Mediterranean Water Eddies Research Articles

Meddies and Their Sea Surface Expressions: Observations and Theory

Abstract The sea surface expressions of Mediterranean Water eddies, known as “meddies,” are observed in satellite data, and their main characteristics are measured. Satellite altimeter observations of surface expressions are detected over the meddies observed in situ using the MEDTRANS meddy dataset (1950–2013). In this study 209 observed meddy cores in the North Atlantic Ocean, selected over the period of the 22 years of sea surface height measurements with satellite altimetry (1993–2013), were analyzed. Results show relatively good agreement between the theoretical estimates of the meddy surface signals as reported by Bashmachnikov and Carton and the measured surface expressions. It was found that, on average, the theoretical results underestimate the measured sea surface elevations of the meddy surface expressions by a factor of 2. Although the variability of the measured expressions is reasonably well described by the combination of meddy core and the ocean background parameters of the theoretical expression, we cannot define a single individual parameter of the meddy core, which chiefly shapes the magnitude of the meddy surface signal. Interestingly, the overall distribution of characteristics of meddy surface expressions in the Atlantic shows that the sea level anomalies formed by meddies intensify westward, growing both in magnitude and radius. This opposes the expected theoretical decrease of meddy surface signals due to a known progressive decay of the meddy cores with distance from their generation region at the Iberian continental slope. This observed tendency is attributed to meddy interaction with the upper-ocean currents and other eddies (in particular in the region of the North Atlantic Current and Azores Current) that are not considered by the theory.

Temperature and Salinity Inverted for a Mediterranean Eddy Captured With Seismic Data, Using a Spatially Iterative Markov Chain Monte Carlo Approach

Ocean submesoscale dynamics are thought to play a key role in both the climate system and ocean productivity, however, subsurface observations at these scales remain rare. Seismic oceanography, an established acoustic imaging method, provides a unique tool for capturing oceanic structure throughout the water column with spatial resolutions of tens of meters. A drawback to the seismic method is that temperature and salinity are not measured directly, limiting the quantitative interpretation of imaged features. The Markov Chain Monte Carlo (MCMC) inversion approach has been used to invert for temperature and salinity from seismic data, with spatially quantified uncertainties. However, the requisite prior model used in previous studies relied upon highly continuous acoustic reflection horizons rarely present in real oceanic environments due to instabilities and turbulence. Here we adapt the MCMC inversion approach with an iteratively updated prior model based on hydrographic data, sidestepping the necessity of continuous reflection horizons. Furthermore, uncertainties introduced by the starting model thermohaline fields as well as those from the MCMC inversion itself are accounted for. The impact on uncertainties of varying the resolution of hydrographic data used to produce the inversion starting model is also investigated. The inversion is applied to a mid-depth Mediterranean water eddy (or meddy) captured with seismic imaging in the Gulf of Cadiz in 2007. The meddy boundary exhibits regions of disrupted seismic reflectivity and rapid horizontal changes of temperature and salinity. Inverted temperature and salinity values typically have uncertainties of 0.16°C and 0.055 psu, respectively, and agree well with direct measurements. Uncertainties of inverted results are found to be highly dependent on the resolution of the hydrographic data used to produce the prior model, particularly in regions where background temperature and salinity vary rapidly, such as at the edge of the meddy. This further advancement of inversion techniques to extract temperature and salinity from seismic data will help expand the use of ocean acoustics for understanding the mesoscale to finescale structure of the interior ocean.

Effect of Subsurface Mediterranean Water Eddies on Sound Propagation Using ROMS Output and the Bellhop Model

Ocean processes can locally modify the upper ocean density structure, leading to an attenuation or a deflection of sound signals. Among these phenomena, eddies cause significant changes in acoustic properties of the ocean; this suggests a possible characterization of eddies via acoustics. Here, we investigate the propagation of sound signals in the Northeastern Atlantic Ocean in the presence of eddies of Mediterranean Water (Meddies). Relying on a high-resolution simulation of the Atlantic Ocean in which Meddies were identified and using the Bellhop acoustic model, we investigated the differences in sound propagation in the presence and absence of Meddies. Meddies create sound channels in which the signals travel with large acoustic energy. The transmission loss decreases to 80 or 90 dB; more signals reach the synthetic receivers. Outside of these channels, the sound signals are deflected from their normal paths. Using receivers at different locations, the acoustic impact of different Meddies, or of the same Meddy at different stages of its life, are characterized in terms of angular distributions of times of arrivals and of energy at reception. Determining the influence of Meddies on acoustic wave characteristics at reception is the first step to inverting the acoustic signals received and retrieving the Meddy hydrological characteristics.

