Progressive Vector Diagrams Research Articles

Structure and dynamics of convective mixing in Lake Onego under ice-covered conditions

ABSTRACTHydrophysical studies conducted in Petrozavodsk Bay of Lake Onego under ice-covered conditions in March 2016 and 2017 detected radiatively driven convection and revealed specific structural and dynamic parameters for the convectively mixed layer (CML). Analysis of time series, spectral energy distribution, and vertical velocity profiles indicated the presence of a mean current, seiches, and convective motion. Because of their similar spatial and temporal scales, these processes were investigated using progressive-vector diagrams (PVDs). Despite low water velocities, the CML hydrodynamic regime remained close to that of fully developed turbulence, and convective cells at a range of different scales were expected. Signal resolution constraints limited detection to only the largest cells. We investigated the horizontal structure of the CML using individual and combined observations from 3 acoustic velocity profilers located within a radius of a few tens of meters. This novel setup collected data indicating that the CML’s large-scale horizontal flow structure consists of a continuum of quasi-deterministic cells. Cell parameters necessary for estimating turbulent transfer were derived from PVD and hodograph curves.

Estimating nitrogen loading and far-field dispersal potential from background sources and coastal finfish aquaculture: A simple framework and case study in Atlantic Canada

Far-field nutrient impacts associated with finfish aquaculture have been identified as a topic of concern for regulators, managers, scientists, and the public for over two decades but disentangling aquaculture impacts from those caused by other natural and anthropogenic sources has impeded the development of monitoring metrics and management plans. We apply a bulk, steady-state nitrogen loading model (NLM) framework to estimate the annual input of Total Dissolved Nitrogen (TDN) from point and non-point sources to the watershed surrounding Port Mouton Bay, Nova Scotia (Canada). We then use the results of the NLM together with estimates of dissolved inorganic nitrogen (DIN) loading from a sea-cage trout farm in the Bay and progressive vector diagrams to illustrate potential patterns of DIN dispersal from the trout farm. Our estimated anthropogenic nitrogen contribution to Port Mouton Bay from all terrestrial and atmospheric sources is ∼211,703 kg TDN/year with atmospheric deposition accounting for almost all (98.6%). At a stocking level of ∼400,000 rainbow trout, the Port Mouton Bay sea-cage farm increases the annual anthropogenic TDN loading to the bay by 14.4% or 30,400 kg. Depending on current flow rates, nitrogen flux from the trout farm can be more than double the background concentrations of TDN near the farm site. Although it is unlikely that nitrogen loading from this single fish farm is saturating the DIN requirements of the entire bay, progressive vector diagrams suggest that the dispersal potential may be insufficient to mitigate potential symptoms of eutrophication associated with nitrogen fluxes. We present an accessible and user-friendly tool for managers to estimate baseline nutrient loading in relation to aquaculture and our use of progressive vector diagrams illustrate a practical and simple method for characterizing potential nutrient dispersal based on local conditions and spatial scales. Our study joins numerous studies which have highlighted the need for more effective monitoring and assessment methods to improve the detection of aquaculture effects at far-field scales and to assess those effects in relation to other natural and anthropogenic factors impacting coastal habitats.

Effect of seasonal and cyclonic winds on internal tides over the Bay of Bengal

The influence of seasonal and cyclonic winds is studied on the characteristics of internal waves (IWs) over the western Bay of Bengal (BoB) by using MITgcm model. As the BoB experiences reversal of seasonal winds and also tropical cyclones during pre-monsoon and post-monsoon months, its effect is seen through the computation of spectral estimates of the IWs. It is seen that the peak estimate is associated with the semidiurnal frequency at all the depths and is found higher in May compared to November. This is attributed to the presence of shallow mixed layer depth and deep thermocline due to the upwelling favorable winds. The computation of isopycnal displacement infers that the internal tides are present from 40 to 120 m depth in case of upwelling favorable winds of May, whereas, the presence of internal tides is restricted between 90 and 120 m for the downwelling favorable winds of November. During May, the available potential energy is also seen in a narrow coastal stretch, whilst it is absent in November. During the Hudhud cyclone period of October 7–14, 2014, it is noticed from the spectral estimates that the IWs of tidal frequency are replaced by inertial frequency with a periodicity of about 2 days as a consequence of strong cyclonic winds. The progressive vector diagram shows the mean current is initially westward up to October 17, 2014 and then northeastward with well-defined clockwise circulation. The maximum radius of inertial oscillation of 15 km is observed. After the cyclone ceases, the estimate associated with inertial frequency slowly diminishes and enhances the estimates related to internal tides. The simulations also suggest that the internal tides are absent for about 6 weeks as a response of the cyclonic winds.

