Sea Surface Velocity Research Articles

A numerical study on circulation and volume transport in Singapore coastal waters

The circulation in Singapore coastal waters is driven by the variable tidal forcing of the surrounding seas, complex bathymetry, irregular coastlines, seasonal monsoons and local winds. An unstructured-grid SUNTANS model, with an average resolution of 100–200 m around Singapore, is applied to Singapore coastal waters. The model is forced at the three open boundaries, located to the west, south and east of Singapore, using tidal constituents as extracted by the OSU Tidal Prediction Software (OTPS) and Absolute Dynamic Topography (ADT) as derived from satellite imagery. The model is also forced by hourly observed wind data at 59 stations in the domain. Our calibration results show that the model accurately predicts sea surface elevations and velocities at locations throughout the model domain. Model results are used to delineate circulation patterns in waters around Singapore, and these results show significant seasonal variation. To examine the effects of different forcing terms on volume transport, a new decomposition method is proposed. It is found that: (1) ADT is the predominant factor that drives monthly mean volume transport, especially in Malacca Strait; (2) the combined effects of tides and local winds influence monthly mean volume transport, especially in Java Sea; (3) SLA affects monthly mean volume transport throughout the whole year; (4) volume transport induced by the combined effects of tides and local winds is highly correlated with tidal-induced volume flux; and (5) the residual effects, which can be attributed to the nonlinear interactions between different forcing terms, tend to reduce the total volume transport.

A three-dimensional variational data assimilation system for the South China Sea: preliminary results from observing system simulation experiments

A three-dimensional variational data assimilation (3DVAR) system based on the Regional Ocean Modeling System (ROMS) is established for the South China Sea (SCS). A set of Observing System Simulation Experiments (OSSEs) are performed to evaluate the performance of this data assimilation system and investigate the impacts of different types of observations on representation of three-dimensional large-scale circulations and meso-scale eddies in the SCS. The pseudo-observations that are examined include sea surface temperatures (SSTs), sea surface heights (SSHs), sparse temperature/salinity (T/S) profiles, sea surface velocities (SSVs), and sea surface salinities (SSSs). The results show that SSHs can extend their impacts into the subsurface or even the deep ocean while other surface observations only have impacts within surface mixed layer. SSVs have similar impacts though confined to their spatial coverage, suggesting that SSVs could be a substitute of SSHs nearshore where SSHs are of poor quality. Despite their sparseness, the T/S profiles improve the representation of the temperature and salinity structures below the mixed layer, and a combination of T/S profiles with surface observations leads to a better representation of the meso-scale eddies. Based on the OSSE results, an affordable observing network for the SCS in the near future is proposed.

Short‐term sea ice forecasting: An assessment of ice concentration and ice drift forecasts using the U.S. Navy's Arctic Cap Nowcast/Forecast System

AbstractIn this study the forecast skill of the U.S. Navy operational Arctic sea ice forecast system, the Arctic Cap Nowcast/Forecast System (ACNFS), is presented for the period February 2014 to June 2015. ACNFS is designed to provide short term, 1–7 day forecasts of Arctic sea ice and ocean conditions. Many quantities are forecast by ACNFS; the most commonly used include ice concentration, ice thickness, ice velocity, sea surface temperature, sea surface salinity, and sea surface velocities. Ice concentration forecast skill is compared to a persistent ice state and historical sea ice climatology. Skill scores are focused on areas where ice concentration changes by ±5% or more, and are therefore limited to primarily the marginal ice zone. We demonstrate that ACNFS forecasts are skilful compared to assuming a persistent ice state, especially beyond 24 h. ACNFS is also shown to be particularly skilful compared to a climatologic state for forecasts up to 102 h. Modeled ice drift velocity is compared to observed buoy data from the International Arctic Buoy Programme. A seasonal bias is shown where ACNFS is slower than IABP velocity in the summer months and faster in the winter months. In February 2015, ACNFS began to assimilate a blended ice concentration derived from Advanced Microwave Scanning Radiometer 2 (AMSR2) and the Interactive Multisensor Snow and Ice Mapping System (IMS). Preliminary results show that assimilating AMSR2 blended with IMS improves the short‐term forecast skill and ice edge location compared to the independently derived National Ice Center Ice Edge product.