Surface signature of Mediterranean water eddies in a long-term high-resolution simulation

We study the surface signatures of Mediterranean water eddies (Meddies) in the context of a regional, primitive equations model simulation (using the Regional Oceanic Modeling System, ROMS). This model simulation was previously performed to study the mean characteristics and pathways of Meddies during their evolution in the Atlantic Ocean. The advantage of our approach is to take into account different physical mechanisms acting on the evolution of Meddies and their surface signature, having full information on the 3D distribution of all physical variables of interest. The evolution of around 90 long-lived Meddies (whose lifetimes exceeded one year) was investigated. In particular, their surface signature was determined in sea-surface height, temperature and salinity. The Meddy-induced anomalies were studied as a function of the Meddy structure and of the oceanic background. We show that the Meddies can generate positive anomalies in the elevation of the oceanic free-surface and that these anomalies are principally related to the Meddies potential vorticity structure at depth (around 1000m below the sea-surface). On the contrary, the Meddies thermohaline surface signatures proved to be mostly dominated by local surface conditions and little correlated to the Meddy structure at depth. This work essentially points out that satellite altimetry is the most suitable approach to track subsurface vortices from observations of the sea-surface.

Interaction between a surface jet and subsurface vortices in a three-layer quasi-geostrophic model

This study focuses on the interaction between mid depth vortices and surface jets and fronts in a three-layer quasi-geostrophic model. Such vortices may be regarded as an idealisation of meddies, eddies of Mediterranean Water in the Northeastern Atlantic Ocean, interacting with the Azores j t and front. Successively, a single vortex, a vortex doublet and a vortex pair (in the middle layer) are studied. When a single vortex is considered, the jet has a critical effect of its motion, temporarily slowing down its zonal drift and accelerating it meridionally as the vortex crosses the front. On the contrary, if the vortex does not cross the front, it can drift fairly rapidly along it. The merger of a vortex doublet (two like-signed vortices) below a surface jet is possible whatever the relative position of this doublet with respect to the jet axis. Nevertheless, doublets initially located below the front, will undergo stronger shear and merger efficiency will be diminished. The merged vortex will be circled at the surface by a large meander of the jet. Finally, eastward jet-dipole interaction experiments are performed with various orientations of the vortex dipoles. Eastward propagating dipoles below the jet follow it without deformation. Southeastward drifting dipoles finally join the previous evolution. Southward and southwestward directed dipoles cross the surface jet southeastward. The presence of meanders initially on the jet does not prevent its crossing by a single vortex. Characteristics of the surface jet meanders are also described for a possible remote detection of this process.

Properties and pathways of Mediterranean water eddies in the Atlantic

Data from ship vertical casts (NODC data-set), ARGO profiling floats (Coriolis data-set) and RAFOS-type neutral density floats (WOCE data-set) are used to study characteristics of meddies in the Northeast Atlantic. In total 241 Mediterranean water eddies (meddies) and 236 parts of float trajectories within meddies are selected for detailed analysis. The results suggest that the meddy generation rate at the southern and southwestern Iberian Peninsula (Portimao Canyon, cap St. Vincent, Estremadura Promontory, Gorringe Bank) is 3 times that at the northwestern Iberian Peninsula (Porto-Aveiro Canyons, Cape Finisterre and Galicia Bank).Meddies generated south of Estremadura Promontory (the southern meddies), as compared to those generated north of it (the northern meddies), have smaller radii, smaller vertical extension, higher aspect ratio, higher Rossby number and higher stability (stronger potential vorticity anomaly). These latter properties result from the southern meddies higher relative vorticity and stronger buoyancy frequency anomaly.Away from the generation regions, meddy drift concentrates along four main paths: three quasi-zonal paths (Northern, Central, Southern) and a path following the African coast (Coastal). The quasi-zonal paths are aligned to the isolines of the ambient potential vorticity field. Several cross-path exchanges, identified in this work, are aligned to topographic rises. Northward translation of the northern meddies within the North Atlantic Current to the subpolar gyre is detected.Within the first 600km from the coast, meddy merger is proved to be a common event. This explains the observed difference in radii between the newly generated meddies and those away from the Iberian margin.The decay of the southern meddies proceeds mainly via the loss of their skirts and does not affect meddy cores until the latest stages. The decay of the northern meddies goes in parallel with the decay of their cores. In average meddy decay is achieved within 1–2years, although may take over 3years. Collisions with the Mid-Atlantic Ridge and seamounts sensibly decrease meddy lifetimes. Meddy decay also speeds up when meddies meet the Azores Current or the North Atlantic Current. A rapid drop in the number of meddies south of the Azores Current proves that it represents a dynamic barrier for weak meddies.