Peculiarities of Multiscale Wind Regime Variability at Coastal Water Area of the North-Eastern Black Sea

Peculiarities of variability of wind regime in the period from 1935 to 2014 are discussed. More detailed analysis of variability of meteorological condition on different scales during the last years (2010−2014) is conducted. Progressive vector diagrams are used for visualization of time series of wind vectors. Changes of prevalent winds from NE to S-E-SE (warm period waves) are reflected on a climatic progressive vector diagram from 1935 to 2014. Maximum wave amplitude on a diagram was registered in a period of 2004– 2006. In 2007–2014, a distinctive feature of the wind regime was a dominance of SE winds during warm and abnormally warm years. Major types of atmospheric processes that give rise to dominating winds are outlined. It is demonstrated that differences of the wind regime during particular years with various temperature characteristics of years and winters are caused by a variability of atmospheric synoptic processes on different scales. Using annual wind diagrams and pictures of temporal wind variability, as well as air and water monthly temperature (observation time every 3 hours), periods of prolonged wind forcing (during several months and during 1−2 decades of every month) are distinguished. Role of the winds of SE bearings with high repeatability during several months in forming of the vertical water structure of coastal zone is demonstrated. Analysis of the results of the experimental data acquired in 2010–2014 (research vessel “Ashamba”) has shown that during stable winds of SE bearings advection of cold intermediate waters from the area of their formation (central divergence zone) to the coastal zone becomes stronger.

서해 천수만 중앙부의 하계 조류/비조류 특성

이 연구는 서해 천수만에서 2010년 7월 29일부터 8월 30일까지 33일동안 관측한 ADCP 자료와 바람(기상청), 서산AB지구의 방류량(한국농어촌공사 천수만사업단)자료를 분석하였다. 기술통계, 조류의 조화분석을 통해 조류의 특성을 파악했고, 스펙트럼(spectrum)과 코히런시(coherency) 분석, 콤플렉스 코릴레이션(complex correlation)과 진행 벡터(progressive vector) 등을 통해 비조류 특성을 파악하여 바람과 담수 유입이 잔차류에 미치는 영향을 알아보았다. 관측 유속의 범위는 -30~41 cm/sec였고, 표준편차는 저층에서 1.7 cm/sec, 표층에서 18.7 cm/sec로 계산되었다. 조화분석 결과 유향은 북북서-남남서 방향으로 주된 흐름을 보였다. M2의 조류타원 장축과 단축은 각각 9.4~14.8, 0.1~0.5 cm/sec의 범위를 보였고, S2의 경우 각각 4.4~7.0, 0.4~1.4 cm/sec의 범위를 보였다. 잔차류 전 수층의 대한 스펙트럼 분석 결과 3~6개의 유의한 주기가 2~8일 주기에 포함되어 있었으며, 바람도 유사한 결과를 보였다. 바람과 전 수층 잔차류의 코히런시 분석 결과 3~5개의 유의한 주기가 2.8일 이내의 위상차로 나타났고, 담수 유입량과 표층 잔차류의 코히런시 분석 결과 4.6일 주기가 가장 유의했으며, 위상차는 1.2일이었다. 잔차류의 진행 벡터는 전 층에서 북향했고, 표층보다 중층에서 북향하는 거리가 컸다. 잔차류의 북향은 바람의 계절적 요인에 기인했다고 볼 수 있으나, 만 북부에서 남하하는 담수의 영향(밀도류)에 의해 표층 잔차류 일부가 남향하는 흐름을 보였다. This study analyzed the ADCP records along with wind by KMA and discharge records at Seosan A-, B-district tide embankment by KRC for 33 days obtained in the Chunsu Bay, Yellow Sea, Korea spanning from July 29 to August 30, 2010. Various analyses include descriptive statistics, harmonic analysis of tidal constituents, spectra and coherence, complex correlation, progressive vector diagram and cumulative curves to understand the tidal and sub-tidal current characteristics caused by local wind and discharge effect. Observed current speed ranges from -30 to 40 (cm/sec), with standard deviation from 1.7 (cm/sec) at bottom to 18.7 (cm/sec) at surface. According to the harmonic analysis results, the tidal current direction show NNW-SSE. The magnitudes of semi-major axes range from 9.4 to 14.8 (cm/sec) for M2 harmonic constituent and from 4.4 to 7.0 (cm/sec) for S2, respectively. And the magnitudes of semi-minor axes range from 0.1 to 0.5 (cm/sec) for M2 and from 0.4 to 1.4 (cm/sec) for S2, respectively. In the spectral analysis results in the frequency domain, we found 3~6 significant spectral peaks for band-passed wind and residual current of all depth. These peak periods represent various periodicities ranging from 2 to 8 (days). In the coherency analysis results between band-passed wind and residual current of all depth, several significant coherencies could be resolved in 3~5 periodicities within 2.8 (days). Highest coherency peak occurred at 4.6 (day) with 1.2-day phase lag of discharge to band-passed residual current. The progressive vector of wind and residual current travelled to northward at all layers, and the travel distance at middle layer was greater than surface layer distance. The Northward residual current was caused by a seasonal southern wind, and the density-driven current formed by fresh water input effected southward residual current. The sub-tidal current characteristics is determined by seasonal wind force and fresh water inflow in the Chunsu Bay, Yellow Sea, Korea.

