laboratory experiments on mesoscale vortices colliding with a

Laboratory experiments on mesoscale vortices colliding with

a seamount

Claudia CenedesePhysical Oceanography Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

Received 21 August 2000; revised 18 June 2001; accepted 3 October 2001; published 19 June 2002.

[1] Interaction between a vortex and a right vertical cylinder was investigated in thelaboratory for both a self-propagating vortex and one advected by a backgrounduniform flow. In the former case, experiments were carried out with a sloping bottomin order to simulate the b plane. In the latter case the bottom was flat and a cylinderwas towed, with a uniform speed, through a fluid otherwise at rest and into astationary vortex. In both cases, after a cyclonic vortex came in contact with thecylinder, fluid peeled off the outer edge of the vortex and went around the cylinderwith a counterclockwise velocity vs as predicted by the circulation equation. This fluidformed a new cyclonic vortex in the wake of the cylinder, and bifurcation of theoriginal vortex into two vortices occurred provided 400 � Re � 1100, where theReynolds number Re = vsLmax/n and Lmax is the larger of the vortex or the cylinderdiameter. This result is in agreement with previous studies of uniform flow past acylinder in a rotating environment, and therefore we suggest that the new vortex in thewake of the cylinder was formed like those in the well-known Karman vortex street.Experiments have been carried out systematically by varying D/d, the ratio of thecylinder diameter to the vortex diameter and the geometry of the encounter. The resultssuggest that the presence of a background flow enhances the bifurcation mechanism. Agood agreement between the laboratory experiments and the observation of a meddybifurcating after collision with the Irving Seamount in the Canary Basin, suggests thatthe oceanic vortex-bifurcation process is similar to that observed in the laboratoryexperiments. INDEX TERMS: 4520 Oceanograpny: Physical: Eddies and mesoscale processes; 4508

Oceanography: Physical: Coriolis effects; KEYWORDS: bifurcation, collision, seamount, laboratory

experiment, mesoscale vortices, meddy

1. Introduction

[2] Intense subsurface vortices of warm, salty Mediterra-nean water (meddies) are generated near the Strait ofGibraltar, primarily near Cape St.Vincent [Bower et al.,1997], and translate westward into the Atlantic. Meddies arecharacterized by their high salinity and temperature relativeto the surrounding water and have a radius ranging between10 and 50 km, a vertical extent of about 1 km, and a coredepth in the range 1100–1200 m, and they are approx-imately symmetric about a horizontal plane passing throughthe core. The azimuthal velocity of these lenses is approx-imately 0.2 m s�1 at radii between 30 and 45 km. Becausetheir core fluid is protected from external mixing within aswirling vortex, meddies can survive for several years [Armiet al., 1988] while transporting anomalous water massproperties over thousands of kilometers. Meddies arethought to play an important role in the maintenance oflarge-scale temperature and salinity distributions in themiddepth North Atlantic, especially the Mediterranean salttongue, but it is not clear how important. For example,

Arhan et al. [1994] suggested that meddies may be respon-sible for more than 50% of the westward salt flux at thelevel of the Mediterranean water. Recent studies [Bower etal., 1997; Richardson et al., 2000] gave good informationon the number of vortices that form per year, their sizes andtheir life histories, giving insights into the role of meddies inthe salinity distribution in the middepth North Atlantic. Oneof the most interesting results from this study, analyzing atotal of 27 meddies, was that most meddies (�90%)collided with major seamounts after a mean life of 1.7years and that the collision often resulted in a majordisruption of the vortical structure. The large number ofmeddy collisions with seamounts and the observed dispersalof subsurface floats during and after collisions suggest thatmuch of the warm, salty Mediterranean outflow wateradvected by these meddies is dispersed into the surroundingregion. A decrease in temperature measured by floats loop-ing in meddies, and especially the cold spikes observedduring collision, is interpreted to be the entrainment ofcolder background water into cores of meddies where it israpidly mixed with warmer meddy water. The effect ofmeddies on the salt tongue could be very different depend-ing on whether meddies remain clear of seamounts andslowly decay over long periods of time (�5 years) as they

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C6, 3053, 10.1029/2000JC000599, 2002

Copyright 2002 by the American Geophysical Union.0148-0227/02/2000JC000599$09.00

6 - 1

slowly translate through the ocean or crash into seamountsand are rapidly destroyed over periods of a few weeks to afew months. In the first case, meddies presumably leavea dilute trail of salty water in their wake. In the second casea much stronger concentration of warm salty water wouldbe injected locally in the vicinity of seamounts.[3] Previous studies by Lucas and Rockwell [1988],

Orlandi [1993], Voropayev and Afanasyev [1994], andVerzicco et al. [1995] investigated vortex dipoles interactingwith a circular cylinder. Those studies were motivated byproblems of central importance in the area of aeroacoustics/hydroacoustics, drag reduction, and heat transfer in practicalapplications, e.g., the sound generation by a jet impingingon a wall, the free-surface signature of trailing vortices ofsubmarines, or the behavior of vortices produced by anaircraft during landing or takeoff. While dynamically veryinteresting, the Re in these studies is usually much higherthan that involved in the collision of a mesoscale vortexwith a seamount. In addition, these studies focused only ondipoles, while mesoscale vortices are monopole features.Hence it is difficult to extend these studies’ results to theoceanic case of interest herein. As discussed in section 4, aseemingly endless list of studies have been performed onflow past a cylinder with von Karman [1954] being a primeexample. The extension of these studies to a rotating

environment [Boyer and Kmetz, 1983; Boyer et al., 1984]and in the presence of a b plane [Boyer and Davies, 1982]has also been done. However, the problem of a monopolevortex interacting with a cylinder has not yet been inves-tigated. Recent studies by Simmons and Nof [2000] ana-lyzed the interaction of a monopolar vortex with a thinmoving straight wall. In order for a zero potential vorticitylens (with a radius r) to split into two equal lenses the walllength must be at least 1.19r. The interaction of a vortexwith a thin wall is different than a vortex interacting with acylinder; the most striking difference is that in the formercase the horizontal scales have different orders of magni-tude, but in the latter case they are the same. Furthermore, acylinder is a more realistic representation of most of theseamounts encountered by meddies in their westward drift.[4] In order to examine the physical processes that

