The Influence of Loop Current Perturbations on the Formation and Evolution of Tortugas Eddies in the Southern Straits of Florida

University of South Florida Scholar Commons

Marine Science Faculty Publications College of Marine Science

10-15-1998

Paula S. Fratantoni University of Miami

Thomas N. Lee University of Miami

Guillermo P. Podesta University of Miami

Frank E. Muller-Karger University of South Florida, [email protected]

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Scholar Commons Citation Fratantoni, Paula S.; Lee, Thomas N.; Podesta, Guillermo P.; and Muller-Karger, Frank E., "The Influence of Loop Current Perturbations on the Formation and Evolution of Tortugas Eddies in the Southern Straits of Florida" (1998). Marine Science Faculty Publications. 52. https://scholarcommons.usf.edu/msc_facpub/52

This Article is brought to you for free and open access by the College of Marine Science at Scholar Commons. It has been accepted for inclusion in Marine Science Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. Cll, PAGES 24,759-24,779, OCTOBER 15, 1998

The influence of Loop Current perturbations on the formation and evolution of Tortugas eddies in the southern Straits of Florida

Paula S. Fra•an•oni Woods Hole OceanographicInstitution, Woods Hole, Massachusetts

Thomas N. Lee and Guillermo P. Podesta Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Frank Muller-Karger University of South Florida, St. Petersburg, Florida

Abstract. Large cyclonic eddies on the northern edge of the Florida Current are the dominant mesoscale features within the southern Straits of Florida. The most prominent of these features is a quasi-stationary eddy that forms near the Dry Tortugas. Our observations,compiled from 3 years of advancedvery high resolution radiometer measurementsin the Straits of Florida and Gulf of Mexico, demonstratea strong relationshipbetween the generationof anticyclonicrings from the Gulf of Mexico Loop Current and the evolution of Tortugas eddies within the southernStraits of Florida. In six cases,Tortugas eddies evolve from cyclonicfrontal eddies which form along the boundary of the Loop Current. The eddies remain stationary near the Dry Tortugas until they are impacted by an approachingLoop Current frontal eddy. The length of time an eddy spends near the Dry Tortugas is increasedwhen the Loop Current shedsan anticyclonic ring. The involvement of a Loop Current frontal eddy in the ring-sheddingprocess results in a delay in its, and hencethe Tortugas eddy's, downstreampropagation. Results suggestthat the lifetime of a Tortugas eddy can be as long as 140 days when a ring-shedding event occurs,or as short as 50 days in the absenceof any ring-shedding events. Upon entering the Straits of Florida, the Tortugas eddies are deformed by the narrowingtopography and shrink to approximately55% of their original size as they propagateddownstream. The shrinkingof these eddiesis accompaniedby an acceleratedtranslation from 5 km/d in the westernStraits of Florida to 16 km/d in the east.

1. Introduction tied between the coast and a meander in the current. These features have been referred to as Tortugas gyres The Straits of Florida is a narrow channel between in the literature, after the Dry Tortugas region where the Florida Peninsula and Cuba in which circulation they are believedto form (Figure 2). In this study we is dominatedby the Florida Current (Figure 1). The refer to the Tortugas gyres as Tortugas eddies to re- northernedge of the Florida Current, closeto the Florida flect their transient nature and upstream genesis.The Keys, is host to an ensembleof cycloniceddies ranging eddiesremain stationary for up to 100 days, and their from 15 to 200 km in diameter and lasting from days longevity,coupled with other climatologicalfactors in to months[Lee, 1975; Lee and Mayer, 1977;Lee et al., the Straits of Florida, maintains a larval retention zone 1992, 1995]. Large cycloniceddies with diametersof near the Florida Keys National Marine Sanctuary[Lee 100- 200 km have been identified as dominant features et al., 1992; 1995]. The cycloniccirculation, between in the circulation within the southern Straits of Florida the coast and a swift boundary current, combined with [Leeet al., 1992,1995]. The eddiespropagate along the prevailingdownwelling favorable winds createsa region northernboundary of the Florida Current (FC), nes- of enhancedsurface convergenceand may help to recruit fish and lobster larvae as well as concentrate man- Copyright1998 by the AmericanGeophysical Union. inducedpollutants transported by the FC [Lee et al., Papernumber 98JC02147. 1992]. Thesecyclonic features have been observed by 0148-0227/98/98JC-02147509.00 many investigatorsover the past 10 years,and several

24,759 24,760 FRATANTONI ET AL' TORTUGASEDDIES IN THE STRAITS OF FLORIDA

30'N

Gulf of Mexico

25'N

Straits Florida

Campeche Bank

20'N 90'W 85'W 80'W Figure 1. Bathymetryof the easternGulf of Mexicoand Straitsof Florida.

hypothesesabout their formation and evolution have been proposed. Our results, basedprimarily on satellite measurements,provide new information about the formation, evolution, and translation of these features. Specifically,the data suggestthat the Tortugas eddies evolvefrom cyclonicfrontal eddieswhich form alongthe edgeof the Loop Current (LC) in the Gulf of Mexico. 26øN Here we describethe evolutionof six Tortugas eddies over a 3-year period spanning 1993 through 1996, using daily sea surfacetemperature fields derived from advancedvery high resolutionradiometer (AVHRR) satellite data. Satellite observations are combined with Dry Tortugas in situ observations from a moored acoustic Doppler current profiler near the 200 m isobath in the southern Straits of Florida (SSF). All six eddieswere influ- 24'N encedby mesoscalevariability associatedwith the sur- roundingcurrent systems.Each of the Tortugaseddies evolvedfrom LC cold perturbationslike thosedescribed by earlierinvestigators [e.g., Vukovich et el., 1979].The meandercrest described by Lee et el. [1995],which is

84'W 82'W 80'W responsiblefor prompting the initial movementof the stationaryTortugas eddies into the SSF, is identifiedin Figure 2. Coastline-followingcoordinate system. all six casesas the leadingedge of a new Tortugaseddy The location of a bottom-mounted 300 kHz acoustic propagatinginto the SSF along the LC. Dopplercurrent profiler (ADCP) is markedby the tri- Following,we providebackground information on the angle. circulation of the Gulf of Mexico and SSF. In section 3, FRATANTONI ET AL.: TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,761

we discussthe processingof the AVHRR and in situ ern Gulf of Mexico, even after the ring has separated. measurements.In section 4, patterns of eddy evolution The northward penetration of the LC has significant and eddy interactions within the Gulf of Mexico and year-to-year variability with a mean period around 8.5 Straits of Florida are identified and the kinematics of months[Sturges, 1992]. the eddiesare quantified. Finally, a discussionof results Early observational programs in the Gulf of Mex- follows in section 5. ico were primarily focused on quantifying the cycle of northward penetration and anticyclonicring shed- 2. Background ding by the LC. However, observationsrevealed a wide range of smaller-scale features that were related to the Flow in the eastern Gulf of Mexico and southern large-scaleprocesses. For instance,Leipper [1970] in- Straits of Florida is constrainedby the straits' configu- vestigated current patterns in the northeastern Gulf of ration and by two broad shallowshelves, the Campeche Mexico and found meandersand eddies located along Bank and the west Florida Shelf (Figure 1). The the northern and eastern edgesof the LC. This led to CampecheBank extendsapproximately 280 km north of the first evidencethat anticyclonic ring sheddingmay the Yucatan Peninsula,bounding the flow entering the be preceded by the westward propagation of cyclonic Gulf of Mexico through the Yucatan Channel. Like- eddiesacross the Yucatan Channel [Cochrane,1972]. wise, the west Florida Shelf steers the flow along the Severalinvestigators have presentedhydrographic data west coast of Florida. Both shelvesare clearly marked showinga closedcyclonic feature near the Dry Tortugas, by steep escarpmentson their seawardedge, plunging which occasionallyappears during ring separationin the to depths beyond 3000 m in the central Gulf of Mexico. LC [Nowlinand McLellan, 1967;Molinari, 1977]. The Sill depths in the Yucatan Channel, the channel con- dynamicheight maps of Ichiye et al. [1973]in the north- necting the Caribbean with the Gulf of Mexico, reach easterngulf showevidence of cycloniceddies embedded approximately 2000 m. The SSF is bounded on the in the edgeof the LC just north of the CampecheBank. north by the Florida Keys and on the south by the No mention of the eddies is made by the authors, but coast of Cuba. A shallow coral bank, Cay Sal Bank, they appear to have rough diameters of 50-100 km. is positioned where the channel and Florida coastline Curiously, although the Gulf of Mexico and SSF make a turn toward the north and is separated from are geographically and dynamically connected by the the island of Cuba by the Nicholas Channel and from LC, previousstudies concerning the mesoscalevariabil- the Bahamas by the Santaren Channel. The width of ity associated with each have remained independent. the SSF decreases from 150 km at the western entrance The most comprehensive studies of LC frontal eddies to approximately 85 km near Cay Sal Bank, while water (LCFEs) and perturbationstook placein the late 1970s depths decreasefrom 2000 m at the western entrance to early 1980swith the studiesof Maul [1977],Vukovich to 800 m in the east. et al. [1979],Paluszkiewicz and Atkinson[1983], and Circulation in the eastern Gulf of Mexico is domi- Vukovichand Maul [1985]. Vukovichand Maul [1985], nated by the LC, a portion of the Gulf Stream system combiningmore than 10 years of satellite infrared data that closesthe subtropical gyre in the north Atlantic. and several years of coincident hydrographic measure- Warm water flowsfrom the Caribbean through the Yu- ments, identified large cold perturbations on the north- catan Channel where it is called the Yucatan Current, ern extreme of the LC that moved southward along loopsanticyclonically through the northeasternGulf of the LC boundary, adjacent to the west Florida Shelf. Mexico, and exits through the SSF where it becomes These meanders eventually grew into stationary me- the FC. The LC penetrates into the northeastern Gulf anders with closed cyclonic streamlines and cold cores of Mexico to varying degreesthroughout the year, and near the Dry Tortugas. Upon reaching the SSF, they occasionallyan anticyclonicring will separatefrom the were observedin one of two possibleconfigurations: A current [Leipper,1970; Maul, 1977; Muller-Karger et large southwestwardoriented protrusion of cold water al., 1991; Sturges,1992]. Typically, ring separation near the Dry Tortugas[e.g., Maul et al., 1984]or a cold causesan abrupt southward retreat of the northern LC tongue, bounded to the west by the southwardflowing boundary and, consequently,the establishmentof direct LC and to the east by a ridge of warm water extending flow betweenthe YucatanChannel and the SSF [Leip- overthe west Florida Shelfto as far north as 26øN [cf. per, 1970; Ichiye et al., 1973; Hurlbutt and Thompson, Vukovichand Maul, 1985, Figure 2a]. The cold tongue 1980; Muller-Karger et al., 1991]. After a ring sepa- and the cold meander events observedby Vukovich et rates, the LC may take several months to reestablish ,1. [1•7•], :U,•l •t ,1. [•84], •nd V•ov•ch • •4•, its anticyclonic circulation in the northeastern Gulf of [1985]always displayed the signaturesof closedcyclonic Mexico. However, occasionally,anticyclonic rings with circulation. Geostrophicestimates of velocities within diameters less than 250 km separate from the LC and the LCFEs exceeded100 cm/s on the LC side of the do not produce a significantchange in the northern po- features where horizontal density gradients were max- sition of the LC boundary [Vukovich,1995]. In this imized, while velocitiesreached 20 cm/s over the west scenario,the current still penetrates into the northeast- Florida Shelf. Diameters varied from 80 to 120 km, and 24,762 FRATANTONI ET AL.: TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