Impacts of a 4-dimensional variational data assimilation in a coastal ocean model of southern Tyrrhenian Sea

The impact of the assimilation of satellite sea surface height, sea surface temperature and surface velocity fields observed by a set of high-frequency (HF) radars is studied using a three-dimensional ocean circulation model configured for the southern Tyrrhenian Sea. The study period is October–December 2010 covered by a large number of data. The nonlinear model is based on the Regional Ocean Modeling System (ROMS) and the data assimilation component on the four-dimensional variational (4D-Var) algorithm.Assimilation proceeds in a series of 7-day windows, providing an analysis solution in each window.The assimilation of surface velocity combined with other observations has more utility in recovering the density fields based on the theory of geostrophic adjustment and a strong impact both on near-surface horizontal volume fluxes and subsurface flows, constraining surface geostrophic currents in the area not covered by the HF radar data.The adjoint of the 4D-Var gain matrix was used to quantify the impact of individual observations and observation platforms on different aspects of the 4D-Var circulation estimates during both the analysis and subsequent forecast cycles. In this study, we focus on the alongshore transport of the surface and intermediate waters in the eastern zone of southern Tyrrhenian Sea. The majority of the observations available during any given analysis cycle are from HF radar, and on average these data, together with SSH data, exert the largest controlling influence on the analysis increments of coastal transport. Also, observations from satellite platforms in the form of SST have a considerable impact on analyses and forecasts of coastal transport, even though these observations represent a relatively small fraction of the available data at any particular time.During 4D-Var, the observations are used to correct for uncertainties in the model control variables, namely, the initial conditions, surface forcing, and open boundary conditions. It is found that correcting for uncertainties in the initial conditions and only secondarily in the boundary conditions has the largest impact on the analysis increments in alongshore transport.Finally, we note that both the control vector and the observation impact calculations are a useful way for monitoring the performance of the data assimilation system, as well as quantifying the impact of the observations on the circulation estimates.

Variability in water properties and predictability of sea surface temperature along Sanriku coast, Japan

We investigated the main controlling factors and predictability of extreme sea surface temperature changes along the Sanriku coast (the east coast of the northern part of Japan's main island). We analyzed distributions of water properties and flow fields via intensive observations using a conductivity–temperature–depth profiler and a coastal water-temperature monitoring system from January 1998 to December 2012. Satellite altimetry and tide gauge data were also analyzed to investigate more widespread horizontal and temporal variation of the sea surface flow field. Anomalous temperature events (2°C lower and higher than climatological monthly values) were observed in winter 2006 and fall 2010 and 2012 along the Sanriku coast. In winter (fall) 2006 (2010, 2012), we observed both unusually thick and wide cold/fresh (warm/saline) waters, corresponding to the Oyashio (Tsugaru Warm Current) waters. At that time, sea surface velocities of the Oyashio (Tsugaru Warm Current) along the Hokkaido coast (Tsugaru Strait) were also high. We propose new methods for predicting extreme temperature changes a few months in advance, based on current observations.

The salinity signature of the cross-shelf exchanges in the Southwestern Atlantic Ocean: Satellite observations.

