Properties and pathways of Mediterranean water eddies in the Atlantic
Introduction
Mediterranean water (MW) represents one of the major water masses in the Subtropical North Atlantic extending from the continental slope of Iberia to the Mid-Atlantic Ridge, and further on to the Western Atlantic. The outflow of the MW through the Gibraltar Strait presents exceptionally strong anomalies of temperature, salinity, nutrients, sediments, etc. for Atlantic mid-depths, which makes it unique in the World Ocean. It largely influences the physical and chemical properties, as well as the stratification of the water column from the sea-surface to, at least, 2000 m depth (Mauritzen et al., 2001). Studies of the mechanisms of the MW spreading in the Atlantic bring up the two main hypothesis (Bower et al., 1997, Mazé et al., 1997, Iorga and Lozier, 1999a, Iorga and Lozier, 1999b, Sparrow et al., 2002, Lozier and Stewart, 2008): the advective transport and the integrated transport by Mediterranean water eddies.
Mediterranean water eddies (meddies) are generated from instabilities of the Mediterranean Undercurrent (MUC) and are encountered across the North-East Atlantic basin from Iberia to the Mid-Atlantic ridge and from tropical to mid-latitudes (Richardson et al., 2000). Meddies are mesoscale vortices, 10–50 km in radius, with one or two vertically aligned cores typically centered between 800 and 1200 m and detected in temperature-salinity sections as strong lens-like anomalies in temperature (up to 4 °C) and salinity (up to 1) (Richardson et al., 2000). Meddies rotate anticyclonically with azimuthal velocities up to 50 cm s−1 (Richardson et al., 2000). Since the MUC water is rather homogeneous as compared to the North Atlantic Central Water, meddy cores are characterized by a strong potential vorticity anomaly, further enhanced by the anticyclonic rotation of their cores. The gradient of potential vorticity at a meddy boundary presents a barrier for particle exchange with the ambient fluid, which is a reason for long lifetimes of meddies.
A single meddy transports 109–1011 tons of salt (Shapiro et al., 1995b, L’Hégaret et al., 2014) hundreds of kilometers away from the Iberian coast, with an average velocity of 1–5 cm s−1, releasing the salt and heat to the ocean. The estimates of the role of meddies (and MW cyclones) in the formation of the Mediterranean salt tongue in the Atlantic vary widely. Bower et al. (1997), based on one year of observations, calculated that 15–20 meddies generated per year, is enough to support about 50% of MW salt flux. This corresponds to the estimate of 17 meddies generated per year by Richardson et al. (2000), as well as to the independent estimate of the meddy salt flux by Arhan et al. (1994). Meanwhile, some of the formation regions, like Portimao Canyon and the Gorringe Bank, were not taken into account (Serra and Ambar, 2002) and the meddy flux is possibly underestimated. For example, in the model study by Barbosa Aguiar et al. (2013) the annual meddy formation rate has been evaluated to reach 26 meddies per year. Mazé et al. (1997), using 3 hydrographical surveys in the Iberian basin, concluded that 100% of the westward MW spreading is due to meddies. Sparrow et al. (2002), using trajectories of RAFOS drifters, inferred that the MW advection plays an important role north of 36°N, while south of this latitude, the transport is mainly due to meddies. On the other hand, Barbero et al. (2010) did not find any persistent advective MW transport in the region 39–45°N and 16–21°W.
As mentioned above, Meddies are formed from the MUC. The main formation regions are: Portimão Canyon (Serra and Ambar, 2002), Cape St. Vincent (Prater and Sanford, 1993, Richardson et al., 2000), Lisbon and Setubal Canyons just south of Estremadura Promontory and Nazare Canyon just north of it (Richardson et al., 2000). A possible meddy generation at the Gorringe Bank (Serra and Ambar, 2002), Porto and Aveiro Canyons (Chérubin et al., 1997) has been discussed in literature. In this paper we argue that meddies may also form at Cape Finisterre and Galicia Bank. Meddies might also form north or northeast of the Iberian Peninsula. Thus, a meddy observed just north of the Iberian Peninsula is thought to be formed in the Gulf of Biscay (Carton et al., 2013). Paillet et al. (2002) suggested that their meddy Ulla could have been formed near Cape Ortegal (at the northern coast of the Iberian Peninsula).
Two main mechanisms of meddy formation have been proposed: the baroclinic instability of the MUC or of its seawards branches (McWilliams, 1985, Chérubin et al., 2007) and the detachment of the shear lateral/bottom boundary layer at capes and canyons (D’Asaro, 1988, Aiki and Yamagata, 2004). Computations suggest that already 100 km downstream of the Gibraltar Strait (i.e. east of Portimão Canyon) bottom friction on the continental slope can generate a relative vorticity in the MUC, similar to that observed in the newly formed meddies: that is 0.2–0.3f, where f is the Coriolis parameter (Prater, 1992).