govern what happens when meddies collide with seafloortopography a series of idealized laboratory experimentshave been performed. These focused in particular onmonopole vortex interaction with an obstacle extendingthroughout the fluid depth in order to simulate the meddies’drift and collision with large topographic features. Thismodel simplifies a number of important features of theoceanic flow, and it is necessary to understand what rolethey play in the meddy interaction with a seamount.Although the experiments reproduce the collision of avortex with an island (defined as a cylinder extendingthroughout the whole fluid depth), we believe they approx-imate well the oceanic case where the seamounts usuallyextend up to 250 m below the free surface, much higherthan the level of interaction with the meddy (approximatelybetween 500 and 1500 m below the free surface), as clearlyshown by Richardson et al. [2000, Figure 9]. These sea-mounts present a side slope that is quite steep (�4%), whichis therefore well represented in the laboratory by the verticalstraight walls of the cylinder. This has been confirmed witha simple scaling analysis. Furthermore, the vortex generatedduring the experiments extends for almost the whole fluiddepth as described in section 2 and is embedded in anhomogeneous fluid, while meddies are interior features andmove within a stratified environment. This is possibly theweakest point of the model, but the simplicity of thehomogeneous versus the stratified experiments made itmore appropriate for this initial study. The good agreementsbetween the results obtained and the oceanic observation ledus to believe that stratification does not invalidate therelevance of the results discussed herein.[5] The vortex interaction with a cylinder was investi-

gated both by allowing the vortex to self-propagate over asloping bottom (first configuration, Figure 1a) and toimpinge on the cylinder and by moving the cylinder intothe vortex (second configuration, Figure 1b). The first caseapproximates the situation of a meddy self-propagating inthe Atlantic Ocean because of the b effect. In the secondcase the moving cylinder approximates the situation of ameddy swept by a uniform background flow into a sea-mount. Two important geometrical parameters regulated theflow: the ratio D/d of the cylinder to the vortex diameter andthe ratio Y/R measuring the geometry of the encounter(Figure 2).[6] The experimental apparatus is described in section 2.

Theoretical consideration of the circulation around an island

Figure 1. Sketch of the experimental apparatus: (a) firstconfiguration(self-propagatingvortex) and(b) secondconfig-uration(vortexadvectedbyabackgroundflow).

6 - 2 CENEDESE: MESOSCALE VORTICES COLLIDING WITH A SEAMOUNT

and the flow past a cylinder are presented in sections 3 and4, respectively. The results for the self-propagating vortexand the vortex advected by a background flow are given insections 5 and 6, respectively. We discuss the results insection 7 and compare them with oceanographic observa-tion in section 8. Finally, the conclusions of the work arediscussed in section 9.

2. Experiments

[7] The experiments were conducted in a glass tank ofdepth 60 cm and base 61 cm. This was mounted on a directdrive, 1 m diameter, rotating turntable with a vertical axis ofrotation. We used a square tank to avoid optical distortionfrom side views associated with a circular tank. The tankwas centered on the vertical rotation axis of the table.[8] A sketch of the apparatus used in the first config-

uration (self-propagating case) is shown in Figure 1a. Thetank had a bottom slope s to simulate the b effect. Theshallowest part of the tank corresponded to the ‘‘northern’’shore of the Northern Hemisphere topographic b plane. Eastwas to the right looking onshore; west was to the left; andsouth was the deepest end. The tank was filled with freshwater. A cylinder of diameter D was positioned in thecentral part of the tank. The cylinder bottom was sliced atan angle so it rested flush with the sloping bottom. Acyclonic vortex was generated by placing an ice cube inthe water [Whitehead et al., 1990], with the diameterdetermined by the ice cube size. Cyclogenesis arises fromconduction of the low ice temperatures to the surroundingwater, which then sinks as a cold plume and forms a colddense lens on the bottom. Pellets placed on the free surfaceclearly showed an inward velocity toward the ice cube thatconsequently influenced by the Coriolis force, gave rise tocyclonic velocities. This mechanism takes place during aninitial transition period and makes large and energeticcyclonic vortices. After this initial period, vortex proprietiessuch as radius and vorticity are conserved, the melting isvery slow (possibly because of the formation of a zone ofcold water around the ice cube), and the fluid is hydrostaticsince vertical velocities have been observed to be negligible

as compared to horizontal azimuthal velocities. Althoughmeddies are anticyclonic vortices, in the laboratory wecould not reproduce barotropic anticyclones since they tendto be centrifugally unstable [see Kloosterziel and van Heijst,1991] and become nonaxisymmetric in few rotation periods.Furthermore, the generation of a baroclinic anticyclonicvortex extending for most of the total fluid depth is lesspractical and does not give satisfactory results in compar-ison to its cyclonic analog. The use of cyclonic vorticesdoes not limit the generality of the results, which can beeasily extended to anticyclones as discussed in section 8.[9] The main interest of this paper is concentrated on the

water column spinning cyclonically above the dense lens.The inward velocities initially present along the entirecolumn depth above the bottom lens are balanced, in orderto conserve mass, by radially outward velocities within thebottom dense lens. Since the lens depth is approximately atenth of the whole column depth, the outward velocities are10 times larger than the inward velocities. Dense fluid flowsradially outward from the bottom dense lens with a velocityvery large in comparison to the rotation period and thereforedoes not form an anticyclonic vortex. When a slopingbottom is present, the fluid flowing radially outward fromthe bottom lens moves slowly downslope within the Ekmanlayer as shown in Figures 6a and 7a and then veers rightinfluenced by the Coriolis force. The fluid within the denselens moves downslope together with the whole cycloniccolumn above it. Influenced by the Coriolis force, the coldlens, together with the cyclonic column above it, veers rightand starts drifting westward as shown in Figures 6a–6b andFigure 7a–7b. In the presence of a flat bottom the fluidsimply spreads radially, and on the outer edge it spreadswithin the Ekman layer as shown in Figure 10.[10] In the second configuration (background flow case)

the bottom of the tank was flat, simulating an f plane, and acyclonic vortex was generated with an ice cube as describedabove (Figure 1b). A cylinder was towed by a step motorthrough the tank from left to right. A vortex advected by auniform background flow current interacting with a cylinderis entirely equivalent to a cylinder towed with a uniformspeed through a fluid otherwise at rest and into a stationaryvortex. The only difference between the two situations is theframe of reference in which the flow is being observed.[11] The diameter of the cylinder,D, was varied between 1

and 11 cm, and the vortex diameter d was varied between 3and 13 cm. The radius of the cylinder and the vortex arereferred to as R and r, respectively. The depth of the water atthe center of the cylinder was h0 = 10 cm, and the Coriolisparameter f was fixed at 0.25 s�1. In the first configurationthe slope s of the bottom was set at s = tana = 0.50, where ais the angle between the slope and the horizontal (seeFigure 1a). The choice of such value was determined bytwo factors: first, we wanted the westward speed of theself-propagating vortex to be the same as the towing speedU = 0.2 cm s�1 set by the capability of the available stepmotor used to move the obstacle; second, we wanted theself-propagating eddy to move at a speed that would allowthe experiment to be completed before the spin down timet = h0/(nf )

0.5 = 200 s. This ‘‘practical’’ choice does notallow the translation timescale td = d/2U to be the same asin the real site (lab, td � 0.5 day; ocean, td � 6.5 days).However, we believe this should not affect this study since

Figure 2. Sketch illustrating the geometry of the encoun-ter between the vortex and the cylinder. The ratio Y/R is ameasure for the geometry of the interaction. For Y/R > 0 theinteraction will take place on the northern side (firstconfiguration) or top side (second configuration) of thecylinder. For Y/R = 0 the interaction will be central, and forY/R < 0 the interaction occurs on the southern side (firstconfiguration) or bottom side (second configuration) of thecylinder.