Vertical migrations of reef fish larvae in the Straits of Florida and effects on larval transport

The goals of this study were to measure vertical distributions of pelagic coral reef fish larvae, identify significant vertical migrations, and estimate the effects of vertical migrations between depths with different ambient currents on larval transport in the Straits of Florida. Spring, summer, and fall time‐series of plankton net tows were conducted at the edge of the Florida Current offshore of Miami. In each time‐series the upper 100 m of the water column at a fixed station was sampled repeatedly at 3‐h intervals for two diel cycles, with different nets sampling at five discrete depth ranges. Simultaneously, currents‐at‐depth were recorded by shipboard current profilers. Mean and standard deviation of larval depth and statistically significant vertical migrations were determined by nonparametric resampling and tree regression techniques. In 10 taxa, larvae showed ontogenic vertical migrations, gradually moving deeper with growth and development. Vertical distributions of six taxa varied between day and night, revealing diel vertical migrations. Larval transport was estimated by generating progressive vector diagrams from current measurements and larval vertical distributions. The dominant alongshore component of the Florida Current resulted in rapid larval transport through the Straits of Florida, while cross‐shore current was an order of magnitude weaker and highly variable. Larvae at depths > 70 m experienced a 15–75% reduction in transport; thus, deep vertical distributions and downward vertical migrations potentially favor retention and settlement in the Straits of Florida.

How useful are progressive vector diagrams for studying coastal ocean transport?

Progressive vector diagrams (PVDs) have been used to estimate transport in the coastal ocean from point measurements of velocity time series, although they strictly only approximate true particle trajectories in regions where the currents are spatially uniform. Currents in most coastal regions vary significantly in both space and time, making coastal transport estimates from PVDs questionable. Here, we used synoptic surface currents measured over periods of several months by HF radars in two coastal areas with distinctly different circulation features (the Gulf of Eilat, a deep, semi‐enclosed basin, and the Delaware Bay mouth, a coastal estuarine outflow) to assess the time scales over which PVD paths computed from velocity time series at fixed points separate from particle trajectories computed from two‐dimensional measured horizontal currents that vary in both space and time. In both study regions, PVDs and particle trajectories separate by 1 km over a mean time period of 7–10 h with no significant month‐to‐month variation in either region. The separation time statistics presented here should serve as a strong caution for investigators motivated to estimate transport using only point measurements.

