Surface Drifter Data Research Articles

Variation of the Kuroshio intrusion pathways northeast of Taiwan using the Lagrangian method

The seasonal variations of the Kuroshio intrusion pathways northeast of Taiwan were investigated using observational data from satellite-tracked sea surface drifters and a numerical particle-tracking experiment based on a high-resolution numerical ocean model. The results of sea surface drifter data observed from 1989 to 2013 indicate that the Kuroshio surface intrusion follows two distinct pathways: one is a northwestward intrusion along the northern coast of Taiwan Island, and the other is a direct intrusion near the turn of the shelf break. The former occurs primarily in the winter, while the latter exists year round. A particle-tracking experiment in the high-resolution numerical model reproduces the two observed intrusion paths by the sea surface drifters. The three-dimensional structure of the Kuroshio intrusion is revealed by the model results. The pathways, features and possible dynamic mechanisms of the subsurface intrusion are also discussed.

Mesoscale surface circulation and variability of Southern Indian Ocean derived by combining satellite altimetry and drifter observations

High resoultion Eulerian mean velocity field has been derived by combining the satellite tracked surface drifter data with satellite altimetry and ocean surface winds. The drifter data used in this study includes Argos and surface drifter data from Global Drifter Program. Maps of Sea Level Anomaly (MSLA) weekly files with a resolution of (1/3)° in both Latitude and Longitude for the period 1993–2012 have been used. The Ekman current is computed using ocean surface mean wind fields from scatterometers onboard ERS 1/2, Quikscat and ASCAT. The derived mean velocity field exhibits the broad flow of Antarctic Circumpolar Current with speeds up to 0.6 m/s. Anomalous field is quite significant in the western part between 20° and 40°E and in the eastern part between 80°E and 100°E with velocity anomaly up to 0.3 m/s. The estimated mean flow pattern well agrees with the dynamic topography derived from in-situ observations. Also, the derived velocity field is consistent with the in-situ ADCP current measurements. Eddy kinetic energy illustrates an increasing trend during 1993–2008 and is in phase coherence with the Southern Annular Mode by three month lag. Periodic modulations are found in the eddy kinetic energy due the low frequency Antarctic Circumpolar Wave propagation.

Northern South China Sea Surface Circulation and its Variability Derived by Combining Satellite Altimetry and Surface Drifter Data

The present study analyses the mean and seasonal mesoscale surface circulation of the Northern South China Sea (NSCS) and determines the influence of El Nino/Southern Oscillation (ENSO). High resolution Eulerian velocity field is derived by combining the available satellite tracked surface drifter data with satellite altimetry during 1993 - 2012. The wind driven current is computed employing the weekly ocean surface mean wind fields derived from the scatterometers on board ERS 1/2, QuikSCAT and ASCAT. The derived mean velocity field exhibits strong boundary currents and broad zonal flow across NSCS. The anomalous field is quite strong in the southern part and the Seasonal circulation clearly depicts the monsoonal forcing. Eddy Kinetic Energy (EKE) distribution and its spatial and temporal structures are determined employing Empirical Orthogonal Function (EOF) analysis. The ENSO influence on NSCS surface circulation has been analyzed using monthly absolute geostrophic velocity fields during 1996 - 1999.

Eulerian and Lagrangian Correspondence of High-Frequency Radar and Surface Drifter Data: Effects of Radar Resolution and Flow Components

