Lagrangian Surface Drifters Research Articles

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.

The Water-Following Performance of Various Lagrangian Surface Drifters Measured in a Dye Release Experiment

Abstract Many different surface drifter designs have been developed recently to track near-surface ocean currents, but the degree to which these drifters slip through the water because of mechanisms associated with the wind is poorly known. In the 2020 Tracer Release Experiment (TReX), 19 drifters of eight different designs, both commercially available and home-built, were simultaneously released with a patch of rhodamine dye. The dye rapidly spread vertically through the mixed layer but also more slowly dispersed horizontally. Although winds were light, drifters moved downwind from the dye patch at speeds of 3–17 cm s−1 (0.6%–4% of wind speed) depending on the design type. Measurements were made of wind and ocean conditions, and these were incorporated into a boundary layer model at the air–sea interface to estimate complete velocity profiles above and below the surface. Then, a steady-state drag model is used with these profiles to successfully predict drifter slip. Drogued drifters (those with a subsurface drag element) can be affected by Eulerian shear in the upper 0.5 m of the water column, as well as the Stokes drift, but undrogued drifters are in addition greatly affected by direct wind drag, and possibly by resonant interactions with waves. The dye, cycling vertically in the mixed layer, is largely unaffected by all of these factors; therefore, even “perfect” surface drifters do not move with a mixed layer tracer. Significance Statement Surface drifters are used by oceanographers to measure ocean surface currents. However, different designs also slip downwind through the water at rates that are poorly known but are typically around a few percent of the wind speed. In 2020 we simultaneously deployed drifters of eight different designs along with rhodamine dye in a field experiment to see how well the different designs track the water. Here we independently and successfully model drifter slippage for the different designs. Slip factors are then estimated for a range of wind and ocean conditions.

Estimates of Near-Inertial Wind Power Input Using Novel In Situ Wind Measurements from Minimet Surface Drifters in the Iceland Basin

Abstract The Minimet is a Lagrangian surface drifter measuring near-surface winds in situ. Ten Minimets were deployed in the Iceland Basin over the course of two field seasons in 2018 and 2019. We compared Minimet wind measurements to coincident ship winds from the R/V Armstrong meteorology package and to hourly ERA5 reanalysis winds and found that the Minimets accurately captured wind variability across a variety of time scales. Comparisons between the ship, Minimets, and ERA5 winds point to significant discrepancies between the in situ wind measurements and ERA5, with the most reasonable explanation being related to spatial offsets of small-scale storm structures in the reanalysis model. After a general assessment of the Minimet performance, we compare estimates of wind power input in the near-inertial band using the Minimet winds and their measured drift to those using ERA5 winds and the Minimet drift. Minimet-derived near-inertial wind power estimates exceed those from Minimet drift combined with ERA5 winds by about 42%. The results highlight the importance of accurately capturing small-scale, high-frequency wind events and suggest that in situ Minimet measurements are beneficial for accurately quantifying near-inertial wind work on the ocean. Significance Statement In this study we introduce a novel, freely drifting wind measurement platform, the Minimet. After an initial validation of Minimet sea surface wind measurements against independent wind measurements from a nearby research vessel, we investigate their utility in context of the near-inertial work done by the wind on the ocean, which is important for the ocean’s energy budget. We find Minimet near-inertial wind work estimates exceed those estimated using winds from a state-of-the-art wind product by 42%. Our results indicate that capturing storm events happening on time scales less than 12 h is crucial for accurately quantifying near-inertial wind work on the ocean, making wind measurements from platforms such as the Minimet invaluable for these analyses.

