Article

Meddy trajectories in the Canary Basin measured during the SEMAPHORE experiment, 1993-1995

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Abstract

As prt of the Structures des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, four Mediterranean wter eddies (Meddies) were identified in the Canary Basin and trcked with freely drifting RAFOS floats. One lrge and energetic Meddy, discovered 1700 km west of Cpe Saint Vincent, Portugal, setdistancend speed record as it translatednother 1700 km southwestwardt 3.9 cm/s during 1.5 years. This Meddy traveled 57% of the distance from Cpe Saint Vincent toward the spot McDowell and Rossby (1978) found a possible Meddy north of the Dominican Republic. Two Meddies were observed to interact with the Azores Current as they pssed underneath or through it. Three Meddies collided with tM1 semounts, which seemed to disrupt the normal swirl velocity, perhaps fatally in two cases. One Meddy appeared to bifurcte when it collided with seamounts.

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... Another large lens, M3, 150 km in diameter, was detected in October 1993 from R/V "Professor Shtokman" (Ivanov and Filyushkin 1995) south of the Azores Current. In November 1993, buoy 173 was introduced into this lens (Richardson and Tychensky 1998). This lens also had two cores vertically (at 900 and 1200 m) (Richardson and Tychensky 1998, figure 4). ...
... In November 1993, buoy 173 was introduced into this lens (Richardson and Tychensky 1998). This lens also had two cores vertically (at 900 and 1200 m) (Richardson and Tychensky 1998, figure 4). The presence of very large MW lenses near the Azores Current can support the hypothesis of MW lens merger in the AFZ zone. ...
... The presence of very large MW lenses near the Azores Current can support the hypothesis of MW lens merger in the AFZ zone. Lenses M2 and M3 directly interacted with the jet current: M2 crossed it, while M3, coupled with a meander; this coupling then yielded a complex structure comprised of a cyclone and of fragments of the lens (Richardson and Tychensky 1998, figure 2). ...
Article
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.
... PSU moving to the south form a latitudinal zone (30°-35° N) from the MAR to the coast of Africa. Arrows in Fig. 2b show the drift direction of two ITL: (H) [27] and (S1) [26], and the salinity values in the cores of these lenses are given by numbers. Drifting to the south, the lenses transport heat and salt to the surrounding waters, changing the position of the isohalines in these directions. ...
... The most remarkably the relationship between the propagation of MW and currents at a depth of 1000 m can be traced in the region of the Azores frontal zone (Fig. 3). Figure 2b shows the position of the SEMA-PHORE experiment survey area [27]. It is in its northern part that the Azores current flows eastward, which limits distribution of more saline waters and lenses to the south. ...
... These results differ remarkably from the previous maps presented in [7,11,23] by a higher reliability of measurements and data processing methods, as well as accuracy in determining the MW boundaries. [3] and SEMAPHORE [27] survey areas; B [22], H [27], MP [6], and S1 [26] are areas of detected isolated intrathermoclinic lenses. Salinity value in lenses at depth of 1000 m is given in parentheses (1-6, see text). ...
Article
Peculiarities of the spatial distribution of intermediate Mediterranean waters (MW), which are the main source to maintain the heat and salt budgets at depths of 600–1500 m in the Atlantic Ocean, have been studied using the ARGO floats measurements database. About 75000 temperature and salinity profiles recorded by 900 ARGO floats in 2005–2014 in the Atlantic Ocean for latitudes from 20° to 50° N were used. To process these data, we used the ARGO-Based Model for Investigation of the Global Ocean (AMIGO). This technique allowed us for the first time to obtain a complete set of oceanographic characteristics up to a depth of 2000 m for different time averaging intervals (month, season, years). Joint analysis of the temperature, salinity, and velocity distributions at 700–1000 m depths made it possible to revise the distribution of MW and their penetration into the western part of the ocean across the Mid-Atlantic Ridge (MAR). It is shown that at depths of 700 and 1000 m, the Mid-Atlantic Ridge is a barrier to advective propagation of salty waters (>35.5 PSU) to the west and is transparent to fragments of destroyed intrathermocline lenses (ITL) with lower salinity (<35.4 PSU). In the Atlantic region, from 20° to 35° N and from 30° to 70° W, individual lens profiles with an anomalous salinity distribution were sought using ARGO measurements to detect ITL and its separate fragments. About 24 000 measurements from 370 ARGO floats were analyzed, and only about 3% of them showed weak salinity anomalies at 800–1200 m depths. No ITL were found from these observations. Analysis of long-term drifting of individual floats recording temperature and salinity profiles with anomalous layers made it possible to study the nature of MW transport through the MAR.
... For a wide background flow, a sufficient condition for trapping at its southern limit is shielding the meddy at the south with an area of enhanced ambient negative relative vorticity (Bell, 1990). The analysis of meddy trajectories (Richardson and Tychensky, 1998;Tychensky and Carton, 1998;Bashmachnikov et al., 2009b) demonstrate a fast meddy translation across the Azores Current (meddies Hyperion and Ceres), as well as a meddy trapping, for at least 6 months, at the southern boundary of the Azores Current (meddy Encelade). ...
... of which also form a part of the meddy data-set above (Armi and Zenk, 1984;Käse et al., 1989;Zenk et al., 1992;Hinrichsen et al., 1993;Prater and Sanford, 1993;Pingree and Le Cann, 1993a,b;Schultz Tokos et al., 1994;Shapiro et al., 1995a;Pingree, 1995;Käse and Zenk, 1996;Bower et al., 1997;Richardson and Tychensky, 1998;Tychensky and Carton, 1998;Richardson and Wooding, 1999;Chérubin et al., 2000;Paillet et al., 2002;Carton et al., 2002). Porto/Aveiro Canyons and 45 N the mode is 1 (1-core is predominantly observed). ...
... This may be a sign that, while crossing the Azores Current, meddies loose their kinetic energy together with the loss of their mass (Fig. 10d). For example, meddy Ceres was observed to strongly weaken after crossing the Azores Current (Richardson and Tychensky, 1998;Bashmachnikov et al., 2014). ...
Article
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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 (PortimaoCanyon, cap St.Vincent, Estremadura Promontory, Gorringe Bank) is 3 times that at the northwestern Iberian Peninsula (Porto-Aveiro Canyons, Cape Finisterre and Galicia Bank).
... Much of this water mass is transported by self-contained, rotating eddies, called salt lenses or Meddies, of 50-100 km horizontal extent and several hundred meters vertical extent (Richardson et al., 2000). Meddies move at a nominal speed of about 2-4 cm s 21 and circulate with speeds of about 20 cm s 21 (Richardson & Tychensky, 1998). Since Meddies are both deep and density compensated, they are difficult to detect and observe. ...
... With an initial current direction of CANARY BASIN TOMOGRAPHY 8990 2508-3008 (relative to North) followed by a current direction of 908-1208, these data are consistent with a relatively strong Meddy moving southwestward. Indeed, the movement of Meddies in the Canary Basin is reliably to the southwest (Richardson et al., 2000;Richardson & Tychensky, 1998), and this event would have taken about 30 days to reach the T1-T2 acoustic path. ...
... With these expressions, the simulated Meddy was centered on 1,000 m depth and midway along the acoustic path, and it had a nominal 350 m vertical extent and With this Meddy model, rays were computed using a series of amplitudes: 0.0, 0.5, 1, 2, 4, 6, 8, 10, and 12 m s 21 . Since a given amplitude corresponds to a particular distance from Meddy center, assuming the Meddy moved at a nominal speed of 4 cm s 21 (Richardson & Tychensky, 1998) allows a time to be associated with each amplitude value. The ray travel times associated with this simulation of the passage of a Meddy across the center of the T1-T2 acoustic path is given in Figure 13, lower pattern. ...
Article
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An acoustic propagation experiment over 308 km range conducted in the Canary Basin in 1997–1998 was used to assess the ability of ocean acoustic tomography to measure the flux of Mediterranean water and Meddies. Instruments on a mooring adjacent to the acoustic path measured the southwestward passage of a strong Meddy in temperature, salinity, and current. Over 9 months of transmissions, the acoustic arrival pattern was an initial broad stochastic pulse varying in duration by 250–500 ms, followed eight stable, identified-ray arrivals. Small-scale sound speed fluctuations from Mediterranean water parcels littered around the sound channel axis caused acoustic scattering. Internal waves contributed more modest acoustic scattering. Based on simulations, the main effect of a Meddy passing across the acoustic path is the formation of many early-arriving, near-axis rays, but these rays are thoroughly scattered by the small-scale Mediterranean-water fluctuations. A Meddy decreases the deep-turning ray travel times by 10–30 ms. The dominant acoustic signature of a Meddy is therefore the expansion of the width of the initial stochastic pulse. While this signature appears inseparable from the other effects of Mediterranean water in this region, the acoustic time series indicates the steady passage of Mediterranean water across the acoustic path. Tidal variations caused by the mode-1 internal tides were measured by the acoustic travel times. The observed internal tides were partly predicted using a recent global model for such tides derived from satellite altimetry.
... However, later studies have cast some doubt on this hypothesis. At any plausible drift rate across the Atlantic and assuming it was formed off the Iberian Peninsula, 5500 km to the east (Käse et al. 1989; Bower et al. 1995), the original meddy must have been at least 5 years old (Richardson and Tychensky 1998 ). Furthermore, analysis of historical hydrographic data has revealed no meddies west of the Mid-Atlantic Ridge (Richardson et al. 1991). ...
