Surface Drifter Trajectories Research Articles

Multiple Lagrangian Jet‐Core Structures in the Malvinas Current

AbstractThe Malvinas Current (MC) in the Southwestern Atlantic is characterized as a system of multiple Lagrangian jets. This is done by applying geodesic theory of Lagrangian Coherent Structures (LCSs) on altimetry‐derived velocity. The number and spatial disposition of jet‐cores organizing the MC are found to vary in time influenced by topographic waves propagating along the current. The Lagrangian analysis reveals that permanent jet‐cores tend to approach one another upstream, yet do not merge into a single jet as the Eulerian analysis of the data suggests. Independent support for the existence of multiple jets is provided by trajectories of surface drifters from a deployment of unparalleled dimensions for the region and ocean color images. The drifters develop the characteristic boomerang‐shaped patterns into which passive tracers straddling jet‐cores deform and exhibit Lagrangian metrics that are largely consistent with those derived from altimetry data. Ocean color imagery reveals similar “chevrons” and suggest upwelling along the jets, which is consistent with a new solution of arrested topographic wave dynamics. This observation and the finding that the MC jet system is influenced by propagating waves suggests that the MC variability may have major implications in the modulation of primary productivity on synoptic timescales. Drifters flowing into the open ocean, exhibit organizing patterns that are largely shaped by mesoscale circulation and essentially differ from those observed along the MC. Evidence of this observation is provided on the basis of LCS analysis and the calculation of a quasi‐objective Lagrangian metric derived directly from drifter trajectories.

Lagrangian surface drifter observations in the North Sea: an overview of high-resolution tidal dynamics and surface currents

Abstract. A dataset of 85 Lagrangian surface drifter trajectories covering the central North Sea area and the Skagerrak from 2017–2021 of 17 deployments is presented. The data have been quality-controlled, uniformly structured, and assimilated in a standard NetCDF format (https://doi.org/10.1594/PANGAEA.963166, Meyerjürgens et al., 2023a). Using appropriate methods presented in detail here, surface currents were calculated from the drifter position data. Based on a drifter deployment in the Skagerrak, it is demonstrated that the Lagrangian measurements can be converted into an Eulerian representation by calculating mean current velocities. Tidal energy spectra were analyzed separately for the southern and northern areas of the North Sea, and tidal ellipses were calculated to determine the tidal impact on surface currents. Significant differences between the shallow shelf and the deeper areas of the North Sea are evident. While the shallow nearshore areas are dominated by tidal currents, deeper areas such as the Skagerrak record a high mean residual circulation driven by high-density gradients. Measurements using Eulerian approaches and remote sensing methods are restricted in temporal and spatial coverage, in particular, to capture fine-scale dynamics. For this reason, Lagrangian measurements, to a large extent, provide new insights into the complex submesoscale dynamics of the North Sea. Exemplarily, the Skagerrak region is used to demonstrate that high-resolution drifter observations capture both mesoscale and small-scale current patterns. This unique dataset, covering the entire southeastern North Sea and the Skagerrak, offers further analysis possibilities and can be used for the investigation of various hydrodynamic and environmental issues, e.g., the analysis of submesoscale current dynamics at ocean fronts, the determination of the kinetic eddy energy, and the propagation of pollutants in the North Sea.

Evaluating altimetry-derived surface currents on the south Greenland shelf with surface drifters

Abstract. The pathways and fate of freshwater in the East Greenland Coastal Current (EGCC) are crucial to the climate system. The EGCC transports large amounts of freshwater in close proximity to sites of deep open-ocean convection in the Labrador and Irminger seas. Many studies have attempted to analyze this system from models and various observational platforms, but the modeling results largely disagree with one another, and observations are limited due to the harsh conditions typical of the region. Altimetry-derived surface currents, constructed from remote-sensing observations and applying geostrophic equations, provide a continuous observational data set beginning in 1993. However, these products have historically encountered difficulties in coastal regions, and thus their validity must be checked. In this work, we use a comprehensive methodology to compare these Eulerian data to a Lagrangian data set of 34 surface drifter trajectories and demonstrate that the altimetry-derived surface currents are surprisingly capable of recovering the spatial structure of the flow field on the south Greenland shelf and can mimic the Lagrangian nature of the flow as observed from surface drifters.