AbstractThis study investigated the correspondence between the near-surface drifters from a mass drifter deployment near Martha’s Vineyard, Massachusetts, and the surface current observations from a network of three high-resolution, high-frequency radars to understand the effects of the radar temporal and spatial resolution on the resulting Eulerian current velocities and Lagrangian trajectories and their predictability. The radar-based surface currents were found to be unbiased in direction but biased in magnitude with respect to drifter velocities. The radar systematically underestimated velocities by approximately 2 cm s−1 due to the smoothing effects of spatial and temporal averaging. The radar accuracy, quantified by the domain-averaged rms difference between instantaneous radar and drifter velocities, was found to be about 3.8 cm s−1. A Lagrangian comparison between the real and simulated drifters resulted in the separation distances of roughly 1 km over the course of 10 h, or an equivalent separation speed of approximately 2.8 cm s−1. The effects of the temporal and spatial radar resolution were examined by degrading the radar fields to coarser resolutions, revealing the existence of critical scales (1.5–2 km and 3 h) beyond which the ability of the radar to reproduce drifter trajectories decreased more rapidly. Finally, the importance of the different flow components present during the experiment—mean, tidal, locally wind-driven currents, and the residual velocities—was analyzed, finding that, during the study period, a combination of tidal, locally wind-driven, and mean currents were insufficient to reliably reproduce, with minimal degradation, the trajectories of real drifters. Instead, a minimum combination of the tidal and residual currents was required.

A study of transport and impact strength of Fukushima nuclear pollutants in the north pacific surface

Based on the statistics of surface drifter data of 1979–2011 and the simulation of nuclear pollutant particulate movements simulated using high quality ocean reanalysis surface current dataset, the transport pathways and impact strength of Fukushima nuclear pollutants in the North Pacific have been estimated. The particulates are used to increase the sampling size and enhance the representativeness of statistical results. The trajectories of the drifters and particulates are first examined to identify typical drifting pathways. The results show that there are three types of transport paths for nuclear pollutants at the surface: 1) most pollutant particles move eastward and are carried by the Kuroshio and Kuroshio-extension currents and reach the east side of the North Pacific after about 3.2–3.9 years; 2) some particles travel with the subtropical circulation branch and reach the east coast of China after about 1.6 years according to one drifter trajectory and about 3.6 years according to particulate trajectories; 3) a little of them travel with local, small scale circulations and reach the east coast of China after about 1.3–1.8 years. Based on the particulates, the impact strength of nuclear pollutants at these time scales can be estimated according to the temporal variations of relative concentration combined with the radioactive decay rate. For example, Cesium-137, carried by the strong North Pacific current, mainly accumulates in the eastern North Pacific and its impact strength is 4% of the initial level at the originating Fukushima area after 4 years. Due to local eddies, Cesium-137 in the western North Pacific is 1% of the initial pollutant level after 1.5 years and continuously increases to 3% after 4 years. The vertical movement of radioactive pollutants is not taken into account in the present study, and the estimation accuracy would be improved by considering three-dimensional flows.

Toward Improved Estimation of the Dynamic Topography and Ocean Circulation in the High Latitude and Arctic Ocean: The Importance of GOCE

The Arctic plays a fundamental role in the climate system and shows significant sensitivity to anthropogenic climate forcing and the ongoing climate change. Accelerated changes in the Arctic are already observed, including elevated air and ocean temperatures, declines of the summer sea ice extent and sea ice thickness influencing the albedo and CO2 exchange, melting of the Greenland Ice Sheet and increased thawing of surrounding permafrost regions. In turn, the hydrological cycle in the high latitude and Arctic is expected to undergo changes although to date it is challenging to accurately quantify this. Moreover, changes in the temperature and salinity of surface waters in the Arctic Ocean and Nordic Seas may also influence the flow of dense water through the Denmark Strait, which are found to be a precursor for changes in the Atlantic meridional overturning circulation with a lead time of around 10 years (Hawkins and Sutton in Geophys Res Lett 35:L11603, 2008). Evidently changes in the Arctic and surrounding seas have far reaching influences on regional and global environment and climate variability, thus emphasizing the need for advanced quantitative understanding of the ocean circulation and transport variability in the high latitude and Arctic Ocean. In this respect, this study combines in situ hydrographical data, surface drifter data and direct current meter measurements, with coupled sea ice–ocean models, radar altimeter data and the latest GOCE-based geoid in order to estimate and assess the quality, usefulness and validity of the new GOCE-derived mean dynamic topography for studies of the ocean circulation and transport estimates in the Nordic Seas and Arctic Ocean.