Tracking the Surface Flow in Lake Champlain

Understanding the hydrodynamics of Lake Champlain is a basic requirement for developing forecasting tools to address the lake‘s environmental issues. In 2003 through 2005, surface drifting buoys were used to help characterize the circulation of the main body and northeast region (Inland Sea) of the lake. Progressive vector diagrams of over-lake winds when compared to drifter trajectories suggest the presence of gyre-like circulation patterns. Drifter statistics suggest average current speeds of 10 cm s −1 and were predominantly northward (+ V) due to northerly-directed winds and lake geometry. Singleparticle eddy diffusivities on the order of 10 6 cm 2 s −1 were calculated which is consistent with results from the Great Lakes and in some oceanic regions. However, the Lagrangian length and time scales, a measure of flow decorrelation scales, were in general smaller than seen in the Great Lakes, which is a natural consequence of the smaller basin size of Lake Champlain relative to the Great Lakes.

Physical processes impacting passive particle dispersal in the Upper Florida Keys

Physical processes affecting the dispersion of passive particles (e.g., coral larvae, pollutants) in the Upper Florida Keys are investigated through in situ observations (acoustic Doppler current profilers and surface drifters) and numerical ocean circulation modeling (horizontal resolution: 800 m, vertical resolution: 0.1–1 m). During the study period in August 2006 (set to coincide with an annual coral spawning event), Lagrangian trajectories in the vicinity of the reef tract indicate that alongshelf advection was mainly poleward and due to the subtidal flow of the Florida Current, while cross-shelf advection was mainly onshore and due to wind-driven currents. Tidal currents resulted in predominantly alongshelf displacements, but did not contribute significantly to net passive particle transport on a weekly timescale. Typical advection distances were of the order of 10 to 50 km for pelagic durations of 1 week, with significant variability linked to geographical location. In contrast, the direction of transport from the offshore reefs remained essentially constant (i.e., potential dispersion pathways were limited). In addition, Lagrangian trajectories and progressive vector diagrams in the vicinity of the reef tract indicate that alongshelf variations in the cross-shelf velocity gradient associated with the FC are relatively weak on an alongshore scale of ca. 50 km. For August 2006, the highest particle concentrations typically occur inshore of the reef tract, thereby suggesting that onshore transport associated with wind-driven currents contributes significantly to the local retention of passive organisms (and other tracers) in the Upper Florida Keys. Overall, the results illustrate the necessity of conducting targeted in situ observations and numerical model predictions to quantify the physical processes affecting reef-scale advection, especially in an effort to understand local retention and dispersion mechanisms for larval marine organisms.

Tracking the Surface Flow in Lake Champlain

Understanding the hydrodynamics of Lake Champlain is a basic requirement for developing forecasting tools to address the lake‘s environmental issues. In 2003 through 2005, surface drifting buoys were used to help characterize the circulation of the main body and northeast region (Inland Sea) of the lake. Progressive vector diagrams of over-lake winds when compared to drifter trajectories suggest the presence of gyre-like circulation patterns. Drifter statistics suggest average current speeds of 10 cm s −1 and were predominantly northward (+ V) due to northerly-directed winds and lake geometry. Singleparticle eddy diffusivities on the order of 10 6 cm 2 s −1 were calculated which is consistent with results from the Great Lakes and in some oceanic regions. However, the Lagrangian length and time scales, a measure of flow decorrelation scales, were in general smaller than seen in the Great Lakes, which is a natural consequence of the smaller basin size of Lake Champlain relative to the Great Lakes.

Effects of waves and currents on the siltation problem of Damietta harbour, Nile Delta coast, Egypt