... One is that there are no imposing topographic barriers between the NWC and the Bahamas comparable to the Mid-Atlantic Ridge. In fact, three of the seven meddies that were seeded with SOFAR (Richardson et al. 1989) or RAFOS (Richardson and Tychensky 1998) floats in the eastern Atlantic well away from Cape St. Vincent were fatally disrupted by interactions with seamounts well before the Mid-Atlantic Ridge was even encountered . Secondly, the mean flow west of Portugal at the depth of eastern Atlantic meddies is less than 0.01 m s 1 to the south (Stramma 1984), whereas in the western Atlantic there are several recirculation cells associated with the NAC and the Gulf Stream that are able to advect an eddy at speeds up to 0.10 m s 1 . ...
... One is that there are no imposing topographic barriers between the NWC and the Bahamas comparable to the Mid-Atlantic Ridge. In fact, three of the seven meddies that were seeded with SOFAR (Richardson et al. 1989) or RAFOS (Richardson and Tychensky 1998) floats in the eastern Atlantic well away from Cape St. Vincent were fatally disrupted by interactions with seamounts well before the Mid-Atlantic Ridge was even encountered . Secondly, the mean flow west of Portugal at the depth of eastern Atlantic meddies is less than 0.01 m s 1 to the south (Stramma 1984), whereas in the western Atlantic there are several recirculation cells associated with the NAC and the Gulf Stream that are able to advect an eddy at speeds up to 0.10 m s 1 . ...
Article
A hypothesis is presented that the original salt lens, or "meddy," observed off the Bahamas in the fall of 1976 may have been formed, not near the Mediterranean outflow, but instead in the vicinity of the northwest corner (51°N, 43°W) of the North Atlantic Current. An eddy was observed near the northwest corner by an isopycnal RAFOS float deployed during the 1993-95 North Atlantic Current Experiment, and had nearly identical temperature/salinity properties as those of the Bahamas lens. Hydrographie evidence of thick homogeneous layers with similar properties near the northwest corner suggest a possible formation mechanism by which surface eddies containing warm and saline waters are cooled and subducted. A plausible scenario is made whereby a northwest corner eddy might be advected southward in the Newfoundland Basin by the flow around the high pressure ridge east of the North Atlantic Current and then enter the recirculation gyre immediately south of the Gulf Stream. Such an eddy could be advected to the site of the Bahamas lens in just three years, perhaps much more quickly than an eddy of Mediterranean origin and without encountering the topographic barrier of the Mid-Atlantic Ridge. This conclusion is ironic because the Bahamas lens is considered the first observation of an eddy of Mediterranean origin, and led to the coining of the term "meddy."
... They can exhibit a double structure in temperature and salinity with warmer waters at 800 m (originating from the upper MW current) and saltier waters near 1200 m (from the lower MW current). Meddies often drift southwestward from their formation sites, under the influence of the planetary vorticity gradient; but vertically sheared currents, topographic effects or the influence of a cyclonic partner can induce drifts in other directions, or even stationarity [Käse and Zenk, 1986;Shapiro and Meschanov, 1996;Richardson et al., 1989Richardson et al., , 1991Richardson et al., , 2000Richardson and Tychensky, 1998;Paillet et al., 1999Paillet et al., , 2002. Away from their generation region, meddies have been observed most often in isolation (i.e., remote from other meddies or MW cyclones), except near the Azores Current. ...
... Away from their generation region, meddies have been observed most often in isolation (i.e., remote from other meddies or MW cyclones), except near the Azores Current. During their evolution, meddies can live for several years; eventually, they slowly dissipate by mixing or rapidly disappear by erosion on seamounts Richardson et al., 1989;Colin de Verdière, 1992;Richardson and Tychensky, 1998;Richardson et al., 2000]. ...
... These Mediterranean eddies or Meddies [25], as they are often called, are rapidly rotating double convex lenses that contain a warm, highly saline core of Mediterranean water 200-1,000 m thick. Compared to the background water in the Canary Basin, Meddy salinity and temperature anomalies reach 1 psu and 2-4 • C higher, respectively [3,33]. Because of the water differences between a Meddy and the north Atlantic, isopycnal amplitude reach 6 cm [36,37]. ...
... We can, therefore, investigate the pathways of the O&M in the North Atlantic by using data with better spatial and temporal resolution from space. Yan et al. [43] reported how well O&M signals agree with in situ float experiments, A Mediterranean Undercurrent Seeding Experiment (AMUSE) [3] and the Structures des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) [33], and XBT measurements during WOCE. In addition to Yan et al.'s [43] annual and monthly comparisons with RAFOS floats, we placed all floats and XBT measurements in seasonal O&M map (Fig. 8). ...
Article
Full-text available
Since there are no direct salinity measurements using remote sensing from space, we developed a new method to estimate salinity variations in the water column using satellite multi-sensor measurements. The technique for estimating vertical salinity variations (and thus salt steric height) is derived from sea surface height variation measured by satellite altimetry after removing the thermal and other steric components. We call this technique Integrated Multi-Sensor Data Analysis (IMSDA). We apply the IMSDA to estimate salinity variation from the Amazon River outflow in the tropical Atlantic, the Yangtze River outflows in the East China Sea (ECS), and Mediterranean Outflow and Meddies (O&M) in the North Atlantic.
... In these mesoscale flow domains, we are concerned with how the existence of topographic obstacles affects the motion of a large eddy, which is a flow domain with a circulation, since the eddy transports mass, momentum and substances such as salt and minerals in oceans. For instance, Richardson and Tychensky observed a collision of a subsurface eddy to seamounts in the Canary Basin [26]. Fratantoni et al. [9] investigated whether a big ocean current, called North Brazil current ring, can pass the multiple gaps of southeastern Caribbean islands into the Gulf of Mexico. ...
... The complex potential for the uniform flow in the unbounded complex plane without boundary is represented by U z for a complex constant U , which indicates that it has a pole singularity in the neighborhood of infinity. Owing to i = (H (∞)), (14) and (15), the function H (z)))) (26) has the pole singularity at infinity and the boundary value of Im[W U (z)] becomes constant on each E j . Hence, the function represents the complex potential for a uniform flow in the z-plane. ...
Article
We present a fast and accurate numerical method for constructing incompressible, inviscid and irrotational flows in two-dimensional coastal domains, which are unbounded multiply connected domains above an infinitely long coastline boundary. In the numerical method, we utilize a numerical conformal mapping method based on a boundary integral equation with the generalized Neumann kernel in order to construct conformal mappings from coastal domains onto four of Koebe’s canonical domains. The numerical method is fast and accurate, since it just requires O((m+1)nlnn) operations and it converges with O(e−cn) for coastal domains of connectivity m+1, where n is the number of nodes in discretizing each smooth boundary component and c is a positive constant. With some examples, we also show that it is applicable to arbitrary coastal domains with high connectivity and complex geometry.
... Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (Tychensky and Carton, 1998;Richardson and Tychensky, 1998) and Pinball (Pingree, 1995;Richardson et al., 2000) satisfied those requirements. ...
... These two interactions were presumably responsible for the variations of the meddy surface signal during a period shorter than a month (Fig. 2c). During an the initial period, the RAFOS floats in the meddy core showed a fast outward drift, in particular as the meddy interacted with the Plato seamount ( Fig. 2a and c, see also Richardson and Tychensky, 1998). Later on (from November 1993 to November 1994), the RAFOS floats rotated at about 30-40 km from the meddy center. ...
Article
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Meddies, intra-thermocline eddies of Mediterranean water, are often visible 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. Then, theoretical estimates of the surface signature of a meddy are derived, based on geostrophy and potential vorticity balance. The intensity of the surface signature is 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. Estimates suggest that the southernmost limit for detection in altimetry of small meddies (with radii on the order of 15 km) should lie in the northern subtropics, while large meddies (with radii of 25–30 km) could be detected as far south as the northern tropics. During the initial period of meddy acceleration after meddy formation or a stagnation stage, a cyclonic signal also is generated at the sea-surface, but mostly the anticyclonic surface signal follows the meddy.
... The radial structure of the velocity in the core of these vortices is compatible with solidbody rotation (Schultz Tokos and Rossby, 1991). Many observations of meddies in the Canary Basin have been reported since the late 1970s (Armi and Zenk, 1984;Richardson et al., 1989;Richardson and Tychensky, 1998) and more recently in the Iberian Basin (K. ase et al., 1989;Zenk et al., 1992;Pingree and Le Cann, 1993;Schultz Tokos et al., 1994;Pingree, 1995) and off the south (Prater and Sanford, 1994) and southwest (Bower et al., 1997) coasts of Portugal. ...
... The floats that traveled within these two structures were detrained off the respective meddy, showing that a disruption of some kind and a release of some of the vortex fluid had to occur. Similar meddy collisions with seamounts have been previously observed (Richardson et al., 1989;Schultz Tokos et al., 1994;Shapiro et al., 1995b;Richardson and Tychensky, 1998). ...
Article
In the framework of the European Union MAST III project Canary Islands Gibraltar Azores Observations, 24 RAFOS floats were deployed in the Mediterranean Water (MW) undercurrent off south Portugal between September 1997 and September 1998. A preliminary analysis of this Lagrangian approach, complemented with XBT and current-meter data, show some of the major aspects of the flow associated with the undercurrent as well as associated eddy activity. Floats that stayed in the undercurrent featured a downstream deceleration and a steering by bottom topography. Three meddy formations at Cape St. Vincent could be isolated from the float data. The dynamical coupling of meddies and cyclones was observed for a considerable period of time. The generation of two dipolar structures in the Portimão Canyon region also was observed with the float data. A major bathymetric relief—Gorringe Bank—was not only an important constraint to the eddy trajectories and of the flow at the MW levels but also a site for meddy formation.
... Entre la SACW et la TSW, des intrusions d'eau salée issue de la NACW peuvent également être observées [Pérez-Rodríguez et al., 2001;Stramma et al., 2005]. Dans les couches plus profondes, la présence de la langue d'eau Méditerranéenne (MW, Mediterranean Water) qui s'écoule dans l'Atlantique par le détroit de Gibraltar peut influencer le CANUS jusqu'à 25°N environ [Richardson and Tychensky, 1998]. Cette masse d'eau s'étend entre 800 et 1200 m de profondeur [Barton, 1989] et est relativement chaude (T = 7 -13°C) et salée (S > 35,5) Machín et al., 2006]. ...