Mean and Seasonal Circulation of the South Indian Ocean Estimated by Combining Satellite Altimetry and Surface Drifter Observations

The mean velocity field of south Indian Ocean has been derived by combining high resolution maps of sea level anomalies and the surface drifter data from the Global Drifter program from 1993 to 2012 with a resolution of 1/3 × 1/3 degrees in latitude and longitude. The estimated mean velocity field exhibits strong western boundary currents, zonal currents and eastern boundary currents. The Agulhas Current shows a velocity of above 1.5 m s-1 at around 35°S. The distribution of energy associated with the fluctuating motion and the mean flow illustrates that mesoscale variability are particularly relevant in the Mozambique Channel, south of Java and around 40°S. Advection of mesoscale features along the western boundary is evident in the distribution of eddy fluxes. The long-term average monthly surface velocity field exhibits large variations in surface currents. The most change is observed in the South Equatorial Current which shows spatial and temporal variations.

Cyclonic activity in the eastern Gulf of Mexico: Characterization from along-track altimetry and in situ drifter trajectories

The shedding sequence of the Loop Current (LC) inside the Gulf of Mexico (GoM) is strongly influenced by cyclonic frontal eddies around its edge. Along-track altimetry data, analyzed based on a wavelet decomposition to provide estimates of individual cyclones’ diameter, amplitude and relative vorticity, and in situ surface drifter data from the Global Drifter Program, are used to investigate the cyclonic activity in the eastern Gulf of Mexico, where the LC extends and retracts. By analyzing this ∼20year long (1992–2011) combined set of observation data records, we were able to complement previous findings, to confirm results from modeling studies and to provide new insights on the LC frontal dynamics.Drifter data indicate, for the first time, that Loop Current Frontal Eddies (LCFEs) are in solid-body rotation close to their core. This property makes relative vorticity the most robust diagnostic from along-track altimetry for characterizing LCFEs in the eastern GoM, based on consistent comparisons with drifter data. Both data sets are complementary for describing the LCFEs’ regional variability.LCFEs observed in the deep southeastern GoM show intense relative vorticity, but they are not frequently observed. The study of an unprecedented, long drifter trajectory suggests that they are not intensified locally. This implies that, among LCFEs coming from the northern GoM, only intense ones reach the deep southeastern GoM.The observation datasets provide, for the first time, quantitative evidences of processes so far only identified with models: LCFEs are intensified when they are advected over the Mississippi Fan in the northern GoM; a small area north of Campeche Bank shows intense LCFE activity.The altimetry and drifter data confirm and complement results from more limited datasets: LCFEs forming the Tortugas Eddies, at the entrance of the Straits of Florida, are the most intensely observed LCFEs in altimetry. Coming from the GoM interior, they can be modified, in size and intensity, just before or during their stay in the Dry Tortugas area, whereas they were long considered to be modified only after they were advected in the Straits of Florida. A single drifter trajectory illustrates in detail the erosion of a LCFE in the southeastern GoM, presumably under the influence of the LC, which leads to the re-arrangement of vorticity and the formation of a distinct LCFE, just upstream the Dry Tortugas area.The study also confirms that LCFEs have diameters of 80–120km, generally larger east and north of the LC. They tend to stay for long periods in the northeastern GoM, and thus may undergo vortex merging with incoming eddies, as previously noted on individual episodes. Altimetry reveals that the largest LCFEs reside in the central eastern GoM, in a location where they are known to block the LC extension; however, our long altimetry dataset shows that these large eddies are not always involved in such LC blocking.Finally, this study led to improvements in data treatment, for both along-track altimetry and drifter trajectories. The wavelet approach used to derive eddy characteristics from along-track altimetry allows robust estimates of the eddy relative vorticity. The drifter dataset has been extended to include data from drifters even after they lost their drogue; the latter data treatment can be applied in areas of moderate winds.