Surface dispersion of coastal discharges in North America towards the Great Pacific Garbage Patch

This research addresses the surface dispersion of coastal discharges in North America that end up at the Great Pacific Garbage Patch. The evolution of the discharged concentration is calculated through statistical simulations using transition matrices and dispersion ellipses, both based on historical records of surface drifter trajectories. The discharge points are adjacent to urban areas distributed along the coast. The preferential routes, times of arrival, and relative contributions of each site to the accumulation area are quantified. A new statistical delimitation of the position, area and orientation of the garbage patch is proposed. Additional experiments suggest that the tracer retention during the summer is influenced by the low-level atmospheric anticyclone in the Northeastern Pacific, which enhances the Ekman drift and hence favours debris convergence. This effect is reduced when the anticyclone weakens in winter, thus decreasing the debris retention and promoting its westward dispersal by the trade winds.

Effects of Eulerian current, Stokes drift and wind while simulating surface drifter trajectories in the Baltic Sea

The simulation of Lagrangian drift is an important task in applications such as dispersion of pollutants, larvae and search and rescue activities. In this study, the Eulerian current, Stokes drift and wind effect on the simulation of observed drifters were analysed. The Lagrangian OceanParcels model was used, and the surface trajectories were assessed by comparison with 9 GPS drifter trajectories in the Gulf of Finland, Gulf of Riga and Lithuanian coast. The Normalised Cumulative Lagrangian Separation (NCLS) distance between the simulated and the satellite-tracked drifter trajectories, and the mean absolute error (MAE) were used as comparison metrics. The present study suggests the need to consider the Stokes drift and the wind factor in addition to the modelled Eulerian currents to obtain a better description of the trajectories of particles. By making these considerations, the OceanParcels model could adequately simulate particle trajectories in the sub-basins within the Baltic Sea. The realized model tests showed that motion of surface drifters are strongly controlled by the Stokes drift when the significant wave height is >1 m, whereas the wind component and the Eulerian currents are crucial when the significant wave height is <0.6 m or the wave (Stokes drift) directions do not match the wind direction.

Circulation structure and dynamic characteristics of Western Tropical Indian Ocean associated with monsoon transitions

The circulation structure in the western tropical Indian Ocean (WTIO) was investigated based on surface drifters and Simple Ocean Data Assimilation (SODA), with particular attention to the characteristics of the circulation structure associated with seasonal monsoon transitions. Surface drifter trajectories manifest three distinct seasonally dependent patterns of western boundary currents in the WTIO. The northward Somali Current (SC) flows across the equator to ∼10 °N in the boreal summer (June to August; JJA). The northward SC turns offshore near the equator during monsoon transitions. The southward SC feeds the South Equatorial Countercurrent together with the northward East African Coast Current in the boreal winter (December to February; DJF). While during the monsoon transition (MAM and SON), drifters prefer to turn eastward and feed the strong equatorial currents. Based on satellite observations, the results of a balanced diagnostic model demonstrate that the geostrophic component Uζ and the wind-driven componentUτ predominate the mean state and the seasonal variations of circulation in the WTIO. The vertical motion shows that upwelling primarily occurs along the coasts of Somali and Oman and is induced by offshore Ekman transport. Ekman transport and associated vertical motions constitute meridional overturning circulation. The SODA results confirm the existence of an equatorial roll in the boreal summer and winter. The results elucidate the dynamic relationships between 3-D circulation and monsoon transitions in the WTIO.

Atlas of surface currents in the Mediterranean and Canary–Iberian–Biscay waters

ABSTRACT Sea surface currents probably are the most relevant essential ocean variable affecting diverse societal challenges concerning the marine environmental (as, for example, safe and efficient navigation, marine pollution and ecological connectivity). This work introduces a climatological Atlas (monthly resolution) of currents in the Mediterranean and Canary–Iberian–Biscay basins, based on today's state of the art reanalyses of the ocean circulation. The focus is on surface and subsurface reanalyses (here understood as and m, respectively) provided by the Copernicus Marine Environment Monitoring Service (CMEMS). The climatological values are computed from the median of the empirical probability density functions and the Atlas also includes the variance matrix and a bimodality index to have quantitative information on their variability. For both domains, the subsurface climatological fields are reasonably consistent with circulation schemes proposed in the previous literature but clearly improving the time and space resolution of the emerging patterns. For the Canary–Iberian–Biscay domain, the monthly climatological surface currents capture accurately the characteristic seasonal signal and its transition between a favourable and non-favourable upwelling regime. In the Mediterranean basin, differences between the near-surface and the 15 m velocity fields suggest a non-negligible role of winds over the variability of the uppermost ocean layer, specially in the Eastern Mediterranean basin. This is, up to our knowledge, the first time that such near-surface climatological patterns are computed. It has been found that, in general, the resulting patterns agree with surface drifter trajectories. In several regions, interannual variability foster bimodal and multimodal probability distributions. The Atlas has been conceived with the purpose of providing a first quantitative assessment on the surface circulation, thus being a complementary tool of real-time ocean forecasting systems. The Atlas is distributed following the FAIR principles and is accompanied with a digital version, with enhanced visualization capabilities for both research and assessment.