Vorticity in the Wake of Palau from Lagrangian Surface Drifters

Abstract Wake eddies are important to physical oceanographers because they tend to dominate current variability in the lee of islands. However, their generation and evolution has been difficult to study due to their intermittency. In this study, 2 years of observations from Surface Velocity Program (SVP) drifters are used to calculate relative vorticity (ζ) and diffusivity (κ) in the wake generated by westward flow past the archipelago of Palau. Over 2 years, 19 clusters of five SVP drifters ∼5 km in scale were released from the north end of the archipelago. Out of these, 15 were entrained in the wake. We compare estimates of ζ from both velocity spatial gradients (least squares fitting) and velocity time series (wavelet analysis). Drifters in the wake were entrained in either energetic submesoscale eddies with initial ζ up to 6f, or island-scale recirculation and large-scale lateral shear with ζ ∼ 0.1f. Here f is the local Coriolis frequency. Mean wake vorticity is initially 1.5f but decreases inversely with time (t), while mean cluster scale (L) increases as L ∝ t. Kinetic energy measured by the drifters is comparatively constant. This suggests ζ is predominantly a function of scale, confirmed by binning enstrophy (ζ2) by inverse scale. We find κ ∝ L4/3 and upper and lower bounds for L(t) are given by t3/2 and t1/2, respectively. These trends are predicted by a model of dispersion due to lateral shear. We argue the observed time dependence of cluster scale and vorticity suggest island-scale shear controls eddy growth in the wake of Palau.

Impacts of the Lagrangian Data Assimilation of Surface Drifters on Estimating Ocean Circulation during the Gulf of Mexico Grand Lagrangian Deployment

Abstract Satellite-tracked in situ surface drifters, providing measurements of near-surface ocean quantities, have become increasingly prevalent in the global ocean observation system. However, the position data from these instruments are typically not leveraged in operational ocean data assimilation (DA) systems. In this work, the impact of an augmented-state Lagrangian data assimilation (LaDA) method using the local ensemble Kalman transform filter is investigated within a realistic regional ocean DA system. Direct positioning data of surface drifters released by the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment during the summer 2012 Grand Lagrangian Deployment Experiment are assimilated using a Gulf of Mexico (GoM) configuration of the Modular Ocean Model, version 6, of the Geophysical Fluid Dynamics Laboratory. Multiple cases are tested using both 1/4° eddy-permitting and 1/12° eddy-resolving model resolutions: 1) a free running model simulation, 2) a conventional assimilation of temperature and salinity profile observations, 3) an assimilation of profiles and Lagrangian surface drifter positions, and 4) an assimilation of the profiles and derived Eulerian velocities. LaDA generally produces more accurate estimates of all fields compared to the assimilation of derived Eulerian velocities, with estimates of surface currents notably improving, when transitioning to 1/12° model resolution. In particular, LaDA produces the most accurate estimates of sea surface velocities under tropical cyclone conditions when Hurricane Isaac (2012) impacted the GoM. Further experiments applying a vertical localization while assimilating surface drifter positions improve the estimates of temperature and salinity below the mixed layer depth. Cases including the surface drifter positions in the DA show better Lagrangian predictability than the conventional DA.

Connectivity Analysis Applied to Mesoscale Eddies in the Western Mediterranean Basin

The Western Mediterranean basin (WMED) is characterized by the presence of energetic and dynamic mesoscale cyclonic and anticyclonic eddies. They mainly originate along the Algerian and the Northern currents and have a large influence on the basin circulation. Eddies can last for months, with longer lifetimes associated with the anticyclones, which can move far from their areas of origin. As they partially isolate and transfer water masses, they also have an impact on water properties (physical, chemical and biological), pollutant’s dispersion and transport of eggs, larvae and planktonic organisms. In this study, a connectivity analysis method is applied to the anticyclonic eddies (AEs) identified by an automated hybrid detection and tracking algorithm south of 42°N in the WMED. The same methodology is also applied to the trajectories of Lagrangian surface drifters available in the study area. The purpose is to highlight the connections between different areas of the basin linked to eddy activities in addition to the connectivity due to the mean surface circulation. Drifter data analysis showed that all the WMED sub-basins are strongly interconnected, with the mean surface circulation allowing a shortcut connection among many areas of the basin. The connectivity analysis of the AEs tracks shows that although AEs are ubiquitous in the WMED, their connectivity is limited to well-defined regions, depending on their origin location. Three main regions: the south-western, the south-eastern and the northern parts of the basin are characterized by AEs recirculation, with sporadic export of eddies to the other WMED zones.