... Le sillage des Canaries a ainsi été nommé le "Canary Eddy Corridor" par Sangrà et al., [2009] [Sangrà et al., 2007]. De même, les anticyclones appelés "Swesties" (Shallow Subtropical Subducting Westward-propagating Eddies), formés dans le sillage de l'archipel des Canaries et observés jusqu'à 50°W, transportent de l'eau relativement chaude et salée entre 100 et 250 m de profondeur [Richardson and Tychensky, 1998;Tychensky and Carton, 1998;Oliveira et al., 2000;Richardson et al., 2000;Carton et al., 2002;Bashmachnikov et al., 2012;Barbosa Aguiar et al., 2013]. (Figure 1.14) [Shannon, 2001]. ...
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... ity anomalies and in approximate solid body rotation. Typical radii range 25 from 10 km (Bower et al., 1997) to 60 km (Richardson and Tychensky, 1998) 26 and the larger ones are usually older too. While anticyclones exhibit a core 27 that can extend from 500 m down to 1500 m, cyclones tend to be shallower 28 vortices with their core centered at about 600 m (Carton et al., 2002). ...
Article
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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 90 days is of 12 Meddies yrÀ1 of which $12% are cyclones. The rate of formation reaches 40 Meddies yrÀ1 (30% cyclones) when considering all the Meddies living over 15 days. 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 30 km. The thickness (depth-difference between isopycnals 27.2 and 27.5) of anticyclones born near Cape St. Vincent contracts by approximately 100 m, after travelling 1000 km from their source; their mean swirl velocities range from about 21 cm sÀ1 (at z = 1000 m) up to 27 cm sÀ1 (at z = 600 m). 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 600 m depth where longer trajectories are recorded. In the vicinity of Portimão Canyon, cyclones outnumber anticyclones while the reverse happens downstream of Cape St. Vincent.
... Since, in this paper, we 68 are interested in the long-term variations of meddy surface signatures, only the meddies tracked with sub-69 surface drifters for a number of months, propagating in relatively calm background dynamic conditions and 70 at least once thoroughly characterized with in situ data (CTD sections) are selected. Meddies Hyperion 71 [Tychensky and Carton, 1998;Richardson and Tychensky, 1998], Pinball [Pingree, 1995;Richardson et al., 2000], 72 and Ulla satisfy those requirements. Those meddies were chosen since they cover a wide 73 range of background conditions. ...
Article
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In 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 Rm ≥ 30 km are always seen at the sea-surface with AVISO altimetry, that medium-size meddies with Rm = 20 km may be seen at the sea-surface only if they are sufficiently shallow and strong, while small meddies with Rm = 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 f/N 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 meddies the upper limit should be used. Numerical experiments show that surface signals of meddies translated with β- 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.
... Therefore, the two problems mentioned above have quite real applications. [414,761,926]; 2 -the domain of detected merging of two lenses [816]; 3 -the domain of formation of lenses with dipole structure [259]; 4 -the domain where the large-scale Mesopolygon experiment was conducted [1016]; SM1 -the trajectory of a lens in the zone of SEMAPHORE experiment; M1, M3 -the trajectory of Sharon lense [762]. ...
Chapter
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In this chapter, we consider the stability of an isolated finite-core (or distributed) two-layer vortex with respect to relatively small and finite perturbations. An analogy between a distributed heton and A-symmetrical structure of discrete hetons is demonstrated. The specific features of the nonlinear stage of evolution of unstable vortices, and the interaction between two distributed hetons or antihetons are considered. The model is shown to be promising for the description of deep-convection processes,water mass mixing in the ocean, and the formation of new quasistationary vortex structures. We study the effect of external flow and of an isolated hill on heton motion. The results obtained for a three-layer, quasigeostrophic model are given; in particular, specific features of the dynamics of meddies are studied. The role of baroclinicity in the formation of the kinematic and thermohaline structure of the ocean is analyzed.
... One can see a strong signature (|∆η′|) toward west and south from July to November. Generally, southward Meddies are formed near 36°N by separation of the frictional boundary layer at sharp corners (29) and in the Canary Basin (30,31,32). The southward travelling Meddies can be explained by the strongest low frequency zonal motions driven by baroclinic instability (33) and the influence of the neighboring mesoscale features (cyclonic vortices or Azores Current meanders) in the regions (34). ...
... The crossing of the AzC by the tracked vorticity surface structure closely corresponds to what is described in Vandermeirsch et al. (2003) for a deep eddy crossing a jet. The same behaviour of a meddy and its surface signal was observed as the RAFOS-tracked meddy Hyperion (Richardson and Tychensky, 1998) was crossing the AzC (Bashmachnikov et al., 2009a). ...
Article
A Mediterranean water eddy (meddy), initially singled out in the south-western part of the Iberian basin with AVISO altimetry data, was later surveyed with 75kHz ADCP data collected by RV "Almirante Gago Coutinho" at the end of August 2010. The azimuthal velocity of the surface anticyclonic vortex was of 15 cm s-1 and was strongly intensified below 600 m depth, reaching 45 cm s-1. The deep dynamic structure was identified as the core of a meddy. Assuming that the meddy is circular, it was estimated that the ADCP section passed about 5 km north of the meddy center. The dynamic radius of the meddy core was estimated to be 10-15 km, which is the second baroclinic Rossby radius of deformation (see also Paillet et al., 2002). Above the meddy, in-between 500 and 600 m depth, the dynamic radius of the anticyclonic structure sharply increased to 15-20 km, and then was gradually increasing further up to reach 20-25 km at the sea-surface (close to the first baroclinic Rossby deformation radius). This transition layer at 500-600 m was also marked with the maximum vertical gradient of the horizontal velocity, suggesting in vertical the double-core eddy structure. The axis of the surface eddy was shifted 15-20 km to the west from the meddy center. The dynamic structure of the upper 500-m layer was identified as a "meddy surface signal", which has been suggested to be a stand-alone structure coupled with a meddy (Bashmachnikov et al., 2009). The intensity of the meddy surface signal is a function of the background conditions and strongly varies in time. During the 3 months preceding the cruise, the meddy surface signal, identified as a local sea-level anomaly (SLA) in the gridded AVISO altimetry as well as in the along-track data, was detected moving south-west, approximately along an altimetry track. During May 2010, before interaction with the Azores Current (AzC), the SLA over the meddy was of the order of 5-9 cm. It decreased to 3-5 cm in the end of June, as the meddy first interacted with a meander of the AzC. During July-August the meddy became aligned with the meander and its surface signal reached its maximum of 10 cm. At the time of the cruise the downward intensification of the anticyclone structure above the meddy suggests that it was not an AzC meander, but rather a dynamic structure generated by the moving meddy. Soon after the cruise the meddy got expelled from the AzC jet westwards (see Vandermeirsch et al., 2003) and its surface signal became undistinguishable from the background noise. Therefore, during interaction of a meddy with a jet current, the meddy surface signal, after its initial enhancement, may be lost.
... Since, in this paper, we 68 are interested in the long-term variations of meddy surface signatures, only the meddies tracked with sub-69 surface drifters for a number of months, propagating in relatively calm background dynamic conditions and 70 at least once thoroughly characterized with in situ data (CTD sections) are selected. Meddies Hyperion 71 [Tychensky and Carton, 1998;Richardson and Tychensky, 1998], Pinball [Pingree, 1995;Richardson et al., 2000], 72 and Ulla satisfy those requirements. Those meddies were chosen since they cover a wide 73 range of background conditions. ...
Article
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Mediterranean Water eddies (meddies) represent rotating bodies of modified Mediterranean Water propagating large distances below the main thermocline in the subtropical Atlantic. Though the phenomenon has been discovered quite long ago, its climatic importance is not sufficiently known due to difficulty in detection and tracking of these deep-water features. The methodology to follow the meddies with remote sensing data, developed by the authors, permits to obtain uninterrupted tracks for a number of meddies for long periods of time. The tracks are confirmed by in-situ profiling probes observations. The results suggest that meddies can be relatively securely tracked with altimetry data in dynamically calm regions. The problems arise in the areas of meddy formation, those with intensive surface circulation structures, and steep topographic rises. Ocean Colour and Sea Surface Temperature signatures of meddies are less obvious, but at times quite clear. Re-analysis of several meddy tracks and historical data permitted to create a picture of major meddy pathways in the North-East Atlantic.
... Stern and Bidlot (1994) showed that an anticyclonic vortex can cross an eastward zonal jet and detach a cyclone from it. This was revisited by Vandermeirsch et al. (2003a) who used a 2½-layer model to study the interaction of meddies with the Azores front [see Tychensky and Carton (1998) and Richardson and Tychensky (1998) for related observations]. They showed that the cyclone formed by the jet generally pairs with the incoming anticyclone to form a dipole that propagates away from the current. ...
Article
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The possibility for a surface intensified anticyclone to subduct beneath a surface front is investigated using an isopycnal numerical model. Subduction occurs for strong coherent vortices, yet is usually accompanied by strong dissipation. Two main mechanisms cause the erosion of the vortex core. The first one is induced by the velocity shear associated with the front. It results in the peeling of the vortex potential vorticity (PV) core, sometimes leading to its complete disappearance. The second mechanism occurs once the vortex has subducted. Entrainment of high PV fluid parcels from the front above the vortex low PV core modifies the stability properties of the latter. Meanders are observed to grow at the rim of the structure, which favor further filamentation. The erosion rates caused by each mechanism are discussed in relation to the jet and vortex characteristics, and to the background stratification. The trapping of high PV fluid parcels above the vortex is also shown to be partly responsible for the decrease and eventual loss of the eddy altimetric signal, once it has subducted.