subsurfacesignatures extended to 1000m [ Vukovichand culationof FC water near Key West during a 1-mont' Maul, 1985]. Surprisingly,in 10 yearsof observations, period of dropsondemeasurements in 1972. Through- the featureswere neverobserved moving into the SSF. out this 1-month period, they observedthat the axis of Oncethe perturbationsreached the Dry Tortugas,they the FC remained approximately 80 km offshorewhile a either dissipatedor grew westward,across the width of persistent countercurrent was present nearshore. the LC. Vukovichand Maul [1985]postulated that dis- Lee et al. [1992]were the first to launchan in-depth sipationof LCFEs may involvekinetic energytransfer study of the circulation patterns within the SSF with to the mean flow, similar to the behaviorof spin-off ed- emphasisplaced on the recirculatingfeatures reported diesdescribed by Lee [1975]in the northernStraits of by previousinvestigators. Participants in the Southeast Florida. The lack of prior observationsof suchpropaga- Florida and CaribbeanRecruitment (SEFCAR) study tions may be an artifact due to aliasingof an imperfect beganmonitoring the circulationoffshore of the Florida sea surfacetemperature time seriesderived from early Keys in 1989 with moorings,hydrography, drifters, and satellite images. infraredsatellite data. Lee et al. [1992]reported obser- In a subsequentstudy, Vukovich[1988a] used a wave- vations of a cold, cyclonicgyre located over the Pour- staff technique on 5 years of infrared satellite data to tales Terrace, thus named the Pourtales Gym. They quantify the boundary variations associated with the postulatedthat the gyre was causedby the offshoreme- LCFEs. Perturbationsalong the northward flowing anderingin the FC in the vicinity of the PourtalesTer- limb of the LC were 20-30 km near 25øN and grew to 90 race and, as a result, the enhanced cyclonic vorticity km near 27oN, suggestingthat perturbationsgenerated input to the system where the channel curves toward at or entering through the Yucatan Channel grow as the north. Circulation velocitieswithin the gyre ranged they propagatetoward the northern sectionof the LC. from 20 to 50 cm/s, and upwellingvelocities in the cen- The eddiesobserved by Maul et al.. [1974]and Maul ter (2 m/d) causeda 50- to 75-m upliftingof colder [1977],proposed to be generatedby shearinstabilities, watersonto the terrace. Subsequently,Lee et al. [1995] and by Cochrane[1965], proposed to be a resultof topo- concludedthat the Pourtales Gyre is a late stage in graphic vortex generation,were all of the order of 10-20 the evolution of larger features that form near the Dry km. Two-layer model experimentsgenerated cyclonic Tortugas, Tortugas eddies. eddiesdownstream of the Yucatan Channel, east of the Tortugaseddies have only been observed during peri- CampecheBank [Hurlbutt, 1986]. The model eddies odswhen the LC penetratesinto the northeasternGulf were generatedby baroclinicinstabilities in the vicinity of Mexico. Thereforeit is anticipatedthat the life cycle of the steeptopography of CampecheBank [Hurlbutt, of Tortugaseddies is affectedby the variabilityasso- 1986]. ciated with anticyclonicring sheddingby the LC. Lee To summarize, cycloniceddies are commonalong the et al. [1995]proposed that Tortugaseddies are formed outer LC boundary, and observationshave suggested whenthe LC, flowingsouth along the westFlorida Shelf, that they may be important in the LC anticyclonicring- overshoots the entrance to the Straits of Florida before sheddingprocess. Model studiesand observationssug- turning cyclonicallyinto the channel. During the pe- gestthat theseLCFEs form alongthe northwardflowing riod followingthe separationof an anticyclonicLC ring, branch of the LC and grow in an unstable manner as when the northern boundary of the LC has retreated to- they propagate downstream. There are no prior pub- ward the south, Lee et al. [1995]did not observeany lished reports suggestingthat these eddiespropagate Tortugaseddies within the SSF. They concludedthat into the SSE upon reachingthe Dry Tortugas. the orientation of the LC entering the SSF the mean- Downstream in the SSE, flow variability is dominated dersin the FC axis, and the curvatureof the topography by the cross-streammeandering of the F'G axis which at the western entrance to the SSF, contribute to the falls within a broad, 30- to 70-dayband in the SSF [Lee e! al., 1995]. Currentoscillations at the shelfbreak, pro- generationof Tortugaseddies near the Dry Tortugas. ducedby the meanderingof the EC, are associatedwi•h Leeet al. [1995]suggested that the LCFEsobserved by a myriad of cycloniceddies found on the northern edge Vukovichand Maul [1985]may providean additional of the FC. The relatively long-period meandersin the sourceof cyclonicvorticity to the eddiesnear the Dry EC axis are associatedwith the passageof large cyclonic Tortugas;however, no direct evidenceof this process eddieshaving diametersof 100-200km. These features was presented. The Tortugas eddiesobserved by Lee originate near the Dry Tortugas, near the southwest et al. [1995]were elliptical with alongshorediameters edgeof the west Elorida Shelf, and propagateeastward reaching180 km and cross-shorelengths reaching 100 into the SSF. Chew[1974] reported large offshore shifts km. The eddies persisted near the Dry Tortugas for of the FC axis associated with fluctuations in the LC approximately 100 days until an approachingmeander upstream. Using hydrographicdata and drifter trajec- crestin the LC prompteda 5 km/d (6 cm/s) translation tories, he providedevidence for a large eddy-like feature downstream[Lee et al., 1995]. The convergenceof flow nestled inshore of a meander in the FC. Later, Brooks near Cay Sal Bank typically shearedthem apart [Leeet aridNiiler [1975]reported the presenceof cyclonicrecir- al., 1995]. FRATANTONI ET AL.: TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,763

