THE MORPHOLOGY OF SHELFBREAK EDDIES - CCPOklinck/Reprints/PDF/garvineJGR1988.pdf15,594 Garvine et al.: The Morphology of Shelfbreak Eddies passages per unit time were sufficient to

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. C12, PAGES 15,593-15,607, DECEMBER 15, 1988

THE MORPHOLOGY OF SHELFBREAK EDDIES

R. W. Garvine, K.-C. Wong, G. G. Gawarkiewicz, and R. K. McCarthy

College of Marine Studies, University of Delaware, Newark

R. W. Hough ton and F. Aikman III

Lamont-Doherty Geological Observatory, Palisades, New York

Abstract. We used a combination of buoy the temperature and salinity gradients are largely tracking, intensive hydrography, satellite thermal offsetting in their effect on density. The imagery, and moored current meters to resolve the resulting density difference across the front in structure of eddies at the shelfbreak front in the at is only about 0.5 kg/m s, while the temperature Middle Atlantic Bight south of New England. difference is typically 50C and the salinity Eddylike features were always present at the front difference about 2 g/kg. in our study area throughout the 15-day period of The mean and low-frequency structure of the observations in June 1984. We found that hydrographic and current fields at the front are hydrographic features in our across-shelf relatively well known. We omit an account of them hydrographic transects that appeared to represent here and refer the interested reader to the recent the detached parcels of shelf water often reported papers of Beardsley and Flagg [1976], Voorhis et in the literature were, in fact, part of the al. [1976], Mooers et al. [1979], Gordon and three-dimensional structure of shelfbreak eddies. Aikman [1981], Houghton and Marra [1983], Adequate alongshelf resolution, in particular, Beardsley et al. [1985], Aikman et al. [1988], enabled us to determine that no detached parcels Houghton et al. [1988], and Burrage and Garvine were present. The two prominent features of the [1988]. eddy groups we found were plumes of lighter shelf For shelf water budgets and ecosystem dynamics water that protruded into slope water, curling the across-frontal exchange of such properties as "backward" opposite the direction of mean shelf heat, salt, and nutrients is vital. The flow, and neighboring cyclones with warmer, persistence of the front, despite its large saltier slope water in their cores, partly or h©rizontal salinity and temperature gradients, wholly encircled by the plumes. The plumes have implies that it is a zone of reduced exchange. the potential especially for producing vigorous There is, nevertheless, conclusive evidence that across-front exchange of heat, salt, and nutrients some exchange does occur from observations of an and may play roles analogous to the "squirts" increase in mean salinity and oxygen isotope ratio found on the California shelf. in shelf water from Cape Cod to Cape Hatteras

[Fairbanks, 1982; Chapman, 1986]. The only source 1. Introduction for such increases is slope water. The amount of

exchange and the mechanisms that drive it are For nearly a century a water mass boundary at important to determine, for the front forms the

the shelfbreak in the Middle Atlantic Bight off seaward edge of the entire 1000-km length of the the U.S. east coast has been recognized. Libbey Middle Atlantic Bight shelf. [1891], Bigelow [1933], and Miller [1950] The amounts and mechanisms of this frontal published the earliest studies. This boundary we exchange are known with far less certainty than call the shelfbreak front, as its locus traces the the mean frontal structure. Three principal shelfbreak, though some others have used the term mechanisms have been studied in field shelf-slope front. It separates cooler, fresher observations: fine-scale interleaving at the shelf water from warmer, saltier slope water, front of the shelf and slope water with subsequent stretching alongshelf between Georges Bank to the exchange by small-scale turbulence and double northeast and Cape Hatteras to the southwest where diffusion, contortion of the front by Gulf Stream it merges with the Gulf Stream front. The warm-core eddies that entrain shelf water shelfbreak front reaches upward from the bottom at streamers, and local detachment of shelf water about the 80-m isobath on the shelf, tilting parcels at the front into water of the upper seaward with a slope of about 2xlO -s. In winter slope. Voorhts et al. [1976] studied the it extends to the surface and imposes a strong interleaving process in detail and estimated that surface temperature gradient there. By summer, it could alone supply the onshore flux of salt increased surface heating has produced a strong needed to balance the freshening of shelf water by seasonal thermocline of about 25-m depth overlying river discharge. Houghton and Marra [1983] the shelf, front, and slope regions [Aikman, 1984] further studied the interleaving process in that confines the strongest horizontal gradients summer, but concluded that double diffusion to the subsurface waters. In all seasons the produced only a weak flux of salt and heat. front has strong thermohaline compensation where Entrainment of shelf water by warm-core eddies