Satellite-derived sea surface salinity (SSS) data from Aquarius and SMOS are used to study the shelf-open ocean exchanges in the western South Atlantic near 35°S. Away from the tropics, these exchanges cause the largest SSS variability throughout the South Atlantic. The data reveal a well-defined seasonal pattern of SSS during the analyzed period and of the location of the export of low-salinity shelf waters. In spring and summer, low-salinity waters over the shelf expand offshore and are transferred to the open ocean primarily southeast of the river mouth (from 36°S to 37°30′S). In contrast, in fall and winter, low-salinity waters extend along a coastal plume and the export path to the open ocean distributes along the offshore edge of the plume. The strong seasonal SSS pattern is modulated by the seasonality of the along-shelf component of the wind stress over the shelf. However, the combined analysis of SSS, satellite-derived sea surface elevation and surface velocity data suggest that the precise location of the export of shelf waters depends on offshore circulation patterns, such as the location of the Brazil Malvinas Confluence and mesoscale eddies and meanders of the Brazil Current. The satellite data indicate that in summer, mixtures of low-salinity shelf waters are swiftly driven toward the ocean interior along the axis of the Brazil/Malvinas Confluence. In winter, episodic wind reversals force the low-salinity coastal plume offshore where they mix with tropical waters within the Brazil Current and create a warmer variety of low-salinity waters in the open ocean.Key PointsSatellite salinity sensors capture low-salinity detrainment events from shelves SW Atlantic low-salinity detrainments cause highest basin-scale variability In summer low-salinity detrainments cause extended low-salinity anomalies

Mapping sea surface velocities in the Changjiang coastal zone with advanced synthetic aperture radar

Range Doppler velocities derived from the Envisat advanced synthetic aperture radar (ASAR) wide swath images are analyzed and assessed against the numerically simulated surface current fields derived from the finite volume coastal ocean model (FVCOM) for the Changjiang Estuary. Comparisons with the FVCOM simulations show that the European Space Agency (ESA) Envisat ASAR based Doppler shift anomaly retrievals have the capability to capture quantitative information of the surface currents in the Changjiang Estuary. The uncertainty analysis of the ASAR range Doppler velocity estimates are discussed with regard to the azimuthal and range bias corrections, radar incidence angles, inaccuracy in the wind field corrections and the presence of rain cells. The corrected range Doppler velocities for the Changjiang Estuary area are highly valuable as they exhibit quantitative expressions related to the multiscale upper layer dynamics and surface current variability around the East China Sea, including the Changjiang Estuary.

Long‐term trends in the East Australian Current separation latitude and eddy driven transport

AbstractAn observed warming of the Tasman Sea in recent decades has been linked to a poleward shift of the maximum wind stress curl, and a strengthening of the poleward flow along the coast of southeastern Australia. However, changes in the East Australian Current (EAC) separation latitude, as well as in the contribution of the EAC, the EAC extension and its eddy field to the total southward transport due to such a strengthening remain unknown. This study uses 30 years (1980–2010) of the Ocean Forecast for the Earth Simulator (OFES) sea surface height and velocity outputs to obtain a three decade long‐time series of (i) the EAC separation latitude, (ii) the southward transport along the coast of southeastern Australia (28°S–39°S), and (iii) the southward transport across the EAC separation latitude. A Lagrangian approach is implemented and the spin parameter Ω is used to provide a quantitative distinction between the transports occurring outside and inside (cyclonic and anticyclonic) eddies. Significant positive trends of the low pass southward transports indicate that the intensification of the poleward flow has occurred both within the EAC and in the EAC extension. In addition, a significant increase in southward transport inside and outside eddies is found. Importantly, the contribution of eddy driven transport has a large temporal variability and shows a sharp increase from 2005 onward. Finally our results show that the EAC has not penetrated further south but it has separated more frequently at the southernmost latitudes within the region where it typically turns eastward.

Field measurements and scaling of ocean surface wave‐breaking statistics

Deep‐water breaking waves provide a mechanism for mass, momentum, and energy transfer between the atmosphere and ocean. Microscale breaking is particularly important, but notoriously difficult to measure in the field. In this paper, measurements from a new technique, using a stereo pair of long‐wave infrared cameras to reconstruct the sea surface shape and velocity field, are presented. Breakers are detected using an image texture‐based algorithm and then tracked on the reconstructed surface. These waves range from large air‐entraining breakers to microbreakers that are undetectable by traditional visible video‐based techniques. This allows measurements of breaker length distributions, Λ(c), that extend to velocities near the gravity‐capillary transition. These distributions are compared with measurements from the literature and from visible video imagery. A nondimensional scaling is proposed which collapses Λ(c). Finally, estimates of energy dissipation and stress based on Λ(c) are found to agree well with wave energy dissipation and wind stress models.