In their model study, Aiki and Yamagata (2004) also showed that a rapid change in direction of a coastline (i.e. a cape or a promontory) may not be indispensable for the early generation of a meddy, but is absolutely necessary for meddies to separate from the continental shelf. Therefore, meddy detached at capes may be generated upstream (relative to the direction of the MUC). As a meddy detach from the continental slope, a surface intensified cyclone is often formed upstream (Richardson et al., 2000, Serra et al., 2002, Aiki and Yamagata, 2004, L’Hégaret et al., 2014). Being intensified, the cyclone advects a meddy around its boundary, forcing it away from the continental slope (Richardson et al., 2000). Aiki and Yamagata (2004) relate formation of such cyclones with the instability of the surface upwelling current at the capes. Such cyclones may also be formed as a consequence of the upper ocean potential vorticity conservation at the lee side of a drifting meddy (Bashmachnikov and Carton, 2012), or, for of the MUC instabilities in canyons, as a part of coupled mushroom-like structures (Serra et al., 2005).
After detachment from the continental slope, meddies propagate westwards. Four major meddy pathways have been identified in regional studies (Shapiro and Meschanov, 1996, Demidov et al., 2012): from the Portimao-St. Vincent region to the Canary Islands; from St. Vincent-Estremadura region along the southern flank of Josephine seamount and farther southwest; from St. Vincent-Estremadura region along the northern flank of Josephine seamount and farther west; and northwestward along the southern flank of the Galicia Bank (Demidov et al., 2012). Demidov et al. (2012) mention that about 70% of meddies leave the Iberian Peninsular to travel west-southwest and only a third of them drifts north or northeast.
For mesoscale eddies their sufficiently large horizontal scales allow the planetary -effect to efficiently affect their propagation (Nof, 1983). One mechanism of meddy self-translation is the generation of secondary circulations in its core (β-gyres) in a spatially varying background potential vorticity field (Cushman-Roisin et al., 1990). After a meddy has started its motion, the surrounding fluid is forced sidewise, and the same background potential vorticity gradient generates the ambient vorticity torques, which enhance the translation. As a result, a meddy self-translates along the isolines of the background potential vorticity, on the planetary -plain – to the west, with a characteristics velocity (Marshall, 1988, Cushman-Roisin et al., 1990). Here is the Rossby radius of deformation taken as the measure of the eddy horizontal scale, is the mean thickness of the density layer where the eddy resides and is the isopycnal deviation at the eddy center (Cushman-Roisin et al., 1990, Morrow et al., 2004).
The entrainment of the surrounding fluid in the gradient of the background potential vorticity also results in a southward drift of a meddy, which is at least an order of magnitude smaller than its westward self-translation (Cushman-Roisin et al., 1990). This southwards drift can also result from the dissipation of a meddy via lateral intrusions or via the radiation of Rossby waves (Flierl, 1984, Colin de Verdière, 1992): the consequent decrease of the relative vorticity in the meddy core induces the drift by virtue of potential vorticity conservation in the core.
The dynamic similarity of the topographic -effect or the baroclinic -effect to the planetary -effect for quasi-geostrophic motions, means that the eddy propagation direction/velocity depends on the relative intensity of the -effects and on the direction of the corresponding potential vorticity gradients. The intensity of the -gyres, generated by the baroclinic -effect of the mean flow, increases with the increase of the intensity of the flow (Morel and McWilliams, 1997). This self-translation mechanism is directed against eddy advection by a zonal flow and can move mesoscale eddies (including meddies) against the dominant flow (Morel and McWilliams, 1997). This counterflow propagation holds for a zonal current invariant with depth (van Leeuwen, 2007), as well as for a vertically sheared one (Vandermeirsch et al., 2001). It is not observed for submesoscale vortices, for which the pure advection by background current dominates (Dewar and Meng, 1995). On another hand, in a southward background current the baroclinic -effect is directed with the mean flow, and this flow can be efficient in advection of meddies (Morel, 1995).
Additionally, an anticyclonic vortex in a large-scale shear background flow tends to migrate towards the current axis in the area of opposite background vorticity and away from the current axis in the area of the like-signed background vorticity (Bell, 1990). The similar behavior is obtained for a meddy interacting with a jet-like flow, the horizontal scale of which is comparable with the meddy diameter (Vandermeirsch et al., 2003). In particular, for a meddy approaching from north the eastwards Azores Current, this means crossing the jet. The mechanism of crossing the jet lies in generation of an anticyclonic meander upstream and a cyclonic meander downstream in the jet. The cyclone, being intensified, pushes the meddy south. But only a sufficiently strong meddy can cross a jet, i.e. if its maximal azimuthal velocity exceeds that in the axis of the jet (Vandermeirsch et al., 2003). After crossing, the meddy may be expelled to the south, or stay trapped at the southern boundary of the jet. This depends on the relative intensity of the cyclone and the anticyclone simultaneously formed in the jet by the meddy (Vandermeirsch et al., 2003). 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).
