Surface Current Speed Research Articles

Wind-Driven Overturning, Mixing and Upwelling in Shallow Water: A Nonhydrostatic Modeling Study

Using a nonhydrostatic numerical model, this work demonstrates that onshore winds are a principal agent of overturning and vigorous vertical mixing in nearshore water of lakes and inner continental shelves. On short (superinertial) timescales of a few hours, onshore winds create surface currents pushing water against the shore which, via the associated pressure gradient force, creates an undercurrent. The resulting overturning circulation rapidly becomes dynamically unstable due to the Kelvin-Helmholtz instability mechanism, internal gravity waves form, and vigorous vertical mixing follows. The vertical extent of the overturning cell depends on the speed of surface currents and density stratification (which is influenced by other processes such as tidal mixing). In smaller enclosed water bodies, wave reflection in conjunction with dynamical instabilities support rapid mixed-layer deepening and overturning of the entire water column. Based on these findings, the author postulates that dynamic instabilities following from onshore wind events are of fundamental importance to biogeochemical cycles and ecological processes in shelf seas and lakes.

An ocean current inversion accuracy analysis based on a Doppler spectrum model

Microwave remote sensing is one of the most useful methods for observing the ocean parameters. The Doppler frequency or interferometric phase of the radar echoes can be used for an ocean surface current speed retrieval, which is widely used in spaceborne and airborne radars. While the effect of the ocean currents and waves is interactional. It is impossible to retrieve the ocean surface current speed from Doppler frequency shift directly. In order to study the relationship between the ocean surface current speed and the Doppler frequency shift, a numerical ocean surface Doppler spectrum model is established and validated with a reference. The input parameters of ocean Doppler spectrum include an ocean wave elevation model, a directional distribution function, and wind speed and direction. The suitable ocean wave elevation spectrum and the directional distribution function are selected by comparing the ocean Doppler spectrum in C band with an empirical geophysical model function (CDOP). What is more, the error sensitivities of ocean surface current speed to the wind speed and direction are analyzed. All these simulations are in Ku band. The simulation results show that the ocean surface current speed error is sensitive to the wind speed and direction errors. With VV polarization, the ocean surface current speed error is about 0.15 m/s when the wind speed error is 2 m/s, and the ocean surface current speed error is smaller than 0.3 m/s when the wind direction error is within 20° in the cross wind direction.

Impact of data assimilation on Eulerian versus Lagrangian estimates of upper ocean transport

AbstractUsing four‐dimensional variational analysis, we produce an estimate of the state of a coastal region in Northern Norway during the late winter and spring in 1984. We use satellite sea surface temperature and in situ observations from a series of intensive field campaigns, and obtain a more realistic distribution of water masses both in the horizontal and the vertical than a pure downscaling approach can achieve. Although the distribution of Eulerian surface current speeds are similar, we find that they are more variable and less dependent on model bathymetry in our reanalysis compared to a hindcast produced using the same modeling system. Lagrangian drift currents on the other hand are significantly changed, with overall higher kinetic energy levels in the reanalysis than in the hindcast, particularly in the superinertial frequency band.

Towards wide-swath high-resolution mapping of total ocean surface current vectors from space: Airborne proof-of-concept and validation