3. Data Sources The SST observations were used to locate the FC front and the LC front and to track the position, size, The primary data sourcefor this study is sea surface and shapeof cycloniceddies flanking these currents. Er- temperature(SST) observationsderived from the ad- rors in derivedSST valuesvary regionallybut have been vanced very high resolution radiometer, an infrared ra- estimatedat 0.6øC [McClain et al., 1985]. This study diometer flown on National Oceanic and Atmospheric emphasizespattern recognition in SST fields rather Administration(NOAA) polar-orbitingsatellites. In- than the quantitative use of absolute SST values, and frared satellite observations have enhanced our under- the AVHRR is highly sensitiveto relative differencesin standingof oceanicsystems by providingsynoptic-scale SST. A frontal position, identified with AVHRR mea- pictures of SST patterns which are the signaturesof surements, is based on temperature differences in the mesoscaleoceanic features. A full description of the al- upper few millimeters of the ocean surface, whereas gorithms used to calculate SST from infrared data is the same frontal position identified with hydrography givenby McClain ½tal. [1985]. Difficultiesarise when is based on horizontal temperature differencesdeeper the atmosphereis cloudy or contains high concentra- in the watercolumn. Paluszkiewiczand Atkinson[1983] tions of water vapor as the infrared signal emitted by found a 5-to 15-kin disagreementbetween frontal posi- the sea surfaceis attenuated by atmosphericabsorption tions identified with AVHRR data and those identified [May et al., 1993]. In the SSF and the Gulf of Mexico, using hydrographicdata for the LC. the sea surface becomes isothermal in the summer due Participantsin the SEFCAR program (1989-1995) to increasedheating [May et al., 1993]. This causes maintained current meter and acoustic Doppler current difficulties in detecting gradients in SST and renders profiler(ADCP) mooringsat severallocations along the AVHRR data lessuseful during summermonths (typ- shelf, in addition to conductinghydrographic surveys in ically June through September[Muller-Karger et al., the SSF. All the current meter mooringswere deployed 1991]. at or shoreward of the 30-m isobath. As a result, the Infrared observations were collected for the period velocity measurementsrepresent a combinedresponse July 1993 through July 1996. The measurementsen- to forcing mechanismson a variety of timescales.This compassthe area 79øW to 84øW and 22.75øN to 27.75øN makes it difficult to isolate the variability associated at approximatelyi km resolution(Figure 2). As this with the passageof Tortugas eddies from that associ- study progressed,it was evident that information about ated with wind events and tides. SEFCAR also main- the circulation in the Gulf of Mexico was essential to tained a bottom-mounted ADCP mooring near Looe understandingthe evolution of Tortugas eddiesin the Key Reef that was positioned in deeper water, near the SSF. Frontal analysis maps produced by NOAA ana- 150-misobath (Figure 2). Variabilityat thissite is dom- lysts for the Gulf of Mexico and Straits of Florida were inated by large onshore-offshoreshifts in the FC axis obtained for the period July 1993 through June 1995. due to the passageof Tortugas eddies. The charts identify dominant fronts and features from compositesof AVHRR data and are used to provide in- 4. Results formation about the circulation in the Gulf of Mexico. AVHRR measurementscollected by the University of An eddy is defined in the SST observationsby a South Florida for the Gulf of Mexico are used to iden- cooler water mass embedded in the northern edge of tify fronts and featuresduring July 1995 through July the warmer FC (Plate 1). When a Tortugaseddy is 1996. present, the FC meandersoffshore to the southwestof In order to enhancethe existinggradients in the SST the eddy and returns to hug the coastline to the east fields, the data were subjectedto a seriesof processing of the eddy. Often, streamersof warm water from the steps. First, clouds were identified and flagged using FC are pulled around the northern edge of the eddy, a combination of spatial homogeneitytests and SST clearly tracing the cycloniccirculation within the fea- thresholds.Second, the imageswere examined for par- ture. Therefore large changesin the position of the tial coverageresulting from being on the edge of the FC front are good indicators of the passageof a Tor- scan of the satellite and for distortion during satellite tugas eddy. Vukovichand Maul [1985]and Vukovich passeswith wide scan angles. The polar-orbiting satel- [1988a,b] monitoredthe evolutionof LCFEs by quanti- lites provided up to two passesper day, per satellite. fying perturbationsalong the boundaryof the LC, and During the study period there were at least two opera- we employa similar method to monitor the evolutionof tional satellitesat all times, providingup to four passes Tortugas eddies. A coast-followingcoordinate system per day. With a few exceptions,during most summer was chosenwith 17 wave staffs oriented perpendicular months(June - September)Tortugas eddies were unde- to the coastlinein the SSF (Figure2). The locationof tectable in the SST observations. The final processed the northern edgeof the FC front was recorded,when- data included 1898 images,corresponding to wintertime ever detectable,along each staff in the coordinatesys- observationsbetween the months of October and May, tem for each SST snapshot. The front was identified which were used to identify and track oceanic features by visually locating the maximum gradient in temper- such as Tortugas eddies. ature along a staff. The time seriesof FC positions 24,764 FRATANTONI ET AL- TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

28-Mar-96 '

2b q

24M'

Plate 1. Sea surfacetemperature (SST) for March 23, 1995, derivedfrom advancedvery high resolutionradiometer (AVHRR) measurements.Cooler temperatures are representedby shades of blue and green, while warmer temperatures are representedby shadesof orangesand reds, as depicted by the color scale. Clouds in the domain are representedby gray patches. A Tortugas eddy is locatednear 83øW, as evidencedby the cooler(green) surface waters north of the meanderin the warmer (orange)Florida Current (FC). Streamersof warm FC water are advected cyclonically into the interior of the eddy.

along each staff were used to derive variousquantities the time frame in which each of the six eddies discussed that describe the first-order characteristics of the eddies herein were observed in the SSF. For convenience, we as they propagatedthrough the SSF. These quantities refer to specificeddies in the text accordingto the eddy included translation speed, along-channel and cross- number assignedin the first column of Table 1. channel sizes,and eddy frequency and duration. Limi- All six of the Tortugas eddiesthat were tracked using tations exist due to the spatial and temporal disconti- the SST observations remained stationary near the en- nuities inherent in the SST fields due to cloud cover and trance to the SSF until they were impacted by a LCFE. undetectablegradients during summermonths. Exami- However, two distinct circulation patterns were recur- nation of the entire 3-year record of SST fields revealed rent in the Gulf of Mexico, which had an effect on the evolution of 11 Tortugas eddies. However, the de- the continuedevolution of the Tortugas eddies. Each tailed evolution of only six of these eddies is described modeinvolved a differentphase in the anticyclonicring- here. The remaining five eddies were not examined, sheddingcycle by the LC and is summarizedby the as the quality of the data was not consistentlyclear schematicillustration in Figure3. The first beginswith enoughin order to detail their evolution. Table 1 shows a stationary Tortugaseddy near the Dry Tortugas. In FRATANTONI ET AL.: TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,765

Table 1. Time framesfor TortugasEddies Observed in the Straitsof Florida as InferredFrom AdvancedVery High ResolutionRadiometer Data

Eddy Mode First First Stationary Last Total detected Movement Time, days Detected Duration, days

I I Jan. 11, 1994 Feb. 6, 1994 27 March 18, 1994 67 2 I March 3, 1994 March 15, 1994 12 April 24, 1994 53 3 2 Jan. 24, 1995 April 23, 1995 90 May 8, 1995 105 4 I Nov. 8, 1995 Nov. 28, 1995 20 Jan. 9, 1996 63 5 I Dec. 17, 1995 Jan. 11, 1996 26 Feb. 7, 1996 53 6 2 Jan. 14, 1996 May 13, 1996 121 May 31, 1996 140

Thequantities reported for each eddy are representative of (1) theevolutionary mode, described in section 4; (2) thedate wheneach eddy was detected near the Dry Tortugas,(3) the dateof first movementinto the southernStraits of Florida (SSF);(4) the totaltime that eacheddy remained stationary near the Dry Tortugas;(5) the datewhen each Tortugas eddywas last detectednear Cay Sal Bank;and (6) the eddy'stotal residencetime in the SSF.