CENEDESE: MESOSCALE VORTICES COLLIDING WITH A SEAMOUNT 6 - 3

both in the lab and the ocean [Richardson and Tychensky,1998] the eddy reduces its translation velocity drasticallyin proximity of the seamount. In the second configurationthe towing speed U was fixed at 0.20 cm s�1. The towingspeed U was chosen to be equal to the westward speed ofthe self-propagating vortex in the first configuration. Theequal values of the vortex advection velocity whileapproaching the cylinder ensured that the differencesobserved between the first and second configurations weredue solely to the presence of a background flow. Fur-thermore, we verified experimentally that the cylinder,towed at a speed U, did not generate any vortices in itswake when a stationary vortex was absent. The largestReynolds number Re = UDn�1 associated with the towedcylinder is Re = 180 (for U = 0.2 cm s�1, D = 9 cm, andn = 0.01 cm2 s�1), and according to Boyer and Kmetz[1983, Figure 9], for this value of Re, vortices were notobserved in the wake of the cylinder (see also section 4).The geometry of the encounter is described by theparameter Y defined to be the distance, in the y direction,between the center of the vortex and the horizontal linepassing through the center of the cylinder in the xdirection (see Figure 2). The parameter Y was variedbetween �3R and 3R. Therefore the ratio Y/R is a measurefor the geometry of the interaction. For Y/R > 0 theinteraction takes place in the northern side (first config-uration) or top side (second configuration) of the cylinder.For Y/R = 0 the interaction is central, and for Y/R < 0 theinteraction occurs on the southern side (first configuration)or bottom side (second configuration) of the cylinder.[12] A video camera was mounted above the tank and

fixed to the turntable so that the flow could be observed inthe rotating frame. The vortex was made visible by dyingthe ice cube with food coloring and by adding buoyantpaper pellets on the surface. The dye in the ice cube sank

and was diluted as it sank. The lower-layer flow wasobserved by injecting dyed tracer with a syringe. Themotion of dye was also observed from a side view.

3. Circulation Around a Cylinder

[13] In order to understand the mechanism involved inthe interaction of a vortex with a right cylinder we begin bycalculating the circulation around a cylinder for a singlelayer of homogeneous fluid that satisfies the horizontalmomentum equation in the form

@ u

@ tþ zþ fð Þk̂ � u ¼ �r p

rþ uj j2

2

!þ Diss uð Þ; ð1Þ

where k̂ is a vertical unit vector and Diss (u) is therepresentation of the dissipation of horizontal momentum.Here it is due primarily to either bottom friction or lateralfriction or some combination of the two. The verticalcomponent of the vorticity is z, and f is the Coriolisparameter. The external forcing term has been excluded inequation (1) as it is negligible in both the laboratory and theoceanographic context.[14] In the presence of a cylinder a fundamental dynamic

constraint can be derived when integrating the tangentialcomponent of the momentum equation around the cylinder[Godfrey, 1989; Pedlosky et al., 1997]. This yields anequation for the circulation around the cylinder. Since thevelocity normal to the cylinder is zero, the tangentialcomponent of the total vortex force (second term in equa-tion (1)) vanishes on the cylinder. The tangential componentof the Bernoulli function (first term on the right-hand side ofequation (1)) is a perfect differential and will vanish whenintegrated around a closed circuit. Therefore, integratingequation (1) around a closed circuit C around the cylinder’sboundary (dashed line in Figure 3), we obtain

@

@t

IC

u � t̂ds ¼IC

Diss uð Þ � t̂ds: ð2Þ

[15] Note that if dissipation could be ignored, equation(2) would reduce to the classic Kelvin’s theorem. In thatcase the circulation would be conserved following a particlemotion. In the case of a no-slip boundary condition thevelocity u is identically equal to zero on the boundary and,consequently, on C, and equation (2) reduces to

IC

Diss uð Þ � t̂ds ¼IC

nr2u � t̂ds ¼ 0; ð3Þ

where only lateral friction has been considered, Diss (u) =nr2u, since this has been observed to be the case in thelaboratory experiments for the water column spinningcyclonically above the dense cold lens (see section 2).[16] When a cyclonic vortex interacts with a cylinder,

equation (3) must hold. As shown in Figure 3, the flowwithin the outer edge of the cyclone encounters the circuit Caround the boundary of the cylinder and stagnates some-where on C. Therefore it separates from the vortex formingwhat we define as a ‘‘streamer’’ and advects around the

Figure 3. Sketch illustrating the interaction of a cyclonicvortex with a cylinder. The flow within the outer edge of thecyclone encounters the circuit C (dashed line) around theboundary of the cylinder and stagnates somewhere on C.Therefore this fluid bifurcates and some fluid forms astreamer (light shading) and goes around the cylinder with acounterclockwise velocity vs covering an angle qs. Theremaining fluid remains part of the original vortex (darkshading) with a cyclonic azimuthal velocity ve interactingwith the cylinder over an angle qe.


cylinder with a counterclockwise velocity vs through anangle qs. The remaining fluid continues as part of theoriginal vortex with a cyclonic azimuthal velocity ve andinteracts with the cylinder over an angle qe. Scaling thehorizontal thickness of the streamer with the boundary layerthickness d, the tangential component of the velocity uwithin the boundary layer thickness with a characteristicvelocity v and, since the cylinder arc element ds = Rdq,equation (3) becomes