... Inspecting the altimetry data from 1993 to 1996, Schouten et al. (2000) have studied large-scale Agulhas anticyclones with the vertical extent of several kilometers crossing the Walvis Ridge in the southeastern Atlantic Ocean and found that some of them split in the direct vicinity of the Verna Seamount which rises from the deep ocean floor to within 50 m below the sea surface. Anticyclonic lenses of warm salty Mediterranean Water, the so-called meddies, travelling westward through the North Atlantic between 500 and 1500 m depth, show similar behavior when encountering the chain of Great Meteor Seamounts (Shapiro et al. 1995;Richardson and Tychensky 1998). However, majority of meddies either passed over the seamount without splitting or completely destroyed. ...
Article
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Long-lived and large-scale anticyclonic Aleutian eddies (AEs), detaching from the Alaskan Stream to the west of 180∘ and propagating southwestward or almost zonally in the Western Subarctic Gyre, have been investigated during 1993–2020 through Lagrangian analysis of altimetry data and Argo float profiles. All such AEs in the altimetry era have been identified, tracked and documented providing the first systematic census of this class of eddies. Inspecting daily computed Lagrangian maps of the origin of water masses and of the distance travelled by fluid particles, we followed as the AEs moved from the formation area toward the Kamchatka Peninsula. The Lagrangian diagnostics allowed us to document the main events in the lifecycle of each AE including interaction with bottom topography, deformation, splitting, merge, erosion and eventual decay. Just after formation, the AEs stagnate for a while over the Aleutian Trench before isolating from the Stream. They experienced strong deformation when crossing the Detroit Rise, and some have been found to split when passing the Rise. Kinematics and vertical structure of a few previously sampled AEs have been considered in detail using the altimetry-based Lagrangian maps and Argo float observation. The observations reveal typical subarctic vertical structure with the specific warm and saline mesothermal water in the intermediate layer. The Aleutian eddies transport warm and saline water under a cold upper layer that eventually contributes to the mesothermal layer off Kamchatka.
... Using this technique, Bower et al. (1997) detected ten Meddy-formation events, as well as the trajectory of several Meddies formed at Cape São Vicente and Estremadura Promontory. Similarly, the use of research vessels allows the detailed description of Meddy structures via CTD surveys, deployment of XBT, profilers, etc. (McDowell and Rossby, 1978;Pingree and Le Cann, 1993b;Richardson and Tychensky, 1998;Tychensky and Carton, 1998;Carton et al., 2002). The only disadvantage of these strategies is the lack of synopticity, allowing the analysis of only a few Meddies at a time. ...
Article
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 1000. m 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.
... The purpose of this research was to propose a theoretical framework for the study of the interaction of Mediterranean Eddies (meddies) with the Azores jet and front. The SEMAPHORE campaign [7][8][9] ("Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale") of 1993-1995 indeed showed the interaction between the Azores jet and one or several meddies and dipolar structures where a meddy (anticyclone) also interacted with a cyclone. From these measurements, the diameter of the vortices is shown to be of the same order of or smaller than the width of the jet. ...
Article
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This paper addresses the dynamical coupling of the ocean’s surface and the ocean’s interior. In particular, we investigate the dynamics of an oceanic surface jet and its interaction with vortices at depth. The jet is induced by buoyancy (density) anomalies at the surface. We first focus on the jet alone. The linear stability indicates there are two modes of instability: the sinuous and the varicose modes. When a vortex in present below the jet, it interacts with it. The velocity field induced by the vortex perturbs the jet and triggers its destabilisation. The jet also influences the vortex by pushing it under a region of co-operative shear. Strong jets may also partially shear out the vortex. We also investigate the interaction between a surface jet and a vortex dipole in the interior. Again, strong jets may partially shear out the vortex structure. The jet also modifies the trajectory of the dipole. Dipoles travelling towards the jet at shallow incidence angles may be reflected by the jet. Vortices travelling at moderate incidence angles normally cross below the jet. This is related to the displacement of the two vortices of the dipole by the shear induced by the jet. Intense jets may also destabilise early and form streets of billows. These billows can pair with the vortices and separate the dipole.
... density field and currents, are likely to be least valid. The possibility that the eddy was ultimately destroyed because of background shear or interaction with topography also cannot be ruled out (Brickman and Ruddick 1990;Richardson and Tychensky 1998;Cenedese 2002;Torres and Gomez-Valdes 2017). ...
Article
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Automated feature tracking and vehicle navigation have the potential to facilitate autonomous surveys of ocean eddies by increasing sampling quality and/or decreasing operator workload. During an observational campaign in late 2013 and early 2014, methods for automated tracking were used to direct multiple ocean gliders during persistent surveys of a California Undercurrent eddy in Washington and British Columbia coastal waters over a three-month period. Glider observations of depth averaged currents in the ocean’s upper kilometer and vertical separation of selected isopycnals were assimilated into a simple model describing eddy position, size, strength, and background flows using an Extended Kalman Filter. Though differing in detail from observations, results show the assumed eddy structure was sufficient to describe its essential characteristics and stably estimate eddy position through time. Forecast eddy positions and currents were used to select targets automatically to guide multiple gliders along transects through the eddy center as it translated. Transects performed under automated navigation had comparable or better straightness and distance from the eddy center when compared to navigation based on manual interpretation of the eddy scale and position. Tracking results were relatively insensitive to model choices at times when the eddy was well-sampled, but were more sensitive during sampling gaps and redundancies or rapid eddy translation. Overall, results provide evidence that automated tracking and navigation is feasible with potential for widespread application in autonomous eddy surveys.
... A seamount may in fact slow down the propagation of an eddy (Herbette et al., 2005). In the presence of chaotic topography, eddies can even remain trapped in the area for several weeks (Richardson and Tychensky, 1998;Herbette, 2003;Sutyrin et al., 2011). ...
Article
Based on satellite and in situ data, the dynamic characteristics and vertical structure of a surface intensified mesoscale dipole recently expelled from the South East Madagascar Current (SEMC) is described for the first time. The dipole was surveyed 250 nautical miles south of Madagascar between 14 and 23 November 2016, during west-east and south-north transects carried out over the northern Madagascar Ridge. The dipole consisted of two counter-rotating vortices of similar size (100 km) and intensity (0.7 f), and an intense southwestward jet (150 cm s⁻¹) in the frontal region between the two eddies. The cyclonic eddy was lying on the western side of the anticyclonic eddy. With azimuthal velocities reaching 100 cm s⁻¹ at the surface and decreasing slowly with depth (40 cm s- 1 at -600 m), this MAD-Ridge dipole was defined as a highly non-linear (Ro∼0.7) isolated eddy-type structure (cβ ∼ 11 cm s – 1 and U/cβ ∼ 0.7) capable of trapping and advecting water masses over large distances. The enhanced concentration of chlorophyll-a found in the cyclone relative to the anticyclone could be tracked back to the spin-up phase of the two eddies and attributed to eddy-pumping. The eddy cores were located above the pycnocline (1026.4 kg m⁻³), within the upper 600 m, and consisted of varieties of Subtropical Underwater (STUW) found within the SEMC. The STUW found in the anticyclone was more saline and oxygenated than in the cyclone, highlighting mixing with the inshore shelf waters from the southeastern coastal upwelling cell off Madagascar. Observations suggest that the dipole interacted strongly with the chaotic bathymetry of the region, characterized by a group of five seamounts lying between -240 m and -1200 m. The bathymetry blocked its westward advection, trapping it in the vicinity of the seamount for more than 4 weeks, so enhancing the role of the eddy-induced velocities in stirring the surrounding water masses. Squeezed between the southern Madagascan shelf and the northern flank of the anticyclone, two filament-like dynamic features with very different water-mass properties could be observed on the south-north transect: i) one filament highly concentrated in chlorophyll-a demonstrating the capacity of the eddy to export shelf water offshore; ii) intrusions of a more southern-type of STUW generally found south of the South Indian Counter Current (SICC) recirculating on the external flanks of the anticyclone. Although the observed circulation and hydrography were largely constrained by the presence of the mesoscale eddy dipole, unmistakable fine-scale dynamics were also observed in the vicinity of the MAD-Ridge seamount, superimposed onto the mesoscale eddy flow.
... In any case, both vortices can move far enough from the original location (Section 3.1). Note that loop-like motions of SOFAR floats seeded in lenses are often observed in the ocean (for example, see [70][71][72]). This loop-like float motion can be explained by the position of the float on the periphery of the lens. ...
Article
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The theory of point vortices is used to explain the interaction of a surface vortex with subsurface vortices in the framework of a three-layer quasigeostrophic model. Theory and numerical experiments are used to calculate the interaction between one surface and one subsurface vortex. Then, the configuration with one surface vortex and two subsurface vortices of equal and opposite vorticities (a subsurface vortex dipole) is considered. Numerical experiments show that the self-propelling dipole can either be captured by the surface vortex, move in its vicinity, or finally be completely ejected on an unbounded trajectory. Asymmetric dipoles make loop-like motions and remain in the vicinity of the surface vortex. This model can help interpret the motions of Lagrangian floats at various depths in the ocean.
... In the world's oceans, vortices typically have long lifetimes and thus can propagate large distances via background currents or via the planetary vorticity gradient [21]. In doing so many will encounter topological barriers including coastlines [22,23,24] or seamounts [25,26], and as these vortices transport significant amounts of heat, momentum, mass and salt in the world's oceans, understanding their motion close to, and interaction with, these barriers is important. In this work we investigate the interaction of two vortices with the coastline of a circular island, and in particular we identify a criteria for the periodic leapfrogging motion of these vortices, using an approach similar to [19]. ...