Mapping Ivorian Continental Shelf Surface Current Using Modis Ocean Color Images: A Maximum Cross Correlation Approach

Mapping Ivorian Continental Shelf Surface Current Using Modis Ocean Color Images: A Maximum Cross Correlation Approach Water motion in the Gulf of Guinea is to a certain extent the basis of the pelagic species distribution and on the other hand, is also the pollution vector. The purpose of the study is to thoroughly understand the sea surface current of the Ivorian continental shelf on the local scale. For this study, the Maximum Cross Correlation (MCC) method is applied on chlorophyll concentration data at 1.1 Km resolution, extracted on selected MODIS satellite imageries acquired from the orbiting AQUA and TERRA satellites and are thereafter filtered to produce an image of composite vectors showing sea surface velocities.

Variational assimilation of HF radar surface currents in a coastal ocean model off Oregon

The impact of assimilation of sea surface velocity fields observed by a set of high-frequency (HF) radars is studied using a three-dimensional ocean circulation model configured along the Oregon coast. The study period is June–July 2008 featuring upwelling and separation of the coastal currents into the adjacent interior ocean. The nonlinear model is based on the Regional Ocean Modeling System (ROMS) and the data assimilation (DA) component on the AVRORA system utilizing the representer-based variational algorithm. Assimilation proceeds in a series of 3-day windows, providing an analysis solution in each window and a 3-day forecast into the next window. Experiments with two different initial condition error covariances are compared (one is dynamically balanced, based on the linearized equation of state, temperature-salinity relation, and geostrophic and thermal wind balance relations and the other is multivariate unbalanced). While the assimilation impact is statistically better in the case of the balanced covariance, the case with the unbalanced covariance also provides sensible improvement in terms of surface velocity and sea surface temperature (SST) model-data forecast statistics. The analysis of representer functions shows that even if the initial condition error covariance is unbalanced, the correction fields at the model initial time are partially balanced after each dynamical field is smoothed independently, due to inherent dynamical properties of the adjoint model. Assimilation of the HF radar surface currents improves not only surface velocity forecasts, but also geometry of the upwelling SST front and the sea surface height (SSH) slope near the coast, as verified against unassimilated satellite SSH and SST data. The assimilation also alters the latitudinal distribution of the time-averaged offshore transport. Combined HF radar velocity and other observations, e.g., altimetry, is needed to better constrain surface geostrophic currents in the entire model domain, including the area not covered by the HF radar data.

A high-resolution pre-operational forecast model of circulation on the Texas-Louisiana continental shelf and slope

A new pre-operational model of circulation over the Texas-Louisiana continental shelf and slope, based on the Regional Ocean Modelling System (ROMS) is presented. The model is designed with a number of practical applications in mind: to predict oil spill trajectories, investigate the mechanisms controlling seasonal hypoxia, and to understand the origins of harmful algal blooms on the Texas-Louisiana continental shelf and slope. This model consists of two parts: the hindcast and pre-operational components. The hindcast model is forced with the observed freshwater fluxes from the nine major Texas and Louisiana rivers, surface winds, and heat and salt fluxes from North American Regional Reanalysis (NARR) dataset. The hindcast simulations are carried out over an eight-year period (February 2003–June 2011). The pre-operational model, which provides nowcast and a five-day forecast, starts from July 2011 with a different surface momentum and heat flux forcing from the Global Forecast System (GFS). Both hindcast and forecast models are nested to the Gulf of Mexico HYCOM (Hybrid Coordinate Ocean Model) which provides an estimate of the circulation in the deep Gulf. Model performance is quantified based on the model skills calculated using measurements from the Texas Automated Buoy System (TABS), coastal water level stations and satellite altimetry data. The model is able to reproduce not only the seasonal pattern of sea surface height (SSH), temperature, and velocity fields, but also the strong sea-breezedriven near-inertial surface currents, which have been found dominant in this region during the summer months.