Two-dimensional high-resolution maps of total surface current vectors obtained for the first time with an airborne demonstrator of the innovative Wavemill instrument concept are validated against HF radar data and compared with output from the POLCOMS high-resolution coastal ocean circulation model. Wavemill is a squinted along-track interferometric SAR system optimized for ocean surface current vector retrieval that operates at moderate incidence angles (∼30°) and is compatible with spaceborne implementation. This paper represents the first comprehensive validation of the current retrieval capabilities of squinted along-track SAR interferometry in support of its development as a future European Space Agency Earth Explorer mission.Wavemill airborne data were acquired in October 2011 in Liverpool Bay off the west coast of Great Britain in light southerly wind (5.5m/s) and maximum tidal ebbing flow (0.7m/s) conditions. Contributions to the measured SAR interferometric phase by surface gravity waves, known as the Wind-wave induced Artefact Surface Velocity (WASV), were removed using our best estimate of wind conditions and the (Mouche et al., 2012) empirical correction derived from Envisat ASAR. Validation of the 1.5km resolution Wavemill current vectors against independent current measurements from HF radar gives very encouraging results, with Wavemill biases and precisions typically better than 0.05m/s and 0.1m/s for surface current speed, and better than 10° and 7° for current direction.The sensitivity of the current retrieval to the wind vector used to compute the WASV is estimated. A ± 1m/s error (bias) in wind speed has minimal impact on the quality of the retrieved currents. In contrast, the choice of wind direction is critical: a bias of ± 15° in the direction of the wind vector degrades the accuracy of the airborne current speed against the HF radar by about ± 0.2m/s. This highlights the need for future instruments to provide calibrated SAR Normalised Radar Cross Section data to support retrieval of wind and current vectors simultaneously.Comparisons of POLCOMS surface currents with HF radar data indicate that the model reproduces well the overall temporal evolution of the tidal current (correlation of spatial fields against HF radar over two tidal cycles of 0.9) but that the model features a systematic 1-h delay in the timing of the maximum ebbing flow in eastern parts of the domain near the Mersey Bar Light buoy. At the maximum ebb flow, the model underestimates the current speed (bias of −0.2m/s) with respect to the HF radar and Wavemill data at the time of the flights. Both the HF radar and Wavemill data reflect much greater snapshot spatial variability of the ocean surface current field than is present in the model, resulting in poor correlation of instantaneous spatial fields (< 0.5) between POLCOMS and the HF radar data. The Wavemill data reveal high spatial variability of ocean surface currents at fine scales, which are not visible in the 4km resolution HF radar data. Wavemill detects several strong (1–1.5m/s) localized current jets associated with deeper bathymetry channels in shallow waters (< 10 m) that are too narrow or too close to land to be observed by the HF radar. The study confirms the value of synoptic wide-swath maps of high-resolution ocean surface current vectors for coastal applications and to validate and develop high-resolution ocean circulation models.

RANCANG BANGUN DAN UJI KINERJA DRIFTER BUOY

Penggunaan drifter untuk penelitian di bidang oseanografi telah cukup lama dilakukan, namun perancangan, dan pengembangan dari instrumen ini masih terus dilakukan. Rancangan yang murah, ringan dan stabil merupakan beberapa syarat yang dituju oleh para perancang drifter. Di Indonesia penggunaan drifter sebagai alat penelitian oseanografi masih jarang dilakukan dan drifter yang dilepas oleh ARGOS pun hampir tidak ada yang memasuki perairan Indonesia, sehingga penelitian dan analisis data berbasis data drifter inipun masih jarang dilakukan di Indonesia. Ohlman, (2007) mengembangkan drifter yang dirancang untuk perairan pesisir. Beberapa syarat yang ditemukan yaitu drifter pesisir harus memiliki resolusi spasial beberapa meter dan sampel posisi dilakukan setiap beberapa menit. Pengukuran near-real-time data telemetri diperlukan sehingga drifter dapat dilacak dan dipulihkan. Dalam makalah ini diuraikan hasil desain dan konstruksi sistem drifter, serta hasil ujicoba lapang. Diharapkan dari data pengamatan drifter dapat dikembangkan lebih lanjut drifter yang handal dan berkualitas

Examining Lagrangian surface transport during a coastal upwelling in the Gulf of Finland, Baltic Sea

Abstract We employ in-situ surface drifters and satellite derived sea surface temperature data to examine the impact that an upwelling event may have on mixing and Lagrangian transport of surrounding surface waters. The test area is located near the southern coast of the Gulf of Finland where easterly winds are known to trigger intense coastal upwellings. The analysis is based on the comparison of motions of three drifters that follow the currents in the uppermost layer with a thickness of 2 m with MODIS-based sea surface temperature data and high-quality open sea wind time series. The presence of an upwelling event superseded the classic Ekman-type drift of the surface layer and considerably slowed down the average speed of surface currents in the region affected by the upwelled cold water jet and its filaments. The drifters tended to stay amidst the surrounding surface waters. The properties of mixing were evaluated using the daily rate of temperature change along several transects. The upwelled cooler water largely kept its identity during almost the entire duration of the upwelling event. Intense mixing started at a later stage of the upwelling and continued after the end of the event when the winds that have driven the entire process began to subside.