Vortex stability in a multi-layer quasi-geostrophic model: application to Mediterranean Water eddies

The stability of circular vortices to normal mode perturbations is studied in a multi-layer quasi-geostrophic model. The stratification is fitted on the Gulf of Cadiz where many Mediterranean Water (MW) eddies are generated. Observations of MW eddies are used to determine the parameters of the reference experiment; sensitivity tests are conducted around this basic case. The objective of the study is two-fold: (a) determine the growth rates and nonlinear evolutions of unstable perturbations for different three-dimensional (3D) velocity structures of the vortices, (b) check if the different structure of our idealized vortices, mimicking MW cyclones and anticyclones, can induce different stability properties in a model that conserves parity symmetry, and apply these results to observed MW eddies. The linear stability analysis reveals that, among many 3D distributions of velocity, the observed eddies are close to maximal stability, with instability time scales longer than 100 days (these time scales would be less than 10 days for vertically more sheared eddies). The elliptical deformation is most unstable for realistic eddies (the antisymmetric one dominates for small eddies and the triangular one for large eddies); the antisymmetric mode is stronger for cyclones than for anticyclones. Nonlinear evolutions of eddies with radii of about 30 km, and elliptically perturbed, lead to their re-organization into 3D tripoles; smaller eddies are stable and larger eddies break into 3D dipoles. Horizontally more sheared eddies are more unstable and sustain more asymmetric instabilities. In summary, few differences were found between cyclone and anticyclone stability, except for strong horizontal velocity shears.

Characteristics of surface signatures of Mediterranean water eddies

AbstractIn this work, we obtain new results on the manifestation of meddies (or of other deep eddies) at the sea‐surface, further developing the results by Bashmachnikov and Carton (2012). The quasi‐geostrophic equations are used to describe a near‐axisymmetric vortex in the upper ocean, forced at its lower boundary by the isopycnal elevation of a moving meddy. The solution thus obtained provides a better approximation of the characteristics of meddy surface signals. The results show that in subtropics large meddies with dynamic radius ≥ 30 km are always seen at the sea‐surface with AVISO altimetry, that medium‐size meddies with = 20 km may be seen at the sea‐surface only if they are sufficiently shallow and strong, while small meddies with = 10 km generally cannot be detected with the present accuracy of altimetry data. The intensity of meddy surface signals decreases to the south with the decrease of the ratio. The seasonal variation in intensity of the surface signal for northern meddies (45°N) is on the order of 2–3 cm, but for subtropical meddies (35°N) it can be on the order of 5–10 cm. The radii of meddy surface signals range from 1 to 2 times the radii of the corresponding meddies. For most of the observed subtropical meddies, the upper limit should be used. Numerical experiments show that surface signals of meddies translated with ‐drift are efficiently dispersed by the radiation of Rossby waves. At the same time, for meddies translated by a background current, the surface signal does not show strong dissipation.

