse experiments should clearly be made on the basis of the same wind product. Uncertainty in mixed layer depth of ∼10% arose from mixed layer deepening at high wind speed and limited resolution of vertical sampling. However the assumption of equal mixing of the two tracers is borne out by the experiment. Two dual tracer releases were carried out during SAGE, and showed no significant difference in transfer velocities using QuikSCAT winds, despite the differences in wind history. In the SAGE experiment, duration limitation on the development of waves was shown to be an important factor for Southern Ocean waves, despite the presence of long fetches.     

Resuspension of benthic fluff by tidal currents in deep stratified waters, northern North Sea

Suspended particulate matter (SPM) concentration and properties (particle size and settling velocity), water column and boundary layer dynamics were measured during a 60-d period at a site in 110 m water depth in the northern North Sea. The site was in stratified waters and measurements were made during September–November as the seasonal thermocline was progressively weakening. SPM concentration was low, c. 1 mg dm⁻³ in the surface mixed layer and maximum values of 2 mg dm⁻³ in the bottom mixed layer. The bottom layer was characterised by larger mean particle size. SPM signals in the two layers were decoupled at the start of the period, when the thermocline was strong, but were increasingly coupled as the thermocline progressively weakened. A spring-neap cycle of resuspension and deposition of SPM was observed in the bottom mixed layer. Bed shear stresses were too small to entrain the bottom sediment (a fine sand) but were competent to resuspend benthic fluff: threshold bed shear stress and threshold current velocity at 10 mab were 0.02–0.03 Pa. and 0.18 m s⁻¹, respectively. Maximum SPM concentration in the bottom layer preceded peak spring tide currents by 3 d. Simulation of fluff resupension by the PROWQM model confirms that this was due to a finite supply of benthic fluff: the fluff layer was stripped from the seabed so that fluff supply was zero by the time of peak spring flow. SPM was redeposited over neap tides. Fluff resuspension must have been enhanced by intermittent inertial currents in the bottom layer but unequivocal evidence for this was not seen. There was some resuspension due to wave activity. Settling velocity spectra were unimodal or bimodal with modal values of 2×10⁻⁴–2×10⁻³ mm s⁻¹ (long-term suspension component) and 0.2–5.7 mm s⁻¹ (resuspension component). The slowest settling particles remained in suspension at peak spring tides after the fluff layer had been exhausted. There was evidence of particle disaggregation during springs and aggregation during neaps.     

ICESat-2激光卫星与光学遥感影像融合水深测量

针对浅海测深的数据特点和应用需求,以我国南海甘泉岛为例,研究了利用ICESat-2(Ice,Cloud,and Land Elevation Satellite-2)激光卫星数据和光学遥感影像开展主被动融合水深测量的方法。首先通过信号点提取、水面/水底识别、水下点折射改正等步骤处理ICESat-2数据,获得水深值,随后以激光点作为控制,计算光学水深反演模型参数,最后由点及面地获取大范围高精度水深。实验表明,甘泉岛区域主被动融合测深中误差优于1.30m,基于激光卫星数据的主被动融合测深方法能够为浅水水深测量提供新手段。     

Observations of surfzone alongshore pressure gradients onshore of an ebb-tidal delta

The relative importance of radiation stress gradients and alongshore pressure gradients to surfzone dynamics is investigated using observations of water levels, waves, and flows measured onshore of a large ebb-tidal delta. Incident wave heights measured along the ~ 11-m depth contour varied about 10% over a 1.2-km alongshore transect, resulting in alongshore wave setup differences on the order of 10 cm over the 600-m extent of the surfzone instrument array in 1.5-m depth. Despite the moderate alongshore variability in wave heights, the southerly alongshore pressure gradient, associated with the alongshore variability of wave-driven set-up, was typically twice as large as the northerly radiation stress gradient forcing, consistent with the observed southerly currents during the week-long experiment. The magnitude of the alongshore forcing and resulting alongshore velocity is reproduced by the two-dimensional depth-averaged numerical model of Shi et al. (JGR-Oceans, 2011). These observations, together with the numerical results, indicate that moderate alongshore wave height gradients (O(10^− 4)) outside the surfzone owing to alongshore variations in the offshore bathymetry can result in alongshore pressure gradients that are larger than radiation stress gradients.     

洱海生态环境恶化及综合治理对策研究

由于７０年代中期西洱河电站建成发电引起洱海水位下降，加之流域植被破坏，水土流失及酷渔监捕，不合理引种等原因，使洱海生态环境发生了变迁。湖泊面积缩小５％左右，容积减少１４％左右；浮游动植物，底栖动物的种群结构，数量发生明显变化；土著鱼种类和数量急剧减少；水在维管束植物覆盖面积达２７％，生物总量达５６．３万ｔ。为此提出保持水位植树造林，控制点，面源污染，适度捕捞及加强管理等综合治理对策。