Simultaneous assimilation of surface drifter data, satellite and in situ observations for improved estimates of meso-scale variability in the Kuroshio Extension Region

In order to better estimate meso-scale variabilities in the energetic Kuroshio Extension (KE) region, simultaneous assimilation of drifter-derived velocity data, together with satellite and in situ hydrographic data, is attempted by using a high-resolution 4-dimensional variational data assimilation (4D-VAR) system. Our experimental results, both with and without assimilation of drifter data (Exp. Drf and Exp. Ref, respectively) for the period during Aug–Oct 2005, show that the reproduced fields in Exp. Drf better reflect the observed meso-scale features such as the KE meandering jet and associated eddies. The adjoint sensitivity analysis indicates that our 4D-VAR system has the ability to provide a more realistic timeseries of the meandering jet structures that play a key role in the intergyre exchange between the subtropical and subarctic gyres in the North Pacific. In addition, the observed information from the surface drifters works to improve the subsurface structure. These results illustrate the advantage of our 4D-VAR simultaneous assimilation with the addition of drifter-derived surface velocity information.

A small-scale oceanic eddy off the coast of West Africa studied by multi-sensor satellite and surface drifter data

A small-scale oceanic eddy, which was generated in autumn 2011 at the headland of Cap-Vert off the coast of Senegal, West Africa, and then propagated westward into the open North Atlantic Ocean, is studied by multi-sensor satellite and surface drifter data. The eddy was generated after a sudden increase of the trade winds causing an enhanced southward flow and upwelling at the coast of Senegal. After this wind burst event, an extremely nonlinear cyclonic eddy with a radius of about 10 to 20km evolved downstream of Cap-Vert with Rossby number larger than one. Our analysis suggests that the eddy was generated by flow separation at the headland of Cap-Vert. The eddy was tracked on its way into the open North Atlantic Ocean from satellites over 31days via its sea surface temperature and chlorophyll-a (CHL) signature and by a satellite-tracked surface drifter. The satellite images show that this small-scale eddy transported nutrients from the upwelling region westward into the oligotrophic North Atlantic thus giving rise to enhanced CHL concentration there. Maximum CHL concentration was encountered few days after vortex generation, which is consistent with a delayed plankton growth following nutrient supply into the euphotic zone within the eddy. Furthermore, the eddy was imaged by the synthetic aperture radar (SAR) onboard the Envisat satellite. It is shown that the radar signatures of cold eddies result from damping of short surface waves by biogenic surface films which arise from surface-active material secreted by the biota in the cold eddy as well as by the change of the stability of the air–sea interface.

Use of Altimeter and Wind Data to Detect the Anomalous Loss of SVP-Type Drifter’s Drogue

Abstract The study presented in this paper is motivated by the unexplained time dependency of the Ekman model recently computed by Rio et al. using a dataset of “drogued” drifting buoy velocities distributed by the Surface Drifter Data Assembly Center (SD-DAC). This suspicious behavior is found to be fully explained by the presence of undrogued buoys in the drogued drifters database (Grodsky et al.). Altimeter and wind data are used to extract the direct wind slippage from the total drifting buoy velocities over the period 1993–2010. The obtained wind slippage values are then analyzed to identify probable undrogued data among the drifting buoy velocities dataset. The proportion of probable undrogued data among the drogued buoy velocities distributed by SD-DAC is found to be 48%, with an important spatial and temporal variability. When averaged over the full 1993–2010 period, the wind slippage affecting these undrogued buoy velocities reaches up to 13 cm s−1 in the Antarctic Circumpolar Current. A simple procedure is applied to produce an updated dataset, including a drogue presence flag as well as a wind slippage correction. The drogued drifter data from this new dataset are finally used to estimate a new, stable over time Ekman model for the global ocean by latitudinal band and by month.