Characteristics of Surface Currents in a Shallow Lagoon–Inlet–Coastal Ocean System Revealed by Surface Drifter Observations

The circulation in a shallow lagoon–inlet–coastal ocean system is significant to material transports (e.g., debris, pollutants, and larvae). To study surface flows in this system, we deployed 35 surface drifters at various tidal phases and wind conditions during 2017 and 2018 in the Maryland Coastal Bays system (MCBs). Given that winds and tides are two important drivers of estuarine and coastal circulation, their influences on surface drifter trajectories were analyzed. Observations indicate that surface drifters exit (enter) the lagoon mostly during ebb (flood) currents, and clockwise circular movements at a length scale of 1.5 km formed at the outer edge of Ocean City Inlet (OCI). Under weak wind conditions, tides are primarily responsible for drifter movements near OCI, whereas both the long-fetch winds and tides are important near the relatively larger Chincoteague Inlet and backbays. Under strong wind conditions, surface drifter movements generally follow wind directions. In the shallow lagoonal system, relative effects of winds on surface drifters gradually become stronger in the regions further away from the adjacent inlet as tides are weaker. Further investigations indicate that the fastest and slowest surface drifters are near the small OCI and backbays with the weakened tides, respectively. The direct surface drifter observations that cover a wide spatial range and long time series can provide strong support to surface current simulations. Enhanced understanding of coastal physical oceanography in the MCBs can be beneficial to similar systems and coastal ocean communities.

A Study on the Prediction of the Surface Drifter Trajectories in the Korean Strait

In order to improve the accuracy of particle tracking prediction techniques near the Korean Strait, this study compared and analyzed a particle tracking model based on a seawater flow numerical model and a machine learning based on a particle tracking model using field observation data. The data used in the study were the surface drifter buoy movement trajectory data observed in the Korea Strait, prediction data by machine learning (linear regression, decision tree) using the tide and wind data from three observation stations (Gageo Island, Geoje Island, Gyoboncho), and prediciton data by numerical models (ROMS, MOHID). The above three data were compared through three error evaluation methods (Correlation Coefficient (CC), Root Mean Square Errors (RMSE), and Normalized Cumulative Lagrangian Separation (NCLS)). As a final result, the decision tree model had the best prediction accuracy in CC and RMSE, and the MOHID model had the best prediction results in NCLS.

Oceanic Mesoscale Cyclones Cluster Surface Lagrangian Material

AbstractAn asymmetry in the clustering of oceanic surface material has been observed at the submesoscales. Energetic and ephemeral submesoscale cyclonic fronts are associated with convergence zones, hence cluster surface material. Their anticyclonic counterparts do not feature such an effect. Yet, at the mesoscale, literature has been contradictory about such an asymmetry. Here, we combine surface drifter trajectories with an altimetry‐derived mesoscale eddy database in the North Atlantic to show that mesoscale cyclones contain 24% more drifters than anticyclones. A numerical Lagrangian experiment using a mesoscale‐resolving model quantitatively reproduces the observational results. It reveals that particles preferentially cluster in cyclonic regions, both in fronts and eddies. The model further suggests that ageostrophic cyclonic fronts concentrate particles a few days before the eddy formation and detection.

Rapid Freshening of Iceland Scotland Overflow Water Driven by Entrainment of a Major Upper Ocean Salinity Anomaly

AbstractNewly available mooring observations from the Overturning in the Subpolar North Atlantic Program (OSNAP) show an abrupt decline in Iceland Scotland Overflow (ISOW) salinity from 2017 to 2018 summer. Previous declines in ISOW salinity of similar magnitude have largely been attributed to changes in convectively formed deep waters in the Nordic Seas on decadal time scales. We show that this rapid decline in salinity was driven by entrainment of a major upper ocean salinity anomaly in the Iceland Basin. This is shown by tracking the propagation of the upper ocean anomaly into ISOW using a combination of mooring and Argo observations, surface drifter trajectories, and numerical model results. A 2‐year total transit time from the upper ocean into the ISOW layer was found. The results show that entrainment allows for rapid modification of ISOW, and consequently the lower limb of Atlantic Meridional Overturning Circulation, on subdecadal timescales.

