JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO. C14, PAGES 9639-9651, NOVEMBER 20, 1983
Observationsof a Loop Current Frontal Eddy IntrusionOnto the West Florida Shelf
THERESAPALUSZKIEWICZ AND LARRY P. ATKINSON
SkidawayInstitute of Oceanography
ERIC $. POSMENTIER
SouthamptonCollege
CHARLES R. M CCLAIN
NASA GoddardSpace Flight Center
Hydrographicand satellitedata from the westFlorida shelf between April 1-7, 1982showed the intrusionof a LoopCurrent frontal eddy onto the shelf. Data were examined to describethe structure of thisfeature and studyeffects of its intrusionon watermasses in the outershelf region. A frontaleddy, consistingof a warmfilament separated from the main current by a regionof coolerwater, propagated southeastwardat 30 cm/sintruding onto the shelf near 26øN between April 4 and6. Temperature-salinity (T-S)properties revealed that water in thefilament was Loop Current water that had been contiguous with 80 m deeperLoop Current water in the mainbody of the current;water in the coldregion was ContinentalEdge water, a transitionalwater mass with cooler, fresher T-S characteristics.Upwelling of deeperLoop Current water occurred under this region, and elevated nutrient concentrations were found in theupwelled dome under the cold region. Interleaving occurred along water mass boundaries enabling theexchange of heatand salt. This mixing and the supply of cool,nutrient-rich water to theouter shelf wasthe majorconsequence of the intrusion.The lengthscale and speedof thisLoop Current frontal eddywas similar to GulfStream frontal eddies through the upwelling was not as intense as in intrusions of Gulf Stream frontal eddies.
INTRODUCTION cyloniceddies usually move northward or westward[Leipper, The interactionof oceanicboundary currentsand adjacent 1970] the cycloniceddies appear to impinge on the west Flo- shelfwaters dominates the hydrography,circulation, and pro- rida shelf [lchiye et al., 1973]. The boundary of the Loop ductivityof outershelf waters. Boundary current variability is Current is perturbed by finger and eddy-like features which a major causeof accompanyingvariations in adjacentconti- possiblyfacilitate the interaction and exchangeof Loop Cur- nental shelfwaters and to a large extent thesecurrent/shelf rent waters and shelf waters [Merrell et al., 1976; Austin and water interactionsoccur through the intrusion of eddies, Jones, 1974; Rinkel, 1971]. Other evidence of interactions of meanders,and other fluctuationsof the mean currentposition the Loop Current with the west Florida shelf waters has been and flow. There is evidenceof thesetypes of interactionsalong shownby Niiler [1976], Vukovichet al., [1979], and Huh et al. the southeastern U.S. shelf and indications are that similar [1981]. Evidencefrom current meterssuggests the region be- processesoccur along the westFlorida shelf. The objectiveof tween 24øN to 27øN along the west Florida shelffrom 200 to this studyis to describea boundarycurrent/shelf water inter- 100 m is stronglyinfluenced by low frequencyturbulence from action on the west Florida shelf. the Loop Current system[Niiler, 1976]. He proposedthat A specificexample of a boundarycurrent/shelf water inter- Loop Currenteddies were imbedded in a long meanderas a action is the intrusion of meanders of the Gulf Stream onto kinematicdescription of the wavesmoving northward and the southeasternU.S. coast [Stefanssonet al., 1971; Webster, alongshelf.Large meanders (in the cross-shelfdirection) along 1961; Lee et al., 1981; Bane et al., 1981; Lee and Atkinson, the west Florida shelf were seen in satellite data during 1983]. Theseintrusions result in exchangesof heat, momen- periodsof Loop Current retreat [Vukovichet al., 1979]. tum, and nutrientsbetween the Gulf Stream and the shelf Meanders were found in all months for which satellite data waters.Upwelling associated with theseintrusions also causes were available. Two "eddies" on the west Florida shelf were localizedareas of high near-surfaceand near-bottombiologi- detectedby Maul [1977] usingLandsat data. He concluded cal production[Yoder et al., 1981]. that theseeddies were similar to "spin-off"eddies described by Oceanic currents,such as the Loop Current, can induce Lee [1975]. Other evidencefor intrusionscomes from studies steadyupwelling of continentalshelf waters and the westFlo- of the intrusion of modified Loop Current water into DeSoto rida shelf was identified as an area where this processcould Canyonnear Pensacola, Florida with a resultingin situmodi- occur [Hsueh and O'Brien, 1971]. Anticylonicand cyclonic ficationof approximatelyhalf the intrudedwaters [Huh et al., eddiesdetach from the Loop Current and while the anti- 1981].Evidence from currentmeters on the continentalslope off Tampa indicatesa gyremay existwhen the Loop Current intrudes onto the shelf and the current bifurcates with the Copyright 1983by the American GeophysicalUnion. main transportto the southbut with sometransport to the Paper number 3C 1315. north [Molinari and Mayer, 1982]. Strong northwestward 0148-0227/83/003C- 1315505.00 flow was associatedwith a large tongueof warm water found