Article
The interaction of two line vortices of differing strengths in the presence of a circular cylinder is considered. Explicit criteria are derived, a function of vortex strengths (including strengths of opposite signs) and the cylinder radius, which separate different behaviors of the system. If the initial position of the vortices satisfies these criteria, they will undergo a periodic leapfrogging motion as they rotate around the cylinder; otherwise, the vortices still interact weakly with one another except without leapfrogging. This is in contrast to the planar wall case where if no periodic leapfrogging occurs, the vortices move apart and do not interact with each other. Numerical results for initial vortex positions which do and do not satisfy these criteria are presented to demonstrate the different motions available, as well as the robustness of the criteria.
... Meddy formation takes place in a relatively small area of the continental slope west of the Strait of Gibraltar and has been investigated in depth (Ambar et al. 2008 and references therein). Shapiro and Meshchanov (1991) and Richardson and Tychensky (1998) characterized the tracks of meddies in the Atlantic statistically on the basis of a variety of measurements. Some specific dynamic effects, such as the collision of a meddy with a sea mountain, have also been investigated (Shapiro et al. 1995, Bashmachnikov et al. 2009). ...
Article
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Despite the considerable impact of meddies on climate through the long-distance transport of properties, a consistent observation of meddy generation and propagation in the ocean is rather elusive. Meddies propagate at about 1000 m below the ocean surface, so satellite sensors are not able to detect them directly and finding them in the open ocean is more fortuitous than intentional. However, a consistent census of meddies and their paths is required in order to gain knowledge about their role in transporting properties such as heat and salt. In this paper we propose a new methodology for processing high-resolution sea surface temperature maps in order to detect meddy-like anomalies in the open ocean on a near-real-time basis. We present an example of detection, involving an atypical meddy-like anomaly that was confirmed as such by in situ measurements.
Article
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The existence of energetic anticyclonic mid-depth vortices of Mediterranean Water (meddies) questions the validity of a conventional advective–diffusive balance in the eastern Atlantic subtropical gyre. A mesoscale experiment in the Azores–Madeira region reveals a link of these meddies to large-scale subsurface meanders. For the first time it is shown that meddies may have strong surface vorticity, indicative of a generation process involving the Azores Current—a deep reaching near-surface jet.
Article
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The influence of the subtropical gyre on the spread of Mediterranean Water in the Atlantic is discussed in terms of a simple horizontal advection-diffusion model. The northern, southern and western boundaries of a rectangular ocean are treated as salt sinks while the distribution of salinity on the east coast representing the highly saline Mediterranean Water is a sine curve. The velocity distribution for the subtropical gyre is that given by Stommel and includes westward intensification. Salinity distributions are calculated for various values of the Peclet number, and for oceanographically reasonable values they indicate that the gyre passes through the high-salinity tongue and advects it toward the south and west. The model is consistent with the observed salinity distribution of the mid-layers of the North Atlantic.
Article
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We report evidence of two deep cyclonic and two deep anticyclonic submesoscale eddies from World Oceanographic Circulation Experiment hydrographic casts made in the Brazil Basin. We infer that three of these were likely formed in or near the Deep Western Boundary Current (DWBC) of North Atlantic Deep Water (NADW), and thus had traveled eastward after formation. These eddies appear to be a new way for transporting NADW away from the DWBC to the ocean interior. One of the apparent cyclonic eddies appeared to be laterally in contact with one of the anticyclonic eddies. About 10 days later an attempt was made to resample the apparent eddies that had been in contact. These observations, although limited, are interpreted to indicate that they survived the encounter, that the cyclonic eddy had now moved to be beneath the anticyclonic one with each being somewhat thinner, and that they produced a new anticyclonic eddy by partially merging. Deep float observations [Hogg and Owens, 1999] partially support the second inference.
Article
Point vortex and finite-difference methods are used to study baroclinic eddies advected into weak and strong encounters with topography. It is argued that weak interactions often scatter radially symmetric eddies into generalized hetons. The dipole moments so generated within the eddy result in eddy propagations at various angles to the current. Strong interactions can result in the complete separation of the upper- and lower-layer circulations. Subsequent evolution in this case depends on many factors, although strong topographic obstacles (i.e., seamounts) permit a recognization of the centers into a coherent structure. Weaker topography, confined to the deep ocean, can disrupt the lower center, although the upper center typically survives. Disassociation of the centers with both retaining their integrity is also possible. Heton generation can occur for eddies with weak lower-layer expressions, demonstrating a potentially strong control of shallow eddy propagation by deep sea bathymetry. Analytical and numerical estimates of the induced propagation speeds are sizable, arguing topographic scattering is a potentially powerful mechanism influencing eddy propagation.
Article
The Mediterranean Outflow (MO) off the southwestern Iberian Peninsula has been simulated using a realistic model. As the MO flows along the southwestern Iberian slopes it becomes unstable, originating vortices with a core of Mediterranean Water (MW) centered between 600 and 1000 m. Several of these eddies have been studied in the ocean using in situ measurements and floats. They are long-lived structures (2 yrs on average) that often break up or merge while interacting with currents and seamounts. Are these vortices the main mechanism driving the Mediterranean salt tongue? The output from a high-resolution and long-term MO simulation is analysed here to provide a census of modelled MW eddies. For the whole domain of study, the formation rates of eddies that survived for at least 90 days with minimum salinity anomalies of 0.12, 0.2 and 0.3 psu are of 12, 9 and 6 MW eddies yr-1, respectively; 12%, 5% and 3% of which are cyclones. About 70% of the population is born in the southwestern Iberian slopes, but several robust MW eddies originate in points of convergence of the main pathways into the open ocean. The longest-lived cyclones propagate northwestwards, while shorter-lived cyclones either recirculate in the Gulf of Cadiz or move to west into the Horseshoe seamounts. As the MW eddies drift away from their birthplace, their radius tends to increase gradually from 15 to 30 km. The thickness of a meddy contracts by approximately 100 m, by the end of a 1000 km journey away from its origin. In general, the properties of eddies born in the slopes upstream and downstream of the Cape St. Vincent are distinct.
Article
Satellite-tracked drifting buoys, deployed in the Canary Basin as part of the Subduction Experiment between July 1991 and October 1993 and the French Semaphore Experiment during October 1993, were used to obtain a description of surface currents and temperature in the Canary Basin. The study focuses on surface water convergence, eddy energy production, and heat transport. The Azores Current associated with the subtropical convergence zone is clearly visible at 34°N, and bifurcates around 22°W, with the major branch of the current circling the Madeira plateau and joining the Canary Current along the continental slope. Eddy kinetic energy maxima are found along the Azores Current. The mean current revealed a region of maximum convergence north of the Azores Current around longitude 29°W occurring with a negative heating anomaly and positive work done by the Reynolds stress. The southward meridional temperature fluxes in the Ekman layer (0-50 m) between 37°W and the African and European coast are estimated between -0.076+/-0.022×1015W, produced by mean southward volume transport in our study area. The residual between local surface heat fluxes and horizontal convergence of heat implies a vertical heat convergence process associated with mesoscale temperature and flow fields.
Article
Previous studies focusing on the remote detection of Mediterranean Eddies (Meddies) have reported that the isopycnal surface changes derived from satellite multi-sensor measurements at the approximate depth of 400m can be used to sense the presence of underlying Meddies. While the isopycnal surface near that depth does indeed reveal the locations of Meddies, an analysis of isopycnal surface changes in response to the evolution of Meddies has yet to be made. Accordingly, this research focuses on analyzing the relationship between isopycnal surface changes and the evolution of Meddies. The vertical isopycnal surface variability of Meddies, which is directly related to contributions from rotational velocity, interior thermal variation, and vertical displacement of Meddies, is observed and studied using float observations from A Mediterranean Undercurrent Seeding Experiment (AMUSE). The contributions of each of the three aforementioned parameters are estimated, enabling us to understand their relative role in changing the isopycnal surface above Meddies. Furthermore, in order to further understand Meddies' evolution and their associated forcing, the dominant frequencies of their horizontal and vertical displacements, as well as the sea surface height variability above the Meddy, are analyzed using the Hilbert – Huang Transform. Finally, the horizontal and the vertical eddy viscosity dissipation of Meddies is computed and compared with a theoretical model. The empirical horizontal and the vertical eddy viscosities are found to be 7x106cm2sec−1 and 200cm2sec−1, respectively. This study will therefore contribute to understanding how the isopycnal surface is related to the presence of Meddies, what frequencies are dominate its variability, and the values of eddy viscosity which can be used for a numerical model. This article is protected by copyright. All rights reserved.
Article
To investigate the northern Canary Current system (NCCS), results from four numerical experiments of increasing complexity are examined. Experiment 1, which uses seasonal wind forcing only, shows that as expected, wind forcing is the key generative mechanism for the current, upwelling, meander, eddy, and filament structures. Experiments 2 and 3, which have the additional effects of irregular coastline geometry, show the following: capes are areas for enhanced upwelling, extensive filaments, maximum current velocities, and enhanced growth of cyclonic meanders and eddies; an embayment like the Gulf of Cadiz is a primary region for anticyclonic meander and eddy development. The results from the complex flow regime of Experiment 4, which has the additional effects of thermohaline gradients and Mediterranean Outflow, highlights the major characteristics and unique features (such as the generation of Meddies) of the NCCS with relatively close similarities to field observations.
Article
A 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 analysed 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.