This study evaluates the effect of prevailing dynamic factors on the sedimentation process in Damietta Harbour along the Nile delta coast of Egypt. The monitoring program spanned the period between 1978 and 1999 and included measurements of waves, currents and bathymetric profiles. The evaluation was based on determination of erosion and accretion rates, current regime, sediment transport, wave characteristics and wave refraction. Results revealed that the predominant wave direction from N-NW sector (86 %) throughout the year is responsible for generation of a longshore eastward current. Less frequent waves from the N-NE sector generate an opposing longshore westward current. The refraction pattern for the prevailing wave direction indicates that the harbour and its navigation channel are located within a divergence of wave orthogonal and in an accretion sediment sink area. The annual net rate of littoral drift on the western side of the harbour is about 1.43 * 105 m3 (accretion), while the annual net rate of littoral drift on the eastern side is about 2.54 * 105 m3 (erosion). Currents fluctuate tremendously in speed and direction, especially during the winter months. Hence, sediment transport takes place in offshore, eastward, and onshore directions. Progressive vector diagrams show that the largest near bottom offshore, onshore and easterly net drift occurs during summer, spring and winter respectively. The onshore sediment transport generated during spring and summer plays an important role in the redistribution of eroded sediments during the winter. The overall study of dynamic factors indicated that the harbour site is characterized by eastern, western, offshore and onshore sediment movements. Therefore, the north-south orientation of the navigation channel, with its depth greater than the surrounding area, interrupts sediment drift from different directions and reduces the current speed. Consequently, the sediments sink within the navigation channel from different directions. The sources of sediments contributing to the siltation process of the harbour and its navigation channels are mainly derived from the Rosetta promontory, Burullus beaches, Damietta promontory and from offshore and the dumping area.

Variability in Measured Current Structure on the Southwest Continental Shelf of India

Analyses of short-duration, current meter records collected in the coastal waters off Beypore on the southwest coast of India have been made in order to understand the tidal and nontidal variability during premonsoon and summer monsoon seasons of the year 2000. The region's tidal and nontidal flows were described by means of time-series displays, simple statistics, harmonic analysis, correlation coefficients, and progressive vector diagrams. The nontidal component of current was extracted using the X0 filter. The study showed that the tidal currents were generally weak (<7 cm/s) and dominated by tidal signals only in a few records. Diurnal and semidiurnal constituents were found to be responsible for the rest of the variance. The signatures of the currents were different at the surface and bottom of the water column in the records collected during the summer monsoon season, suggesting a baroclinic nature of motion, whereas the premonsoon records showed a barotropic nature. The stratification of the water column was strong during the summer monsoon season but was well mixed during the premonsoon season. There was a strong signature of diurnal solar forcing in the surface-temperature records, which was totally lacking in the bottom records.

Tidal and Sub-tidal Current Characteristics in the Kangjin Bay, South Sea, Korea

This study analyzed the current meter records along with wind records for over 500 days obtained in the Kangjin Bay, South Sea, Korea spanning from March, 2003 to Nov. 2005. Various analyses include descriptive statistics, harmonic analysis of tidal constituents, spectra and coherence, the principal axis, progressive vector diagrams. These analyses can illustrate the response of residual current to the local wind resulting in the net drift with rotational motion. Current speed ranges from -28 to 33 (cm/sec), with standard deviations from 6.5 to 12.9 (cm/sec). The harmonic analyses of the tidal current show the average form number, 0.12 with semi-diurnal type and the recti-linear orientation of the major axis toward northeast. The magnitudes of the semi-major range from 12.7 to 17.7 (cm/sec) for M2 harmonics, while for S2 harmonics, they range from 6.3 to 10.4 (cm/sec), respectively. In the spectral and coherency analysis of residual current and wind, a periodicity of 13.6 (day) is found to be most important in both records and plays an important role in the net drift of residual current. The progressive vector diagrams of residual current and wind show two types of behaviors such as unidirectional drift and rotational motion. It was also found that 3 % rule holds approximately to drive 1 (cm/sec) drift current by 30 (cm/sec) wind speed based on the correlation of the semimajor axis of wind and residual current.

Deep flow field in the Japan/East Sea as deduced from direct current measurements