9639 9640 PALUSZKIEWICZ ET AL.' INTRUSION OF LOOP CURRENT ON SHELF
•'•.•:'• ,. t• Tampa28ø 25¸ to 27øN during April 1-7, 1982. Alternating CTD and XBT observationswere obtained every 5-6 nautical miles in sevencross-shelf transits at three differentlocations (Figure 1). i ' '• Sect,on .... 27'ø • • .. [ ' Salinity and temperature were determinedby a Plessey9400 CTD interfacedwith a Hewlett Packard 9825 desktop com- puter. Water sampleswere taken at selecteddepths during the upcastwith Niskin bottles mounted on a rosettesampler cou- pled to the CTD. Surface temperature,salinity, and relative surfacechlorophyll fluorescencewere recordedalong transects • • • ( CapeSablel / 1 usingthe CTD and a Turner Designsmodel 10 fluorometerin ...... :::.,{ •' / line with a flow of surfacewater. Phosphate,silicate, and ni- trate were analyzed with a Technicon Auto Analyzer II [Gli- ...... ';%?.....! •'•"'"'"::•'1bert and Loder, 1977]. The standard error of the mean for 85 ø 84 ø 83 ø 82 ø 81øW phosphate, nitrate and silicate were ___0.008,__+0.11, and Fig. 1. Sectionlocations for April 1-7, 1982. Section1 was oc- +__0.02#m, respectively.Dissolved oxygen concentrations were cupiedon April 2-3; section2 on April 3-4; section4 on April 4; determined using the methods of Strickland and Parsons section5a on April 4-5; section6 on April 6 and section8 on April 7. Data from section5b and 7 are not presentedhere. [1965]. Sea surfacetemperatures were derived from infrared data from the Advanced Very High Resolution Radiometer where the Loop Current ran onto the west Florida shelf near (AVHRR) onboardthe NOAA-7 polar orbiting satellite.These 25øN. data were calibrated by NASA using the two-channel algo- One consequenceof boundary current and adjacent shelf rithm of McClain [1981] along with observedsea surface tem- water interaction is the local modification of water masses. In peratures.The imageswere then rectifiedfor geometricdistor- this respect,the west Florida shelf can be thought of as a tion and enhancedto reveal temperaturegradients more clear- transition region. In the off-shelfregion temperature-salinity ly. The SST intercomparisonsindicate a 1ø-1.5øCbias which (T-S) characteristicsdisplay a salinity maximum (36.6-36.8•) is attributed to aerosol loading by the E1 Chichon eruptions near 22.5øC in the upper 200 m. Waters with this character- (R. W. Barbieri et al., manuscriptin preparation, 1983). Com- istic have been called Yucatan Water [Wennekens, 1959], parison of the location of surfacetemperature fronts identified Eastern Gulf Loop Water [Nowlin and McLellan, 1967], Righthand Water [Leipper, 1970], and Loop Current Water [Price, 1976]. The salinity maximum is a characteristicof Sub- tropical Underwater [Wust, 1964]. In general, water in the upper layer (0-200 m) of the Loop Current is due to an influx of Caribbean water which is a mixture of North Atlantic and South Atlantic waters.For simplicitywe will call waterscoin- cident with the subsurfacesalinity maximum Loop Current A D Water (LCW). Waters with fresher, colder T-S characteristics also occur in the region. Wennekens[1959] calls these waters Continental Edge Water (CEW) and notes that the T-S characteristicsare intermediate between Yucatan and western Gulf Water and they are found along the continentalside of the current. Low salinity valuesin watersbounding the Loop Current have also been describedby Nowlin and McLellan [1967] and Morrison North and Nowlin [1977]. We call waters with these characteristics and regional positionCEW. Data from a recenthydrographic study on the west Florida Map Vie w shelf showed that interactions between LCW, CEW, and shelf water do occur, and an intrusion of a Loop Current "frontal eddy" was seen.Hydrographic and satellitedata collectedbe- tween April 1 and 7, 1982,showed a filament of LCW and the region of cold CEW which constituteda frontal eddy, along with the resulting interaction with shelf water as the water intruded onto the west Florida shelf. The structure of the LCW front and filament and the effect of the intrusion on the shelf waters appear similar to Gulf Stream frontal eddieson the east Florida shelf[Lee et al., 1981; Yoder et al., 1981].
OBSERVATIONAL METHODS Prior to the cruise, surface temperature charts (NOAA- , ['.'i' Section NESS, Miami) were examined to determine the position and Fig. 2. (Top) Schematic map view. (Bottom) Schematic north- likely propagation path of frontal events.Based on these ob- facingsection. Section A, Main body of Loop Current Water (LCW); servations,hydrographic data and satellite-derivedsea surface Section B, filament of Continental Edge Water (CEW); Section C, temperatureswere obtained on the west Florida shelf between filamentof LCW; and SectionD, main body of shelfwater. PALUSZKIEWICZ ET AL.' INTRUSION OF LOOP CURRENT ON SHELF 9641
Fig.3. Compositesatdlit• infrared image co'nstructed fromNO'AA-7 A•VHRR data from March 31-April 4 which showsthe entireGulf of Mexico,Florida, the Loop Current,the .Loop Currentfilament. The currentand filamentare lighter, indicatingwarmer water.