Article
[1] The new generation of high-resolution ocean models offers a new way to investigate the characteristics and the evolution of the ocean mesoscale. An analysis of the simulated Mediterranean eddies, the so-called “meddies,” is presented. The model used in this study is the Mercator North Atlantic [9°N, 70°N] and Mediterranean Sea Prototype (PAM), a high-resolution configuration (3.5–8 km horizontal grid) based on the OPA ocean general circulation model. The meddies are coherent structures of warm and salt Mediterranean Water (MW) advected in the northeast Atlantic. A 5 year experiment performed with PAM reproduced the main observed characteristics of the meddies: thermohaline properties (11.8°C, 36 psu), sizes (radius between 25 and 110 km), thickness (between 500 and 1000 m), westward advection velocities (1.4 cm.s−1), angular velocities (a period of 20 days), a good estimate of the number of meddies in the northeast Atlantic (∼22), and their realistic geographical distribution (80% south of 40°N). Moreover, and in agreement with a previous study based on an observation cruise, these modeled meddies represent half of the westward salinity transport of MW.
Article
All Agulhas rings that were spawned at the Agulhas retroflection between 1993 and 1996 (a total of 21 rings) have been monitored using TOPEX/Poseidon satellite altimetry and followed as they moved through the southeastern Atlantic Ocean, decayed, interacted with bottom topography and each other, or dissipated completely. Rings preferentially crossed the Walvis Ridge at its deepest parts. After having crossed this ridge they have lower translational speeds, and their decay rate decreases markedly. Half the decay of long-lived rings takes place in the first 5 months of their lifetimes. In addition to the strong decay of rings in the Cape Basin, about one third of the observed rings do not seem to leave this region at all but totally disintegrate here. The interaction of rings with bottom topography, in particular with the Vema Seamount, is shown frequently to cause splitting of rings. This will enhance mixing of the rings' Indian Ocean water into that of the southern Atlantic. This localized mixing may well provide a considerable source of warm and salty Indian Ocean water into the Atlantic overturning circulation.
Book
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This book presents an extensive analysis of the dynamics of discrete and distributed baroclinic vortices in a multi-layer fluid that characterizes the main features of the large and mesoscales dynamics of the atmosphere and the ocean. It widely covers the case of hetonic situations as well as the case of intrathermocline vortices that are familiar in oceanographic and of recognized importance for heat and mass transfers. Extensive typology of such baroclinic eddies is made and analysed with the help of theoretical development and numerical computations. As a whole it gives an overview and synthesis of all the many situations that can be encountered based on the long history of the theory of vortex motion and on many new situations. It gives a renewed insight on the extraordinary richness of vortex dynamics and open the way for new theoretical, observational and experimental advances. This volume is of interest to experts in physical oceanography, meteorology, hydrodynamics, dynamic systems, involved in theoretical, experimental and applied research and lecturers, post-graduate students, and students in these fields.
Chapter
Stirring in the neutral (i.e., isopycnal) direction drives isopycnal mixing, which acts to homogenise tracers along the neutral plane. Isopycnal mixing does not directly enter the mechanical energy budget, and instead must be studied through a range of fluid dynamical perspectives, including quasigeostrophic turbulence theory, mixing-length theory, and Lagrangian kinematics. We review these frameworks and identify four cross-cutting challenges in the study of isopycnal mixing: inhomogeneity, anisotropy, non-locality, and scale dependence. In this context, we review recent progress and insights gained from observations of isopycnal mixing and high-resolution numerical simulations. We also describe some of the impacts of isopycnal mixing on global ocean circulation, tracer distributions, and biogeochemical cycles. We explain how isopycnal mixing is parameterised in ocean models and conclude with some important open questions for future research.
Chapter
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This chapter deals mostly with the dynamics of discrete vortices in a two-layer fluid and has the following structure. First, two vortices (a simplest heton) are used to demonstrate the characteristic features of a system of baroclinic vortices, after which, the results of analytical and numerical studying of vortex ensembles are studied, starting from relatively complex (systems of A and A + 1 vortices) with arbitrary A to simpler ones with A = 2. A class of motions of axisymmetric vortex structures in an external deformation field. The possibility of formation of chaotic regimes is examined. The last subsection of the chapter gives formulas describing the motion of simplest stationary vortex structures in a three-layer fluid.
Chapter
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In Sect. 1.1 of this chapter we explain the subject of Geophysical Fluid Dynamics, and give the description of main vortex structures that have become objects of the present book: (a) Heton – a two-layer vortex with opposite rotations in different layers, and (b) Intrathermocline lens, which is studied in this work as a vortex patch in the intermediate layer of a three-layer ocean model. In this section, we propose also a short review of works in the related topics. Section 1.2 is a mathematical introduction. It contains all main formulae which are used further in Chaps. 2 and 3.
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This chapter summarizes the main results obtained in the book, defines possible generalizations and evokes perspectives for future investigation as well as related problems.
Article
In the northeastern Atlantic Ocean, intrathermocline lenses (ITL) occur at depths of 600–1600 m. These ITLs are localized vortex patches (anticyclonic and cyclonic), generally of an elliptical shape with hor� izontal axes from 40 to 100 km, vertical axes from 0.4 to 1 km, and volumes of 1000–3500 km3. According to observations, the coexistence of several lenses is a rather common phenomenon in certain ocean areas. Thus, the problem of their interaction, and, in particular, the influence of lenses on larger vortices, is especially important. The aim of the present work is to study, using a three�layer quasi�geostrophic model, the interaction between intrathermocline lenses and synoptic gyres existing in few layers. Simulations show that synoptic gyres change significantly their shape under the effect of ITLs. The authors propose a tentative mechanism of ITLs’ deceleration because of the interaction with synoptic gyres, located at different layers. It is obvious that turbu� lent exchanges at mid ocean depths intensify when vortices collapse.
Chapter
We examine the nonlinear interactions between a zonal jet and a vortex (oceanic eddy), using a one-and-a-half-layer quasi-geostrophic numerical model. The eastward jet is defined by two strips of constant and opposite potential vorticity; it is stable in isolation. The anticyclonic eddy is initialized north of the jet by a circular patch of constant potential vorticity. We numerically determine the physical conditions under which the eddy crosses the jet meridionally, or drifts along its northern side. Three groups of dynamic regimes are found: the first one with the eddy staying north of the jet and drifting eastward, the second one with the eddy crossing the jet and then drifting away as a dipole, and the last one with the eddy also crossing the jet, and progressing eastward. Furthermore, an analytical criterion for the crossing of the jet is established, which remarkably compares with the numerical experiments. The sensitivity of these dynamic regimes to environmental parameters is finally examined.
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Finite-area monopolar vortices which propagate around topography without change in shape are computed for circular seamounts and wells including the limiting cases of each: islands and infinitely deep wells. The time-dependent behaviour of vortex pairs propagating toward circular topography is also examined. Trajectories of point-vortex pairs exterior to the topography are found and compared to trajectories of vortex patches computed using contour dynamics.
Article
The Mediterranean waters (MW) occupy intermediate depths (500–1500 m) of the Northeast Atlantic. These waters are transported by the bottom current to the Atlantic through the Strait of Gibraltar. Salinity of these waters exceeds 37.0 and the temperature is warmer than 13°C. In addition to the MW transport by the quasi-continuous “stream” component there is an irregular “lens” component. This component consists of intra-thermocline eddies or long-living MW lenses, the “Meddies”, which are formed west of the Iberian Peninsula and then spread over long distances. These lenses desintegrated in time and support the salt MW “tongue” far from the Strait of Gibraltar. The goal of this work is to give a brief review of the main research of this phenomenon and to determine the criteria to distinguish MW lenses based on selected field data. Peculiarities of the space and time evolution of the MW lenses are shown based on the T,S-analysis of the lens cores.
Article
The interaction of meddies with a complex distribution of seamounts is studied in a three-layer quasi- geostrophic model on the f-plane. This study aims at understanding if and how this seamount chain can represent a barrier to the propagation of these eddies and how it can be involved in their decay. The eddies are idealized as vortex patches in the middle layer, interacting with a regional cyclonic current and with ten idealized seamounts. The numerical code is based on the contour surgery technique. The initial position, radius, shape, number and polarity of the eddies are varied. The main results are the following: (1) Though they do not describe the unsteady flow, the streamlines of the regional and topographic flow provide a useful estimate of the vortex trajectories, in particular towards the major seamounts, where stronger velocity shears are expected. (2) The tallest and widest seamounts which have the largest vorticity reservoir are able to considerably erode the vortices, but also to draw anticyclones towards the seamount top. The ability of narrower seamounts to erode vortices is related to their multiplicity. (3) Only 1/3 of the anticyclones with about 30-km radius reach the southern boundary of the seamount chain, and their erosion is larger than 50 %. The other anticyclones are either completely eroded or trapped over a wide seamount top. Cyclones are less affected by seamounts because they oppose the topographic draft towards the seamount top and they drift along the side of the seamount. (4) Large vortices resist topographic erosion more efficiently. The rate of erosion grows from a few percent to about 35–50 % as the vortex radius decreases from about 60 to 30 km. Small cyclones are not eroded, contrary to small anticyclones (which completely decay), in relation with the different trajectories of these eddies in the vicinity of the seamounts. (5) The detailed vortex shape does not appear critical for their evolution, if they are close enough to the seamount chain initially. The interaction between a group of vortices initially north of the seamount chain can modify their trajectory to such an extent that they finally avoid collision with seamounts. (6) Finally, meddy trajectories across the Horseshoe Seamounts (data from the AMUSE experiment) show qualitative similarity with the vortex paths in the model. Several events of vortex decay also occur at comparable locations (in particular over the wide and tall seamounts) in the model and observations.
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In January 1990 a Meddy was observed 100 km to the west of the Irving Seamount just after it had passed through a narrow passage between Hyeres and Irving Seamounts located at the western edge of the Canary Basin. Vertically, Meddy "Irving' had two cores centred at 900 and 1150 m depth. In the horizontal plane the Meddy had an elliptical shape with axes of 100 and 60 km. Behind the Meddy, a wake of saline water was found which tracked the lens trajectory. Two small salty patches were observed inside the wake; these were probably split from Meddy "Irving' during the impact with bottom topography. Despite the damage caused by the collision with the seamounts, Meddy "Irving' kept its individuality and coherent structure. -from Authors
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Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediter- ranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent. Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15-20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3-7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast. The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D'Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.