Estimation of sea surface currents based on ocean colour remote-sensing image analysis

ABSTRACTAccurate quantification of the sea surface current speeds with high spatial resolution and in near real-time is beneficial for many applications of physical oceanography, and can be implemented by processing ocean colour remote-sensing imagery data. The robust optical flow (ROF) approach applied to sub-image processing is described in detail in this article, and compared with a conventional maximum cross-correlation (MCC) block matching algorithm with sub-pixel operation. ROF results obtained from Geostationary Ocean Colour Imager (GOCI) imagery are shown to be in good agreement with sea surface currents derived from Ocean Surface Current Analyses-Real time (OSCAR) data, providing a validation of the ROF method.

Trapping and episodic flushing of suspended sediment from a tidal river

Recent studies suggest that tidal forcing can be as important as gravitational circulation in maintaining an estuarine turbidity maximum (ETM). It is further postulated that a long-term mass balance between the import and export of sediment in an estuary may require episodic large river discharges or ‘freshets’ to flush sediment out of the ETM towards the open sea. In this study, we use a 2-month data set from a mooring in a tidal river that drains into a large drowned-river-valley estuary (Kaipara Harbour, New Zealand) to investigate interactions between tidal-current asymmetry and gravitational circulation. During baseflow river discharge and on spring tides, suspended-sediment transport was directed up-channel (landwards), driven by tidal pumping due to tidal-current asymmetry. During neap tides, the suspended-sediment flux was approximately zero. The data suggest that the bed was not locally erodible and that bed sediments at the site were being supplied by an ETM. The ETM only migrated far enough down channel to be observed at the mooring site during spring tides when the tidal excursion was longer. Suspended sediments were effectively trapped and recycled within the ETM. During and after two freshets, high river discharge displaced saline water from the tidal river, water-column stratification strengthened and the surface and bed current speeds increased. As a result, the advective component of the down-channel directed suspended-sediment flux increased. This provided a transport pathway for sediment out of the otherwise tidally pumped, flood-dominant system. We conclude that largest export of sediments out of the tidal river would potentially occur when a large freshet coincides with an apogean spring tide.

Genetic algorithm for investigating flight MH370 in Indian Ocean using remotely sensed data

This study utilized Genetic algorithm (GA) for automatic detection and simulation trajectory movements of flight MH370 debris. In doing so, the Ocean Surface Topography Mission(OSTM) on the Jason- 2 satellite have been used within 1 and half year covers data to simulate the pattern of Flight MH370 debris movements across the southern Indian Ocean. Further, multi-objectives evolutionary algorithm also used to discriminate uncertainty of flight MH370 imagined and detection. The study shows that the ocean surface current speed is 0.5 m/s. This current patterns have developed a large anticlockwise gyre over a water depth of 8,000 m. The multi-objectives evolutionary algorithm suggested that objects are existed on satellite data are not flight MH370 debris. In addition, multiobjectives evolutionary algorithm suggested that the difficulties to acquire the exact location of flight MH370 due to complicated hydrodynamic movements across the southern Indian Ocean.