this case, a cyclonic LCFE comparable in size to the 4.1. Mode I Tortugas eddy approachesfrom the north. The meander crestpreceding the LCFE forcesthe Tortugaseddy Fourof the six Tortugaseddies observed in this study into the SSF. Upon enteringthe SSF, the Tortugaseddy are classifiedas mode i eddies: eddies1, 2, 4, and 5 translatesdownstream, gradually shrinking in sizeas it (Table 1). The satelliteanalysis charts produced by entersthe narrow channel. This scenarioclosely resem- NOAA analystshave been redrawnto depict the posi- bles the observationsof Lee et al. [1995]and is the tion of the LC and FC fronts (Figure 4a). Figure 4a most common scenario representedin these SST obser- containsweekly snapshotsspanning the period January vations. The secondmode involvesthe sheddingof a 20 through April 21, 1994. Eddy i was located near small anticyclonicring by the LC. Here, the LCFE pre- the Dry Tortugas on January 20, the first panel in Fig- viously responsiblefor forcing the Tortugas eddy into ure 4a. The Tortugas eddy is defined on its seaward the SSF never reachesthe Dry Tortugas. Instead, the side by the offshoremeander in the LC as it enters the propagationof the LCFE is sloweddue to the separation SSF. Filaments of FC water advectedwestward, along of an anticyclonicring from the LC. It is important to the north side of the eddy, are evidenceof the cyclonic note that the ring is sufficientlysmall to preventa large circulation within the feature. This effect is well illus- southwarddisplacement of the LC boundary. Therefore trated on February 10, 1994. Here a filament of FC the stationaryTortugas eddy survivesthe ring separa- water has been cyclonically wrapped into the core of tion, supported by the cyclonic flow curvature at the the eddy. Eddy i remained stationary near the Dry entranceto the SSF. In this scenario,the Tortugaseddy Tortugasuntil mid-February. At this time, a LCFE of remainsnear the Dry Tortugasuntil a secondLCFE ap- comparable size approached from the north and forced proachesalong the LC boundary. This LCFE forcesthe the Tortugas eddy into the SSF. As eddy i entered the Tortugas eddy into the SSF where, like the first case,it SSF, it began to shrink, as indicated by the decreasing translatesdownstream, gradually shrinking in size as it meander amplitude in the FC. Meanwhile, the LCFE enters the narrow channel. had replaced eddy i near the Dry Tortugas to become In the following sectionswe describeboth modes in eddy2 (Table 1). The evolutionof eddies4 and 5 was more detail as they correspondto the continual evolu- similar to that of eddy 1. The LCFE that displaced tion of the six eddies identified in the SST observations. eddy 4 into the SSF became stationary eddy 5. Simi- For convenience,eddies that wereobserved during mode larly, a LCFE replacededdy 5 (data not shown). 1 circulation conditions are referred to as mode i eddies Eddy 2 was influenced by mode i circulation pat- while all others are called mode 2 eddies. A third circu- terns throughoutmost of its lifetime. Eddy 2 initially lation pattern has been identified in previous observa- appeared as a LCFE, forcing eddy i into the SSF and tions in which a large anticyclonicring separatesfrom remaining stationary near the Dry Tortugas on Febru- the LC, causing the LC to retreat toward the Yucatan ary 24, 1994 (Figure 4a). On March 3, a LCFE was Channel[Vukovich, 1988a]. The subsequentabsence of visible on the northern limb of the LC, and by March the Loop Current inhibits the formation of a new Tor- 31 it had begun to push eddy 2 into the SSF. How- tugaseddy [Lee et al., 1995]. Each of the anticyclonic ever, after eddy 2 began its translation, the LC bent LC ringsthat was shedduring the 3-year AVHRR mea- westwardand the meander surroundingthe LCFE am- surement period was relatively small. This casewill not plifiedacross the neckof the LC. Simultaneously(April be discussedhere, as it was not observedin these SST 7), a tongueof warmwater formed over the westFlorida observations. Shelf in a configuration similar to that described by 24,766 FRATANTONI ET AL' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

Mode 1 Mode 2 90 85 80 90 85 80 30

25 , TE TE

2O 30

20 30

Figure 3. Schematicsof the two circulationpatterns common in the AVHRR observations.Lines drawn are representativeof the positionof the Loop Current (LC) and Florida Current (FC). The positionsof cyclonicLoop Current frontal eddies (LCFEs), Tortugas eddies, and anticyclonic rings are labeled.

Vukovichand Maul [1985]. In this flow configuration, with the positionof the FC front. The line drawingsde- part of the southward flowing LC recirculatesaround pict the grossposition of the FC front as well as smaller-- the LCFE over the west Florida Shelf while the remain- scaleperturbations along its boundary. A sampleof der of the flow is directedinto the SSF [Vukovichand the frontal charts is presentedfor eddy 1 to illustrate Maul, 1985].The studiesof Vukovichand Maul [1985] the evolutionof mode1 eddieswithin the SSF (Fig- showedthat this cold-tongueflow configurationoften ure 4b). On February7, eddy 1 waspositioned near the leads to anticyclonicring separation. By this time of Dry Tortugas and the FC flowed close to the northern the year, the seasurface had becomeisothermal, and it coastof Cuba. Finger-like intrusionsare presentin the is unclearfrom the SST fieldswhether a ring-shedding frontal tracing, indicativeof the cycloniccirculation in eventensued. However, satellite altimeter data suggest the eddy. As the eddy beganto translatedownstream, that an anticycloneformed approximately 2 weekslater it shrunk considerablyuntil it reached the northward (data not shown). bend in the SSF. There was no recognizablesignature Frontaltracings were generated from the high-resolu- of eddy 1 in the surfacetemperature by March 22, indi- tion SST observationswithin the SSF.The tracingsde- cating that the feature had been shearedapart within pict the locationof the high SST gradientsassociated the narrowingchannel. FRATANTONI ET AL' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,767

24 Feb, 1994 31 Mar, 1994

27 Jan, 1994 3 Mar, 1994 7Apr, 1994

3 Feb, 1994 10 Mar, 1994 14Apr, 1994

10 Feb, 1994 17 Mar, 1994 21 Apr, 1994

17 Feb, 1994 24 Mar, 1994 Figure 4a. Frontaltracings redrawn from National Oceanic and Atmospheric Administration analysesfor theperiod January 20, 1994through April 21, 1994.Tracings depict the location of the highSST gradientsassociated with the positionof the LC andFC.

The detailed evolution of eddy 2 is illustrated in Fig- boundary overlays similarly shaped topography near ure 5. On April 5, 1994, eddy 2 beganpropagating into the southwestcorner of the west Florida Shelf (Fig- the SSF. The meander crest ChaCprompted its move- ure 1). IC is possiblethaC becausethe meandercrest ment extended well Co the north of the Florida Keys, was trapped over the west Florida Shelf in the form of and by April 13 a sharp corner (highlightedwith an a warm tongue, the Tortugas eddy was unable Cotrans- arrow in Figure 5) had formedin the frontal bound- lace coherenClyinto the SSF and the direct, swift flow ary. The location of the sharp corner in the frontal of the FC dissipatedthe eddy through shear stresses. 24,768 FRATANTONI ET AL.' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

83 82 81 8O ter describedby Vt&ovichand Maul [1985],which is fre- 1 quently observednear the Dry Tortugas. By February 25 7 February, 1994 9, a LCFE was positioned along the northern limb of the LC, which was tilted toward the west away from the westFlorida Shelf(Figure 6a). Meanwhile,eddy 3 remainednear the Dry Tortugas, its size and shapecon- 24 tinually changingdue to the passageof smaller frontal eddies. It is difficult to determine from the surface temperature signature whether these smaller eddies were

83 82 81 80 l

25 20February, 1994 __ 25 27 March, 1994

24 -- 24

23 23

25 9 Mamh, 1994 25

24 24

23 23

[

25 22 March, 1994 25 3 April, 1994

24 24

23 23

Figure 4b. Frontaltracings derived from the SST fieldsin the southernStraits of Florida (SSF) for the 25 24 April, 1994 periodspanning the propagationof Tortugaseddy 1.

24 4.2. Mode 2

Eddies 3 and 6 were mode 2 eddies. The frontal anal- yses redrawn in Figure 6a illustrate the evolution of 23 eddy 3. On January 24, 1995, eddy 3 was located near the Dry Tortugas. The axis of the eddy was oriented Figure 5. Frontaltracings derived from the SST fields from northeast to southwest,as indicated by the large in the SSF for the period spanningthe propagationof meanderin the LC front. This configurationresembles Tortugaseddy 2. The arrow pointsto the sharpcorner the largesouthwestward oriented protrusion of coldwa- in the front that formed on April, 13. FRATANTONI ET AL.' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,769

24 Jan, 1995 2 Mar, 1995 11 May, 1995

2 Feb, 1995 9 Mar, 1995 13 Apr, 1995 18 May, 1995

9 Feb, 1995 16 Mar, 1995 20Apr, 1995

16 Feb, 1995 23 Mar, 1995 27Apr, 1995

Figure 6a. Asin Figure4a, but for theperiod January 24, 1995,through May 25, 1995.The positionof the cyclonicLCFE, discussedin text, is labeled.

propagatingaround the peripheryof the Tortugaseddy southwardflowing limb of the LC. However,instead of or whether the smaller eddieswere coalescingwith the continuingdownstream, it beganto enlargewestward, largereddy, resulting in theformation of filamentsalong slicingacross the width of the LC. By March23, 1995, the edgesof the largervortex. The overalleffect was to the surfacesignature of an anticyclonicring appeared, alter the sizeand shapeof the Tortugaseddy but not to indicatingthat it had separatedfrom the LC. The fil- promptdownstream translation. By February23, the amentsthat appear along the northwestedge of the LCFE had propagatedapproximately halfway down the anticyclonicring suggestthat the smallerLCFE may 24,770 FRATANTONI ET AL.' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

-96 -92 -88 -84 -80 -96 -92 -88 -84 -80 -96 -92 -88 -84 -80

30,

28.

26,

24.

22.