IC

nr2u � t̂ds ¼IC

nv

�2R dq ¼ 0: ð4Þ

Hence, considering that R, d, and n are constants, equation(4) gives

IC

v dq ¼ 0; ð5Þ

and, integrating (5) around the circuit C, we obtained

ve qe ¼ vs qs; ð6Þ

where, again, the subscript s indicates the fluid within thestreamer, the subscript e indicates the fluid within thevortex, and qs and qe indicate the angle spanned by the fluidwith velocity vs and ve, respectively.[17] This simple relationship in equation (6) implies that

the dissipation of fluid within the vortex interacting with anarc of the cylinder given by Rqe has to be balanced by thedissipation of fluid within the streamer going around thecylinder in the opposite direction (counterclockwise) overan arc Rqs. Therefore, for a cyclonic vortex interaction thefluid within the vortex moves around the cylinder in aclockwise direction (not to be confused with the velocitywithin the vortex that keeps being cyclonic, i.e., counter-clockwise), while the remaining fluid in the streamer goesaround the cylinder in a counterclockwise direction. Thevalidity of equation (6) has been confirmed experimentallyas discussed in section 7.[18] In the case of an anticyclonic vortex interaction,

equation (6) still holds, but since the fluid within the vortexmoves around the cylinder in a counterclockwise direction,the fluid within the streamer goes around the cylinder in aclockwise direction. To simplify the discussion hereafter forthe self-propagating case (background flow case), we willrefer to the fluid going around the cylinder counterclock-wise as fluid going around the northern (top) side of thecylinder, while we will refer to the fluid going around thecylinder clockwise as fluid going around the southern(bottom) side of the cylinder (see Figure 3).

4. Flow Past a Right Cylinder

[19] The phenomenon of periodic vortex sheddingbehind a symmetrical bluff body and the formation ofvortices in a street have been a concern of experimentalistsand theoreticians for the last century, with von Karman[1954] being a prime example. A two-dimensional uniformflow past a right cylinder is well described by the

dimensionless parameter Re = UDn�1, the Reynolds num-ber. When considering increasing values of Re, the flowwill evolve from laminar potential flow around the cylin-der, to flow forming two attached vortices in the wake ofthe cylinder, to flow forming an unstable wake giving thewell-known Karman vortex street (the reader is referred toVan Dyke [1982] for spectacular pictures). For values ofRe exceeding about 100, vortices are periodically shedfrom the cylinder to form the vortices of the street. Furtherincrease of the Re above 3 � 105 will eventually result ina transition to a turbulent wake.[20] This problem has also been investigated in the

context of rotating flows [Boyer and Davies, 1982; Boyerand Kmetz, 1983; Boyer et al., 1984], and a similarqualitative behavior has been observed, with experimentsindicating that the critical Reynolds number for vortexshedding is somewhat larger than in the absence of back-ground rotation.[21] This classic problem of flow past a cylinder has

been investigated so far only in the case of a uniformflow. What happens when instead of a uniform flow, acoherent structure such as a vortex passes by a rightcylinder? As discussed in section 3 and shown inFigure 3, the flow in the outer edge of the cyclonic(anticyclonic) vortex forms a streamer that goes aroundthe top (bottom) side of the cylinder hugging andfollowing the boundary in a similar fashion as in thecase of uniform flow past a cylinder. Studies of uniformflow around a vertical half cylinder against one wall[Griffiths and Linden, 1983] show that vortices are shedfrom the half cylinder at large Re [Griffiths and Linden,1983, Figure 8]. The sign of these vortices is cyclonic(anticyclonic) in the case of uniform flow going aroundthe top half (bottom half) of the cylinder. Therefore, inthe case of a cyclonic (anticyclonic) vortex interactingwith a cylinder we would expect the streamer to goaround the top (bottom) part of the cylinder and formcyclonic (anticyclonic) vortices in the wake of the cylin-der, provided the Re is large enough. The difference withthe above mentioned Griffiths and Linden [1983] study isthe finite volume of fluid transported by a streamer, asopposed to an infinite uniform flow. The volume of thestreamer is a fraction of the original vortex volume thatdepends on the timescale of the interaction. The finitevolume of the streamer allows only a finite number ofvortices to be shed in the wake. In particular, when thevortices in the wake have a horizontal scale (diameter) ofthe same order of magnitude as the original vortex, onlyone vortex will be present in the wake. This result hasbeen confirmed experimentally (sections 5 and 6 andFigures 6, 7, and 10); in a few experiments the diameterof the original vortex was large enough to produce twovortices in the wake, as shown in Figure 4. We define theoriginal vortex that interacts with the cylinder as vortex 1,the new vortex formed in the wake of the cylinder asvortex 2 (Figures 5 and 9), and the second formed vortexas vortex 3 (Figure 4).

5. Self-Propagating Vortex

[22] We start with the description of the interaction of acyclonic vortex with a cylinder for the case where the vortex


self propagates on a sloping bottom (first configuration,Figure 1a). Experiments have been carried out varying theratio D/d of the cylinder to the vortex diameter and the ratioY/R as defined in Figure 2.

5.1. D/d <<< 0.2

[23] When the cylinder diameter was much smallerthan the vortex diameter, the vortex moved undisturbedpast the cylinder for all the values of Y/R. No majorchanges of the vortex were observed. The size, thewestward velocity, and the azimuthal velocity of thevortex were nearly the same before and after the inter-action with the cylinder.

5.2. 0.2 �� D/d �� 1.0

[24] When the diameter of the cylinder was smaller (butD/d 0.2) or equal to the vortex diameter, differentbehaviors were observed for different geometries of theinteraction, i.e., different values of Y/R. For values Y/R >0 (north hit), after the vortex came in contact with thecylinder, fluid peeled off the outer edge of the vortex andformed a streamer. This streamer went around the north-ern side of the cylinder (Figures 5a and 6b) and startedforming a new cyclonic vortex (2) in the wake of thecylinder. Meanwhile, the original vortex (1) also passedaround the northern side of the cylinder (Figures 5b and6c), overtook and merged with the newly formed vortex(2), and continued its westward drift as a single coher-ent structure (Figures 5c and 6d). A different scenario oc-curred for Y/R � 0. As before, a streamer peeled off thevortex, went around the northern side of the cylinder(Figures 5d and 7b), and formed a new cyclonic vortex(2) in the wake of the cylinder. However, for Y/R � 0 theoriginal vortex (1) passed around the southern side of thecylinder (Figures 5e and 7c). The original vortex (1)

followed a different path from the streamer, did notovertake the newly formed vortex (2), and continued awestward drift independent from the new vortex (2).Meanwhile the new vortex (2) completed its formation(Figures 5f and 7d) and began drifting westward inde-pendent of the original vortex (1). The interaction causeda bifurcation of the original vortex (1) into two vortices,one containing the original core (1) and the other con-taining the fluid (2) of the streamer.