The dynamic responses of flow and near‐bed turbidity to typhoons on the continental shelf of the East China Sea: field observations

AbstractTyphoons are commonly occurring disastrous events in the Western Pacific and have become more severe as the result of global climate changes, increasingly affecting the ocean and seafloor throughout the Western Pacific. However, studies on the influences of typhoon processes and dynamic mechanisms on the continental shelf remain rather limited because of insufficient observational data in the area. This paper uses observational records at a water depth of 110 m on the continental shelf of the East China Sea in the periods when the No. 1323 typhoon ‘Fitow’ and No. 1324 typhoon ‘Danas’ passed the East China Sea to study changes in the water temperature, salinity, current and turbidity. Observation results show that typhoon Fitow had limited effects on the measurement site because of the long distance to the centre of the typhoon, but during typhoon Danas, the measured surface current speed reached 2.95 m/s, and the turbidity of the sea water increased rapidly due to the vertical mixing of sea water. Typhoon Danas causes both near‐inertial oscillations (NIOs) and diurnal currents to increase, which is considered to be the major factor that contributes to bottom water temperature changes and the suspension of bottom sediments after the passage of the typhoon. Typhoons can also cause the Kuroshio Current to shift towards the continental shelf, which may affect the Tsushima Current. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Deep water drilling riser mechanical behavior analysis considering actual riser string configuration

A dynamic analysis model considering the actual riser string configuration is established to analyze the mechanical behavior of a drilling riser. The riser in this model is regarded as a simply supported beam located in the vertical plane and is subjected to axial and lateral ocean environment loadings. The model is solved using the finite element method. The validity of the analysis model has been proved by a similar experiment conducted in a deep water basin and ANSYS. Riser lateral displacement, bending moment, and deflection of the deep water well considering actual riser string configuration in South China Sea are simulated. The effects of top tension, drilling platform drift distance, surface current speed, wave height, wave period, wave length, wind speed and damping coefficient on riser lateral displacement, bending moment, and deflection are also discussed. Results indicate that riser string configuration has a significant effect on riser mechanical behavior, particularly in the distribution of bending moment. Riser string configuration designing according to concrete ocean environment can significantly improve the riser’s mechanical behavior. The distribution of riser lateral displacement, bending moment, and deflection increases with the increase in surface current speed and drilling platform drift distance, whereas decreases with the increasing top tension and damping coefficient. The distribution of riser lateral displacement, bending moment, and deflection significantly increases with the increase in surface current speed and large current speed may lead to large lateral displacement, bending moment, and deflection. Increasing top tension can significantly decrease riser mechanical behavior parameters and avoid large lateral displacement, bending moment, and deflection when suffering extreme ocean environment conditions in deepwater drilling field. Drilling platform should avoid a large drift distance, which may result in the deflection over the flex joint working range and the enlargement of the riser bend moment.

VARIATION IN THE SURFACE CURRENTS IN THE PANAMA BIGHT DURING EL NIÑO AND LA NIÑA EVENTS FROM 1993 TO 2007

Climatic anomalies have changed the ocean circulation pattern and thus the demographic connectivity. However, in many geographical regions there is insufficient evidence of this change. Therefore, comparisons were made between neutral years and years of El Niño and La Niña with moderate intensity, for the North Equatorial Counter Current (NECC), the South Equatorial Current (SEC), the Coastal Current (CoaC) and the main anticyclonic eddy in the Panama Bight. Daily dynamics topography data of the Maps of Absolute Dynamic Topography (MADT) provided by AVISO and daily wind stress data provide by the European Centre for Medium Range Weather (ECMWF) were used to calculate the speed of surface currents (multi-year, quarterly average), during months with the highest number of eggs and larvae released by the species with a pelagic phase (Sept-Nov). It was found that the speed magnitude for the three oceanic currents was statistically different among the compared events, except for the anticyclonic eddy; obtaining higher values of speed for neutral years in relation to years with El Niño or La Niña for the NECC, for the SEC higher values for La Niña years, followed by neutral years and a moderate El Niño years; for the CoaC higher velocity for neutral and La Niña years but the lowest for El Niño years; and a tendency of higher values in La Niña years for the anticyclonic eddy. Additionally, the number of eddies increased in moderate El Niño years. The results suggest that the decreased velocity of the NECC and the potential barriers created by the cyclonic eddies and the anticyclonic eddy near the South American coast could diminish the passive dispersal of larvae and the potential functional connectivity between the Western, Central and Eastern Tropical Pacific. Therefore, there are implications at the evolutionary, biogeographic, and ecologic levels (dispersion rates and population rescue effect). In contrast, during La Niña the SEC could favor teleplanktonic larval transport to the Central Pacific, material which is exported from the South American coast by CoaC, aided by the anticyclonic eddy. In conclusion, anomalous climatic events alter the velocity of oceanic currents in the Panama Bight; consequently these could change the functional potential connectivity from September to November.