20,

18,

30,

18 -96 -92 -88 -84 -80

18 -96 -92 -88 -84 -80 -96 -92 -88 -84 -80

Figure 6b. Frontal tracingsderived from AVHRR data collectedby the Universityof South Florida for the period January 17, 1996, through May 27, 1996.

be orbiting the larger anticyclone. A similar cycle of eddy 3 had propagatedinto the SSF. By the end of the deformation and filament formation has been observed sequence,a large warm tongue of LC water extended in Gulf Stream rings [Hookerand Brown, 1994]. It is over the west Florida Shelf similar to the configuration important to note that the anticyclonic LC ring was rel- observedby Vukovichand Maul [1985]. atively small, approximately 250 km in diameter, and Similar frontal tracings were used to investigatethe thereforeits separation did not causea completesouth- evolutionof eddy 6 (Figure 6b). On January17, 1996, ward retreat of the LC. The northern boundary of the eddy 6 forced eddy 5 into the SSF and remained near the LC was still located north of 25øN, thereby preserv- Dry Tortugas. By January 27, a LCFE appearedalong ing eddy 3 throughoutthe ring-sheddingprocess. The the northern boundary of the LC while a secondLCFE separatedring appears to prevent subsequentLCFEs foliowedclose behind. Twenty dayslater, the largerper- from translating into the Dry Tortugasregion and forc- turbation in the northern LC boundary suggeststhat ing eddy 3 into the SSF. It is possiblethat the LCFEs the two LCFEs had coalescedin the north. Meanwhile, were unable to survive the cyclonic shear region main- a large LCFE had developednear the Yucatan Chan- tained betweenthe lingeringanticyclone and the LC to nel (February16) and begunto propagatedownstream the south. As a result, the residence time of eddy 3 along the northward flowing limb of the LC. With the near the Dry Tortugas approached90 days. On April convergenceof these two features, a small anticyclonic 13, the anticycloneappeared to partially reattach itself ring separatedfrom the LC. As in the above scenario, to the LC, and a week later a smaller ring separated the anticyclonic ring was sufficiently small to prevent from the current. By April 27, there was evidencethat a significantsouthward shift in the LC boundary,and FRATANTONI ET AL' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,771

250

200

150

100

Eddy 2 Eddy 1 _ 50 , I , I , I • I , I • I , I , I • I • I • I , I , I , I 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16

' I I ' 250

200

150

IO0

Eddy 3 Eddy 4 50 , I , I , I • I • I , I , 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16

250 ' I '

200

150

IO0

Eddy 5 50 Eddy 6 , I , I , I , I , I • I , I , I , I , I , I , I , I , I 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 Staff Staff Figure 7. Along-channel(thin line) andcross-channel (thick line) dimensions of the sixTortugas eddies as a function of staff number. The cross-channel width of the eddies is defined by the offshoredisplacement of the front measuredperpendicular to the shoreline. The along-channel length is derivedfrom the total eddy area and the cross-channeldimension.

eddy6 waspreserved throughout the ring-sheddingpro- positionsrecorded along each wave staff from the high- cess.Eventually, by May 27, a LCFE approachedfrom resolutionSST observationsin the SSF (Figure 2). the north and forcededdy fi into the SSF. One notable Changesin the spatial scalesof the eddies were esti- characteristic of the mode 2 eddies was their long res- mated by tracking the along-channeland cross-channel idencetime near the Dry Tortugas,during which time sizesas they evolved(Figure 7). The cross-channel the size and shape of the eddies changed dramatically. diameter of the eddies, oriented perpendicular to the •)nce eddy 3 was forced into the SSF, it took only 2 coastline,was definedby the offshoremeander in the FC weeks to reach Cay Sal Bank. Similarly, eddy fi re- frontal boundary. Paluszkiewiczand Atkinson [1983] mained stationary for 121 days compared with its 19- found a 5-to 15-kin disagreementbetween LC frontal day propagation through the SSF. positionsidentified using AVHRR data and thoseiden- tified from hydrographicdata. Halliwell and Mooers 4.3. Kinematic comparisons [1979]conclude that the positionof the Gulf Stream Changesin the size, shape,and speedof the Tortu- could be determined to within 5 km using AVHRR data gaseddies were monitored using a time seriesof frontal under optimum conditions. The error for the cross- 24,772 FRATANTONI ET AL.' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

channel dimension of the eddies here is estimated at 4-5 first six staffs,followed by a gradual decreasethrough km. Followinga methodsimilar to Vukovich[1988b], the remainderof the SSF. On average,eddies entering the area between the FC meander and the coastline was the SSF had a cross-channelwidth of approximately calculated and, assumingthe Tortugas eddy is ellipti- 200 km and an along-channellength of approximately cal in shape[Lee et al., 1992,1995],the along-channel 115km (Figure7 and Figure8). Thesedimensions are dimensionof the eddy was computed. The boundaries comparableto thosereported by Vukovich[1988b] who over which to integrate the frontal curve were identified found that as the LCFEs approachedthe Dry Tortu- by the inflection points in the FC front upstream and gas, their along-stream to cross-streamsize ratio in- downstreamof the eddy. The error associatedwith this verted. Once the eddiesenter the SSF, this ratio is in- choice,coupled with uncertainties in the frontal digiti- vertedagain due to the narrowingof the channelso that zation process,produces a combinederror of approxi- the along-channeldimensions are larger. The width of mately 15% in our along-channelcalculations. the SSF narrowsconsiderably beginning around 83øW The changesin the cross-channelsize of all six Tor- (staffs4 through8; Figure2). In this portionof the tugas eddiesare remarkablysimilar (Figure 7). The channel,the large volumeof the FC constrictsthe cross- cross-channel diameter of all six eddies decreasesexpo- channelsize of the eddies.The averagetotal area of the nentially from 200 km near the entrance of the SSF to eddiesis greatestnear the entranceof the SSF, imme- approximately 50 km near Cay Sal Bank. The along- diately after the axis-lengthinversion has taken place channel dimensionsof the eddiesare not as predictable; (Figure 8). By the time the eddiesreach the narrow- however,some salient characteristicsare evident in the est portion of the SSF (north of Cay Sal Bank, near evolution of the six eddies. The most dominant char- staff 14; Figure 2), their dimensionshave decreasedto acteristic is the increasein along-channel size over the approximately170 km and 80 km in the along-channel and cross-channeldirections, respectively, about 55% of their originalsize (Figures 7 and 8). The steadyde-

22O creasein averageeddy area at the surfaceimplies that size either the eddiesare continuallylosing mass to the FC 2OO as they propagatedownstream or that they spin faster 180 and deepen.Observations by Lee et al. [1995]did not 160 showan increasein swirl velocityas the featuresprop- agate downstream. If we assume that the eddies are 140 cylindricaland penetrateto a depth of 300 m and that 120 theypropagate through the SSFin 38 days(the average 100 propagationtime for modei eddies),then the effective

80 rate of fluid lostby the eddiesas they propagatethrough the SSF is approximately 0.7 Sv. 60 , - 0 2 4 6 8 10 12 14 16 The eddiesexperienced changes in their translation speed coincidingwith the changesin eddy shape and size. Translation speed, defined as the time it takes 220 2.2 for the center of a Tortugaseddy to advancefrom one 200 staff to another, was estimated for each of the eddies ... - _ ., as they propagatedthrough the SSF. In order to avoid •, 180 - .Meanalongshore size '. R=.87 1.8 spuriouslyhigh velocityestimates due to changesin the • 160 ._o sizeor shapeof the features,velocities were only calcu- Mean area '- 140 R=.97 1.6 lated betweenstaffs separated by at least55 km (i.e., E ,_ / 1.4 'o 120 _ I skippingat leastone staff). In this way,small deforma- i 1.2 tionsin the shapeof a slowlymoving eddy had minimal "' 100 effect on translationcalculations. Figure 9 showsthe 80 Meancross-shore size 1.0 R=.99 translationspeed of eddiesgrouped by mode(Table 1), 60 , I , I , I , I , I , I , I , 0.8 with error bars representingan estimated 10% uncer- 0 2 4 6 8 10 12 14 16 tainty in the calculations.Staff 8 arbitrarily separates Staff the westernSSF from the easternSSF (seevertical line Figure 8. (top) Mean Tortugaseddy dimensionscal- in Figure9). Velocitycurves are incompletefor someof culated for the ensembleof eddies presentedin Fig- the eddies due to intermittent cloud cover. Results in- ure 7. Error bars reflect 1 standard deviation. Solid dicate that translation speedsincreased as all six of the lines denote the third order polynomial fit to the mean points. (bottom) Mean Tortugaseddy dimensions com- Tortugas eddiespropagated into the middle SSF. Their pared with the mean area of the ensembleof eddies as maximum downstreamspeed coincided with the maxi- a functionof downstreamposition (staff number). The mum decreasein eddy area, suggestingthat eacheddy correlation coefficientR is displayedcorresponding to acceleratedwhile shrinkingwithin the narrowingchan- each polynomial fit. nel and had greater interaction with the FC. The mode FRATANTONI ET AL' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,773

, 2.2 eddies 1 and 2 are not greater than the differencesin speed among mode 1 eddies. The important result is 16 - 2.0 that all six of the eddiesappear to acceleratethrough the middleSSF. Leeet al. 's [1995]estimate of 5 km/d is 1.8 > comparableto translation speedsobserved in the west- - 1.6 • ern SSF; however, they did not report any downstream x acceleration in their observations. 1.4 The most dominant correlation between the circula-