5.3. D/d >>> 1

[25] When the cylinder diameter was larger than thevortex diameter, bifurcation of the original vortex was nolonger observed for any values of Y/R. The streamerpeeled off the outer edge of the vortex, went around thenorthern side of the cylinder very slowly, and did not forma vortex in the wake of the cylinder. The original vortex(1) was slightly modified by the loss of fluid into thestreamer. However, the original vortex passed by theobstacle as a single coherent structure either around thenorth or south side for values of Y/R > 0 and Y/R � 0,respectively.[26] In summary, a self-propagating cyclonic vortex

interacted with a cylinder and bifurcated into two vorticesonly for south and central hits (circa Y/R � 0) and for valuesof the ratio close to 0.2 � D/d � 1.0, as shown in Figure 8.When the original vortex did not bifurcate, it passed by thecylinder undisturbed (D/d < 0.2) or with only minormodification (D/d > 1.0).

6. Vortex Advected by a UniformBackground Flow

[27] The same parameters D/d and Y/R were variedsystematically to model a vortex advected by a uniform

Figure 4. Experiment where the diameter of the original vortex was large enough that the streamerproduced two vortices (vortices 2 and 3) in the wake. The original vortex (1) moved around the southernside of the cylinder, while the streamer that generated the two vortices went around the northern side of thecylinder. The larger of the two newly formed vortices (vortex 2) has begun to be deformed by the tank wall.


background flow and interacting with a cylinder (secondconfiguration, Figure 1b).

6.1. D/d <<< 0.2

[28] When the cylinder diameter was much smaller thanthe vortex diameter, the vortex behaved exactly as in thecase of a self-propagating vortex described in section 5.1and moved undisturbed past the cylinder for all the valuesof Y/R.

6.2. 0.2 �� D/d �� 1.3

[29] Bifurcation of the original vortex into two separatevortices was observed for values of the cylinder diameterequal or smaller (but still D/d 0.2) than 1.3d, providedthe ratio Y/R � 1. When the interaction occurred along thebottom or central part of the cylinder (i.e., Y/R � 0), thevortex behaved in a way similar to that described in section5.2 (Figures 9d, 9e, and 9f) and bifurcated into twovortices. A different behavior from the self-propagatingvortex was observed for 0 < Y/R � 1. After the cyclonicvortex came in contact with the cylinder, fluid peeled offthe outer edge of the vortex and went around the top side ofthe cylinder (Figures 9a and 10b). This streamer began toform a new vortex in the wake of the cylinder, as describedpreviously. This newly forming vortex (2) was advected bythe background flow left, away from the cylinder. Mean-while, the original vortex (1) passed around the top side ofthe cylinder (Figures 9b and 10c) but did not overtake ormerge with the newly formed vortex (2), and both the new(2) and the original (1) vortices were advected as independ-ent structures (Figures 9c and 10d). For values Y/R > 1 thevortex did not bifurcate; a streamer peeled off the vortex and,after going around the top side of the cylinder, started

forming a new vortex (2) in the wake of the cylinder.However, as described in section 5.2, the original vortex(1) also passed around the top side of the cylinder (Y/R > 1),overtook, and merged with the newly forming vortex (2) andwas advected left as a single coherent structure.

6.3. D/d >>> 1.3

[30] The original vortex was no longer observed to bifur-cate for any values of Y/R. The behavior was similar to thatof the self-propagating vortex described in section 5.3.[31] In summary, a cyclonic vortex advected by a

background flow against a cylinder bifurcates into twovortices not only for south and central hits, but also fornorth hits provided approximately Y/R � 1, and forvalues of the ratio very nearly to 0.2 � D/d � 1.3(Figure 11). Therefore a vortex interacting with a cylinderis more likely (wider range of Y/R and D/d) to bifurcateinto two vortices when embedded into a background flowthan when self-propagating. Similar to the self-propagat-ing case, when the original vortex did not bifurcate, itpassed by the cylinder undisturbed (D/d < 0.2) or withminor modification (D/d > 1.3).

7. Discussion

7.1. Self-Propagating Vortex

[32] The interaction of a cyclonic vortex with a cylinderresulted in the formation of a streamer that went around thenorthern side of the cylinder while the original vortex (1)moved around either the northern (Y/R > 0) or the southernside of the cylinder (Y/R � 0), choosing the path ofminimum changes in its potential vorticity. As predictedby equation (6), the streamer was always observed to go

Figure 5. Sketch illustrating a self-propagating cyclonic vortex interacting with a cylinder. The vortexself-propagated at a velocity U indicated by the dashed arrow. (a)–(c): Y/R > 0, north hit. The originalvortex (1) passed around the northern side of the cylinder and overtook and merged with the newlyformed vortex (2) and continued to drift westward as a single coherent structure. (d)–(f ) Y/R � 0, southand central hits. The original vortex (1) passed around the southern side of the cylinder and did notovertake the newly formed vortex (2). The original vortex (1) and the new vortex (2) drifted westward asindependent structures.


around the northern side of the cylinder. The dissipation dueto the vortex was balanced by the dissipation due to thestreamer. Approximate measures of the angles qe, qs, and veand vs (Table 1) satisfied equation (6) within experimentalerror. For a fixed value of the vortex diameter d andazimuthal velocity ve an increase of the cylinder diameterD gives an increase of qs and a reduction of qe. Therefore,from equation (6) the azimuthal velocity vs of the fluidwithin the streamer would be reduced with increasing D,and this behavior was observed during the experiments.Hence, since the value of the boundary layer thickness dwas approximately constant, the streamer transport washighly reduced for large values of D; that is, less fluid fromthe original vortex (1) went around the northern side of thecylinder.[33] The streamer was observed to form a new cyclonic

vortex (2) in the wake of the cylinder provided thevelocity vs was large enough. As discussed in section 4,we can think of the streamer as a finite volume flow pastthe northern side of the cylinder, and for a high enoughReynolds number, i.e., large enough velocities, we wouldanticipate the formation of a vortex in the wake. Qual-itatively, the experiments confirmed this. The transition toa vortex-shedding regime is expected to occur at Resimilar to the case of uniform flow past a cylinder in a

rotating environment, which is somewhat larger than theRe = 100 found for nonrotating flow. We defined aReynolds number for the interaction of a vortex with acylinder as

Re ¼ vsLmax

n; ð7Þ

where vs is the velocity of the streamer going around thecylinder, n is the kinematic viscosity, and Lmax = max [D, d ]is the larger lengthscale between the cylinder and the vortexdiameter. In equation (6), ve can be expressed as ve = �er,where �e is the angular velocity of the vortex. By singlesubstitution, equation (7) becomes

Re ¼ �e

qeqs

rLmax

n¼ �e

qeqs

r

R

RLmax

n; ð8Þ

where the Reynolds number is now a function of the ratioof the vortex to the cylinder radius (or diameter).Generation of a vortex (2) in the wake of the cylinderand consequent bifurcation of the original vortex (1) wasobserved for 0.2 � D/d � 1.0, provided Y/R � 0(Figure 8). The value of Re for the experiments in which

Figure 6. Self-propagating vortex. Experiment for Y/R = 0.93 (north hit) and D/d = 0.79. After thevortex came in contact with the cylinder, fluid peeled off the outer edge of the vortex and formed astreamer. (b) This streamer went around the northern side of the cylinder and started forming a newcyclonic vortex (2) in the wake of the cylinder. (c) Meanwhile, the original vortex (1) passed around thenorthern side of the cylinder. (d) Hence the original stronger vortex (1) overtook and merged with thenewly formed vortex (2) and continued to drift westward as a single coherent structure. The time intervalbetween successive frames is given in rotation periods T: t = 0T (Figure 6a); t = 1.2T (Figure 6b); t = 1.8T(Figure 6c); and t = 2.5T (Figure 6d). The white dashed lines represent the approximate location andextent of the vortices. The dark fluid moving south and west outside the white dashed line is the densefluid within the Ekman layer as discussed in section 2.