Variability of surface velocity in the Kuroshio Current and adjacent waters derived from Argos drifter buoys and satellite altimeter data

By combining Argos drifter buoys and TOPEX/POSEIDON altimeter data, the time series of sea-surface velocity fields in the Kuroshio Current (KC) and adjacent regions are established. And the variability of the KC from the Luzon Strait to the Tokara Strait is studied based on the velocity fields. The results show that the dominant variability period varies in different segments of the KC: The primary period near the Luzon Strait and to the east of Taiwan Island is the intra-seasonal time scale; the KC on the continental shelf of the ECS is the steadiest segment without obvious periodicity, while the Tokara Strait shows the period of seasonal variability. The diverse periods are caused by the Rossby waves propagating from the interior ocean, with adjustments in topography of island chain and local wind stress.

Eulerian mean sea-surface velocity field of the South Indian Ocean derived by combining satellite altimeter and drifter data

Eulerian mean sea-surface velocity field of the South Indian Ocean derived by combining satellite altimeter and drifter data

Estimation of model errors generated by atmospheric forcings for ocean data assimilation: experiments in a regional model of the Bay of Biscay

The characterization of model errors is an essential step for effective data assimilation into open-ocean and shelf-seas models. In this paper, we propose an experimental protocol to properly estimate the error statistics generated by imperfect atmospheric forcings in a regional model of the Bay of Biscay, nested in a basin-scale North Atlantic configuration. The model used is the Hybrid Coordinate Ocean Model (HYCOM), and the experimental protocol involves Monte Carlo (or ensemble) simulations. The spatial structure of the model error is analyzed using the representer technique, which allows us to anticipate the subsequent impact in data assimilation systems. The results show that the error is essentially anisotropic and inhomogeneous, affecting mainly the model layers close to the surface. Even when the forcings errors are centered around zero, a divergence is observed between the central forecast and the mean forecast of the Monte Carlo simulations as a result of nonlinearities. The 3D structure of the representers characterizes the capacity of different types of measurement (sea level, sea surface temperature, surface velocities, subsurface temperature, and salinity) to control the circulation. Finally, data assimilation experiments demonstrate the superiority of the proposed methodology for the implementation of reduced-order Kalman filters.

Loggerhead turtle by-catch depends on distance to the coast, independent of fishing effort: implications for conservation and fisheries management

Drifting longline is considered one of the most dangerous fishing gears for marine tur- tles and is their principal threat in the western Mediterranean Sea. During the summers of 2000 to 2003, 179 fishing operations were observed onboard a traditional Spanish drifting longline boat in the Balearic Sea (western Mediterranean) to analyse the effects of fishing effort and ecogeographical fac- tors on the by-catch of loggerhead turtles Caretta caretta. We observed 4074 swordfish captured and 675 loggerhead turtles by-caught. The probability of catching at least 1 loggerhead was related to the distance of the fishing-ground to the coast and, to a lesser extent, depth, both independent of fishing effort. The average number of loggerheads captured was spatially structured only according to mean distance to the coast. However, the number of swordfish captured was correlated with fishing effort but was not correlated with distance to the coast or depth. Distance to the coast was correlated with gear retrieval time, sea surface temperature and velocity, and phytoplankton concentration. As the number of loggerheads captured was independent of the number of hooks, the number of fishing operations was a better unit than the number of hooks to assess loggerhead by-catch. Limiting drift- ing longline fishing activity to within 35 international nautical miles from the coast could reduce log- gerhead by-catch substantially without affecting swordfish captures and with little resistance from fishermen.