- tion patterns within the Gulf of Mexico and eddy kine- 1.2• matics is found in the longevity of the features. The -• TE2 lifetimes of each of the six Tortugas eddies are pre- •- TE4 - 1.0 o TE5 a sentedin Figure 10. Tortugas eddies 1, 4, and 5 were , I • I I I I i I , I t I , 0.8 all mode 1 eddies and had residence times in the SSF 0 2 4 6 8 10 12 14 16 of approximately 50-65 days. Through •nost of its life, eddy 2 behavedlike a mode i eddy, and its life span, 53 days, reflects this similarity. Tortugas eddies 3 and 6, both mode 2 eddies, had much longer residencetimes 2.22.0 of 105-140 days. This analysis seemsto suggestthat mode i eddies tend to take much longer to propagate downstream, approximately 30-40 days, than the mode 2 eddieswhich pass through the SSF in approximately 1.6• 15-20 days. However, both of the mode 2 eddies were tracked into late spring when temperatures within the 1.4ø• SSF had become isothermal and when feature identification becameincreasingly difficult. Therefore it is likely '1.,?. • that the date of last detection underestimates their to- 1.o tal lifespan when compared with other eddiesthat were observedduring the fall and winter months. 0.8 This analysis is aliased due to the limitations of us- 0 2 4 6 8 10 12 14 16 ing the AVHRR sensorin this regionduring the warmer Staff number summer months. Satellite-derived ocean color measure- Figure 9. Translationspeed as a functionof down- ments would probably be helpful in monitoring these streamposition (staff number) for (a) mode1: eddy1, eddies during summer months, as phytoplankton can eddy2, eddy4, and eddy5, and (b) mode2: eddy3 andeddy 6. Errorbars are drawn to denotea 10%error in the translationspeed calculations. The thick line de- notesthe meaneddy area calculatedfrom the ensemble of eddies. stationaryLifetime(days)time (days) I 140 I- 1 eddyspeeds range from 5 km/d in the westernSSF to 16 km/d in the easternSSF. Eddy 2 stoppedtranslating 120 by the time it reached the middle SSF due to the warm- 100 tongue formation over the west Florida Shelf. However, its translation speed was steadily increasingas it en- 80 tered the westernSSF. Mode 2 eddy speedsrange from 2 to 12 km/d (Figure9b). By the time the Tortugased- 60 diesreached the middleSSF (definedby staff8), all the 40 eddieshad similar translation speeds.However, eddy 3 (Figure9b) had significantlyslower propagation speeds 20 in the western SSF compared with the mode 1 eddies.

This difference may be attributed to the difference in 1 2 3 4 5 6 size between eddy 3 and the mode 1 eddies. In the Eddy westernSSF, eddy 3 is as muchas 70% largerthan the Figure 10. Lifetime (days) from detectionnear the other eddies(Figure 7). However,it is likely that these Dry Tortugas until last recognizablesurface tetnpera- differencesin translationspeeds are not statisticallysig- ture signaturenear Cay Sal Bank (solidbars) compared nificant. The differencesbetween the translation speed with the time that each eddy remained stationary near of eddy 3 in the western SSF and those calculated for the Dry Tortugas(shaded bars). 24,774 FRATANTONI ET AL.' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

100 cm/s

25 rn

100 cm/s

50 rn

O0 cm/s

lOO rn

1993 1994

Aug Sep O• Nov Dec Jan Feb May Jun Jul Aug Sep O• Nov Dec

210 240 270 300 330 360 25 55 85 115 145 175 205 235 265 295 325

Figure 11. Current velocitiesat 25m, 50m and 100mdepth binsrecorded by an upwardlooking 300-kHzADCP mooredat 150mnear LooeReef (seeFigure 2). The velocityrecords have been 40-hour low-passfiltered to removethe high-frequencyvariability associatedwith tides. Current vectorshave beenrotated to align with the local isobathsso that vectorspointing toward the top indicate eastwardflow. Shadedbars highlight the period when eddy i and eddy 2 passedover the mooring line. serve as tracers of the circulation in the Gulf of Mexico ward to westwardvelocities throughout the water col- [Muller-Kargeret al., 1991]. However,there wereno umn. The westward flow event that occurred in late ocean color satellites operating during the period cov- January1994 was caused by the passageof a Tortugas ered by this study. In situ observations were used to eddythat propagatedthrough the SSFprior to the pas- corroborate results inferred from the satellite data and sageof eddy 1. This Tortugaseddy waspresent in the to fill in the data gaps during summermonths. Veloc- frontal maps on January 20, near the easternbound- ity measurementsfrom a moored ADCP located at the ary of the SSF (Figure4a). There were12 suchevents 150-m isobath near Looe Reef overlapthe SST observa- duringthe 17-monthrecord or an averageof about 10 tionsfrom July 1993through December 1994 (Figure2 eventsper year, althoughall of theseTortugas eddies and Figure 11). Current vectorswere rotated to align couldnot be trackedusing AVHRR observationsdue to with the isobathsand filtered with a 40-hour low-pass the isothermalnature of SST duringsummer months. filter to removetidal signals.The passageof a Tortugas This velocityrecord was divided into winter(October eddy is markedby significantcurrent reversalsthrough- - May) andsummer (June- September)time series to out the water column that can last from i to 4 weeks. correspondwith the usable and unusableportions of Westwardflow reached50 cm/s for severalof the eddy the satelliteobservations. Variance-conserving spectra eventsin the record. Shadedboxes (Figure 11) high- were computedfor the two time seriesand compared light the period when eddies i and 2 passedover the (Figure12). The spectraare similar,with the highest instrument. The passageof the eddiesis suggestedby variabilitycentered around 40 days.This suggeststhat the significantvelocity reversalsfrom dominantly east- conclusions derived from the wintertime SST fields are FRATANTONI ET AL.' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,775

2500 ...... takes 60-90 days for the anticyclonicring-shedding pro- cessto complete,during which time a Tortugas eddy is present near the Dry Tortugas, and assumingit takes Winter 2000 another 65 days for a new LCFE to propagate into position to interact with the Tortugas eddy, then the total

; stationary time of the original Tortugas eddy would be approximately 125-145 days. This is within the range 1500 of mode 2 eddy lifetimes. The lifetime of mode i eddies (Figure10) is slightlyshorter than the 65-dayperiod associatedwith the evolution of the LCFEs as reported by 1000 Maul et al. [1978]. Mode i eddieswere more common during this study period. On the basisof the residence times estimated for mode i eddies,ranging from 50-65

500 days(Figure 10), we couldexpect six to sevenTortugas eddiesin the SSF per year. This is slightly lessthan the ß " 10 eddy per year estimatefrom the ADCP data (Fig- ure 11); however,it is difficulito isolatereversals in the i i i i i i i i I ...... 1000 1 O0 10 1 velocity recordsthat are causedpurely by the passageof Period (days) Tortugas eddiesfrom the effectsof relatively high frequency,submesoscale eddies [Shay, 1997; Graber and Figure 12. Variance-conservingspectra computed Limouzy-Paris, 1997]. from the alongshorevelocities at 30 rn as measuredby the ADCP in Figure 11. The velocityrecord has been An apparent inconsistencyexists between the num- dividedinto winter (October- May) andsummer (June ber of eddieswe describehere and the number predicted -September)time series, chosen to correspondwith the by the velocity time seriesmeasured by the ADCP and usableand unusableportions of the satellite data. inferred from the residence times of the eddies observed in this and previousstudies. We have argued basedon reversals in the velocity record that 12 eddies propagated through the $SF in a period of 17 months. This applicableto the entireyear and that thereis no obvi- suggeststhat 17 Tortugas eddies should have propa- ousseasonality in the eddysignals. The 40-day spectral gated through the SSF during the 24 months of winter peakappears to resultfrom the passageof the Tortugas AVHRR observations. Similarly, independent estimates eddies.An averageof 10 Tortugaseddies per yeargives based on our AVHRR data and results from previous a fluctuation timescaleof about 36 daysand is the dom- studiespredict that betweensix and sevenTortugas ed- inantmode of currentvariability at the site(Figure 11). diespass through the SSF in a givenyear. This implies This is consistentwith the 30-to 70-day periods previ- that we should have observed approximately 13 mode ouslyreported for the dominantcurrent variability in i eddies or nine eddies if one third of those observed the SSF [Leeet al., 1992,1995]. were mode 2 eddies. The number of eddies that we have actually described here is significantly less than the numberspredicted above. However,up to three ad- 5. Discussion ditional eddies were identified in the SST fields prior The lifetimeof a Tortugaseddy may be controlledby to the appearance of eddy i in January 1994 and two severalgeographic and oceanographicfactors. However, eddies were identified prior to the appearanceof eddy the most important factor appearsto be the eddy's rela- 3 in January 1995. These eddies were not described tionship with the LC and its anticyclonicring-shedding in detail here becausethe quality of the data between cycle. For instance, it appears from the SST obser- October and January was not consistentlyclear enough vations that Tortugas eddiesrequire the impact of an in order to detail their evolution. However,taking into LCFE to initiate their movement into the SSF. Each account these additional eddies, the total number of ed- eddy was forced into the SSF by an LCFE of compa- dies that we identified during the 24 months is 11. This rable size which quickly replacedthe displacedTortu- number is consistent with the 9-13 eddies inferred from gas eddy. In the absenceof an LCFE, all six Tortu- the AVHRR results. However,this number is still sig- gaseddies remained stationary near the Dry Tortugas. nificantly smaller than the 17 eddies predicted by the Mode 2 circulation patterns were characterizedby the ADCP velocity record. The passageof Tortugas ed- diversionof an approachingLCFE into the anticyclonic dies was identified in the velocity record by significant ring-sheddingprocess. Maul et al. [1978]reported a reversalsfrom dominantly eastward to westward veloc- 65-day period associatedwith LC perturbations,while ities throughout the water column. However, current Vukovichand Maul [1985]reported a 60-to 90-daytime fluctuations may also be causedby the passageof sub- period from the generation of a detectable LCFE to mesoscalefrontal instabilities having diameters of 10 - identifiable anticyclonic ring separation. Assuming it 20 km which propagatethrough the $SF in timescales 24,776 FRATANTONI ET AL.' TORTUGAS EDDIES IN THE STRAITS OF FLORIDA of days to weeks. These eddies would tend to cause 30N I I I I I smaller magnitude reversals that are confined to the upper 50 - 80 rn in the water column [Shay, 1997]. 29N- - It is possible that we have overestimated the number 4/26/94 of eddiesthat propagated through the SSF during the end•3/29 17-month deployment by including reversalsforced by smaller frontal eddies. -.• ß• 27N- - (•28N- 4/19•7 ß - 1/20/94 - Some puzzling questions remain. Foremost, what 2/15 start would cause a cyclonic LCFE to contribute to an anticyclonic ring-shedding event, rather than translating 26N- - downstreaminto the SSF? LC anticyclonicring shed- a 25N ding has been characterizedby Hurlburr and Thompson I I I I I [1982]as a cycleof northwardpenetration and westward 90W 89W 88W 87W 86W 85W 84W bending of the LC, whereby the LC reachesan unsta- 30N • ble configuration. Model results suggestthat small cy- 3/7 clonic frontal eddies spin up through barotropic insta- 29N- bility processesduring the final stagesof ring separation