Observed and modeled surface eddy heat fluxes in the eastern Nordic Seas

Large‐scale budget calculations and numerical model process studies suggest that lateral eddy heat fluxes have an important cooling effect on the Norwegian Atlantic Current (NwAC) as it flows through the Nordic Seas. But observational estimates of such fluxes have been lacking. Here, wintertime surface eddy heat fluxes in the eastern Nordic Seas are estimated from surface drifter data, satellite data and an eddy‐permitting numerical model. Maps of the eddy heat flux divergence suggest advective cooling along the path of the NwAC. Integrating the flux divergence over temperature classes yields consistent estimates for the three data sets; the waters warmer than about 6°C are cooled while the cooler waters are warmed. Similar integrations over bottom depth classes show that regions shallower than about 2000 m are cooled while deeper regions are warmed. Finally, integrating the flux divergence along the core of the NwAC suggests that the highest eddy‐induced heat loss at the surface is along the steepest part of the continental slope, east of the Lofoten Basin. The model fields indicate that cooling of the current by lateral eddy fluxes is comparable to or larger than the local heat loss to the atmosphere.

Gyration effect of the large-scale turbulence in the upper ocean

A new quantity, gyration vector, measuring the gyration effect in turbulent flow field is suggested. While the formerly introduced spin characterizes the trajectories of fluid particles within the Lagrangian stochastic turbulence models, then the suggested gyration vector is defined as an average Eulerian characteristic of the turbulent flow field. The relation between the gyration vector and the average spin is analyzed. The meridional distributions of the gyration vector and of the average spin estimated from the global ocean surface drifter data in the Pacific Ocean are presented. Although the estimates of values of both quantities differ substantially for certain latitude intervals, their meridional distributions exhibit extensive qualitative similarities. The gyration vector as well as the average spin estimated for the unaltered original data on the velocity of drifters display the anticyclonic orientation of rotation at all latitudes, while for the low-pass filtered velocity data with the cut-off frequency value corresponding to 1.5 inertial periods the both quantities show a cyclonic orientation of rotation in narrow zonal bands around 10–20°N and S. The estimates of meridional distribution of two other quantities related to the gyration vector are presented and some aspects of theoretical context of the gyration vector are discussed.

Warm and saline events embedded in the meridional circulation of the northern North Atlantic

Ocean state estimates from 1958 to 2005 from the Simple Ocean Assimilation System (SODA) system are analyzed to understand circulation between subtropical and subpolar Atlantic and their connection with atmospheric forcing. This analysis shows three periods (1960s, around 1980, and 2000s) with enhanced warm, saline waters reaching high latitudes, alternating with freshwater events originating at high latitudes. It complements surface drifter and altimetry data showing the subtropical -subpolar exchange leading to a significant temperature and salinity increase in the northeast Atlantic after 2001. The warm water limb of the Atlantic meridional overturning cell represented by SODA expanded in density/salinity space during these warm events. Tracer simulations using SODA velocities also show decadal variation of the Gulf Stream waters reaching the subpolar gyre and Nordic seas. The negative phase of the North Atlantic Oscillation index, usually invoked in such variability, fails to predict the warming and salinization in the early 2000s, with salinities not seen since the 1960s. Wind stress curl variability provided a linkage to this subtropical/subpolar gyre exchange as illustrated using an idealized two ]layer circulation model. The ocean response to the modulation of the climatological wind stress curl pattern was found to be such that the northward penetration of subtropical tracers is enhanced when amplitude of the wind stress curl is weaker than normal. In this case both the subtropical and subpolar gyres weaken and the subpolar density surfaces relax; hence, the polar front moves westward, opening an enhanced northward access of the subtropical waters in the eastern boundary current.