Ship Detection With TanDEM-X Data Extending the Polarimetric Notch Filter

Synthetic aperture radar plays a vital role in ship detection due to the possibility of acquiring high-resolution images at nighttime and under cloud cover. This letter is focused on improving ship detection, exploiting the capability of TanDEM-X to collect interferometric data. Currently, along-track interferometry is used to estimate the speed of ocean surface currents or vessels. The detection of ships plays an important role in the retrieval of vessel speed and is mostly executed exploiting only one of the TanDEM-X images (i.e., not taking advantage of the availability of a second interferometric image). The aim of this study is to extend the capabilities of a ship detector previously developed by the authors, namely, geometrical perturbation-polarimetric notch filter (GP-PNF), to include single-pass interferometric information acquired by TanDEM-X. Interestingly, such enhancement makes it possible to employ the GP-PNF with single-polarization data as well. The proposed algorithms and their statistical behavior are tested on five Tandem-X dual-polarimetric HH/VV scenes acquired in the North Sea. The detection results are validated, exploiting the Automatic Identification System location of vessels. All of the new GP-PNF versions show good performance and provide larger vessel–sea contrast compared with single-channel detectors.

Observation and modeling of tide- and wind-induced surface currents in Galway Bay

A high-frequency radar system has been deployed in Galway Bay, a semi-enclosed bay on the west coast of Ireland. The system provides surface currents with fine spatial resolution every hour. Prior to its use for model validation, the accuracy of the radar data was verified through comparison with measurements from acoustic Doppler current profilers (ADCPs) and a good correlation between time series of surface current speeds and directions obtained from radar data and ADCP data. Since Galway Bay is located on the coast of the Atlantic Ocean, it is subject to relatively windy conditions, and surface currents are therefore strongly wind-driven. With a view to assimilating the radar data for forecasting purposes, a three-dimensional numerical model of Galway Bay, the Environmental Fluid Dynamics Code (EFDC), was developed based on a terrain-following vertical (sigma) coordinate system. This study shows that the performance and accuracy of the numerical model, particularly with regard to tide- and wind-induced surface currents, are sensitive to the vertical layer structure. Results of five models with different layer structures are presented and compared with radar measurements. A variable vertical structure with thin layers at the bottom and the surface and thicker layers in the middle of the water column was found to be the optimal layer structure for reproduction of tide- and wind-induced surface currents. This structure ensures that wind shear can properly propagate from the surface layer to the sub-surface layers, thereby ensuring that wind forcing is not overdamped by tidal forcing. The vertical layer structure affects not only the velocities at the surface layer but also the velocities further down in the water column.

Remotely sensed spatial and temporal variability of the Leeuwin Current using MODIS data