[Hurlbuttand Thompson,1982]. Model results and ob- 28N- - servations have also shown evidence of closed circulation ½) /24/95 in subsurfacelayers before surface signaturesof a ring ß'1•• 27N- ß•start - '• 6•20•95 • • separationappear [Hurlburr and Thompson,1982; Moli- _1 end 5/,•0'"'"'"'• nari, 1977]. Vukovichand Maul [1985]speculate that 4/18 3/28 26N- - the cold featuresintensify through instability processes, thereby leadingto the formation of an anticyclonethat b 25N has closed circulation below the surface. They propose 88W 8•7W 8%W 85W that the cold features then move westward in the shear 30N I I I I I zone betweenthe ring and the LC to the south. There is evidence both in model experiments and observations that LCFEs are continuously formed due 29N- - to baroclinic instabilities near the steep topography of the Campeche Bank. Two-layer models generate -• 28N- 2/•/17/96- LCFEs downstream of the Yucatan Channel, east of the CampecheBank [Hurlbutt,1986]. The modeleded- '• 27N- 11/22/95 dies have comparable sizes and speeds to the LCFEs start• 12/8 4/17 observedby other investigators. Vukovich[1988a] re- 26N- - ported perturbations along the boundary of the northward flowing limb of the LC which grow from 20 to 90 C 25N km by the time they reach the northern boundary of 90W 8•9W •8W 8•7Wd6W •5W 84W the LC. The data from this study show that cyclonic Longitude LCFEs are found not only preceding a ring-shedding event but also during the entire extensionprocess. Figure 13. Northernmostposition of the LC axis as Figure 13 illustrates the changingorientation of the determinedfrom the SST fieldsduring (a) January20, 1994, through April 26, 1994, the period spanningthe LC during the periods January 1994 through April evolutionof eddy i and eddy 2, (b) January24, 1995, 1994, January 1995 through June 1995, and November through June 20, 1995, the period spanning the evo- 1995 through May 1996. The dots indicate the north- lution of eddy 3, and (c) November22, 1995through ernmost position of the LC axis, as determined from May 27, 1996, the period spanning the evolution of ed- the SST fields at various times during each clear im- dies4, 5, and 6. Squaresindicate the beginningand end age sequence.During the period January 1994 through of a sequence,and arrows indicate the direction of LC April 1994, the northern LC boundary grew into the boundary movement between realizations. The time of each anticyclonicring sheddingis indicated by a jagged northern Gulf of Mexico and showedsigns of westward line between two points. bending(Figure 13a). During this time, eddies1 and 2 propagatedinto the SSF. Recall that during April 1994, a warm tongue of LC water extended over the west Florida Shelf, hinderingthe coherentpropagation the southeastthat an anticyclonic ring separatedfrom of eddy 2. During the period January 1995 through the LC as indicated by the jagged line. Eddy 3 was po- June 1995, the LC boundary gradually extendednorth- sitioned near the Dry Tortugas throughout this entire ward and westward, followed by an immediate retreat to sequence.The LCFE upstream of eddy 3 reachedthe the southeast(Figure 13b). It wasduring this retreat to southwardflowing limb of the LC during the northwest- FRATANTONIET AL.: TORTUGASEDDIES IN THE STRAITSOF FLORIDA 24,777 ward extensionperiod and began to propagatewest- SSFfrom upstreamsources in the Caribbeanand re- wardacross the LC. Duringthe periodNovember 1995 cruitedinto the FloridaKeys using the Tortugaseddies throughMay 1996, the LC boundaryshifted northward as a mechanismfor transportout of the FC [Leeet al., and then bent westward(Figure 13c). This westward 1992].This may also be a mechanismfor entrainment shiftwas immediately followed by a largeeastward shift, of red tide organismsinto the FloridaKeys waters. duringwhich time an anticycloneseparated from the Surprisingly,previous studies found no evidence that current. Eddies 4 and 5 were forced into the SSF dur- LCFEs enteredthe SSF. However,our resultsindicate ingthe stages preceding the large westward bending of that LCFEs becomeTortugas eddiesin six separate the LC while eddy6 remainedstationary near the Dry cases. In all cases,the departingTortugas eddy was Tortugasthroughout the westward bending period. The quickly replacedby a LCFE which subsequentlyre- subsequenteastward retreat of the LC coincidedwith mained stationary near the Dry Tortugas. It has been theseparation of a smallanticyclonic ring from the LC. proposedthat the longevityof stationaryTortugas ed- Our resultssuggest that if the LC is in the unstable diesenables a longeradvection pathway for thoselarval configurationdefined by Hurlbutt and Thompson [1982], speciesthat havelonger planktonic stages [Lee, 1998]. bentwestward away from the westFlorida Shelf, and if These results suggestthat the stationary period of a a LCFE ispresent along the eastern edge of theLC, then Tortugaseddy may varywidely, with mode2 eddiesre- that LCFE may be absorbedinto the ring-shedding mainingstationary for twice the amountof time that process.From the SST observations,it appearsthat mode 1 eddies do. The mode 1 scenario was most com- the LCFE is locally strengthened,perhaps due to the mon in these observations.This is to be expected in a instabilitymechanism proposed by Vukovichand Maul largersample if the evolutionarypatterns of Tortugas [1985].In bothof the casespresented here, the anticy- eddiesare truly determinedby the separationperiod clonethat separatedfrom the LC wasrelatively small, of anticyclonicrings by the LC. The periodassociated suggestingthat the positionof the LCFE whenthe cur- with ringseparation by the Gulf of MexicoLC mayvary rent assumesan unstableconfiguration may helpdictate from6 to 25 months[Vukovich, 1988a; Sturges, 1992], the size of the anticyclone.The separationof a small whereas the evolution of LCFEs has been associated anticyclonedoes not causethe total retreatof the LC with a 65-day period in the spectrumof LC variability to the south,thereby preserving the stationaryTortu- [Maulet al., 1978].This would imply that theresidence gaseddy. In the caseof Tortugaseddy 2, the thermal time of Tortugaseddies near the Dry Tortugasshould contrast in SST was lost before we could tell what hap- approach65 daysa majorityof the time whileonly oc- penedto the warmtongue configuration that formed casionallyexceeding 125 days. This variabilityin res- near the Dry Tortugas. However,on the basisof the idencetimes could have profoundimplications for the positionof the LC and the ideaspresented above, we successfulrecruitment of certain speciesinto the SSF. mightexpect a largeanticyclone to develop.Satellite On the basis of these results, the longest recruitment altimetry observationsconfirm the formationof an an- pathwaywould occur when a modei eddyis propagat- ticycloneon May 15, 1994. ing throughthe SSF while a mode 2 eddy is present Mesoscalemotion playsa crucialrole in the recruit- near the Dry Tortugas. It is possible,however, that a ment of a larval community,as the hatchedfish or lob- largersampling of Tortugaseddies might defineother sterlarvae may be retainedin or advectedaway from fa- patterns of eddy evolution. vorablenursery grounds [Okubo, 1994]. The life spanof a Tortugaseddy may dictate which larval species spend 6. Conclusions their planktonicstage in the FloridaKeys region. For instance,slipper lobster (Scyllarus) and grouper(Ser- Observationscompiled from 3 yearsof AVHP•P•data ranidae)and snapper larvae (Lutjanidae) have plank- in the Straits of Florida and Gulf of Mexico were used tonicstages that canlast at least1 month[Robertson, to identify patternsin eddy evolutionand relate them 1968;Leis, 1987]. Tortugas eddies have been observed to the anticyclonicring-shedding cycle by the Gulf of in the middleSSF, stalled over the PourtalesTerrace for Mexico LC and to the evolution of LCFEs propagating up to I month,and it hasbeen proposed that theseed- alongthe LC boundary. Theseobservations demon- diesmay provide a localrecruitment pathway for these strate a strongrelationship between the generationof species[Lee et al., 1992].Model studies suggest that in anticyclonicrings from the Gulf of MexicoLC and the the presenceof a cycloniceddy shoreward of a strong evolutionof Tortugaseddies within the SSF. Two com- boundarycurrent, winds blowingagainst the current mon patternsin eddy evolutionemerged from the ob- induce unbalancedsurface stress patterns [Roothand servationsof six separateTortugas eddies: A Tortugas X ie, 1992].This situation may produce enhanced sur- eddyis approachedfrom upstreamby a LCFE of com- faceconvergence in the region between the FC axisand parablesize and forced into the SSFby its impactor the a Tortugaseddy, thereby providing a meansfor surface southwardpropagation of an approachingLCFE is in- wateradvected by the FC to be entrainedinto nearshore terruptedby the separationof a smallanticyclonic ring recirculations. Specieshaving planktonic stagestoo from the LC. In the second mode, stationary Tortugas longfor localrecruitment may be advectedinto the eddiesmust await impact by subsequentLCFEs before 24,778 FRATANTONI ET AL.: TORTUGAS EDDIES IN THE STRAITS OF FLORIDA