A 28 ø N in the hydrographic data to the locations of SST fronts derivedfrom the imagery indicatedan agreementof 5-15 km. TampaBa]•....•_27ø RESULTS AND DISCUSSION In this section we discussvarious aspectsof the observa- tions such as the propagation of a frontal event, water masses 26 e in the event, and the interaction between water masses.The importance of mixing resulting from the interaction will be
Legend: discussedand the frontal eddy observedwill be compared to -25 ø 1 April eddies in the Gulf Stream off the southeast United States. 2 April 3 April : : • S.atellite-observedsea surfacetemperatures and a fe& hy- 4 April ...... 2( 30 m•' drodynamic calculations show that the main feature of the • 24 ø 8• ø 8'3ø 82ø W region was a propagating convolution of a front separating the LCW, mainly to the west of the front, from CEW water, a [Com•os.eSSTcontour • ... __•[ [ 28 e N mainlytothe east of the front (Figure 2).We refer to the long, I from1-4 April • lampa t•ay •'J narrowfeature, - C," as a filamentof the watertypes found, I satelliteimagery..• t •.. l l
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I -o----ø SST contour • j • • 12 Ictco nsedfromtru hydrographic V •-25e • 20 2 I(•atafor 5-7 April,x marks /•, • • Idata lOcatiOn _ [•• I •. 34 Ii Uncertain, , 200•24rn.-'"' 1 ø • 15 • Fig. 4. (a) A time seriesof SST contours (--, 24øC) for April 1-4 derived from AVHRR data which define the Loop Current/shelf water front87ø and the 86* Loop Current8•ø filament. 84øThe 8'3* length of82 theøW filament I•lO • '• 28• 35.5 36.o 36.5 37.o may be longerthan appearsin this figure and on the imagery which is limited by coverage and cloud cover at the northern extent of the Salinity (%.) filament; (b) the projectedposition of the frontal eddy on April 5-7 Fig. 5. CharacteristicT-S curvesshowing LCW (stations12, sec- based on hydrographic data and the last position from the satellite tion 1 and station79, section6), CEW (station 104, section8) and data. interleaving(station 34, section2). 9642 PALUSZKIEWlCZET AL.' INTRUSION OF LOOP CURRENT ON SHELF
(S-36) x 100 on Sigma-t=25.25 southwardand exitedthe Gulf throughthe Straitsof'Florida. 85 ø 84 ø 83øW The filamentin the northeastpart of the Loop Currentwas 200 27020 ' warm,and the area betweenthe filamentand Loop Current '"':':-'"--•---':•'-'.'-'.:;:...G wascool. The filament'slength was 220 km fromits junction with the Loop Currentto the northerntip of the warmfila- :.•.:-..:.x-•:....::.:.::.....:.;• ment.Figure 3 showssurface temperatures on the shelfwhere coolerwater was closeto the north Florida coast,extended •oo southwardto Tampa Bay, with warmer water off southwest :...... -._....•...... •....•...... ••...... :•:;• ß Florida.A cycloniccurl of coolerwater originatednear the coast and extended to the middle shelf section due west of TampaBay. A bandof cooler(darker shade) water was at the
g"'::",".•..'..'.'::';'•".•.... ::.'.•.•.O outer sloperegion between26 ø and 28øN. o 25o30'N The term "frontal eddy" is usedto describethe filamentof 0 100 2OO Distance East (km} the LCW, the cool region of CEW betweenthe filamentand Fig. 6. The horizontaldistribution of salinityon the sigma- the Loop Current, as well as the distortion of the front associ- t = 25.25 surfaceusing salinity valuesfrom all stations.Darker shad- ated with the gradientbetween Loop Current filamentand ing indicateshigher salinities (> 36.4)with lessdense shading indica- shelfwaters. Figure 4a showsthe changes of thefrontal eddy ting lowersalinities, respectively. overApril 1-4. The frontaledge and the coldregion propa- gatedsoutheastward 95 km asmeasured at the tip of the cold mostlyin the main body, "A." However,"B" is a filamentof regionand eastward 56 km betweenApril 1 and4, placingthe cold region 15 km westof the shelfbreak at the 200 m isobath. CEW whichmay be the shelfwater from muchfarther north The calculatedspeed was 30 cm s-•. The filament became or a transitionalbody of water betweenLCW and shelfwater proper. It is not similar to shelf water "D" in this section. thinner,the widthnarrowed from 62 km on April 1 to 35 km Propagation of the Frontal Event on April 4, andwarmed as indicatedby the infraredimagery. The width of the coldregion increased from 45 to 50 km. The SSTpattern which represents the LoopCurrent is no- Satelliteimagery on the following3 days was not clear ticeablein our thermalimages as a lightershade indicating enoughto trackthe propagation of thefrontal eddy. However, warmer temperatures(Figure 3). It reacheda point as far hydrographicdata, whichwill be discussedlater, obtainedon north as 27ø30'N in the Gulf of Mexico and then turned April 5-7 allowedus to estimatethe positionof the onshore