Mechanisms of Lateral Spreading in a Near-Field Buoyant River Plume Entering a Fjord

Observations collected from a fast-flowing buoyant river plume entering the head of Doubtful Sound, New Zealand, were analysed to examine the drivers of plume lateral spreading. The near-field plume is characterised by flow speeds of over 2 ms−1, and strong stratification (N2 &amp;gt; 0.1 s−2), resulting in enhanced shear which supports the elevated turbulence dissipation rates (ϵ &amp;gt; 10−3 W kg−1). Estimates of plume lateral spreading rates were derived from the trajectories of Lagrangian GPS surface drifters and from cross-plume hydrographic transects. Lateral spreading rates derived from the latter compared favourably with estimates derived from a control volume technique in a previous study. The lateral spreading of the plume was driven by a baroclinic pressure gradient toward the base of the plume. However, spreading rates were underestimated by the surface drifters. A convergence of near-surface flow from the barotropic pressure gradient concentrated the drifters within the plume core. The combination of enhanced internal turbulence stress and mixing at the base of the surface layer, and the presence of steep fjord sidewalls likely reduced the rate of lateral spreading relative to the theoretical spreading rate. The estimates of plume width from the observations provided evidence of scale-dependent dispersion which followed a 4/3 power law. Two theoretical models of dispersion, turbulence and shear flow dispersion, were examined to assess which was capable of representing the observed spreading. An analytical horizontal shear-flow dispersion model generated estimates of lateral dispersion that were consistent with the observed 4/3 law of dispersion. Therefore, horizontal shear dispersion appeared to be the dominant mechanism of dispersion, thus spreading, in the surface plume layer.

Dispersion of Surface Drifters in the Tropical Atlantic

The Tropical Atlantic Ocean has recently been the source of enormous amounts of floating Sargassum macroalgae that have started to inundate shorelines in the Caribbean, the western coast of Africa and northern Brazil. It is still unclear, however, how the surface currents carry the Sargassum, largely restricted to the upper meter of the ocean, and whether observed surface drifter trajectories and hydrodynamical ocean models can be used to simulate its pathways. Here, we analyze a dataset of two types of surface drifters (38 in total), purposely deployed in the Tropical Atlantic Ocean in July, 2019. Twenty of the surface drifters were undrogued and reached only ∼8 cm into the water, while the other 18 were standard Surface Velocity Program (SVP) drifters that all had a drogue centered around 15 m depth. We show that the undrogued drifters separate more slowly than the drogued SVP drifters, likely because of the suppressed turbulence due to convergence in wind rows, which was stronger right at the surface than at 15 m depth. Undrogued drifters were also more likely to enter the Caribbean Sea. We also show that the novel Surface and Merged Ocean Currents (SMOC) product from the Copernicus Marine Environmental Service (CMEMS) does not clearly simulate one type of drifter better than the other, highlighting the need for further improvements in assimilated hydrodynamic models in the region, for a better understanding and forecasting of Sargassum drift in the Tropical Atlantic.

Pathways and Water Mass Transformation Along and Across the Mohn‐Knipovich Ridge in the Nordic Seas

AbstractAtlantic Water takes various pathways through the Nordic Seas, and its transformation to denser waters forms a crucial connection to the lower limb of the Atlantic Meridional Overturning Circulation. Circulation maps often schematize two distinct pathways of Atlantic Water: one following the Norwegian Atlantic Slope Current along the continental slope of Norway and one following the Norwegian Atlantic Front Current along the Mohn and Knipovich Ridges. In this paper, the connectivity between the northward flow along these ridges is investigated. Analyzing trajectories of surface drifters and ARGO floats, we find that only 8% of the floats that travel near the mid‐ocean ridges take the frontal pathway to the north. Indeed, by tracing numerical particles in a realistic numerical simulation, part of the water mass traveling along the Mohn Ridge follows the 2,500 m isobath eastward and joins the slope current, instead of flowing north along the Knipovich Ridge. Furthermore, north of 74°N, frequent exchange between the slope current and the front current is observed. Therefore, the slope current and front current are less isolated than often schematized. Additionally, the observational data set reveals substantial cross‐ridge exchange; 31% of the floats that travel within 60 km from the mid‐ocean ridges cross it. Results from numerical simulations indicate that the cross‐ridge exchange leads to cooling and freshening of the Atlantic Water along the front. Deployments of floats near the mid‐ocean ridges are needed to investigate the pathway of Atlantic Water and its exchange across the ridge in more detail.