Evidence of Mediterranean Water dipole collision in the Gulf of Cadiz

AbstractA collision of Mediterranean Water dipoles in the Gulf of Cadiz is studied here, using data from the MedTop and Semane experiments. First, a Mediterranean Water eddy (meddy) was surveyed hydrologically in November 2000 southwest of Cape Saint Vincent. Then, this meddy drifted northeastward from this position, accompanied by a cyclone (detected only via altimetry), thus forming a first dipole. In February 2001, a dipole of Mediterranean Water was measured hydrologically just after its formation near Portimão Canyon. This second dipole drifted southwestward. The western and eastern meddies had hydrological radii of about 22 and 25 km, respectively, with corresponding temperature and salinity maxima of (13.45°C, 36.78) and (11.40°C, 36.40). Rafos float trajectories and satellite altimetry indicate that these two dipoles collided early April 2001, south of Cape Saint Vincent, near 35°30′N, 10°15′W. More precisely, the eastern meddy wrapped around the western one. This merger resulted in an anticyclone (a meddy) which drifted southeastward, coupled with the eastern cyclone. Hydrological sections across this final third resulting dipole, performed in July 2001 in the southern Gulf of Cadiz, confirm this interaction: the thermohaline characteristics of the final meddy can be tracked back to the original structures. The subsequent evolution of this dipole was analyzed with Rafos float trajectories. A numerical simulation of the interaction between the two earlier dipoles is also presented. We suggest that these dipole collisions at the Mediterranean Water level may represent a mechanism of generation of the larger meddies that finally leave the Gulf of Cadiz.

A census of Meddies in a long-term high-resolution simulation

The output from a high-resolution two-decade long Mediterranean Outflow simulation is analysed here to provide a census of Mediterranean Water eddies (aka Meddies), both anticyclones and cyclones. The formation rate of Meddies that survive for at least 90days is of 12 Meddies yr−1 of which ∼12% are cyclones. The rate of formation reaches 40 Meddies yr−1 (30% cyclones) when considering all the Meddies living over 15days. About 70% of the population is born along the southwestern Iberian slope, but several robust Meddies also originate in points of convergence of the main pathways into the open ocean. The longest-lived Meddies propagate northwestwards, but most of the anticyclones veer southwestwards after a while. As the Meddies drift away from their birthplace, their radius tends to increase gradually from 15 to 30km. The thickness (depth-difference between isopycnals 27.2 and 27.5) of anticyclones born near Cape St. Vincent contracts by approximately 100m, after travelling 1000km from their source; their mean swirl velocities range from about 21cms−1 (at z=1000m) up to 27cms−1 (at z=600m). Mean salinity and temperature anomalies are significantly lower for cyclones, which in general are also more slowly rotating, shallower and thinner than anticyclones. Cyclones are more easily tracked at 600m depth where longer trajectories are recorded. In the vicinity of Portimão Canyon, cyclones outnumber anticyclones while the reverse happens downstream of Cape St. Vincent.

Surface signature of Mediterranean water eddies in the Northeastern Atlantic: effect of the upper ocean stratification

Abstract. Meddies, intra-thermocline eddies of Mediterranean water, can often be detected at the sea surface as positive sea-level anomalies. Here we study the surface signature of several meddies tracked with RAFOS floats and AVISO altimetry. While pushing its way through the water column, a meddy raises isopycnals above. As a consequence of potential vorticity conservation, negative relative vorticity is generated in the upper layer. During the initial period of meddy acceleration after meddy formation or after a stagnation stage, a cyclonic signal is also generated at the sea-surface, but mostly the anticyclonic surface signal follows the meddy. Based on geostrophy and potential vorticity balance, we present theoretical estimates of the intensity of the surface signature. It appears to be proportional to the meddy core radius and to the Coriolis parameter, and inversely proportional to the core depth and buoyancy frequency. This indicates that surface signature of a meddy may be strongly reduced by the upper ocean stratification. Using climatic distribution of the stratification intensity, we claim that the southernmost limit for detection in altimetry of small meddies (with radii on the order of 10–15 km) should lie in the subtropics (35–45° N), while large meddies (with radii of 25–30 km) could be detected as far south as the northern tropics (25–35° N). Those results agree with observations.