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Three Meddies were tracked for up to two years in the Canary Basin using neutrally buoyant SOFAR floats. These Meddies have cores of warm, salty Mediterranean Water and are approximately 100 km in diameter, 800 m thick, and are centered at a depth of 1100 m. Meddy 1 was tracked for two years (1984–86) with five floats as it drifted 1090 km southward with a mean velocity of 1.8 cm s⁻¹. Four shipboard surveys made during these two years revealed the nearly total decay of Meddy 1 by gradual mixing processes. Meddy 2 drifted 530 km southwestward over 8.5 months with a mean velocity of 2.3 cm s⁻¹ until it collided with Hyères Seamount near 31°N, 29°W. The floats trapped in this Meddy then stopped looping abruptly, implying a major disruption of this Meddy. Meddy 3 drifted 500 km southwestward for a year and a half with a mean translation velocity of 1.1 cm s⁻¹. A comparison of the velocity of Meddies to the velocity of nearby floats at 1100 m depth outside of the Meddies shows clearly that all three Meddies moved southwestward through the surrounding water at a speed of about 1.3 cm s⁻¹. The floats inside the Meddies looped anticyclonically in a nearby solid-body rotation with a period of 6 days for Meddy 1, 4 days for Meddy 2, and 5 days for Meddy 3. The rotation period of Meddy 1 appeared to remain constant over nearly two years despite a large decrease in the Meddy's thickness and diameter due to mixing. Rotation velocities in the Meddies were as great as 34 cm s⁻¹ (Meddy 2), much faster than speed of nearby floats outside of the Meddies.
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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.
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The location, spreading pattern, mesoscale structure and the influence of bottom topography on the Mediterranean Water (MW) outflow were studied using historical hydrographie data from the Iberian Basin of the North Atlantic. It was shown on the basis of joint analysis of continuous and discrete component of the MW distribution, that after leaving the Gulf of Cadiz the MW outflow is broken into two main cores: Northwestern and Southwestern ones. The wide, intense Northwestern core spreads in a regular and continuous manner. West off the Tejo plateau it is split into three branches, the most intense of them keeps continuity up to 14 ° W. The less intense Southwestern core passes south of the Gettysburg bank and is split into two branches just after the Gulf of Cadiz. This core has a lenticular intermittent character starting with 11° W. West of 15° W all the branches of MW outflow spread in the form of isolated salty patches. In the Iberian Basin (32 ° − 44 °N, 8 ° − 22 ° W) 30 MW lenses were found out, 12 of which were not mentioned in the literature before.
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The Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) mesoscale experiment took place from July to November 1993 in the northern Canary Basin, where the circulation is dominated by the eastward flowing Azores Current (AC). A large data set was acquired from three hydrographic arrays (phases 1, 2, 3), current meter moorings, surface drifters drogued at 150 m, and 2000 m deep RAFOS floats. The analysis confirmed the large-scale observations previously made in this region but also provided new insights into fine-scale dynamics of the AC. The front was observed over the 6-month period. It was narrow (100 km) and mostly surface intensified (velocities reaching 40-50 cms-1). Whereas at the beginning of the experiment (phase 1) the AC was mainly zonal with weak oscillations, large meridional meanders were observed from phase 2 until the end of the experiment. They seem to be related to the arrival of two Mediterranean eddies (Meddies), which interacted with the AC [Käse and Zenk, 1996; Tychensky and Carton, this issue]. The front had a deep dynamical signature (down to 2000 m), with a 16-18 sverdrup (Sv) volume transport (0-2000 m depth integrated). The southward recirculation branch of the AC near 22°-23°W [Klein and Siedler, 1989] corresponds to meridional transport of 5-6 Sv. Then, 4.5 Sv of these waters are recirculating westward (along 31°-32°N). Some interesting new oceanographic results were obtained by examining the RAFOS float trajectories over the abyssal plain. The circulation is similar to that observed at the surface, with mean velocities of about 1-3 cms-1 and eddy kinetic energy
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A lens of Mediterranean water (Meddy) was tracked in the eastern North Atlantic for two years with SOFAR floats. The Meddy was first found between the Canary Islands and the Azores in October 1984. It center moved in an irregular pattern, at speeds of a few cm s⁻¹, and translated 1100 km to the south in two years. This Meddy was surveyed four times by CTD and velocity profilers, and once with the microstructure profiler EPSONDE. When observed during the first two surveys the Meddy had a core that was stably and smoothly stratified in both salinity and temperature, nearly uniform in the horizontal, and was saltier than the surrounding ocean by 0.65 psu. The Meddy was eroded from its edges, top and bottom, and lost salt and hat with an e-folding time of about one year. The salinity at the center remained at its original value during the first year and decreased during the second year. Evidence was seen for mixing by lateral intrusions, double diffusion, and turbulence; the intrusions are thought to be the most important mode of mixing in terms of salt and heat loss. Radial profiles of azimuthal velocity revealed a core in almost solid body rotation, with a period of 5–6 days corresponding to 0.35 times the local Coriolis parameter. During the October 1984 survey, the azimuthal speed had a maximum of 0.3 m s⁻¹ at a radius of 24 km. Both the radius and magnitude of the velocity maximum decreased with time. The anticyclonic circulation attained a maximum at the radius of the salinity front. As the lens was eroded from the sides, the radius of maximum velocity and the maximum velocity both decreased, but the rotation rate of the core remained fairly steady.
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The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m-2. This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.
Article
Vertical and horizontal austauch coefficients have been obtained from standard station data on the Mediterranean high-salinity tongue by use of a simple model including advection by a constant velocity and three-dimensional diffusion. It is shown that background effects can be reduced by applying the model to the salinity anomaly relative to a linear θ-S relationship. The analysis gives typical values for KH of 1.5 to 3×107 cm2 s−1 and for Kv of 0.3 to 0.7 cm2 s−1. It is argued that such values indicate the diffusion of potential density is not important in the main pycnocline of the North Atlantic anticyclonic gyre.
Article
Richardson, P.L., McCartney, M.S. and Maillard, C., 1991. A search for meddies in historical data. Dyn. Atmos. Oceans, 15: 241-265. A search was made using historical hydrographic data from the eastern North Atlantic to find measurements of very salty layers between 700-1300 m that could be observations of the warm, salty lenses known as meddies (Mediterranean water eddies). Twenty-five stations were found out of a total of 13551 with positive salinity anomalies of at least 0.4 psu, about half that of a strong meddy in the Canary Basin. These possi.ble meddy observations were combined with additional reported observations to show that meddies generally lie in an oval whose long axis extends 3000 km southwestward from the coast of Portugal. Five possible meddy observations were found north of this region, near 44 ° N, where they have previously never been reported.
Article
Two surveys of the absolute velocity field of an eddy of Mediterranean Water (meddy) in the Eastern North Atlantic were conducted one year apart in 1984 and 1985. Two velocity regimes were revealed. Within the radius of maximum velocity, the meddy rotated anticyclonically as a solid body with a depth-dependent rotation period near 6 days at its mid-depth (1000 m). One year later the radius of the core had decreased by one third. The rotation rate of the lens also decreased, except at its mid-depth where there was a small but perceptible increase. There was a sharp (5 km or less) transition between the core and the outer region where the velocity decayed exponentially with radius. A strong potential vorticity front, due to the abrupt change in sign of horizontal shear, kept the core isolated from the outer region. Potential vorticity was nearly constant within the upper confines of the core over the study period, whereas, there was a notable increase in potential vorticity in the lower portion of the core due to erosion from underneath. Although there were significant azimuthal velocities beyond the transition, the potential vorticity was nearly that of the background field. The horizontal uniformity of the potential vorticity field suggests free exchange along isopycnal surfaces.
Article
Isolated compact anticyclonic eddies or salt lenses were found in the Canary Basin. Hydrographic surveys of three such lenses show large anomalies of salinity and temperature (∼0.8, 2.5°C). They are centered at ∼1100 m, have a vertical extent of up to 900 m and radii of ∼50 km. Current meter records indicate anticyclonic velocities up to 29 cm s−1. Fine structure with vertical scales of ∼20 m and less, possibly due to intrusive decay, appears at the outer edges of the lenses whereas the centers are free of such structure. The probability of finding a salt lens at any station in the Canary Basin is fairly high (∼0.08).
Article
Drifting buoy data from the eastern tropical Pacific Ocean are used to evaluate the degradation of sea surface temperature and current information incurred by reducing the number of transmissions from drifting buoys using the ARGOS system for position finding. Buoy locations are interpolated at uniform time intervals using an optimum interpolation method known as Kriging, which provides also an estimate of the rms position error. It is found that the published standard for surface current measurement for the TOGA Program (5 cm s⁻¹) can be met with transmissions on one day of three in the Southern Hemisphere. Due to stronger mesoscale variability in the Northern Hemisphere the standard would be jeopardized by reducing transmissions even to one day of two. The standard for observation of sea surface temperature (0.1°C) can be met in either hemisphere with transmissions on one day of four. The Lagrangian decorrelation times for the Northern Hemisphere region of the eastern tropical Pacific are estimated as 4 days in the meridional direction, and 14 days in the zonal direction. It is recommended that transmissions be made on one day of three, and the time scale for the TOGA standard be revised accordingly.
Article
Mediterranean salt lenses (meddies) are a dominant factor in the salt budget of the Atlantic at middepth. In spite of their important role, their juvenile migration has not yet been directly observed. For the first time, two RAFOS float trajectories show strong evidence of a meddy along the Iberian continental slope off Lisbon. Over six weeks we obtained drift observations from two levels (629, 847 dbar). Both instruments recorded a series of loops with an azimuthal speed O (30 cm s-1) at a radius of about 25 km. Relatively high propagation speeds of several centimeters per second indicate the meddy was probably carried along with the undercurrent of Mediterranean Water. The Tejo Plateau, a prominent feature of the continental slope and a natural obstacle for the spreading Mediterranean Water tongue, appears to act as a deflector for advected meddies possibly formed by interaction of the undercurrent with the canyon-rich topography farther south.