Surface circulation in the Nordic Seas from clustered drifters

We compare two methods for estimating mean velocities and diffusivities from surface drifter observations, using data from the Nordic Seas. The first is the conventional method of grouping data into geographical bins. The second relies on a “clustering” algorithm, and groups velocity observations according to nearest-neighbor distance. Capturing the spatial variability of the mean velocity requires using bins with a length scale of ∼ 50 km . However, because many bins have few observations, the statistical significance varies substantially between bins. Clustering yields sets with approximately the same number of observations, so the significance is more uniform. At the densely sampled Svinøy section, clusters can be used to construct the mean flow field with ≤ 10 km resolution. Clustering also excels at the estimation of eddy diffusivities, allowing resolution at the 20 km scale in the densely sampled regions. Taking bathymetry into account in the clustering process further improves mean estimates where the data is sparse. Clustering the available surface drifter data, extended by recent deployments from the POLEWARD project, reveals new features in the surface circulation. These are a large anticyclonic vortex in the center of the Lofoten Basin and two anticyclonic recirculations at the Svinøy section. Clustering also yields maps of the eddy diffusivities at unprecedented resolution. Diffusivities are suppressed at the core of the Norwegian Atlantic Current, while they are elevated in the Lofoten Basin and along the Polar Front.

Mapping the Agulhas Current from space: An assessment of ASAR surface current velocities

Over 2 years of surface current information collected in the Agulhas Current region and derived from the Doppler centroid anomalies of Envisat's advanced synthetic aperture radar (ASAR) are examined. The sources of errors and potential use of ASAR surface current velocities for oceanographic research are assessed. ASAR surface current velocities are compared to surface drifter data and merged altimetry observations. Maps of sea surface temperature are used to establish the ASAR's capacity to capture the synoptic circulation. Discrepancies between observed and predicted ASAR velocities result predominantly from inadequate wind corrections combined with radar incidence angles below 30°. Occasionally observed wind‐induced outliers cause a bias in the estimated ASAR velocities but do not affect the ability of the ASAR to systematically image regions of strong surface current flow and shear. Time‐averaged maps of ASAR‐derived surface current velocity seem able to accurately capture the position as well as the intensity of the Agulhas Current. The ability of the ASAR to pick up the smaller features of the circulation along the shelf break also shows that variability along the Agulhas Bank is of the same order of magnitude as that observed in the Agulhas retroflection. ASAR surface current velocities offer a very good complement to altimetry in regions where the mean dynamic topography is poorly resolved. The quasi‐synoptic nature of ASAR acquisitions combined with the relatively high resolution of ASAR surface current velocities also make it attractive for studies of submesoscale processes and western boundary current dynamics.

Malvinas-slope water intrusions on the northern Patagonia continental shelf

Abstract. The Patagonia continental shelf located off southeastern South America is bounded offshore by the Malvinas Current, which extends northward from northern Drake Passage (~55° S) to nearly 38° S. The transition between relatively warm-fresh shelf waters and Subantarctic Waters from the western boundary current is characterized by a thermohaline front extending nearly 2500 km. We use satellite derived sea surface temperature, and chlorophyll-a data combined with hydrographic and surface drifter data to document the intrusions of slope waters onto the continental shelf near 41° S. These intrusions create vertically coherent localized negative temperature and positive salinity anomalies extending onshore about 150 km from the shelf break. The region is associated with a center of action of the first mode of non-seasonal sea surface temperature variability and also relatively high chlorophyll-a variability, suggesting that the intrusions are important in promoting the local development of phytoplankton. The generation of slope water penetrations at this location may be triggered by the inshore excursion of the 100 m isobath, which appears to steer the Malvinas Current waters over the outer shelf.