Y/R � 0 is shown in Figure 12 (triangles); bifurcationoccurred for 400 � Re � 1100 (solid triangles). However,for a few experiments within this range, bifurcation waspredicted but not observed (open triangles). These some-what anomalous experiments lie principally close to theedge of the bifurcation region, possibly suggesting that thelimits of this region are not sharp. These results, indicatingthat bifurcation occurs approximately for 400 � Re �1100, are in agreement with previous studies of uniformflow past a cylinder in a rotating environment, andtherefore we suggest that the new vortex (2) in the wakeof the cylinder was formed in a similar fashion as those inthe Karman vortex street. For values of Re < 400,corresponding to larger values of the cylinder diameterD/d >1.0, the streamer velocity vs is too small (see discussionabove), and therefore the streamer goes around thecylinder as a potential flow without forming a vortex inthe wake. For values of the Re > 1100, corresponding tosmall cylinder diameter D/d < 0.2, the streamer velocity istoo large, and the vortex moved undisturbed past thecylinder without major changes. A similar value of the Remarks the passage to a turbulent wake in the classic caseof a uniform flow past a cylinder in a rotating environ-ment. For the case of a cyclone interacting with a cylinder,in the limit of Re > 1100 a turbulent wake was not

observed; therefore the analogy with the classic case isweak.

7.2. Vortex Advected by a Background Flow

[34] The theoretical approach described in sections 3 and4 does not depend on whether the vortex is self-propagatingor advected by a background flow. Therefore it should alsoapply in the case of a vortex advected by a background flowinteracting with a cylinder. As expected, the streamer peeledoff the outer edge of the original vortex upon interactionwith the cylinder and went around the top side of thecylinder, as predicted by equation (6), while the originalvortex (1) moved around either the top (Y/R > 0) or thebottom side of the cylinder (Y/R � 0), following the back-ground flow streamlines. A reduction of the transport withinthe streamer was observed for increasing values of thecylinder diameter, as discussed above in section 7.1, andagain, approximate measures of the angles qe, qs, and ve andvs (Table 1) satisfied equation (6) within experimental error.[35] The streamer formed a new vortex (2) in the wake of

the cylinder as described in section 6, and bifurcation of theoriginal vortex (1) into two vortices was observed for a widerparameter region than in the case of a self-propagatingvortex. For example, in the presence of a background flow,vortex bifurcation was also observed for values 0 < Y/R � 1

Figure 7. Self-propagating vortex. Experiment for Y/R = �0.17 (south hit) and D/d = 0.86. As inFigures 6a and 6b, the streamer peeled off the vortex, (b) went around the northern side of the cylinder,and formed a new cyclonic vortex (2) in the wake of the cylinder. (c) The original vortex (1) passedaround the southern side of the cylinder; consequently, it did not overtake the newly formed vortex (2)but continued its westward drift as an independent structure from the new vortex (2). (d) Meanwhile,the new vortex (2) completed its formation and started drifting westward as an independent structure.The images were taken at the same time intervals as in Figure 6. The white dashed lines represent theapproximate location and extent of the vortices. The dark fluid moving south and west outside thewhite dashed line is the dense fluid within the Ekman layer as discussed in section 2.


(top hit). This behavior can be explained by the fact thatduring the formation process the new vortex (2) wasadvected by the background flow, at speed U, farther down-stream (left of the cylinder), and as a consequence, theoriginal vortex (1), advected by the same background flowat the same speed, could not overtake the new vortex (2). Inthe case of a self-propagating vortex and Y/R > 0 the newlyformed vortex (2) started propagating westward (left of thecylinder) only after the formation process was completed

(not during it). Since the original vortex (1) went around thecylinder faster than the time necessary to form the newvortex (2), the original vortex could overtake the new vortexand merge with it as described in section 5.2.[36] Furthermore, bifurcation occurred for a wider range

of the ratio of the cylinder to the vortex diameter, 0.2 � D/d� 1.3. However, the values of the Reynolds number forwhich bifurcation occurred do not depend on whether thevortex is self-propagating or advected by a backgroundflow. When calculating equation (7) for the experimentsin which the vortex was advected by a background flow,particular care should be taken when obtaining the streamervelocity vs. The experiments reproduced a situation dynam-ically equivalent to an interaction between a cylinder and avortex advected by a background flow by towing a cylinderinto a stationary vortex through a fluid otherwise at rest.During the experiments the streamer velocity vs was meas-ured in the tank’s frame of reference, where the vortex wasstationary. Therefore, in order to obtain a value for thevelocity of the streamer in a topographic frame of reference,where the vortex has been advected by a background flow(of velocity U ), the cylinder uniform speed U needs to beadded to the experimentally measured velocity of thestreamer, vs. The new expression of Re for a vortex advectedby a background flow is

Re ¼ �e

qeqs

r

RR

� �þ U

� �Lmax

n: ð9Þ

Figure 12 shows the value of Re for the experiments inwhich Y/R � 1 (diamonds): as expected, bifurcationoccurred for the same range of Re as that of the self-propagating vortex, approximately 400 � Re � 1100.

Figure 9. Sketch illustrating a cyclonic vortex advected by a background flow and interacting with acylinder. The background flow velocity U is indicated by the dashed arrows. (a)–(c) Y/R > 0, top hit. Thenewly forming vortex (2) was advected by the background flow and moved left, away from the cylinder.The original vortex (1) passed around the top side of the cylinder but did not overtake or merge with thenewly formed vortex (2). The new (2) and the original (1) vortices were advected left by the backgroundflow as independent structures. (d)–(f ) Y/R � 0, bottom and central hits. The original vortex (1) passedaround the bottom side of the cylinder and did not overtake the newly forming vortex (2). The originalvortex (1) and the new vortex (2) were advected left by the background flow as independent structures.