Evaluation of Structured and Unstructured Models for Application in Operational Ocean Forecasting in Nearshore Waters

The oceanography sub-initiative of Canada’s Oceans Protection Plan was tasked to develop high-resolution nearshore ocean models for enhanced marine safety and emergency response, fitting into the multi-scale, multi-level nested operational ocean forecasting systems. For decision making on eventual 24/7 operational support, two ocean models (a structured grid model, NEMO (Nucleus for European Modelling of the Ocean); and an unstructured grid model, FVCOM (Finite Volume Coastal Ocean Model), were evaluated. The evaluation process includes the selection of the study area, the requirements for model setup, and the evaluation metrics. The chosen study area, Saint John Harbour in the Bay of Fundy, features strong tides, significant river runoff and a narrow tidal-river channel. Both models were configured with the same sources of bathymetry and forcing data. FVCOM achieved 50-100 m horizontal resolution in the inner harbour and included wetting/drying. NEMO achieved 100 m resolution in the harbour with a three-level one-way nesting configuration. Statistical metrics showed that one-year simulations with both models achieved comparable accuracies against the observed tidal and non-tidal water levels and currents, temperature and salinity, and the trajectories of surface drifters, but the computational cost of FVCOM was significantly less than that of NEMO.

Coupling of Estuarine Circulations in a Network of Fjords

AbstractThe Kitimat Fjord System in the northern British Columbia coast features a network of deep channels, islands, and shallow sills. In particular, the fresh water leaving Gardner Canal bifurcates at a triple junction: (1) north through a shallow sill (100‐m deep) to Devastation Channel and then out through Douglas Channel and (2) west along Verney Passage then passing over a shallow sill (35 m) to Wright Sound. The partitioning of the fresh water leaving Gardner Canal has substantial implications for interpreting estuarine circulation and water exchange of the system. In this study, the partitioning of the fresh water at the triple junction is quantified by using observations and a circulation model. The model is based on FVCOM with ~200‐m horizontal resolution in the narrow channels and forced by tides, winds, and river discharges. The remotely sensed freshwater plume, surface drifter trajectories, and temperature profiles suggest that the fresh water from Gardner Canal contributes to the estuarine circulation in the Douglas Channel. A passive tracer evolving with the advection and diffusion processes in the circulation model is used as a proxy to quantify the freshwater transport. The results show that the majority of low salinity water leaving Gardner Canal at the triple junction flows northward into the Douglas Channel through the Devastation Channel and the synoptic variability of the freshwater partition is modulated by strong wind events. A synthesis of various circulations in the abovementioned connecting channels around the Hawkesbury Island indicates a net anticyclonic circulation.

The Surface Pathways of the South Atlantic: Revisiting the Cold and Warm Water Routes Using Observational Data

AbstractThe relative contributions of the Atlantic Meridional Overturning Circulation upper limb source waters in the South Atlantic (SA) remain largely unresolved. We contribute to a resolution by using observational data to explore water mass pathways feeding into the Benguela Current, considered a major component of the Atlantic Meridional Overturning Circulation upper limb in the SA. We focus on the pathways from the Pacific Ocean via the Drake Passage (traditionally termed the “cold” water route) and the Indian Ocean via Agulhas Leakage (traditionally termed the “warm” water route). We employ two main observational data sets: (1) surface drifter trajectories from the National Oceanic and Atmospheric Administration's Global Drifter Program and (2) the Ocean Surface Current Analysis Real‐time product. Based on our Lagrangian analysis, the majority of particles originating in the Drake Passage remain in the Antarctic Circumpolar Current, with a small portion reaching the northern limb of the SA subtropical gyre, and a few infiltrating the Agulhas Current region. The majority of particles released in the Agulhas Current follow the Agulhas Return Current and recirculate in its vicinity. Only a small portion leaks into the SA. Observed pathways reveal strong interactions between the two source waters in the Brazil‐Malvinas Confluence, the SA subtropical gyre, the Benguela Current, and the Agulhas Current region. Results from backward trajectories suggest a sizable contribution of waters from the Drake Passage to the Benguela Current but show disagreement between the two datasets in the magnitude of this contribution.