The Leeuwin Current is an anomalous poleward-flowing eastern boundary current that transports warm tropical waters southward and imposes significant ecological influences on the coastal marine ecosystems off Western Australia. In this study, we attempt to map the spatial structure of the sea surface temperature (SST) signature of the Leeuwin Current using MODIS satellite data, which is used to investigate the spatial and temporal variability of the Leeuwin Current for the period July 2002 to December 2012. A topographic Position Index (TPI) method is used to map the Leeuwin Current's SST signature from the MODIS SST images. The semi-automatic classification process involves automatically identifying areas occupied by warm SST waters of the Leeuwin Current and manually discarding detached Leeuwin Current features. The TPI mapping results enabled us to quantitatively examine the spatial and temporal variability of the Leeuwin Current's SST signature, including its spatial structure, extent, and cross-shelf migration, as well as its associated surface chlorophyll a characteristics. This study showed that the TPI mapping is able to capture the complex spatial structures of the Leeuwin Current, including its meanders, offshoots and eddies along its path, as reflected in the SST data. Using an eddy-resolving, data-assimilating numerical ocean model, we were able to relate the Leeuwin Current's SST signature with its surface current speed signature, though generally the speed signature is on the offshore side of the SST signature by about 25 to 30km. Temporal wavelet analyses indicated that the variability of the Leeuwin Current's SST structure is dominated by seasonal cycle. During austral winter, the warm SST signature of the current is close to the shelf break, having greater spatial extent and higher chlorophyll a concentrations. In contrast, during austral summer, the SST signature of the current moves offshore, with reduced spatial extent and chlorophyll a concentrations. The Leeuwin Current also has notable inter-annual variation, to a large extent in response to ENSO variability in the Pacific. In El Niño years the current's SST signature tends to have reduced spatial extent, move further inshore, and increase its chlorophyll a concentrations. The opposite occurs during the La Niña years, especially after the unprecedented 2010–11 La Niña/Ningaloo Niño event when there was a significant reduction of the surface chlorophyll a concentration within the Leeuwin Current's SST signature. The mapping method developed in this study will be also useful in detecting climate driven variability of other ocean boundary current systems.

Tidal stream characteristics on the coast of Cape Fuguei in northwestern Taiwan for a potential power generation site

Sites of high tidal current power potential are mostly found at narrow tidal passages or channels. Not many coastal sites with unbounded ocean with high tidal current speed have been reported. The coastal sea near the Cape Fuguei in northwestern Taiwan is such a site with a high tidal power potential. The purpose of this study is to investigate the tidal current speed at this location for assessing the tidal current power potential. In-situ and remote sensing instruments were deployed simultaneously to measure the temporal and spatial distributions of tidal currents. An Acoustic Doppler Current Profiler (ADCP) was deployed at a 26m depth site and an X-band marine radar was installed at a coastal building. Two measurements lasting over one month were done. Both instruments showed the maximum current speed at the surface was more than 3.5m/s in the measurement period. ADCP measurements showed that the time mean speed in the upper layer (−1.7 to −6m) was 1.5m/s. The current speed decreased with depth slightly. It was also found from both ADCP and radar measurements that in the surface layer, the probability of duration for current speed exceeding 1m/s was more than 70%. In addition, radar measurement showed that the average surface current speed at the area from 2km offshore out to 4km (radar offshore limit) was more than 1m/s. The size of this area is more than 15km2. Our investigations indicated that this area has a high tidal power potential for current turbine deployments.

Winter storm-induced hydrodynamics and morphological response of a shallow transgressive shoal complex: Northern Gulf of Mexico

Using extended deployments during seasons of low and high discharge from the Atchafalaya River, meteorological, hydrodynamic and bottom boundary layer parameters were monitored from Tiger and Trinity Shoal complex, off Louisiana coast, USA. During winter storms, the surface current speed measured at both shoals exceeded 0.5 m/s and the entire water column followed the prevailing wind direction. The current speed close to the bottom exceeded 0.3 m/s during high energy northerly winds. The mean water level in the shoal complex increased during southerly winds and decreased during northerly winds, such that the difference between wind set-up and set-down exceeded 0.7 m in Tiger Shoal and 0.6 m in Trinity Shoal during high energy frontal passages. The swell height was inversely correlated with mean water level, and increased during pre-frontal phase and decreased during post-frontal phase of winter storms. The sea (short waves) height responded quickly to wind direction and speed; and within a few hours after the wind shifted and blowing from the north, the sea height increased during both deployments. Bimodal wave frequency spectrum was observed during wind veering from southerly to northerly, when both sea and swell intensities were significant. The Tiger Shoal bed sediment texture transformed drastically, from mud to shell and shell hash assemblage, within a period of two weeks during the December 2008 deployment. Backscatter signal intensity from a Pulse Coherent Acoustic Doppler Profiler (PCADP) and its velocity estimates were used to determine the vertical extend and timing of mud resuspension and their eventual flushing out from the shoal environment, when exposed to high energy winter storm passages. The computed time frame for a total transformation of bottom sediment texture (from muddy bottom to shell and shell hash assemblage) was supported by the combined wave and bottom current induced shear stress at shoal bed. The bed samples collected from Tiger Shoal before and after the deployment in spring 2009 consisted of more than 80% shell and shell hash, which again confirmed a stable bottom as predicted from the PCADP data. However, the fine sand and mud dominated bed at Trinity Shoal was highly dynamic and experienced a few cm of ephemeral sediment deposition during the passage of each cold front, as revealed from the analysis of acoustic backscatter data from the PCADP. Suspended sediment concentration estimated from Optical Backscatterance Sensors (OBS) and PCADP were in good agreement during low river discharge events in December 2008; but significantly diverged during the spring 2009 deployment, when a high suspended sediment load was discharged into the shelf from the Atchafalaya River, and subsequently pushed farther offshore into the deployment sites by wind-induced strong currents during the passage of cold fronts.