When coupled with a cyclone of the same strength in the same layer, a meddy also tends to drift along isolines of the background potential vorticity (Velasco Fuentes and van Heijst, 1995). Lager and stronger anticyclones or cyclones (including the surface ones) are seen by a weaker meddy as strong local anomalies of potential vorticity. The former are found to sharply change the direction of meddy propagation, advecting the latter around their periphery (Schultz Tokos et al., 1994, Richardson et al., 2000, Carton et al., 2010).
Meddies, separated from the surrounding waters by strong gradients of potential vorticity, are quite stable structures with life times often exceeding 1 year. The mechanisms of meddy decay can include gradual vertical and horizontal diffusive exchange with the background due to instability of meddy periphery (Hua et al., 2013), along-isopycnal lateral intrusions (Hebert et al., 1990), entrainment of meddy periphery by a background flow (Mariotti et al., 1994), internal meddy instability resulting in core deformation and in filaments formation (Ménesguen et al., 2012), energy dispersion into lee Rossby waves (Flierl, 1984), loss of a part of meddy core, splitting or complete decay while interacting with seamounts (Richardson et al., 2000, Cenedese, 2002, Bashmachnikov et al., 2009a, Sokolovskiy et al., 2013).
Hebert et al. (1990), in their 2 year-long periodic re-sampling of meddy Sharon, conclude that lateral intrusions are a more efficient mechanism of meddy decay than horizontal/vertical diffusion. The former mechanism, though, cannot compete in its destructive efficiency with the more rare, but more drastic meddy-seamount interaction. The Meddy seamount interaction, in turn, can be strong and lead to meddy significant or complete erosion, moderate in the case of meddy splitting and possible reunification behind the seamount, or weak in the case of meddy rotation around a seamount flank, leaving the seamount without any significant change of its properties (Shapiro et al., 1995a, Richardson et al., 2000, Cenedese, 2002, Herbette et al., 2003, Adduce and Cenedese, 2004, Sokolovskiy et al., 2013). The strength of the interaction depends on horizontal seamount dimensions in comparison to the radius of the meddy, as well as on the height of the seamount (van Geffen and Davies, 2000, Sokolovskiy et al., 2013) and is enhanced in the presence of a background flow (Cenedese, 2002). In-situ observations and numerical models suggest that during interactions with a seamount a meddy typically loses around 25–40% of its core (Shapiro et al., 1995a, Richardson et al., 2000, Wang and Dewar, 2003, Bashmachnikov et al., 2009a).
In spite of a significant progress in our understanding of meddy pathways and dynamics in the recent decades, the observational evidences of the theoretical and model results mostly cover case studies of some parts of meddy life cycles (Shapiro et al., 1995b, Richardson et al., 2000, Demidov et al., 2012). This work presents the most complete for the moment, systematic and holistic overview of observations of meddies, and gives further details on meddy characteristics and dynamics, as a function of their distance from the generation region and latitude.
Section snippets
Hydrological data
Data from the World Ocean Database 2013 (WOD) were downloaded from the National Oceanographic Data Center (NODC, http://www.nodc.noaa.gov/OC5/WOD/pr_wod.html): OSD (Ocean Station Data, low vertical resolution), CTD (Conductivity–Temperature–Depth, high vertical resolution) and PFL (Profiling Floats, with various vertical resolutions, mainly obtained from the ARGO profiling float array). We only use data collected between 1950 and 2012. The climatological reference state is obtained from the
Meddy distribution and pathways
In this section, the probability to find a meddy in different parts of the Atlantic is computed as a ratio of the number of casts through meddies to the total number of casts in 1° × 1° areas. To avoid local biases in the probability distribution, resulting from in-purpose multiple sampling of the same meddy during a cruise, the data-points, sampled in approximately the same place and approximately at the same time, are clustered to a single point. The clustering is done separately for meddy and
Conclusions and discussion
In this study we combined data from ship vertical casts (NODC data-set), ARGO profiling floats (Coriolis data-set) and RAFOS-type neutral density floats (WOCE data-set) to derive of spatial variations of meddy characteristics in the NE Atlantic. From the overall 26,062 vertical casts available, 775 meddies were identified using Richardson’s criterion (salinity anomaly exceeds 0.2 in the MW layer, Richardson et al., 1991). 241 of the meddies, sufficiently covered with observations, were selected
Acknowledgements
The authors acknowledge the scientific project MEDTRANS (PTDC/MAR/117265/2010), sponsored by the Portuguese Foundation for Science and Technology (FCT) and Marine and Environmental Sciences Center (MARE) of the University of Lisbon (CO-Pest-OE/MAR/UI0199/2011). I.B. also acknowledges the contract C2008-UL-CO-3 of Ciencia 2008 between Foundation for Science and Technology (FCT) and the University of Lisbon (UL). The authors acknowledge Joana Medeiros for collecting meddy characteristics from
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