Brazil Current surface circulation and energetics observed from drifting buoys

The southwestern Atlantic mean surface circulation, its associated variability and energetics are studied through the analysis of 13 years of surface drifter data binned onto a 0.5° × 0.5° grid. Special attention is given to the following three main regional features of the domain: the Brazil Current, the Malvinas Current, and the Brazil‐Malvinas Confluence. For the western boundary currents, the mean field reveals velocities larger than 45 cm s−1, associated with high levels of variability (standard deviation of more than 15 cm s−1), while the highest values of standard deviation are found in the Brazil‐Malvinas Confluence (>25 cm s−1). Conversely, over most of the domain the mean velocities found (20–30 cm s−1) and associated standard deviations (<15 cm s−1) are generally lower. High values of kinetic energy of the mean flow per unit mass (265–1000 cm2 s−2) are estimated in the western boundary currents and their extensions. Eddy kinetic energy along the main jet of the western boundary currents is generally lower than the kinetic energy of the mean flow, except for some areas along the Brazil Current path. In contrast, the Brazil‐Malvinas Confluence reveals eddy kinetic energy levels comparable or larger (∼2500 cm2 s−2) than the mean kinetic energy. Away from the boundary, most of the kinetic energy of the surface circulation is in the eddy field. Furthermore, the analysis of the kinetic energy conversion term suggests the presence of barotropic instabilities along the Brazil Current. Over most of the Brazil Current core, the kinetic energy conversion term points from the mean to eddy kinetic energy.

Diel vertical migration of Antarctic krill (Euphausia superba) is flexible during advection across the Scotia Sea

We model a summer snapshot of the behavior of Antarctic krill (Euphausia superba) during advection across the Scotia Sea. Individual krill respond to a changing landscape of predation risk and food availability by migrating vertically in the water column and choosing an average distance to their nearest neighbor (swarm density). We determine the optimal behavior of 30, 40 and 50 mm krill using a state-dependent life history model where individuals move along 30-day segments of hypothetical journey tracks in three different regions of the Scotia Sea, with the tracks extracted from a combination of circulation models and surface drifter data. Food availability is based on satellite data for surface Chl a with additional heterotrophic and detritus food components, and mortality is parameterized with respect to distance from shore, daylight and krill swarming-behavior. We predict that proximity to predator colonies has a distinct effect on behavior, particularly on depth choice when food-availability is low. Observations made during an acoustic survey of the region found swarms to be deeper at the Antarctic Peninsula compared with South Georgia, in line with model predictions. Our predictions are also consistent with observations that swarm density changes little on a logarithmic scale across the region. We show that being able to change behavior on short time scales has distinct advantages to krill.

Comparison between VHF radar observations and data from drifter clusters in the Gulf of La Spezia (Mediterranean Sea)

Results from a WERA radar system in very high frequency (VHF) mode in a small coastal area (range of 7 km, resolution of 250 m) are compared with surface drifter data. The measurements are performed in the Gulf of La Spezia, a highly populated area characterized by a complex use of the territory, where monitoring of surface currents can play an important role in correct management. The drifters have been launched in clusters of 3–6 units, aimed at investigating the significant time and space variability of the flow. In particular, two clusters launched at the same location 2 days apart show strikingly different patterns of evolution. The first cluster is swiftly advected with little particle spreading, while the second cluster divides into two branches with significant drifter spreading. Radial velocities from radar and drifter data are compared. The difference is characterized by a RMS ~ 5 cm/s, on the lowest side of the RMS values reported in the literature. In-situ drifter trajectories are then compared with synthetic trajectories computed from VHF radar fields, and a separation rate of ~ 1 km/12 h between individual trajectories is found. Finally, the spreading patterns of the in-situ clusters are qualitatively compared with the synthetic clusters and in all cases the synthetic trajectories are found to correctly reproduce the patterns, including the drastic difference between the two launches from the same location. The in-situ trajectories appear to fall in the envelope of the synthetic ones. The positive results are likely to be due to a combination of factors and primarily to the high frequency and space resolution of the radar fields, as well as to the flow regime characteristic of protected areas. The results confirm that radar data are well suited for the study of coastal flows in limited areas with complex patterns of velocity and transport.