Subtidal surface circulation in lower Cook Inlet and Kachemak Bay, Alaska

Nearly 90 Lagrangian surface drifters were deployed and often recovered and re-deployed in lower Cook Inlet and Kachemak Bay from 2003 through 2007 and from 2012 through 2017. Velocity vectors calculated from the hourly position data by central finite-differences were low-passed filtered to create maps of the mean, sub-tidal surface circulation. The circulation maps, while generally consistent with the results of Burbank (1977) and Muench et al. (1978), provide new detail showing anti-cyclonic circulation around Kalgin Island, a single cyclonic circulation cell in outer Kachemak Bay, and two circulation cells, one cyclonic and the other anti-cyclonic, in inner Kachemak Bay.

Impact of the Monthly Variability of the Trent River on the Hydrodynamical Conditions of the Bay of Quinte, Ontario: A Case Study 2016–2019

The spatial and temporal (monthly) variability of river discharge has a significant effect on circulation and transport pathways within shallow embayments whose dynamics are largely controlled by wind and riverine inputs. This study illustrates the effects of the monthly variation in Trent River discharge on simulated particle transport and settling destination in the Bay of Quinte, Lake Ontario for the years 2016–2019. Observations of Lagrangian surface drifter data were used to derive Trent River discharge forcing for a three-dimensional hydrodynamic numerical model of the Bay of Quinte. Peak monthly flushing was up to three times as much as the lowest monthly flushing in any year, with the Trent River responsible for up to 95% of the flushing in low runoff years. Particle transport simulations showed that particles could be trapped along shorelines, which extended residence times, and Trent River releases suggest that researchers should look for delayed peaks in Total Phosphorous (TP) load measurements in observations between Trenton and Belleville as particles move downstream. Particles with constant settling velocities originating from the Trent River did not move downstream past Big Bay, and particles from the Napanee River were the primary source for Longreach.

Evaluation of Radial Ocean Surface Currents Derived From Sentinel‐1 IW Doppler Shift Using Coastal Radar and Lagrangian Surface Drifter Observations

AbstractOcean surface radial velocities (RVLs) derived from the Sentinel‐1 A/B Interferomic Wide (IW) mode Doppler frequency shift observations are regularly acquired over the Norwegian coastal zone. These data can be used to complement existing ocean observation systems with high‐resolution (up to 1.5 × 1.5 km) spatial ocean surface current (OSC) maps. In this study, Sentinel‐1 IW Level 2 OSC retrievals were obtained from 2 months (October–November 2017) of raw Doppler shift observations acquired over the Norwegian Coastal Current (NCC). The results were evaluated using coastal high‐frequency radar (HFR) and Lagrangian ocean surface drifter observations. The analysis shows that distinct patterns of the NCC, with range directed currents reaching up to 0.7 m/s, can be detected in the SAR data. The mean bias between Synthetic Aperture Radar (SAR) and HFR observations was −0.08 m/s and the root mean square deviation (RMSD) was 0.25 m/s. In comparison, the agreement between the SAR‐derived OSC and the trajectories from Lagrangian surface drifters showed a mean bias of 0.02 m/s and an RMSD of 0.24 m/s. The accuracy of the SAR OSC retrievals rely on precise wind‐wave bias correction. Hence, the accuracy of the model wind field (speed and direction) is crucial. The sea state contribution must also be taken into account during the bias correction. A typical required accuracy of the OSC velocity is on the order of 0.1 m/s. Therefore, the comparisons demonstrate that the use of SAR for OSC retrieval is promising.

Surface current measurements using lagrangian Drifters in Anmok

In this study, surface currents measured by small lagrangian GPS drifters (Aquadrifter) in Anmok coastal waters were analysed to account for the variability of nearshore surface current and wave-induced current to understand sediment transport mechanism near the crescentic bars in the surf-zone and near Kangneung breakwater and submerged breakwater in Anmok. The 8 times lagrangian drifter experiments were conducted mostly during in 2nd, 3rd, 4th intensive measurements in winter, summer, and spring seasons with long-term wave observation at the station W1. The analysed surface currents near the breakwaters in Anmok show that wave-induced currents at the middle of the submerged breakwater were separated and flowed toward the shoreline but offshore currents were dominant through the channels between the breakwaters. The longshore currents near the shoreline were flowed to the northwest (southeast) depending on the incoming waves from ENE (NNE). The surface nearshore offshore currents were generated mostly by waves and winds in case of high and low wave energy environments. Using the small-size lagrangian surface drifter experiments, we successfully measured longshore and offshore wave-induced currents in the surf-zone and near submerged breakwater close to Kangneung breakwater. The drifter experiment results show the availability of direct observation of nearshore surface currents to understand the mechanism of sediment transport analysing observed wave-induced current and ebb-current in the surf-zone generated by incoming waves and local winds.