Figure 8. Regime diagram for the self-propagating vortex.Vortex bifurcation occurred (solid triangles) for approxi-mately 0.2 � D/d � 1.0 and Y/R � 0. The open trianglesindicate experiments for which vortex bifurcation was notobserved. The dotted lines represent the northern (Y/R = 1)and southern (Y/R = �1) sides of the cylinder. The dashedlines enclose the region of vortex bifurcation.


However, also in this case for few experiments within thisrange, bifurcation was predicted but not observed (opentriangles). In summary, we suggest that these results supportthe idea that the new vortex (2) formed in the wake of thecylinder is generated by a similar mechanism as in theKarman vortex street.

8. Comparison With Observation ofMeddy Bifurcation

[37] During a major field program called Structures desEchanges Mer-Atmosphere, Proprietes des HeterogeneitesOceaniques: Recherche Experimentale (SEMAPHORE),four meddies were identified in the Canary Basin andtracked with freely drifting RAFOS floats [Richardson andTychensky, 1998; Richardson et al., 2000]. Three meddiescollided with tall seamounts, which seemed to disrupt thenormal swirl velocity, perhaps fatally in two cases. Evi-dence from two floats suggests that one meddy (meddy 3)split into two smaller meddies at the time of collision, asshown in Figure 13. Float 172 (dashed line) was ejectedfrom a different meddy (meddy 2) approximately 2 monthsbefore and then was entrained in meddy 3 and began toloop. In early June, float 173 (solid line) in meddy 3abruptly stopped its translation (while continuing to loop)

Figure 10. Vortex advected by a uniform background flow. Experiment for Y/R = 0.63 (top hit) andD/d = 0.73. After the cyclonic vortex came in contact with the cylinder, fluid peeled off the outer edgeof the vortex and went around the top side of the cylinder. This streamer started forming a new vortexin the wake of the cylinder in the usual fashion. This newly forming vortex (2) was advected by thebackground flow and moved away from the cylinder. Meanwhile, the original vortex (1) passed aroundthe top side of the cylinder but did not overtake or merge with the newly formed vortex (2), and boththe new (2) and the original (1) vortices were advected as independent structures. The images weretaken at the same time intervals as in Figure 6. The white dashed lines represent the approximatelocation and extent of the vortices. The dark fluid moving radially outside the white dashed line is thedense fluid within the Ekman layer as discussed in section 2.

Figure 11. Regime diagram for a vortex advected by abackground flow. Vortex bifurcation occurred (solid dia-monds) for approximately 0.2 � D/d � 1.3 and Y/R � 1.The open diamonds indicate the experiments for whichvortex bifurcation was not observed. The dotted linesrepresent the top (Y/R = 1) and bottom (Y/R = �1) sides ofthe cylinder. The dashed lines enclose the region of vortexbifurcation.


in front of Cruiser Seamount, turned, and translated south-ward over or around it and then east of Hyeres and GreatMeteor Seamounts from mid-June to mid-October 1994,when the float finally stopped looping. Float 172 (dashedline) in meddy 3, positioned in the outer edge of thevortex, followed a different path. The float’s loopsdiverged from the one described above. Initially, the floatlooped around Cruiser Seamount without major changes ordisruption, and then it passed around the southern side ofIrving Seamount and continued looping on the westernside of Irving Seamount and translated northward. Thedifferent paths followed by the two floats implied that themeddy was somehow cleaved by the seamount into tworoughly equally sized smaller meddies, which then sepa-rated. Therefore the observation of meddy 3 bifurcatingafter the interaction with the Irving Seamount was possiblethanks to the unplanned entrainment of the second float(dashed line) into meddy 3 just before it started interactingwith the seamount.[38] In order to compare our results with field observa-

tions it is necessary to recall that the experiments’ resultsapply to cyclonic vortex interacting with a cylinder. How-ever, the theoretical approach described in sections 3 and 4can be similarly followed when an anticyclonic vortexinteracts with a cylinder. The streamer of fluid peeling offthe vortex will go around the cylinder in a clockwisedirection, i.e., around the southern side of the cylinder, aspredicted by equation (6). This result has been confirmed bythe float positioned in the outer edge of meddy 3 (Figure 13)that went around the southern side of the Irving Seamount,as predicted by the experimental results. The streamer willthen form an anticyclonic vortex in the wake of the cylinderprovided 400 � Re � 1100. From Figure 13 it is clear thatthe float, after passing around the southern side of theseamount, started looping again anticyclonically, confirm-ing our prediction. The evaluation of the Re for the observedmeddies is not trivial. Although the velocity of the streamerand the value of Lmax are known, the value of the eddyviscosity n for this particular process is not well known.Hence it is more practical to consider the result obtained insection 7 that links the Re with the geometrical ratio D/d, abetter known quantity in these field observations. Thediameters of the Cruiser and Irving Seamounts at 1000 mare 18 and 36 km, respectively. We chose the diameter at1000 m to be representative since that is the approximatedepth of the meddies’ center. The overall diameter of meddy3 was around 150 km [Richardson and Tychensky, 1998],

giving a value of the ratio D/d for the Cruiser and the IrvingSeamounts of 0.12 and 0.24, respectively. These valuessuggest that the Cruiser Seamount was too small for theoriginal meddy to bifurcate since D/d < 0.2, while the IrvingSeamount’s value of D/d makes it suitably sized to cause thebifurcation of either a self-propagating vortex or a vortexadvected by a background flow as shown in Figures 8 and 11.[39] The mean background flow at 1000 m depth near the

seamounts is negligible when compared to the self-prop-agating velocity of the meddies (�5 cm s�1) [Armi andZenk, 1984]. However, meddy trajectories are influenced bystrong surface-intensified currents, such as the Azores Cur-rent [Richardson and Tychensky, 1998; Tychensky andCarton, 1998]. Therefore, when comparing the results ofthe present study to field observations, a combination ofboth the self-propagating and advected vortex cases shouldbe considered. Furthermore, although tidally driven flows

Table 1. Experimental Values of Quantities in Equation (6)a

ve, cm s�1 qe vs, cm s�1 qs ve qe, cm s�1 vs qs, cm s�1

1.12 37 0.90 50 41 450.66 32 0.34 63 21 211.16 39 0.74 68 45 501.63 35 0.68 80 57 541.06 33 0.62 55 35 340.90 35 0.36 79 31 280.49 42 0.25 64 21 160.84 26 0.28 64 22 18aUnfortunately, we could not measure the velocity with accurate precision, but for some experiments we verified the validity of equation (6) by

performing measurements by eye. These measurements were labor intensive. Therefore we did not perform them for each experiment. The first four rowsare for self-propagating vortices, while the last four rows are for vortices advected by a background flow.