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and offshore front associatedwith the filament. Figure 4b all stations, and although it is not synoptic it does show that shows the last position of the cold region on April 4 and its the same feature that was delineated by temperature was also positionon April 5-7 from hydrographicdata. evident in salinity. The relationshipof the salinitiesto the propagation of the event follows. Upper layer salinitieswere HydrographicObservations of the Frontal Eddy greater than 36.0%0in sections 1 and 2 (Figures 7 and 8, Hydrographicdata from five cross-shelf CTD transectsand respectively)both downstreamfrom the frontal event.How- additional XBT transectsare not synoptic becausethe time ever, salinities were lower in sections traversing the cold scale(5 days)was long with respect to thefrontal dynamicS; region of CEW. In section 5 (Figure 11), during which the however, several of the hydrographic sectionscoincide with frontal eddy intruded, there was a 40 m thick surface layer infrared imagery, providing a three-dimensionalview of the with salinities less than 36.0%0just east of the Loop Current frontal eddy and giving insight into the interaction of Loop front between stations 57 and 59. Similar low salinity surface Current and shelf waters. layers were observedin section6 and section8 at the northern A first step toward understandingthese interactions was to edge of the frontal eddy. The low salinity surface water ap- characterize the T-S relationships (Figure 5). Station 12 peared to be associatedwith the cold region of CEW between showedcharacteristics of LCW. Water in the Loop Current is the LCW filament and LCW front with largest volumesfound easily identified by its subsurfacesalinity maximum, oxygen in the northern section. Wennekens[1959] showed that the minimum, and elevated nutrient concentrations. Salinity shallow low salinity layers in the CEW result from land drain- maxima occasionallyexceeded 36.6%0 in western parts of the age. If so, one would expect the volume to be largest in the sectioncoinciding with the steeplysloping isothermsand in- northern end of the study area, and our data support this. dicating the Loop Current easternwall. Station 79, also LCW, Transect 1 (location given in Figure 1), occupiedon April 2, was located in the filament. Station 104 had characteristics of crossed the shelf (Figure 7) where the front had intruded CEW, and station 34 showed lateral interleaving structures shoreward of the 200 m isobath. A front was indicated by indicatingthat mixing occurredduring the intrusion. temperatureand density sections(Figure 7) near 25ø38'N and The horizontal distribution of salinity on the sigma- 84ø15øW(stations 4-6) with a gradient of •2øC over 10 km. t = 25.25 surface(Figure 6) resemblespatterns of the SST con- This front coincided within • 7 km of the surface front, indi- tours (Figures 4a and 4b). This plot usessalinity values from cated on the infrared imagery at 25ø04'N and 84ø11'W, and 9644 PALUSZKIEWICZET AL.' iNTRUSIONOF LOOP CURRENTON SHELF
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O') '•"•' '•' '•' '•' '•' "•' sured from imagery (25ø39'N, 84ø05'W) was within approxi- mately 15 km of that indicated by hydrographic data A 24• (25ø40'N,83ø55'W; station 31) and was locatedover the 120 m isobath,15 km shorewardof its previousposition. It is unlike-
22 ly that the changein depth of the isothermsresulted from tidal -1='_ IAAA u,•p,a•ment, usually'1It55 .... tlli:tll'-A- I m, or internal tides which, even with intensificationat the shelf break, are only 5 m in T100 amplitude [Koblinsky,1979]. It is more likely that elevationof the isothermswas related to the propagation of the frontal 140 event. After section2, a northward XBT transect(section 3) was TE•PER^TURE (C) 180 run acrossthe cold region of the frontal eddy. The isotherms 15 4APRIL 1982 were domed, with the most intensedoming at depth and less 250 200 150 100 50 0 DIST^NCE ^LONGSHORE (KM) displacementof isothermsin the upper 50 m (Figure 9). A slight front, with a gradient of 0.5øC per 10 km was located
28 ø N over the edges of the cold region. The position of thermal fronts agreed with those derived from the imagery within 5 km. The edge of the cold region was approximately 15 km B TarnpaBayfrom the 200 m contour. The vertical displacementof iso- therms under the cold region indicatesthat upwellig had oc-
... Section 3,(4 AD ",,, curred or was occurring.Without current meter data, we are unable to determine whether this cold dome is a remnant
/ which has been advected along with the frontal eddy or whether the cycloniccirculation implied by surfacethermal 25 o patternsmaintains a local upwelling.Upwelling in this region is what has been seen in other frontal eddies with similar SST contour fromsatellite •mage,(4 April} '% •-200 m• surfacetemperature distributions [Lee et al., 1981]. Following , , , , 24 ø this section a shorewardXBT transect,section 4 (Figure 1), 87 ø 86 ø 85 ø 84 ø 83 ø 82 ø W was occupied.A filament of LCW is evidenced(Figure 10) by Fig. 9. (a) Temperaturesection for XBT section3, April 4, 1982; and (b) sectionlocation and SST from satellite image for reference. the depressionof the isothermsbetween stations 46 and 51. X = XBT.
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352 32• 252 2• 15• 122 350 322 250 200 150 100 0ISTANCE OFFSHORE (KM) DISTANCE OFFSHORE (KM) Fig. 11. (a) Temperature'(b) salinity;(c) sigma-t'(d) oxygen'and (e) nitratefor section5 for April 4-5, 1982.C = CTD, X = XBT.