they are able to propagateinto the SSF. Observations References suggestthat LCFEs propagatingalong the boundary Brooks,I. H., and P. P. Niiler, The FloridaCurrent at Key of the LC were detained by the ring-sheddingprocess West: Summer1972, J. Mar. Res.,33, 83-92, 1975. when the axis of the LC was undergoingnorthwestward Chew,F., The turningprocess in meanderingcurrents: A spreading. A twofold increasein residencetimes was casestudy, J. Phys. Oceanogr.,, d, 27-57, 1974. observed for Tortugas eddies that survived the sepa- Cochrane,J. D., TheYucatan Current, annual report, project ration of a small anticyclonic ring from the LC. The 286, Reft 65-17T,pp. 20-27,Texas A&M Univ., College Station, Tex., 1965. mode i Tortugas eddies were characterizedby shorter Cochrane,J. D., Separationof an anticycloneand subse- residencetimes near the Dry Tortugas, typically 50-60 quentdevelopments in the LoopCurrent (1969), in Con- days, while mode 2 Tortugas eddieswere characterized tributionson the PhysicalOceanography of the Gulf of by longer residencetimes, typically 105-140 days. The Mexico,edited by L. R. A. Capuroand J. L. Reid, pp. absorptionof the LCFE into the ring-sheddingprocess 91-106, Gulf, Houston,Tex., 1972. Graber,H. C., andC. B. Limouzy-Paris,Transport patterns increasedthe residencetime of the Tortugas eddy and of tropicalreef fish larvae by spin-offeddies, Oceanography, lengthenedthe retention timescalefor larvae and pollu- 10, 68-71, 1997. tants near the Florida Keys National Marine Sanctuary. Halliwell,G. R., Jr., and C. N. K. Mooers,The space-time The kinematics of the six Tortugas eddieswere rea- structure and variability of the shelf water-slope water sonably similar. The eddies were deformed upon en- and Gulf Stream surfacetemperature fronts and associated warm-coreeddies, J. Geophys.Res., 8d, 7707-7725, trance into the SSF due to the narrowing topography 1979. and continued to decreasein size as they propagated Hooker,S. B., and J. W. Brown,Warm corering dynam- downstream. The shrinking of the eddies was accom- ics derivedfrom satelliteimagery, J. Geophys.Res., 99, panied by an acceleratedtranslation from 5 km/d in 25,181-25,194, 1994. the westernSSF to 16 km/d in the easternSSF. This Hurlburr, H. E., Dynamic transfer of simulated altimeter data into subsurfaceinformation by a numerical ocean accelerationwas not reported in previous studies. The model, J. Geophys.Res., 91, 2372-2400,1986. size of the eddies ranged from an average 200 km in Hurlburr, H. E., and J. D. Thompson,A numericalstudy the cross-channeldirection and 115 km in the along- of Loop Currentintrusions and eddy shedding,J. Phys. channel direction, near the entrance to the SSF, to 70 Oceanogr.,10, 1611-1651,1980. km and 165 km, respectively,near Cay Sal Bank. These Hurlburr, H. E., and J. D. Thompson,The dynamicsof Loop Current and shed eddies in a numerical model of the dimensionsare comparable to those observed in previ- Gulf of Mexico,in Hydrodynamicsof Semi-Enclosed seas, ous studies and correspond to a decreasein eddy area editedby J.C.J. Nihoul,pp. 243-297,Elsevier Scientific, of approximately55% betweenthe Dry Tortugasand New York, 1982. Cay Sal Bank. The steady decreasein average eddy Ichiye, T., H. Kuo, and M. R. Carnes, Assessmentof cur- area observedat the surfaceroughly correspondsto an rentsand hydrographyof the easternGulf of Mexico, Con- trol 601, TexasA&M Univ., CollegeStation, Tex., 1973. effectivefluid lossby the eddy of approximately 0.7 Sv. Lee,T. N., FloridaCurrent spin-off eddies, Deep Sea Res., By closely monitoring the evolution of several Tor- 22, 753-765, 1975. tugas eddies, we have demonstratedseveral new ideas. Lee, T. N., Mean distributionand seasonalvariability of (1) Tortugaseddies may be the downstreamexpression coastalcurrents and temperaturein the Florida Keyswith implicationsfor larval recruitment,Bull. Mar. $ci., in of LCFEs. (2) A Tortugaseddy will remain station- press, 1998. ary near the Dry Tortugas until it is impacted by a Lee, T. N., and D. A. Mayer, Low-frequencycurrent vari- LCFE. (3) BecauseLCFEs are an integralpart of the ability and spin-off eddiesalong the shelf off Southeast LC's anticyclonicring-shedding process,the longevity Florida, J. Mar. Res., 35, 193-220, 1977. of a Tortugas eddy can be expectedto vary in response Lee, T. N., C. Rooth, E. Williams, M. McGowan,A. F. to the LC's ring-sheddingcycle. BecauseTortugas ed- Szmant,and M. E. Clarke,Influence of FloridaCurrent, gyres and wind-driven circulation on transport of larvae dies have been identified as an important mechanism and recruitment in the Florida Keys coral reefs, Cont. for the transport of larvae within the coastal environ- Shelf Res., 12, 971-1002, 1992. ment surroundingthe Florida Keys, this final point may Lee, T. N., K. Leaman,E. Williams, T. Berger,and L. be central to the identification of recruitment pathways Atkinson,Florida Currentmeanders and gyreformation and retention timescales within the SSF. in the southernStraits of Florida, J. Geophys.Res., 100, 8607-8620, 1995. Leipper, D. F., A sequenceof current patterns in the Gulf Acknowledgments. The authors thank Joanie Splain of Mexico, J. Geophys.Res., 75, 637-657, 1970. at RSMAS, University of Miami, for producingthe Straits of Leis,J. M., Reviewof early life historyof tropicalgroupers Florida SST fields from the AVHRR data. We thank Dou- (•qerranidae)and snappers (Lutjanidae), in Tropical•qnap- glas Myhre of the University of South Florida for providing per and Groupers:Biology and FisheriesManagement, the SST fields for the Gulf of Mexico. This work was funded edited by J. J. Polovina and S. Ralston, pp. 189-237, in part by NOAA/COP SEFCAR and Florida Bay stud- Westview, Boulder, Colo., 1987. ies through CIMAS Grant NA67RJ0149. Research within Maul, G. A., The annualcycle of the Gulf Loop Current,I; the Florida Keys National Marine Sanctuary was conducted Observationsduring a one-yeartime series,J. Mar. Res., underNational Marine Sanctuary Research permit KLNMS 35, 29-47, 1977. and LKNMS- 11-89. Maul, G. A., D. R. Norris, and W. R. Johnson,Satellite FRATANTONI ET AL.: TORTUGAS EDDIES IN THE STRAITS OF FLORIDA 24,779

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