Looping RAFOS floats in the California Current System

Looping motions of RAFOS floats deployed off the Central California coast between 1992 and 2010 are described. Most floats were deployed in the California Undercurrent. Floats at 300m were observed to loop 26% of the time, with anticyclonic rotation observed about twice as often as cyclonic rotation. Characteristics of anticyclonic rotation at 300m included median swirl speeds of 14cm/s for diameters of 59km and 14-day periods. Long-lived (>70 days and at least 8 consecutive loops) anticyclonic loopers are identified as California Undercurrent eddies (or “cuddies”). One cuddy was observed to move southwestward 1650km over a period of 520days before observations ended. Kinematically, cuddies are similar to Mediterranean Water eddies (meddies) in the Iberian Basin, except that meddies are somewhat stronger, larger and deeper.Over the slope, most cuddies were formed between either Pt. Sur and Pt. Reyes or Cape Mendocino and Cape Blanco. The region north of Cape Mendocino is estimated to generate six cuddies each year. These eddies form from waters on the inshore side of the California Undercurrent as they move past Cape Mendocino. Topographic drag is the most likely formation mechanism.Two of eight deep floats were observed to loop, one anticyclonically (1545m) for 279 days and the other cyclonically (1167m) for 192 days. Looping for these floats ended near Taney Seamounts.

Interactions of surface and deep anticyclonic eddies in the Bay of Biscay

In 1990, satellite observations revealed that an anticyclonic surface eddy (a SWODDY, for Slope Water Oceanic eDDY) followed a cycloidal trajectory north of the Iberian coast in the Bay of Biscay. To understand the mechanisms underlying such a trajectory, we study the evolution of an idealized surface eddy in a two-layer flat-bottom quasi-geostrophic model. The effect of several processes is studied, notably the presence of deep anticyclonic vorticity. This deep vorticity may result either from the tilting of the swoddy itself, or from the presence of an anticyclonic eddy of different origins, such as a meddy (Mediterranean Water EDDY). We also study the influence of a zonal coast south of the swoddy, via the “mirror effect”.Firstly, a point-vortex model is used on the f-plane. When the surface and deep vortices lie much farther away from the coast than from each other, their motion is close to the addition of a mutually induced rotation and of a quasi-uniform zonal drift induced by the mirror vortices. The sensitivity of the rotation and translation characteristics to vortex position, strength, thickness and to a surrounding flow, are investigated. Such a surface-deep vortex interaction can reasonably well represent the observed motions of the swoddy, if the deep vortex is far enough from the surface one, and if they have comparable strengths.Then, a numerical code of the two-layer quasi-geostrophic equations is used to model finite-area vortices, again on the f-plane. Vertical alignment of the surface and deep vortices or vortex pairing with the mirror image, are not observed. Vortex splitting due to mutual shearing effects occurs only for vortices with very different strengths. Vortex trajectories similar to the ones observed are reproduced by the model for equal strength vortices.Complementary data, from the CONGAS experiments in 2004–2007, and from the ARGO profiling float database, are used to show that meddies (or at least coherent salinity anomalies at 1000m depth) can originate from the continental slope near 45°N, 8°W and move regularly northeastward toward the region of swoddy generation and drift. These data also show that meddy–swoddy (“deep vortex–surface vortex”) encounters may occur in the southeastern Bay of Biscay.

Stochastic Heterogeneity Mapping around a Mediterranean salt lens

Abstract. We present the first application of Stochastic Heterogeneity Mapping based on the band-limited von Kármán function to a seismic reflection stack of a Mediterranean water eddy (meddy), a large salt lens of Mediterranean water. This process extracts two stochastic parameters directly from the reflectivity field of the seismic data: the Hurst number, which ranges from 0 to 1, and the correlation length (scale length). Lower Hurst numbers represent a richer range of high wavenumbers and correspond to a broader range of heterogeneity in reflection events. The Hurst number estimate for the top of the meddy (0.39) compares well with recent theoretical work, which required values between 0.25 and 0.5 to model internal wave surfaces in open ocean conditions based on simulating a Garrett-Munk spectrum (GM76) slope of −2. The scale lengths obtained do not fit as well to seismic reflection events as those used in other studies to model internal waves. We suggest two explanations for this discrepancy: (1) due to the fact that the stochastic parameters are derived from the reflectivity field rather than the impedance field the estimated scale lengths may be underestimated, as has been reported; and (2) because the meddy seismic image is a two-dimensional slice of a complex and dynamic three-dimensional object, the derived scale lengths are biased to the direction of flow. Nonetheless, varying stochastic parameters, which correspond to different spectral slopes in the Garrett-Munk spectrum (horizontal wavenumber spectrum), can provide an estimate of different internal wave scales from seismic data alone. We hence introduce Stochastic Heterogeneity Mapping as a novel tool in physical oceanography.