The width of the filament measuredfrom the image (• 35 km) 12) the filament was indicatedby the depressionof the 24øC matched closely with that indicated by surfacetemperatures isotherm between stations 78-81 east of the cold region of (40 km). The subsurfaceextent of the filament was • 180 m as CEW. The subsurface extent of the filament is much shallower indicated by the depressionof the isothermsthroughout the (• 20-60 m) in this sectionand during the other sectionsfol- entire water column. The surface front between the filament lowing the intrusionof the filament onto the shelf.Section 8, and shelf waters was pronouncedwith a gradient of the order • 70 km to the north, showsthe doming of isothermson the of about IøC over 10 km. The location of the surface front shelf,the depressionof the 23øCisotherm, and the intersection derivedfrom the imagery(26ø11'N, 84ø09'W) was within 8 km of the 22øC isothermnear 100 m indicatingthe cold region of of the location of the front seenin hydrographicdata stations CEW, filament of LCW, and wall of the Loop Current, respec- 51-53 (26ø10.6'N, 84ø03.4'W).The cold region of CEW was tively (Figure 13). The occurrenceof the domingon the shelf immediately west of this section. The structure in the water to the north suggeststhat the horizontal length scaleof this .column,that is, domingunder the cold regionof CEW and feature is on the order of 100 km. depressionof isothermsacross the LCW filament, indicates Oxygen concentrationsin the upper layers of the Loop Cur- cyclonicflow in the frontal eddy. The current meter observa- rent are characteristically low [Wennekens, 1959]. Low tions of Molinari and Mayer [1982] indicate this type of flow oxygen concentrationswere found in our first sections,coin- when there is an intrusion of a warm Loop Current filament cident with high salinities and high nutrients from the Loop onto the shelf with major transport to the south and minor Current. With time, oxygen concentrationsdecreased in the transport to the north. This relative magnitudewould be rea- near-bottom layer which was about 20-30 m thick. Lowest sonable for the feature discussedhere given the size of the concentrations(3.16 ml/1) were in this lower layer in sections filament with respectto that of the main current. 5, 6, and 8 (Figures 11, 12, and 13) which were nearestvertical In following days, sectionswere repeated in the area of displacementof isothermsassociated with the intrusion and onshore motion. The data showed progressivedeepening of doming under the cold region of CEW. Nitrate concentrations the isothermsto the west and east and doming below the cold also increase in the near-bottom layer with highest con- region of CEW as it intruded onto the shelf.Sections 5 and 6 centrations (16 ttm) in the lower layer under the region of (Figures 11 and 12) show that the domed feature reachedthe strongestdoming (seesections 5, 6, and 8). Silicate and phos- 60 m isobath,the most intensedoming reacheddepths of 40 m phate concentrations (Figure 14) followed similar trends. betweenstations 59-57 (section5) and stations81-85 (section Theseresults confirm the frontal analysisshowing the offshore 6). The 22øCisotherm reached a depth of nearly 100 m, indica- Loop Current movement and propagation of a frontal eddy ting the easternwall of the Loop Current. In section6 (Figure and also indicate upslopemovement of nutrient-rich, oxygen- 9646 PALUSZKIEWICZ ET AL.: INTRUSION OF Loop CURRENT ON SHELF
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. I , , • , , , .... /•/• , , • , •50 25B 2BB 15B 1• 35• 3B• 25• 2BB 15B 1 DISTANCE OFFSHORE (KM) DISTANCE OFFSHORE Fig.12. (a) Temperature;(b)section location and SST from satellite image for reference;(c)salinity; (d) sigma-t- (e)oxygen' and (f) nitratefor section6 for April6, 1982.C = CTD, X = XBT.
poor deepwater beneath the cold region.The watersover the Mixing upper slope are typical of deeper Gulf of Mexico waters [Nowlin and McLellan, 1967; Morrisonand Nowlin, 1977; Advectionand mixingon a localscale are a consequenceof Morrison, 1977; Wennekens,1959] while lower salinitiesbe- theeast/west movements of the LoopCurrent and the passage tween the LCW front and the LCW filament are characteristic of frontaleddies or otherfrontal events. Frontal analyses and of CEW. The doming indicatedin sections3, 5, 6, and 8 hydrographicdata demonstratedlarge scale advection during (Figures9, 11, 12, and 13) occursbeneath the regionwhere the passageof the frontaleddy, and T-S plots(Figure 5; sta- CEW is found.It is notclear whether the cool surface temper- tion 34) indicatefiner scalemixing. In this sectionwe showthe atureswere a resultof the upwardvertical displacement of the impactof theseprocesses on the upperslope and shelfwater. isothermsor the presenceof CEW. Figure 15 showsthat interleavingoccurred along the In summary,satellite and hydrographicdata indicatedthat boundariesof the two water masses.Interleaving was most a frontal eddyconsisting of a cold regionof CEW and warm evident between stations 87 and 83. This location coincided filamentof LCW propagatedsouthward intruding into upper with that of the front between LCW and CEW. Between sta- slopeand outershelf waters. At the sametime the Loop Cur- tions 83-81, interleavingcoincided with the positionof the rent either maintained its position •45 km from the shelf surfacefront betweenthe cold CEW region and the LCW breakor possiblymoved offshore. The edgeof the currentwas filament. Interleavingwas coherentover severalstations and found45 km from the shelfbreak in section1 (Figure7) but strongestwhere isotherms, isohalines, and isopycnalsslope was not detectedat the offshoreend of the followingsections steeply.The interleavingalong the water massboundaries oc- whichaveraged 40-60 km from the shelfbreak. The extrapo- curredin all sectionsimplying mixing and exchangeof heat lated position of the intersectionof 22øC at 100 m was at least and saltcaused by the intrusionof the frontaleddy. 