Figure 12. Re versus D/d. A vortex was observed tobifurcate into two vortices after colliding with a cylinder forvalues around 400 � Re � 1100 (dashed lines). This resultapplied to both a self-propagating vortex (triangles) and avortex advected by a background flow (diamonds). Solid(open) symbols indicate the experiments in which vortexbifurcation was (was not) observed. The open symbolswithin 400 � Re � 1100 are experiments for whichbifurcation, although predicted, did not occur. The numberof such experiments is much smaller than that for whichpredicted behavior occurred. Furthermore, those anomalousexperiments lie principally in the transition region betweenregimes.


over the top of a seamount can have values comparable tothe azimuthal speed of a vortex [Kunze and Toole, 1997],they have been observed not to extend down the sides of theseamount. Therefore this flow has not been simulated in thelaboratory experiments.[40] For the field observations it is difficult to determine

an exact value for Y/R for comparison with our results.However, Figure 13 suggests that meddy 3 interactionoccurs on the northern side of the seamount, supportingour results for a self-propagating vortex applied to ananticyclone. The experiments suggest (section 5) that aself-propagating cyclone would bifurcate only for Y/R �0; therefore an anticyclonic vortex is expected to bifurcateonly for Y/R 0 (north and central hits), in agreement withthe oceanic observation.

9. Conclusions

[41] Laboratory experiments were performed to investi-gate the interaction between a vortex and a right verticalcylinder. Two configurations were used. In the first config-uration the vortex self-propagated over a sloping bottompositioned in the tank in order to simulate the b effect; in thesecond configuration the tank had a flat bottom and asituation dynamically equivalent to an interaction betweena cylinder and a vortex advected by a background flow wasreproduced by towing a cylinder into a stationary vortexthrough a fluid otherwise at rest. Two important geometrical

parameters regulated the flow: the ratioD/d of the cylinder tothe vortex diameter and the ratio Y/R measuring the geom-etry of the encounter (Figure 2). After a cyclonic vortexcame in contact with the cylinder, fluid peeled off the outeredge of the vortex and went around the cylinder with acounterclockwise velocity vs as predicted by equation (6).This fluid formed a new vortex in the wake of the cylinder,and bifurcation of the original vortex into two vorticesoccurred provided Y/R � 0 and 0.2 � D/d � 1.0 for theself-propagating vortex and Y/R� 1 and 0.2� D/d� 1.3 forthe vortex advected by a background flow. This resultsuggests that the presence of a background flow enhancesthe bifurcation mechanism, as discussed in section 7. Theformation of a new vortex in the wake of the cylinder can beexplained in the context of uniform flow past a cylinder in arotating environment. The periodic shedding of vorticesfrom the cylinder, forming the vortices in the Karman vortexstreet, occurs at Reynolds numbers somewhat larger thanwould be expected in the absence of background rotation(Re > 100). The Reynolds number for a vortex interactingwith a cylinder (equation (7)) can be expressed as afunction of the geometrical ratio D/d (equations (8) and(9)), and vortex bifurcation has been observed to occur forapproximately 400 � Re � 1100, provided Y/R � 0, whenthe vortex was self-propagating, and Y/R � 1, when thevortex was advected by a background flow.[42] The theoretical approach described in sections 3 and

4 does not depend on the sign of the vortex, and as

Figure 13. Bifurcation of meddy 3 as shown by two floats that diverged in early June 1994 when themeddy collided with the seamounts [from Richardson and Tychensky, 1998]. The overall diameter ofmeddy 3 was around 150 km, as seen in the largest loop of one float and shown in shading. The darkshading shows depth <1000 m on a chart by Hunter et al. [1983]. The float positioned in the outer edge ofmeddy 3 goes around the southern side of the Irvine Seamount and forms an anticyclonic vortex in thewake of the seamount as predicted by the experimental results.


discussed in section 8, it can be easily extended to ananticyclonic vortex interacting with a cylinder. The exper-imental results have been confirmed by field observations ofmeddy 3 (Figure 13) interacting with the Irving Seamount.The ratio D/d for this particular observation is 0.24, sug-gesting that a vortex bifurcation process similar to thatobserved in the laboratory took place. The value of D/dranges between 0.2 and 0.5 for meddies interacting withmajor seamounts along their southwestward journey, sug-gesting a high likelihood for these meddies to bifurcate intotwo separate vortices. Unfortunately, few observations areavailable for comparison with the experimental results. Thelack of observations of this phenomenon can be explainedby the observation methods used (RAFOS and conductiv-ity-temperature-depth profilers). As indicated in Figure 13,in order to observe the original vortex bifurcating into twovortices the presence of at least two floats is necessary.Usually, meddies are tracked using a single float [Richard-son and Tychensky, 1998; Richardson et al., 2000], makingit impossible to observe a vortex bifurcation. In fact, theobservation of a vortex bifurcation discussed in section 8was possible thanks to the unplanned entrainment of asecond float into meddy 3. An intentional seeding of onemeddy with many floats in order to observe vortex-sea-mount interaction could provide more observational vali-dation of this work.[43] This study suggests that bifurcation of meddies,

although not frequently observed, should frequently occurwhen meddies encounter seamounts. Therefore the effect ofmeddies on the salt tongue should be revisited since theclassic idea of meddies remaining clear of seamounts andslowly decaying over long periods of time is not very likely.Instead, a more realistic scenario is that of a meddyinteracting with a seamount and redistributing its warmerand saltier water signature into two vortices. These twovortices might then each interact with a seamounts again ina scenario in which they first pass by the HorseshoeSeamounts and then the Great Meteor Seamounts. In sucha scenario, four vortices will be the results of those twomultiple interactions. This new scenario yields an enhancedredistribution of heat and salt in the North Atlantic Basinsince a larger number of vortices will correspond to a largernumber of dilute trails of salty and warm water present inthe basin. The investigation of the decay of the vorticesgenerated by vortex bifurcation and consequent mixing withbackground water goes beyond the scope of the presentstudy, but interesting extensions of the present work in alarger laboratory tank could give helpful insights on themechanism by which heat and salinity get redistributed inthe North Atlantic Basin.

[44] Acknowledgments. We wish to thank Jason Hyatt, Phil Richard-son, Amy Bower, and Dave Fratantoni for carefully reading drafts andsubstantially improving the clarity of the manuscript, Phil Richardson, JackWhitehead, and Karl Helfrich for their suggestions and advice, DaveFratantoni for initial encouragement, and John Salzig for providing assis-

tance and help to make the experiments possible. Support for C. C. wasprovided by a WHOI postdoctoral fellowship.

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graphic Institution, Woods Hole, MA 02543, USA. ([email protected])


laboratory experiments on mesoscale vortices colliding with a

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