In situ and remote sensing signature of meddies east of the mid‐Atlantic ridge

Mediterranean Water eddies (meddies) are thought to play an important climatic role. Nevertheless, their dynamics are not sufficiently known because of difficulties encountered in their observation. Though propagating below the main thermocline, a number of pieces of evidence of sea surface manifestation of meddies are collected. The present work is based on joint in situ and altimetry data analyses to prove that the meddies can be followed with remote sensing data for long periods of time. The in situ observations are based on data from an oceanographic cruise, which crossed three meddies, and reanalysis of historical data sets, including RAFOS floats paths. Suggested methodology permitted us to obtain uninterrupted tracks for several meddies for a period from several months to more than 2 years. It was found that the dynamically calm region to the north of the Azores current presents favorable conditions for meddy tracking. The meddy surface signal may become shattered and difficult to follow during interaction with a strong dynamic structures (the Azores current/surface vortexes) or peaking topography. Theoretical considerations support the observations and lead to the conclusion that the dynamic signature of meddies at the sea surface is an intrinsic property of meddy dynamics.

Interaction of Mediterranean water eddies with Sedlo and Seine Seamounts, Subtropical Northeast Atlantic

Abstract During years 2003 and 2004 both Sedlo and Seine Seamounts, lying in the area of influence of the Mediterranean water outflow in the Northeast Atlantic, were investigated in the framework of the EU-funded project OASIS. Analysis of CTD data shows that the levels of major variability in temperature and salinity were confined to the upper ocean layer, which is subject to seasonal air–sea heat exchange, and to the Mediterranean Water (MW) level. In this paper we concentrate on the latter case and show that the variability observed resulted from Mediterranean water eddies (Meddies) colliding with the seamounts. Combined data from CTD surveys, current meter moorings, ship-mounted ADCP and satellite altimetry suggest that during the period of observations there was only one Meddy interacting with each of the seamounts. On Sedlo Seamount intensive interaction resulted in the loss by the Meddy of about 25% of the salt around the seamount, whereas on Seine the Meddy (or portion of it) escaped the seamount after performing several rotations around the summit without any significant interaction.

Seamounts and organic matter—Is there an effect? The case of Sedlo and Seine Seamounts: Part 1. Distributions of dissolved and particulate organic matter

Abstract The temporal and spatial variability in the water-column distribution of dissolved organic carbon (DOC), and the particulate organic matter, collected both with bottles (POM-b) and in situ pumps (POM-p), was studied around Sedlo and Seine, two seamounts located in subtropical waters of the northeast Atlantic Ocean. Seine and Sedlo presented high POM-b concentrations, compared with reported background concentrations from the Azores-Madeira region, although our results show large regional, temporal and intra-seamount variability around the two seamounts. On the contrary, the POM-p pool, which contributed only 25–56% of the POC-b at surface, represented a fairly constant fraction of the suspended POM. Seine was characterized by higher POC-b concentrations than Sedlo, presumably as result of a combined mechanism of advection from the far field, and retention/accumulation of organic carbon, favoured by the secondary circulation around the seamount. Passive accumulation of POM-b was also evident at the southern flank of Sedlo during one the surveys, coinciding with a change in the local circulation around the seamount caused by the intrusion of a Mediterranean water eddy (Meddy). Average PON concentrations at the two seamounts were comparable in magnitude, and hence the C/N ratios were higher in Seine, adding support to the hypothesis of advection of allochtonous highly refractory organic matter into Seine. The contribution of DOC to the apparent oxygen utilization (AOU) in the upper 1000 m was significantly higher in Seine (25%) than in Sedlo (11%). This points to a proportionally more important role of dissolved material to the overall carbon remineralization rates in the water column of Seine.