Annual and Interannual Variability of the Tropical Instability Vortices in the Equatorial Eastern Pacific Observed from Lagrangian Surface Drifters

Abstract This study documents the spatial distributions and temporal variations of anticyclonic eddies with identified radii ≥100 km in the equatorial eastern tropical Pacific Ocean [viz., tropical instability vortices (TIVs)] using Lagrangian surface drifters. The TIVs identified from Lagrangian surface drifters are distributed in a band along 5°N and are closely associated with latitudinal barotropically unstable shear between the westward South Equatorial Current (SEC) and the eastward North Equatorial Countercurrent (NECC). Fewer TIVs are identified from February to June when the shear between the SEC and NECC is weak, whereas more TIVs are found from July to January when the shear is enhanced. The number of identified TIVs also exhibits substantial interannual variability, with fewer TIVs identified during El Niño events and more TIVs found during La Niña events. This relationship is likely associated with the interannual variations of the zonal circulation in the equatorial Pacific modulated by El Niño–Southern Oscillation (ENSO).

Diagnosing cross‐shelf transport along an ocean front: An observational case study in the Gulf of Lion

AbstractExchanges between coastal regions and the open ocean are often associated with intermittent and localized processes such as eddies, fronts, and filaments. Since these features are difficult to observe, their impact has been predominantly investigated using numerical models and remote sensing. In this study, satellite sea surface temperature maps, Lagrangian surface drifter trajectories, and ship‐based surveys of currents and hydrography from the Latex10 campaign are used to quantify cross‐shelf exchanges associated with a temperature front in the western Gulf of Lion. Satellite imagery and thermosalinograph sections provide the characterization of the various water masses associated with the front. Lagrangian drifter trajectories are used to identify the main transport structures and to quantify the velocity components associated with near‐inertial oscillations. These are removed from the instantaneous ADCP observations with which the cross‐shelf exchanges are then computed. The results indicate an average outflow of 0.074 ± 0.013 Sv and an inflow of 0.021 ± 0.006 Sv. Integrated over the 2 week lifetime of the front, such outflow induced a total export of ∼90 ± 14 km3 of water, indicating that three to four of such events are sufficient to completely renew the surface waters of the Gulf of Lion. The total import was ∼25 ± 7 km3, suggesting larger inflows at depth or in the eastern part of the gulf to maintain its volume balance. These in situ estimates represent a key term of comparison for the further development of numerical model‐based and satellite‐based studies of cross‐shelf exchanges associated with this type of processes.

Northward migration of Cape São Tomé rings, Brazil

In the present work, nine years of altimetry data are analyzed in combination with sea surface temperature and ocean color images in order to track Brazil Current eddies generated at Cape São Tomé, Brazil. Direct evidences are presented that at least six eddies detach from the main current during this period giving rise to isolated rings. Two rings move southward, bordering the current, while four migrate northward, and translate in the direction of the Tubarão Bight. These rings eventually reach the bight, leading to a cyclonic circulation in that region that is analogous, at surface levels, to what would be observed during formation of the Vitória Eddy. For one ring, the observations are complemented by in situ measurements: a Lagrangian surface drifter, an expandable bathythermograph section during the ring formation at Cape São Tomé, and acoustic Doppler current profiler data during the ring formation and inside the bight. The cyclonic circulation within the bight in this occasion reaches levels much deeper than those previously reported for locally-generated eddies. These results indicate that properties of shelf and slope waters near Cape São Tomé can be carried upstream the Brazil Current, eventually influencing the coastal waters further north, with possible ecological and environmental implications.