5 km furtheroffshore than the endof thesesections indicating Several order-of-magnitudeestimates were made for ef- possible offshore movement. The cross-sectionalvolume of fectivelateral diffusioncoefficients (Kn, the ratio of a cross- high salinity water within 75 km of the shelf break decreased shelfflux to a cross-shelfgradient) of the processesdiscussed as the frontal eddy passed.At the sametime salinityin the in order to establishthat the hydrographicfeatures were not upperlayer decreased to lessthan 36.0%0with the exceptionof dissipatedby smallerscale mixing processes. A cross-shelfdis- outer shelfwater whereit remained36.2-36.49/00. Oxygen and placementof 100 km of the filamentsfrom their main bodies nutrientconcentrations indicated that underthe regionof cold and eventfrequencies of oneeach month, result in K n = (100 CEW wasadvected or upwelleddeeper LCW. km)2/1month = 4 x 107 cm2 s-•. Froma differentpoint of PALUSZKIEWlCZ ET AL.' INTRUSION OF LOOP CURRENT ON SHELF 9647
U X U X X U •1•1ß N
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Section 8
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,
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OXYGEN, I (ML/L)• I •8•N•R• (•) •oo• 350 300 :>50 200 150 100 350 300 :>50 :>00 150 100 DISTANCE OFFSHORE (KN) DISTANCE OFFSHORE Fig. 13. (a) Temperature;(b) sectionlocation and SST from satelliteimage for reference;(c) salinity'(d) sigma-t' (e)oxygen' and (f) nitratefor section8 for April 7, 1982.C = CTD, X = XBT. view, if six layers of interleaving, each 10 m thick, are dis- strophicspeeds calculated by Nowlin and McLellan [1967], placed 10 km cross-shelfevery day, and if their effecton cross- using large-scaledynamic topography for this region are front exchangesis distributedover 100 m depth, this corre- within 50% of the speedcalculated using our data. spondsto K• = (10 km)2 x 6 x 10 m/100 m/1 day = 0.7 x Another quantity is potential energy associatedwith the 107cm 2 s-t. Yet anotherestimate may be obtainedby scaling verticaldisplacement of the filamentfrom the main body of down the Ka for heat from double diffusiveintrusions studied the Loop Current. The densityprofiles at stationstypical of by Posmentierand Hibbard(1982), 3.7 x 107cm 2 s-t, by a the main body of LCW and of the filament of LCW (for factorof (0.2 ppt/1 ppt)2. The resultis 0.15 x 107 cm2 s-t. example,stations 12 and 79 in Figure 16) are offsetby about The range of valuesof thesethree estimatesis not disturbingly 80 m in depth. The same depth differenceoccurs between large, consideringthe inexactitudeof the parametersused to correspondingpoints in coincidingT-S relationships(Figure derivethem. Their trend•decreasingKa with decreasingspa- 5). We inferfrom this that the filamentof LCW on the shallow tial scaleof correspondingphenomenon--is consistentin that shelfwas contiguous with 80 m deeperLCW in the main body hydrographicfeatures are not dissipatedby the next smaller of LCW to the west.Density differences between the deepand scaleprocess much sooner than the next event occurs.Conse- shallowstations average roughly 1.4 x 10-3 g cm-3. The in- quently, we see that the interleaving (mixing) occurred be- creaseof potentialenergy, which would occurduring 80 m of tween differing water massesbrought into contact by the in- elevationover which the densityanomalies increase gradually trusion of a filament of LCW yet the mixing estimatedby from 0 to 1.4 x 10-3 g cm-3, is equivalentto the kinetic assumingdouble diffusionor interleavingwas not sufficientto energyof water with a speedof 105 cm/s. The kinetic energy eradicatethe hydrographicsignature of the feature. of geostrophicvelocity discussed above is of the correctmag- nitudeto providethe potentialenergy necessary for communi- Kinetics cation between the main body of LCW and the shallower Here we examine the consistencyof some hydrodynamic LCW filament. quantities with circulation inferred from hydrographiccon- The third calculationestimates vorticity differencesbetween siderations. To determine the baroclinicity in the frontal the LCW main body and the filament.We considera scenario region betweenthe main body of LCW and the CEW region in which a currentextending from the surfaceto 160 m depth we use the densities at stations 100 and 102 to estimate the and having no relativevorticity is funnelledinto 80 m depth geostropicshear between the surfaceand 80 m (the signifi- where its combinedplanetary and relative vorticitiesare the canceof this depth is discussedbelow). The resultingspeed is sameas the original planetaryvorticity. The resultingrelative 51 cm/s northward at 80 m relative to the surface.The geo- voriticityis 3.2 x 10- • s-x, whichis equalto 64 cm/sper 20 9648 PALUSZKIEWICZET AL.: INTRUSIONOF Loop CURRENTON SHELF
I I I I I I I I I I I I
14 Sect zon l 2-3 APRIL 1982
ox uxoxoxx xo
i i iT
20
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2O
140