Article
The evolution of a Mediterranean salt lens (Meddy) over a two year period is examined. Several nondimensional numbers can be used to describe the overall decay in the structure of the Moddy. Two Rossby numbers, one using the central relative vorticity and another using the radius and velocity of the azimuthal velocity maximum, did not change over the two year period. However, the Burger number N2H2/(f2L2) increased as the Meddy decayed. Another Burger number, the ratio of total kinetic energy to total available potential energy, decreased from 1.1 to 0.6 over a one year period. The rates at which the Meddy lost salt and heat are consistent with estimates of horizontal fluxes by intrusions. A horizontal diffusivity of O(5 m2 s−1) is needed if this flux by intrusions is parameterized by an eddy coefficient. Simple models of the evolution of an isolated eddy by horizontal and vertical mixing of mass and momentum are examined. These simple attempts to explain the evolution of the Meddy suggest more c...
Article
The Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Expérimentale (SEMAPHORE) oceanographic experiment surveyed a 500 × 500 km2 domain south of the Azores from June to November 1993 and collected hydrological data, float trajectories, and current meter recordings. This data exhibited three intrathermocline eddies of Mediterranean water (Meddies), two of them being repeatedly sampled. Their hydrological and dynamical properties are quantified here by an isopycnic analysis. For the three Meddies, intense temperature and salinity anomalies (up to 4°C and 1.1 practical salinity units (psu)) are observed extending vertically over up to 1000 m and centered around 1000 m. Horizontally, these anomalies spread out to radii of 50–60 km, while the maximum azimuthal velocities (30 cm s−1, as computed by geostrophy) lie only at 35–40 km from the central axis. These Meddies followed curved trajectories, with drift velocities up to 7.5 cm s−1, under the influence of the neighboring mesoscale features (cyclonic vortices or Azores Current meanders). The three-dimensional structure of potential vorticity in and around these features evidences their complex interactions. Northwest of the domain, a Meddy was coupled to a subsurface anticyclone, forming an “aligned” vortex. It later interacted with the Azores Current, creating a large-amplitude northward meander by vertical alignment of vorticity. In the southeastern part of the domain, another Meddy was vertically aligned with an anticyclonic meander of the Azores Current and horizontally coupled with a cyclone of large vertical extent. These two features, as well as a small warm and salty fragment in their vicinity, seem to result from the southward crossing of the Meddy under the Azores Current. These observations illustrate previous theoretical studies of baroclinic vortex dynamics.
Article
This paper reports the results of a hydrographic survey and the successful deliberate deep droguing of a meddy (Pinball) and the seeding of its core with two ALACE floats. The drogued buoy results give important kinematic properties of the eddy core in real timesemicolon the ALACE have allowed the position of the meddy to be tracked for seven months. Pinball was found against the continental slope near Lisbon canyon. The maximum core salinity was 36·564 psu, at a depth of 1260 m, but the maximum rotation rate with period ~2·5 d was in the upper core near 700 m, where temperatures reached 13·2°C. The azimuthal transport to a radius of 50 km was ~13 Sv. Pinball moved from the continental slope near Lisbon to the central Tagus Abyssal Plain, returned towards the continental slope and then moved westwards crossing the central Tagus Abyssal Plain a second time. At times it had a marked remote sensing infra-red sea-surface signature. It moved ~550 km over 204 d and the near real-time data meant that, in principle, this eddy could have been re-surveyed, redrogued or reseeded with floats during this period.
Article
The formation of a Mediterranean Water eddy, or meddy, was observed directly for the first time off the southwestern coast of Portugal near Cape St. Vincent. The formation event is revealed in the 30-day trajectory of a RAFOS float deployed in the lower core of the Mediterranean Undercurrent in the Gulf of Cadiz. For the first several days after deployment, this float was advected westward in the Undercurrent at a speed of about 0.4 m s−1, generally paralleling the topography. Just after passing Cape St. Vincent, where the coast turns abruptly northward, the float began looping anticyclonically, indicating that it had become trapped in the core of a new meddy. The meddy translated first westward, then southwestward along the southern flank of Gorringe Bank. The float rotated around the meddy center with azimuthal speeds of 0.20–0.25 m s−1 at a radius of about 10 km. The rotation period was on the order of three days, and the average translation speed of the meddy over 25 days was 0.08 m s-1. Observations of this and four additional 30-day trajectories indicate (1) persistent westward flow of the Undercurrent along the south coast of Portugal, (2) a tendency for the lower core of the Undercurrent to separate from the continental boundary after passing Cape St. Vincent, and (3) evidence of anticyclonic looping west of Cape St. Vincent. These preliminary results confirm the speculation that the region off Cape St. Vincent is one site of meddy generation.
Article
SOFAR floats that looped in discrete eddies were studied in order to map and describe the distribution and characteristics of eddies in the North Atlantic. One hundred eighteen individual looping float trajectories (loopers) were identified, each consisting of two or more consecutive loops. Each looper was interpreted to be in a discrete eddy, and its characteristics were estimated from the float trajectory. The highest percentage of loopers occured at 700m in the Newfoundland Basin, where roughly half of the float data were in loopers, mostly cyclones. In the Gulf Stream region, approximately 20% of the float data recorded at 700m were in loopers, again mostly cyclones. Overall, 21% of 700m data and 6% of 2000m data were in loopers.
Article
Data from approximately 144,000 hydrographic stations in the North Atlantic have been retrieved from the national Oceanic Data Centre and analysed to produce maps of mean pressure, temperature, salinity and oxygen on selected potential density surfaces for the domain bounded by 0°–85°W and 0°–65°N. The data span the period from 1904 to 1990 with the majority of the data from the last four decades. The data set for this region is 60% larger than that used in the production of Levitus' Climatological World Atlas. This increase in stations, coupled with smoothing scales specific to the North Atlantic rather than the global ocean, considerably improves the resolution of the basin's features. The mean property fields and their associated standard deviations are resolved on a one-degree grid with little smoothing, contrasting with the Levitus Atlas where properties, although presented on a one-degree grid, have been smoothed on the order of 1000km. Another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid. In a significant departure from the Levitus analysis, which averaged on depth surfaces, these data were averaged on potential density surfaces, thus eliminating an artificial mixing of water mass properties. The database is used to describe the baroclinicity of well-known features such as the Gulf Stream, the North Atlantic Current and the Deep Western Boundary Current, and to elucidate the recirculations associated with these currents. It additionally resolves several new features in the intermediate and deep North Atlantic. These features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the North Atlantic Current to the separation point of the Gulf Stream near Cape Hatteras. This recirculation, which spans approximately 2000m of the water column, encompasses more local recirculations and potentially mixes subpolar and subtropical waters. Furthermore, in the upper thermocline, this database reveals a coherent Azores Current that stretches from the Gulf Stream system south of the Tail of the Grand Banks to Madeira. This flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed.
Article
Two stacked outflow cores of the Mediterranean Water undercurrent pass through a broad “gateway” between Cape St. Vincent and Gettysburg Bank entering the Iberian Basin. The upper core (depth ∼750 m, σ1=31.85) shows a strong tendency to follow the contours of the Portuguese continental rise. Yet, the lower core (depth ∼1250 m, σ1=32.25) primarily meanders west and northwestward forming large blobs of Mediterranean Water. The predominance of isolated Meddy structures embedded in a background field is reflected in a long-term current meter record from the deep Iberian Basin.
Article
Originally published in Journal of Marine Research 39 (1981): 31-52 The warm, salty water influenced by the Mediterranean outfow can be observed at mid-depth throughout the Central North Atlantic Ocean. Katz (1970) first noted that rather than a gradual salinity decrease away from the source, large changes were observable over major topographic features such as the mid-Atlantic Ridge, despite the fact that the topography presented no direct physical barrier to the core layer. Two mechanisms are considered which can account for this sub-surface frontal transition: variable eddy diffusivity and horiontal shear induced by submarine topography. The structure of the actual geostrophic currents and water masses southwest of the Azores is explored with CTD, XBT, and float data collected in June 1977 aboard the R.V. Knorr. A CTD section normal to and crossing the ridge axis near 35N, 35W shows the thermocline to be domed up 200 m over the ridge axis compared with stations 200 km to either side. At 1000 meters depth a change in salinity of nearly 0.15% in the Mediterranean Water is observed to occur over a horizontal distance of 100-150 km, and is located west of the ridge near 38N and over the ridge axis near 34N. Near this transition two neutrally buoyant floats were tracked for a period of a day. CTD stations around and over a float at 810 meters depth showed the temperature and salinity intrusions to persist with time and horiontal space scales in excess of 15 hours and 4 km. If the currents observed SW of the Azores are representative of the general circulation of the region, they not only explain the water mass structure in the thermocline and Mediterranean Water but also point out that the historical data base of zonal hydrographic sections does not adequately resolve the baroclinic structure. The research program reported was supported by the Office of Naval Research, contract N00014-76-C-197 NR083-400 with the Woods Hole Oceanographic Institution.
Article
An anticyclonic lens of water in the permanent thermocline off the Bahamas has water mass characteristics representing Mediterranean and eastern Atlantic central waters. This eddy's ability to translate across the Atlantic without losing its identity points to baroclinic eddies as a specific mechanism for large-scale mixing.
Continen-tal margin off northwest Africa, bathymetry of the north-east Atlantic, sheet 5, Hydrographer of the Navy, Hydro-graphic Department
  • P M Hunter
  • R C Searle
  • A S Laughton
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