The mean flows and variability in the abyssal Japan/East Sea are investigated on the basis of direct current observation data. The distribution of the mean flow vectors indicates relatively strong cyclonic circulations along the basin periphery and sluggish flows in the interior region. Strong mean flows of faster than 6.0 cm s−1 are found at the western edge in the Yamato and Ulleung Basins, considered to be the western boundary currents in each basin. The similarity between the distributions of the mean flow vectors in the upper (400–1000 m) and lower layers (1000 m-bottom) indicates common circulation patterns throughout the water column of the Proper Water. Progressive vector diagrams show meandering curves with clockwise and anticlockwise turns due to the passing of mesoscale eddies. Since a dominant background flow is absent in the basin interior, the ratio of eddy kinetic energy to the mean-flow kinetic energy reaches more than 50 in the interior region. This indicates that the flows observed in the interior region are comparable to those in the basin periphery, even though the mean flow is generally weak. Spectral analysis has revealed that tidal fluctuations are prevailing in the Ulleung Basin, while they are absent in the Yamato and Japan Basins, though the inertial oscillation is dominant in all of the basins. This indicates that the tidal energy from the East China Sea is confined within the Ulleung Basin, suggesting the barrier effect of the Yamato Rise and the Oki Spur. Significant energy peaks are also found in the 2–5 and 5–10 day period bands in the Yamato Basin. The former variation represents the shelf-wave activity trapped on the basin edge and the latter is due to eddy migrations associated with the Tsushima Current.

High-frequency observations of wind-forced onshore transport at a coastal site in Baja California

We investigated the effect of the daily sea breeze on the surface flow at Bahia Salsipuedes, Baja California, Mexico. Drifter-tracking experiments were conducted over a 2-week period in July, 1999. Surface drifters were deployed daily and their trajectories tracked from a small boat for 3–7h. Wind speed and direction, as well as the vertical distribution of temperature and current velocities were measured throughout the observation period. Our results indicate that strong winds associated with the daily sea breeze at the study area can produce onshore surface currents with velocities of up to 10cms−1 (net transport of up to 2km in a diurnal cycle). There was a hyperbolic relationship between onshore wind stress and net onshore transport of the drifters. A decrease in stratification and more consistent pattern of onshore transport were associated with sustained winds >6ms−1 towards the end of the study. Progressive vector diagrams computed from near-surface currents failed to predict the trajectory of drifters, especially on days where wind forcing was more consistent in terms of direction and magnitude. We discuss the implications of these results regarding the collection of settlement data and their interpretation in terms of larval supply to coastal populations.

Current measurements and spectral analyses in the upper 450 m and deep layers of the northeastern South China Sea

On the basis of the current measurements from the moored Long Ranger ADCP in the upper 450 m layer and the deep current measurements at 2000 and 2300 m from the moored current meters with the time series data of about 7 months at the mooring station in the northeastern South China Sea, the spectral analyses and calculation have been made. The major results are as follows: (i) From the progressive vector diagrams of the observed daily currents at the water levels from 50 m to 400 m, its temporal variation of velocity rotated counterclockwise in most of the observing time. This agrees basically with the result from the qualitative analysis of the sea surface height data, which was obtained from TOPEX/ERS-2 altimeter data by CCAR. The daily and monthly average velocities are both the largest in November, next in October and minimum in August. (ii) At the 2000 and 2300 m levels, the daily and monthly average velocities are both the largest in January, next in September and minimum in August. From the seasonal change of currents, the current velocity is the strongest in winter (January-March), next in autumn, and weak in summer. (iii) There exists the variation of tidal current with the change of depth. In the upper layer, the height of diurnal peak is higher than that of semidiurnal peak. However, the semidiurnal peak is higher than the diurnal peak at the levels from 200 m to 400 m. In the layers above 450 m the clockwise component is dominant in their fluctuations. In the layers below 1500 m the diurnal peak is again higher than the semidiurnal peak. (iv) There is the prominent periodic fluctuation of more than two months in the layer from 50 m to 2300 m. The period of this prominent peak is 75 d and its fluctuation is counterclockwise in the upper 450 m layers, and is 68 d and 69 d at the depths of 2000 and 2300 m, respectively, and the counterclockwise component is dominant in their fluctuations. (v) There are the variations of periods fluctuating with the change of depth in the upper 450 m layers. For example, when f>0, there are the prominent fluctuations of about 22 d and 15 d period at the 50 and 100 m levels. However, there are no such periods at the layer from 200 m to 400 m, where only the fluctuation of about 13 d period occurs. (vi) There are the fluctuations with periods of more than one month, 23 d and 15 d at the 2000 m and 2300 m levels. (vii) In the layer from 50 m to 2300 m there are the following prominent peaks: i) the fluctuation in the period range of about 4-8 d, which occurs in the weather process; ii) the fluctuation with inertial period, the fluctuation is clockwise; and iii) the fluctuations with short periods of about 8 h and 6 h. (viii) From the cross spectral estimates between two time series, it is shown that there are significant coherence peaks with the periods of more than two months(T = 68.3 d) and more than one month between the two time series of currents at 2000 m and 2300 m depths, and also those with periods of about half a month (15.5 d), 2 d and so on between two time series of currents at 100 m and 2300 m depths.