The impact of high‐frequency current variability on dispersion off the eastern Antarctic Peninsula

We present observations of high‐frequency current variability on the continental shelf and the slope of the Antarctic Peninsula using Lagrangian surface drifters deployed as part of the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project. Here we focus on high‐frequency processes such as tides and inertial oscillations that are typically smoothed out of large‐scale spatially averaged, and/or temporally averaged, observed current fields. We investigate the role that this class of motion plays in the transport of physical or biogeochemical properties. Lateral displacements on the shelf and slope are found to be larger than displacements in deeper waters where tidal currents are negligible. We apply this result in a parameterization of the lateral dispersion during an off‐line drifter modeling study. The outcome is an improvement on the modeling of Lagrangian drifting particles compared with a standard random walk scheme.

Life in the really slow lane: loggerhead sea turtles mature late relative to other reptiles

Summary1. Age at maturity is hard to estimate for species that cannot be directly marked or observed throughout their lives and yet is a key demographic parameter that is needed to assess the conservation status of endangered species.2. For loggerhead turtles (Caretta caretta) in the North Atlantic and North Pacific, juvenile growth rates (c. 10 cm year−1) were calculated by examining size increases during transoceanic journeys; durations of which were estimated from satellite‐tracked Lagrangian surface drifter buoy trajectories.3. Lagrangian‐derived growth estimates were used in a weighted loglinear model of size‐specific growth rates for loggerhead turtles and combined with newly available information on size at maturity to estimate an age at maturity of 45 years (older than past estimates).4. By examining the age at maturity for 79 reptile species, we show that loggerhead turtles, along with other large‐bodied Testudine (turtle and tortoise) species, take longer to reach maturity than other reptile species of comparable sizes. This finding heightens concern over the future sustainability of turtle populations. By maturing at an old age, sea turtles will be less resilient to anthropogenic mortality than previously suspected.

Near-Surface Transport Pathways in the North Atlantic Ocean: Looking for Throughput from the Subtropical to the Subpolar Gyre

Abstract Motivated by discrepancies between Eulerian transport estimates and the behavior of Lagrangian surface drifters, near-surface transport pathways and processes in the North Atlantic are studied using a combination of data, altimetric surface heights, statistical analysis of trajectories, and dynamical systems techniques. Particular attention is paid to the issue of the subtropical-to-subpolar intergyre fluid exchange. The velocity field used in this study is composed of a steady drifter-derived background flow, upon which a time-dependent altimeter-based perturbation is superimposed. This analysis suggests that most of the fluid entering the subpolar gyre from the subtropical gyre within two years comes from a narrow region lying inshore of the Gulf Stream core, whereas fluid on the offshore side of the Gulf Stream is largely prevented from doing so by the Gulf Stream core, which acts as a strong transport barrier, in agreement with past studies. The transport barrier near the Gulf Stream core is robust and persistent from 1992 until 2008. The qualitative behavior is found to be largely independent of the Ekman drift.

Temporal and Spatial Variability of Freshwater Plumes in a Semienclosed Estuarine–Bay System

While the physical forcing mechanisms that govern the outflows of major rivers throughout the world are well documented in the literature, comparably less research has been done to examine the mechanisms that govern the contributions of small rivers and streams to coastal ocean systems. These rivers and streams provide a direct means for the transport of anthropogenic and terrigenous materials from watersheds to coastal oceans. This study describes the temporal and spatial variability of freshwater plumes from Kaneohe Stream, Hawaii, USA, after storm events in the Kaneohe Bay watershed. Freshwater plumes were examined using a combination of fixed moorings, synoptic shipboard surveys, and lagrangian surface drifters. Data sets were collected over the course of 19 months from August 2005 to March 2007 with particular attention paid to storms during the boreal winters. Stream discharge and duration were found to exert a primary control on plume persistence in the southern Kaneohe Bay system. Time series data show a strong coherence between wind forcing and surface currents, which, in combination with data derived from shipboard and aerial surveys, indicate that the spatial variability of freshwater plumes is primarily determined by atmospheric forcing.