Effect of Coastal Waves on Hydrodynamics in One-Inlet Coastal Nador Lagoon, Morocco

Nador lagoon is a coastal system connected to the sea through a narrow and shallow inlet; understanding its hydraulic performance is required for its design and operation. This paper investigates the hydrodynamic impacts of the whole lagoon due to tidal waves using a numerical approach. In this study we use a two-dimensional, depth-averaged hydrodynamic model based on so-called shallow water equations solved within triangular mesh by a developed efficient finite volume method. The method was calibrated and validated against observed data and applied to analyze and predict water levels, tidal currents, and wind effects within the lagoon. Two typical idealized scenarios were investigated: tide only and tide with wind forcing. The predicted sea surface elevations and current speeds have been presented during a typical tidal period and show correct physics in different scenarios.

Changes in the quantitative distribution of Caspian Sea polychaetes: Prolific fauna formed by non-indigenous species

Although considered major players in many ecosystems around the world, the ecology of Caspian Sea polychaetes is currently understudied. This study describes the species composition and quantitative distribution of polychaetes in the southern Caspian Sea and relates the distribution to seasonal changes in environmental parameters of water (temperature, salinity, and surface current speed) and sediments (organic matter and grain size composition) at four depths (2, 5, 9 and 14m) on the Noshahr coast in the southern Caspian Sea in 2010. Four species were found: Alitta succinea, Hediste diversicolor, Hypania invalida, and Streblospio gynobranchiata. Among them, the non-indigenous S. gynobranchiata was the dominant species, accounting for 87% of the total abundance. Significant differences in abundance across the four species were observed among depths and seasons. Additionally, significant correlations between species abundances and the tested environmental factors were evident. This study highlights the potential consequences of established non-indigenous species in the southern Caspian Sea.

A comparison of GOCE and drifter-based estimates of the North Atlantic steady-state surface circulation

Over the last decade, due to the Gravity Recovery And Climate Experiment (GRACE) mission and, more recently, the Gravity and steady state Ocean Circulation Explorer (GOCE) mission, our ability to measure the ocean's mean dynamic topography (MDT) from space has improved dramatically. Here we use GOCE to measure surface current speeds in the North Atlantic and compare our results with a range of independent estimates that use drifter data to improve small scales. We find that, with filtering, GOCE can recover 70% of the Gulf Steam strength relative to the best drifter-based estimates. In the subpolar gyre the boundary currents obtained from GOCE are close to the drifter-based estimates. Crucial to this result is careful filtering which is required to remove small-scale errors, or noise, in the computed surface. We show that our heuristic noise metric, used to determine the degree of filtering, compares well with the quadratic sum of mean sea surface and formal geoid errors obtained from the error variance–covariance matrix associated with the GOCE gravity model. At a resolution of 100km the North Atlantic mean GOCE MDT error before filtering is 5cm with almost all of this coming from the GOCE gravity model.