Westward moving waves or eddies ( Storms) on the Subtropical/Azores Front near 32.5°N? Interpretation of the Eulerian currents and temperature records at moorings 155 (35.5°W) and 156 (34.4°W)

Three Argos buoy-years of Lagrangian data in westward-moving cyclonic eddies, or Storms, near 32.5°N, together with hydrographic measurements, have shown that Storms move westward at nearly 3 km day −1. Water in eddies can be trapped and moved westward by advection within the eddy or by phase propagation of the eddy pattern, so we cannot say that the flow field (or Eulerian mean) is 3 km day −1 westward. Two moorings (155 and 156) deployed in the Storm Corridor have provided a further 8 instrument-years of Eulerian data. The temperature and current records confirmed that two Storms a year passed each mooring over the 2-year measurement period. As expected, there is a lag of ∼1.3 month at mooring 155 (which is 102 km to the west of mooring 156) with respect to conditions at mooring 156. The progressive vector diagrams (PVDs) derived from the current meter records exhibit fairly regular X (east or zonal) and Y (north or meridional) displacement scales that repeat with semi-annual periodicity (SAP). The dominant current signal is the north component of the SAP, which reaches an amplitude of 18 cm s −1 for the upper layer or near surface current record (∼242-m depth). The geostrophic north component values derived from altimetry were in good agreement with the upper layer current meter measurements. The large north component amplitude was not interpreted as evidence for Rossby Waves but rather due to the passage of nine eddies (eight complete) of alternate sign (cyclonic, anticyclonic) passing the mooring rigs during the 2-year deployment period. The Y scale shows that the near surface characteristic or mean maximum azimuthal speed is about 35 cm s −1 for cyclonic eddies or Storms, and that this value is reduced to 4 cm s −1 at 1400-m depth. The residual or mean Eulerian currents range from 8 cm s −1 for the upper layer currents to 1 cm s −1 for the deeper currents at ∼1400-m depth and are predominantly westward. Simple theoretical considerations and idealised numerical simulations show that the mean westward Eulerian current depends markedly on whether the eddy centres pass to the north or south of the rigs. This raises the question as to what do we mean by Eulerian residual currents, even for relatively long records (∼2 years). It is shown that the strong near surface westward current (∼6 km day −1) measured at mooring 155 is largely due to a westward-moving eddy field with variable centre offsets. The magnitude of the near surface east–west component of flow was estimated as eastward at 2 cm s −1. The north–south component of mean flow was southward at 2 cm s −1. The deeper records gave a weak westward flow of ∼1 cm s −1 but did not show a significant southward component for the mean Eulerian flow field. 7.4 float-years of Lagrangian ALACE data in the Subtropical Front region near 740 dbar gave mean east–west flows that were <0.5 cm s −1. Overall, it is shown that the eddy structures propagate westward mainly by phase propagation (i.e. a westward-moving pattern with no westward advection for the current meter to measure), though plane Rossby Wave dynamics appeared inappropriate. Theoretical and modeling considerations show that a speed of 3-km day −1 westward is too large a value for the self-advection of eddies due to the beta effect.

VARIABILITY OF CURRENTS IN BAYS OF TAGALAHT AND UUDEPANGA

The variability of the hydrodynamical fields in the sea around the Tagamoisa Cape, the north-western coast of the Saaremaa Island, has been investigated from August to November 2000 measuring currents and stratification. The time series of the forcing parameters, such as the sea level, wind, and currents are presented, and the primary relationships between them have been established. The horizontal fields and the progressive vector diagrams of the currents are constructed. The measured currents in the Tagalaht Bay are significantly dependent on the forcing factors and reveal predominantly a two-layer character. During the dominating southern winds the surface flow to the north, which carries water out from the bay, is compensated by the undercurrent, causing in such a way intensive ventilation of the bay. The two-layer shallow-water model was used to calculate the vertical structure of the currents by integrating the equation of motion through upper and lower layers and eliminating the local sea level forcing term. In the Uudepanga Bay the dominating southern winds cause anticlockwise circulation and the currents reveal predominantly the one-layer structure.