180[ I • I 60 A 350 300 250 200 150 100 •50 300 250 200 150 1•0 DISTANCE OFFSHORE (KM) DISTANCE OFFSHORE (KM) Fig. 14. (a)-(e) Silicatefor sections1, 2, 5, 6, and 8. (f)-(j) Phosphatefor sections1, 2, 5, 6, and 8. km, a velocityand distancein good agreementwith both the Comparisonof West Florida ShelfFrontal geostrophicspeed and the speedequivalent to the potential Eddiesto Gulf StreamFrontal Eddies energyof 80 m of verticaldisplacement and with the filament half-widths,respectively. This agreementshows that the topo- The intrusionof a Loop Current frontal eddy onto the west graphic effect of mixing 160 m of LCW onto an 80 m deep Florida shelf is evidenceof interaction of the boundary cur- shelf would cause an anticyclonic vorticity realtive to the rent (the Loop Current)with shelfwaters. Similarly, intrusions mean state on the shelf and comparablein magnitudeto the of Gulf Stream frontal eddiesare examplesof the interaction observed vorticities. of the Gulf Stream with a shelf [Lee et al., 1981; Lee and These calculationsgive some indication of the sourceand Atkinson, 1983]. The major featuresof the frontal eddiesare possiblythe formation of the filament of the Loop Current. similar. In both cases,these eddies consist of a counter-flowing Density profilesfrom the west Florida shelffilament indicate warm filament or streamer of the main current. These fila- that it was once contiguouswith the main body of LCW ments are separatedfrom the main current by cooler water. which was 80 m deeperat one time and that sufficientkinetic The filaments are of the same length scale, 100-200 km for energyis availablefrom the baroclinicshear to providefor the Gulf Stream filaments, 200 km for the LCW filament. How- elevation over 80 m. Leipper [1970] attributed differencesin ever, the filament of the west Florida shelf eddy was deep, depth (150-300 m) of a representativepoint on the T-S curve approximately180 m when the filament was still off shelfand between right-hand and left-hand water to the uplifting of the T-S characteristics indicated filament water to 60 m on the left-handwater as it passesacross the shallowerYucatan bank shelf.The Gulf Streamfilaments are relatively shallowfeatures when entering the Gulf. Processessimilar to this could ac- (15-20 m) [Lee et al., 1981;Bane et al., 1981]. count for the observed80 m depth differencebetween T-S Gulf Streamfrontal eddies move from 30 to 70 cm s- x [Lee pointsin the main body of LCW. There are indications(l/uko- et al., 1981; Bane et al., 1981; Lee and Atkinson, 1983] and the rich et al. [1979]; Maul [1977]; GOES imagery examined Loop Current frontal eddy propagated at approximately30 prior to our cruise)of filamentsalong the Yucatan bank and cm s-x. Thesepropagation speeds are of the sameorder of along the westernand northern boundariesof the Loop Cur- magnitude. During the intrusion of the Gulf Stream frontal rent. The uplifting could occur at any time along the bound- eddy, there were large drops in temperature[Lee et al., 1981]. ary betweenthe shelf edge and the Loop Current, or as the We did not witnessthese "before and after" effectsin the Loop frontal eddy intrudesonto the westFlorida shelf. Current frontal eddy intrusionbut the presenceof fine struc- PALUSZKIEWICZ ET AL ' INTRUSION OF LOOP CURRENT ON SHELF 9649
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I--
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PHOSPHATE (•M) 180 i 2-3APRIL 1982
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•50 300 250 200 150 100 350 ' 300 250 200 150 DI STANCE OFFSHORE (KM) DISTANCE OFFSHORE Fig. 14. (continued) ture indicates that changes in the water column properties [1977] and Vukovich et al. [1979] suggestwave-like pertur- may evolve more slowly. No evidence of finestructure was bations of the Loop Current as does the time seriespresented presentedby Lee et al. [1981] in the caseof the Gulf Stream here (Figure 4) On a smaller scale, wave-like perturbations frontal eddy intruson and they have seenlittle evidenceof it in are apparent along the front and along the edge of the fila- other similar intrusions. Gulf Stream frontal eddies form from ment; however, without further evidence we can only suggest amplified wavesof the Gulf Stream front [Lee et al., 1981; Lee that the Loop Current frontal eddiesdevelop from wave-like and Atkinson, 1983]. It. is not clear that the Loop Current perturbationsof the Loop Current front. The satelliteimagery frontal eddy formed in a similar manner. The data of Maul clearly illustatesthat the frontal eddy and its propagation are related to the Loop Current; however,we note that it is possi-
Station Numbers 89 87 85 83 81 79 77 Sigma-t 24 I 21.0 22.0 23.0 24.0 25.0 26.0 27.0 i t i i i i 0-
20-
40- 25 60- Station 79 80- • 26 lOO- •'ß 12o
14o
16o 27 Station 12 40' 20' 84 ø 40' 20'W 18o
Fig. 15. Contours of salinity versusdensity and distance(from 2oo section6, April 6); contourinterval is 0.1 ppt. Note the long, finger- like interleavingstructures. Fig. 16. Density profilesfrom stations12 and 79. 9650 PALUSZKIEWICZ ET AL.: INTRUSION OF LOOP CURRENT ON SHELF ble that the wind regime might also affect the intensity of the of intrusionson productivity.This could be due to lessener- circulationin the eddy, its shapeand possiblyits propagation. getic current velocitiesor to loss of energy due to frictional For the Gulf Stream frontal eddies, upwelling under the dissipation over the wide shelf area. We note that this one cold region uplifted deep waters with high nutrient con- example of a Loop Current frontal eddy may be much centrationsonto the shelf. In the Gulf Stream case,upwelling stronger or weaker than the mean and consequentlymore introduced nitrate concentrationsof 10 #m to the 30 m depth extensivesampling is neededfor a valid comparison.In addi- [Lee et al., 1981; Lee and Atkinson, 1983]. It is not clear tion, further studies are needed to address how these frontal whether the cold region associatedwith the west Florida shelf eddiesare formed,the frequencywith which theyintrude upon frontal eddy is due to