Anomalous Southward Advection During 2002 in the Northern California Current: Evidence from Lagrangian Surface Drifters

Equatorward velocities in the upwelling jet of the northern California Current were 0.05–0.06 m s−1 faster in spring and summer 2002 than on average over 1998–2002. This result is based on a five‐year data set of surface drifters released across the continental margin off central Oregon (44.65°N) during April and July of each year. At this speed, anomalous water displacements of over a degree of latitude can occur in 20–25 days. Given a source of cold, Subarctic water to the north, this anomalous southward displacement is a plausible explanation for the cold, nutrient‐rich halocline water observed off Oregon during the summer of 2002. This interannual variability in the northern California Current and its implications for the ecosystem response, i.e., increased primary productivity, may be contrasted with interannual variability of the opposite sign ‐ increased poleward velocity, warmer temperatures and decreased productivity ‐ observed in this same region during El Niño years.

Mapping mesoscale variability of the Azores Current using TOPEX/POSEIDON and ERS 1 altimetry, together with hydrographic and Lagrangian measurements

The SEMAPHORE mesoscale air/sea experiment was conducted in the Azores‐Madeira region from July to November 1993. TOPEX/POSEIDON (T/P) and ERS 1 were flying simultaneously at that time. The main purposes of this paper are to evaluate the estimation of the oceanic mesoscale circulation from the two different sets of altimetric data (T/P and ERS 1) and to compare the results with in situ measurements provided by the SEMAPHORE hydrographic surveys and surface drifters (three expendable bathytermograph conductivity‐temperature‐depth surveys in a 500‐km2 box and a set of 47 Lagrangian surface drifters drogued at 150 m). Comparisons are carried out through the maps obtained by objective analysis from the four data sets. The mapping accuracy of T/P, ERS 1, T/P and ERS 1 combined, and in situ data is investigated, as well as the sensitivity of the mapping to the correlation functions used. There is a good qualitative agreement between altimetric maps and corresponding drifter and hydrographic maps for the three hydrographic surveys. Correlations are about 0.8, and the regression fit is about 0.6–0.7; the lower values are due to the smooth climatology used to reference the altimetric maps. The correlation for time differences is better, with regression lines not significantly different from 1, especially when ERS 1 and T/P are combined. T/P mapping is almost as good as ERS 1 mapping, which was rather unexpected since the ERS 1 space‐time sampling is better suited for the mesoscale. This may reflect the fact that the signal mapped by the hydrography and drifters does not contain the high frequency/wavenumber components. T/P and ERS 1 combined provide better results, although the improvement is not as large as expected, probably for the same reason.

Structure of Velocity and Temperature in the Northeast Pacific as Measured with Lagrangian Drifters in Fall 1987

In October 1987, 49 Lagrangian surface drifters (TRISTAR-II) were released in a 200-km × 200-km square area southeast of Ocean Station Papa as part of the OCEAN STORMS Experiment. The drifters measured temperature at the drogue level and reported their position through ARGOS approximately 11 times per day. Thirty-one of the drifters retained drogues for longer than three months, and data from those instruments are used to describe the evolving fall 1987 pattern of current and temperature structures at 15 m in the area between 46° and 49°N, 142°W and 132°W. Time variable currents were dominated by mesoscale eddies of anticyclonic rotation with horizontal radii of 53–86 km and rotational speeds of 10–20 cm s−1. These eddies persisted for at least 90 days as evidenced by successive drifter trajectories through the eddies. Currents with periods longer than 1 day had a mean to the east of 4.4 cm s−1 and a mean to the north of 0.7 cm s−1. Background eddy kinetic energy levels were 40 cm s−2. Thus, eddy kinetic energy was four times larger than mean kinetic energy. The eastward single particle diffusivity was 1100 m2 s−1 and northward diffusivity was 1600 m2 s−1. The local change of thermal energy at 15-m depth was −2.9 W m−3, while on average, flow advected cold water to the east at a rate of 0.8 W m−3. Therefore, large-scale advective processes accounted for 28% of the thermal energy balance at 15 m. This horizontal heat convergence in the open ocean is comparable in magnitude to that produced by powerful equatorial currents in the eastern Pacific cold tongue.