Particulate fluxes and transports on the slope between the southern East China Sea and the South Okinawa Trough

This paper presents time-series variations of the particulate fluxes in resolutions of 3–15 d from 10 mooring sites on the continental slope between the southern East China Sea and the South Okinawa Trough, northeast of Taiwan. Three of these sites were located in a canyon cutting across the slope. Temporal and spatial variations in particulate flux in the study area were strongly affected by the bottom topography, the topographically induced eddies, the intensity of tidal currents, the depth and height above bottom where the traps were deployed, and episodic events. The apparent mass fluxes measured at depths from about 300 to 1350 m at different locations varied with time and showed synchronous variations among the traps deployed at the same site for some sites. The time-averaged mass flux for each individual trap deployed at different depth and in different length of mooring time ranged from 0.77 to 53.7 g m −2 d −1. In the canyon the average fluxes were generally greater than 30 g m −2 d −1 with large standard deviations; at other slope sites the average fluxes were generally less than 10 g m −2 d −1, excluding one or two unusually high values which were observed in some individual traps probably reflecting episodic events. The particulate fluxes always increased toward the ocean bottom at each mooring site but the increment was much greater in the canyon. This suggests that the particulates were transported out of the shelf or upper slope area essentially through the canyon where large mass fluxes were observed. The current meter data processed with low-pass filtration and expressed in stick diagrams and progressive vector diagrams showed large temporal and spatial variations which were generally not correlated with those of particulate fluxes. Although strong tidal velocity oscillations in the canyon were found to be associated with high mass fluxes in some instances (Hung and Chung, 1998, Continental Shelf Research 18, 1475–1491), episodic events with very high mass fluxes were generally not related to the current systems. The deduced prevailing flow was consistently along the isobaths of the lower slope toward the southwest even though the current meters were deployed in different seasons for variable length of time. The mean current speed varied from 30 cm s −1 within the Kuroshio at 305 m depth to 3 cm s −1 at about 250 m above the bottom of the deepest site near the South Okinawa Trough. Most values were about 5–10 cm s −1. In contrast to the canyon where the trapped particulates were mostly silt and sand (Hung and Chung, 1998), the particulates collected from the general slope area were mainly silt. High mass fluxes were found to be associated with high sand and silt fluxes in the canyon (Hung and Chung, 1998) probably due to strong tidal currents and episodic events that had occurred in this high-energy environment. Lower mass fluxes with less sand contents observed at general slope sites suggest that no significant amounts of particulates may be transported via the intracanyon areas.

EOF analysis of the velocity fields in the Arabian Gulf

EOF analysis of the along- and cross- axis components of the current fields at various moorings (M 2, M 3, M 4) has been carried out to determine the different modes of variation in the Arabian Gulf. The analysis reveals that the first eigenmode explains about 70 % of the variance and resembles the surface inflow at the various moorings. The second eigenmode explains about 28 % of the variance and represents the bottom outflow. A complex EOF analysis in the vertical direction shows that the first eigenmode represents the more steady current, regardless of the depth and mooring position. It has a shape similar to that of the progressive vector diagrams (PVD) at moorings M 2 and M 3 in the surface layer, thus portraying the inflow to the Gulf. At mooring M 4 this mode resembles the current at the bottom layer and therefore represents the outflow. The second eigenmode resembles the bottom current at mooring M 2 and the surface current at mooring M 4. The results of complex EOF analysis in the horizontal direction reveal that the first eigenmode explains 66 % of the variance in the surface layer and has a shape similar to that of the PVD at mooring M 2. The second mode explains 22 % of the variance and resembles the PVD at moorings, M 3 and M 4 in the surface layer. The first mode explains 71 % of the variance with a similar shape to the PVD at moorings M 2, M 4 and M 7 in the bottom layer.