Shallow Cross‐Equatorial Gyres of the Indian Ocean Driven by Seasonally Reversing Monsoon Winds

AbstractInterior, cross‐equatorial transports are unique phenomena of the Indian Ocean, fluxing heat and salt between hemispheres. We use real and simulated surface drifter trajectories to reveal three cross‐equatorial gyres in the northern Indian Ocean that are driven by the seasonally reversing monsoon winds. Simulated trajectories are computed by advecting hundreds of thousands of particles through a monthly drifter climatology. Gyres are depicted using probabilistic pathlines, defined as the most common trajectories that cross the equator within the Somali Current. Northward pathlines in boreal spring and summer split into two gyres: A 1‐year loop contained west of the Maldives, described as a wind‐driven overshoot of the Southern Hemisphere equatorial gyre, and a 2‐year figure‐of‐eight, encompassing the Great Whirl and reaching the eastern boundary via the equatorial Wytrki jet, to return westward within the North Monsoon Current. Both these gyres include cross‐equatorial flow, driven by southward Ekman drift during the southwest monsoon. Southward pathlines through the Somali Current in winter originate in the dissipating Great Whirl and feed eastward into the South Equatorial Counter Current. Eighty‐five percent of simulated trajectories participate in these gyres, with significant leakage into the Bay of Bengal (14%) and Mozambique Channel (8%). A striking result of our analysis is the lack of pathlines connecting the Somali Current and coastal upwelling regions with the Arabian Basin, in contrast to the continuous coastal current that seasonal streamlines depict. Geostrophic pathlines reveal slow connectivity between these regions below the surface. Consideration of upwelling and subduction is reserved for subsequent modeling studies.

Global Observations of Horizontal Mixing from Argo Float and Surface Drifter Trajectories

AbstractMixing by mesoscale eddies in the ocean plays a major role in setting the distribution of oceanic tracers, with important implications for physical and biochemical systems at local to global scales. Roach et al. (2016; https://doi.org/10.1002/2015JC011440) demonstrated that a two‐particle analysis of Argo trajectories produces robust estimates of horizontal mixing in the Southern Ocean. Here we extend this analysis to produce global 1° × 1° maps of eddy diffusivity at the nominal Argo parking depth of 1,000 m. We also applied this methodology to estimate surface eddy diffusivities from Global Drifter Program (GDP) surface drifters. The global mean eddy diffusivity was 543 ± 155 m2/s at 1,000 m and 2637 ± 311 m2/s at the surface, with elevated diffusivities in regions of enhanced eddy kinetic energy, such as western boundary currents and along the Antarctic Circumpolar Current. The eddy kinetic energy at the equator is high at both the surface and depth, but the eddy diffusivity is only enhanced near the surface. At depth the eddy diffusivity is strongly suppressed due to the presence of mean flow. We used our observational estimates to test the validity of an eddy diffusivity parameterization that accounts for mixing suppression in the presence of zonal mean flows. Our results indicated that this parameterization generally agrees with the directly observed eddy diffusivities in the midlatitude and high‐latitude oceans.

A new perspective on origin of the East Sea Intermediate Water: Observations of Argo floats

The East Sea Intermediate Water (ESIW), defined as the salinity minimum in the East Sea (hereafter ES) (Sea of Japan), is examined with respect to its overall characteristics and its low salinity origin using historical Argo float data from 1999 to 2015. Our findings suggest that the ESIW is formed in the western Japan Basin (40–42°N, 130–133°E), especially west of the North Korean front in North Korean waters, where strong negative surface wind stress curl resides in wintertime. The core ESIW near the formation site has temperatures of 3–4 °C and less than 33.98 psu salinity, warmer and fresher than that in the southern part of the ES. In order to trace the origin of the warmer and fresher water at the sea surface in winter, we analyzed the data in three different ways: (1) spatial distribution of surface water properties using monthly climatology from the Argo float data, (2) seasonal variation of heat and salt contents at the formation site, and (3) backtracking of surface drifter trajectories. Based on these analyses, it is likely that the warmer and fresher surface water properties found in the ESIW formation site are attributed to the low-salinity surface water advected from the southern part of the ES in autumn.