Identification of eddies from sea surface temperature maps with neural networks

This paper describes a new algorithm for automatic detection of Mediterranean water eddies from sea surface temperature (SST) maps of the Atlantic Ocean. The proposed approach looks at satellite images as textural patterns of water temperatures. Given a point of the map, information on the surrounding temperature gradient field is extracted through a binary mask and organized as a numerical vector of gradient angles. An artificial neural network (ANN) is trained to recognize those textural patterns of gradient angles that characterize eddy structures. Tested over a range of different binary extraction masks, the proposed identification system achieves accurate and robust learning results. Results are also characterized by very low rates of detection of false positives. The latter is particularly important because eddies occupy only a small portion of the ocean map area. Experimental comparison is carried out using Laws' method for texture analysis. The results show that the proposed gradient‐based algorithm allows similar performances with a reduced design effort. The complexity of the proposed approach also compares favourably to other methods for identification of mesoscalar phenomena published in the literature. Given the satisfactory accuracy of the results obtained, the gradient‐based approach may be preferable to current techniques as it is simpler and more easily reconfigurable.

Observed mean and mesoscale upper ocean circulation in the midlatitude northeast Atlantic

Lagrangian instruments and altimetry data are used to investigate the upper (0 to ∼500 m) mean and mesoscale circulations in a region of the midlatitude northeast Atlantic (36°–47°N, 13°–23°W), a formation and subduction area of Subpolar Mode Water. Lagrangian data were collected between September 2000 and September 2001 with isobaric subsurface floats, totaling 34.0 float years, and surface drifting buoys, totaling 27.9 buoy years. The near‐surface (0–100 m) mean circulation is dominated by two weak (∼5 cm/s) eastward currents which veer southward in the domain. In the 100‐ to 500‐m‐deep layer, these eastward mean currents retroflect westward and form two anticyclonic circulation cells, opened to the west and separated by a cyclonic cell. Eddy kinetic energy is decreasing from the northwest to the southeast of the area, from ∼100–200 (cm/s)2 to ∼20–30 (cm/s)2. Long‐lived anticyclones and cyclones display mean coherent displacements, both to the southwest. Westward eddy propagation speeds, close to that of long Rossby waves, increase southward and are consistent with a superposition of beta effect and advection by mean currents. Southward eddy displacements are coherent with the observed mean currents. In the north, anticyclones are seen to consist of Central and Subpolar Mode Water eddies, most of them likely originating from the eastern boundary, whereas in the south, most anticyclonic features result from the influence of “Mediterranean Water Eddies” (meddies) on the upper layers. Cyclonic activity is concentrated in “cyclonic zones,” which are seen to exchange floats and water properties.

Meddies and decadal changes at the Azores Front from 1980 to 2000

Twenty years of time series observations from the deep-sea mooring KIEL276 are used to obtain information on the frequency and propagation of meddies (Mediterranean Water eddies), on long-term changes in flow properties, and on a possible relation to the North Atlantic Oscillation. The mooring was set at the nominal position 33°N, 22°W at a water depth of more than 5200 m in the northern Canary Basin. It is located near the southern boundary of the Azores Current (AC), which is part of the North Atlantic subtropical gyre, and also in the large-scale Mediterranean Water (MW) tongue originating from the Strait of Gibraltar. The amplitudes of time-varying vertical quasi-geostrophic modes and the correlation of current and temperature changes at levels in the MW and the North Atlantic Central Water above are used to identify meddies. A total of 10 meddies passed the mooring during the period 1980–2000. Half of the events can be related to earlier observations. Directional changes in meddy-related velocities are used to estimate speeds and directions of meddy propagation. Directions of propagation are very homogeneous, with all the 10 meddies observed propagating with a southward velocity component within a sector of 90°, and typical speeds are 2–3 cm/s. Meddy occurrence was uneven in time, with six meddies found during the first four years and only four meddies during the remaining 16 years. Decadal changes show the annual-mean and the fluctuating kinetic energy levels at the site changing from lower values in the 1980s to high values in the 1990s. This change appears to be correlated with variations in the North Atlantic Oscillation (NAO) index, with a delay in oceanic response of about 3 years. A conceptual model of AC meanders is used to identify meander signals in the upper-layer time series. The AC axis appears to be closer to the site during the 1990s than during the preceding decade and indicates a southward or southwestward displacement of the AC with increasingly positive values of the NAO index. Meddy frequency is lower when the AC gets closer from the north. A reduction in meddy occurrence in the region just south of the AC is possibly caused by the shear-induced blocking of some meddies crossing the front from the north.