upwelling which occurslocally or up- shelves,the impact on shelf nutrient supplies,and the overall stream and is consequentlyadvected downstream and onto impact on the productivity of shelf waters. It would also be the shelf.However, the doming of isothermsunder the CEW is enlighteningto obtain evidenceof these eventsalong other consistentwith the cyclonic circulation of a cold-core eddy current/shelfboundaries and to then identify the common with upwelling of the cold core. Nitrate concentrationsof 10 processto which they are related. /•m were between 80 and 100 m on the west Florida shelf. Generally, the concentrationsof nutrients in the upper 50 m Acknowledgments.This research was funded by the Minerals were not as high during the intrusion of the west Florida shelf Management Service under contract 14-12-0001-29144 (formerly AA851-CTI-45) awarded to Woodward-Clyde Consultants (WCC). frontal eddy as during Gulf Stream intrusions.It appearsthat We thank B. Chandlerand F. Flynn for help in preparationof this the west Florida shelf upwelling is less intense than similar data, and D. Menzel, J. Yoder, and J. Blanton for helpful comments. processeson the other coast.This could be an indication that We would also like to thank D. Barbieri and D. Endres of NASA/ it is "older" upwelling which has been advecteddownstream, GSFC for their work on the satelliteimagery and M. Brown (MMS, or a lessenergetic cyclonic circulation to maintain the upwell- Metairie, Louisiana)and H. Chin (WCC) for their help and cooper- ation. We would like to expressour gratitude to C. Miller, J. Harris, ing. Another possibleexplanation for this is that for a Loop A. Boyette,and S. Mcintosh for the typing and draftingof this manu- Current frontal eddy to affect shelf waters < 100 m it must script. intrude (200 m marks the shelf break) at least 50 km. Gulf Stream frontal eddies need to move 15-25 km onshore to REFERENCES reach the 70 m depth from the mean position of the Gulf Stream [from Bane and Brooks, 1979]. The wider shelf area Austin, H. M., and J. I. Jones,Seasonal variation of physicaloceano- graphic parameterson the Florida middle ground and their rela- that Loop Current frontal eddies must crossmeans there is tion to zooplankton biomasson the west Florida shelf,Florida Sci., more friction and turbulent mixing that could alter energyand 37, 16-32, 1974. erode the pressuregradient that maintainsthe upwelling.Fin- Bane, J. M., Jr., and D. A. Brooks, Gulf Stream meandersalong the ally, we consideredthe conceptual model of a cold dome continentalmargin from the Florida Straitsto Cape Hatteras,Geo- phys.Res. Lett., 6, 280-282, 1979. meander and the joint propagation of the cold dome and Bane,J. M., Jr., D. A. Brooks,and K. R. Lorenson,Synoptic observa- meander downstream [Chew et al., 1982]. In this model, the tions of the three-dimensionalstructure and propagationof Gulf cold water giving shapeto the dome is not trapped within the Stream meandersalong the Carolina Continental Margin, J. 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Brook, The propagation of a cold-dome meander: A conceptual model, in Proceedingsof the cannot be determined from our data that they form in a simi- Workshopon Gulf Stream Structureand Variability, Publisher,pp. lar manner. 63-68, ResearchTriangle Park, N. C., 1982. Glibert, P.M., and T. C. Loder, Automated analysisof nutrientsin CONCLUSION seawater:A manual of techniques,Tech. Rep. WH01-77-47, Woods Hole Oceanogr.Inst., Woods Hole, Mass., 1977. Hydrographicand enhancedinfrared satellitedata showed Hsueh,Y., and J. J. O'Brien, Steadycoastal upwelling induced by an the intrusion of a Loop Current frontal eddy onto the west alongshorecurrent, J. Phys.Oceanogr., 1, 180-186, 1971. Huh, O.K., W. J. Wiseman, Jr., and L. J. Rouse, Jr., Intrusion of Florida shelf betweenApril 1 and 7, 1982. T-S characteristics Loop Current waters onto the West Florida Continental Shelf, J. showed that the filament was of LCW uplifted earlier 80 m Geophys.Res., 86, 4186-4192, 1981. from the deeperLCW source.The regionof cool surfacewater Ichiye, T., H. Kuo, and M. R. Carnes, Assessmentof currents and between the filament of LCW and main body of LCW was hydrography of the eastern Gulf of Mexico, Control 601, Texas identified as CEW by its cooler, fresher T-S characteristics. A&M Univ., CollegePark, 1973. Koblinsky, C. J., Tides on the west Florida shelf, Ph.D. Thesis, Sectionsthrough this region prior to and during the intrusion Oregon State Univ., Corvallis, 1979. showedupwelling beneath this region. Nutrient and oxygen Lee,T. N., Floridacurrent spin-off eddies, Deep Sea Res.,'22, 753-765, concentrationsindicated that the upwelled water was deeper 1975. LCW water. Lee, T. N., and L. P. Atkinson, Low frequencycurrent and temper- Interleaving occurred along both boundariesof the CEW ature variability from Gulf Stream frontal eddiesand atmospheric forcingalong the southeastU.S. outer continentalshelf, J. Geophys. region indicating mixing resulted from the intrusion of the Res., 88, 4541-4568, 1983. LCW filament and front onto the shelf. Lee, T. N., L. P. Atkinson, and R. Legeckis,Observations of a Gulf The Loop Current frontal eddy had similar characteris- Stream frontal eddy on the Georgia continental shelf,April 1977, tics-length scale and speed--to the Gulf Stream frontal Deep Sea Res.,28, 347-378, 1981. Leipper, D. F., A sequenceof current patterns in the Gulf of Mexico, eddies describedearlier [Lee et al., 1981]. Upwelling on the J. Geophys.Res., 75, 637-657, 1970. west Florida shelf was less intense than that associated with Maul, G. 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