Distinct 17- and 33-Day Tropical Instability Waves in Subsurface Observations*

Abstract Tropical instability waves (TIWs) within a half-degree of the equator in the Pacific Ocean have been consistently observed in meridional velocity with periods of around 20 days. On the other hand, near 5°N, TIWs have been observed in sea surface height (SSH), thermocline depth, and velocity to have periods near 30 days. Tropical Atmosphere–Ocean (TAO) Project moored equatorial velocity and temperature time series are used to investigate the spatial and temporal structure of TIWs during 3 years of La Niña conditions from 1998 through 2001. Along 140°W, where the TIW temperature and velocity variabilities are at their maxima, these variabilities include two distinct TIWs with periods of 17 and 33 days, rather than one broadbanded process. As predicted by modeling studies, the 17-day TIW variability is shown to occur not only in meridional velocity at the equator, but also in subsurface temperature at 2°N and 2°S, while the 33-day TIW variability is observed primarily in subsurface temperature at 5°N. These two TIWs, respectively, are shown to have characteristics similar to a Yanai wave/surface-trapped instability and an unstable first meridional mode Rossby wave. One implication of such a description is that the velocity variability on the equator is not directly associated with the dominant 33-day variability along 5°N.

Decadal Spinup of the South Pacific Subtropical Gyre

Abstract An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment (WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170°W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40°S, 170°W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and Argo float trajectories at 1000 m confirms the gyre spinup during the 1990s. The signals in sea surface height, dynamic height, and velocity all peaked around 2003 and subsequently began to decline. The 1990s increase in wind-driven circulation resulted from decadal intensification of wind stress curl east of New Zealand—variability associated with an increase in the atmosphere’s Southern Hemisphere annular mode. It is suggested (based on altimetric height) that midlatitude gyres in all of the oceans have been affected by variability in the atmospheric annular modes on decadal time scales.

Modes of mesoscale sea surface height and temperature variability in the East Australian Current

Mesoscale variability where the East Australian Current (EAC) separates from the coast is studied using sea surface temperature and surface velocity streamfunction observed by satellite and a regional numerical model. The mean circulation simulated by the model (the Regional Ocean Modeling System (ROMS)) is compared to a high‐resolution regional climatology, and the realism of the simulated mesoscale variability is tested by comparison to statistical analyses of the satellite data. Both ROMS and data show spectral peaks in the mesoscale energy band at periods between 90 and 180 days. Complex Empirical Orthogonal Function (EOF) analysis identifies two significant modes of mesoscale variability in the data; an Eddy Mode, for which the variability propagates southwestward along the coast, and a Wave Mode, for which phase propagation is predominantly onshore. The regional model open boundary conditions include only annual and semiannual harmonics of variability so remote mesoscale forcing is absent. The Eddy Mode is represented well in the model indicating this aspect of the circulation results from local instabilities of the flow and that its underlying dynamical process is simulated well. While the observed and modeled Wave Modes have some similarities, their differences suggest the model is deficient in representing westward propagation of mesoscale period variability in the region. Whatever the source of this energy, the orthogonality property of the EOF analysis indicates the Wave Mode does not interact significantly with eddy processes in the EAC separation.

Observations of divergence and upwelling around Point Loma, California

Historical records of near‐surface water temperatures in the Southern Californian Bight often show a preferential cooling in the lee of headlands such as Point Dume, Palos Verdes, and Point Loma. At times, this cooler water is associated with an increase in chlorophyll‐a as is evident in satellite images of ocean color from the region. Here we combine hydrographic data from a 1 day cruise aboard the RV Roger Revelle (a precursor to the 0304 California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise) with high‐frequency (HF) radar (CODAR) measurements, satellite images, and long‐term thermistor records of near‐surface temperature to identify a small‐scale, isolated upwelling in the lee of Point Loma (32.5°N). Associated with the more saline water downstream of the headland are higher nutrient concentrations, an increase in chlorophyll‐a concentration, and a bloom of chain‐forming diatoms, indicative of a mature upwelling system. It is suggested that this upwelling is not primarily due to local or remote wind forcing but rather to the divergence of the prevailing southerly flow as it passes the Point Loma headland. Time series of surface vorticity calculated from HF radar measurements of sea surface velocity show that as the flow separates from the headland, relative vorticity increases offshore of the cape. Inshore, the time series of divergence/convergence shows a tendency toward divergence at the surface, indicating a preferential upwelling which appears to raise the thermocline, thus resulting in a flux of cold nutrient‐rich water to the surface. In the presence of high nutrients and light, photosynthetic organisms bloom in these upwelled waters as they are advected away from the headland and offshore by the prevailing surface currents.