appears to offer an efficient, but highly intermittent exchange mechanism. Smith [1978]

Copyright 1988 by the American Geophysical Union. used satellite imagery to study warm-core eddy interaction with the shelfbreak front along the

Paper number 88JC03455. ScotJan shelf and concluded that the amount of 0148-0227/88/88JC-03455505.00 exchange per eddy passage and the number of

!5,593

15,594 Garvine et al.: The Morphology of Shelfbreak Eddies

passages per unit time were sufficient to make The purpose of our study was to conduct eddies an important contribution to climatic mean intensive field observations of the current and exchange. Morgan and Bishop [1977], Ramp et al. hydrographic fields of the shelfbreak front south [1983], and Churchill et al. [1986] have provided of New England. We were especially hopeful to excellent case studies of eddy-front interactions resolve the structure of shelfbreak eddies that in the Middle Atlantic Bight. Cresswell [1967] might be present. In this paper we present a first suggested that detachment of shelf water description of the morphology of the eddies we parcels into slope water along the front was a found. Because the eddy morphology was common process that could produce significant complicated, our presentation of it is mainly exchange. He likened the detachment process to visual, and we make extensive use of color the "calving" of icebergs into coastal waters from graphics. In a subsequent paper we will provide a an ice shelf. He inferred the existence of statistical summary, including estimates of such parcels from well-sampled across-shelf temperature properties as diffusivity and heat and salt flux sections he obtained at the front south of New associated with the eddies we observed.

England. His interpretation assumed such parcels were and remained detached from their place of 2. Data Sources origin on the shelf. Wright [1976] studied records of their presence using extensive The results we present here are based on a historical data and found evidence for them at all combination of dat• sets from three field

times of year. Houghton and Marra [1983] programs: the Shelfbreak 3 experiment conducted encountered similar features, but despite their by the University of Delaware, the Shelf Edge use of conductivity-temperature-depth (CTD) Exchange Processes (SEEP-I) program conducted by profiling with high horizontal resolution, they Lamont-Doherty Geological Observatory in could not determine whether the parcels were cooperation with the Woods Hole Oceanographic detached or not. Houghton et al. [1988] analyzed Institution and Brookhaven National Laboratory yearlong temperature and current records from an [Aikman et al., 1988], and the North Atlantic array across the front. They found that 11 Slope and Canyon Study by the U.S. Geological parcels were clearly present, but only from May to Survey [Butman, 1988]. The latter two programs November, i.e., only during the stratified season. began earlier and continued longer than Shelfbreak

The rank order of these three exchange 3. Our primary data sources were records from mechanisms and quantitative analyses of their mean moored current meters, tracks of drogued buoys, contributions to exchange are thus still lacking. hydrography from CTD and expendable To this uncertainty we add still a further bathythermograph (XBT) profiles, and satellite sea candidate mechanism, only recently discovered. surface temperature (SST) imagery. Small-scale eddies that we call shelfbreak eddies We conducted the Shelfbreak 3 study from June at times contort the front. Figure 1 presents a 12 to 27, 1984, when (as we expected) storm winds particularly striking view of a number of them were absent but shelfbreak eddies were present. derived from a NOAA 7 satellite thermal image of Furthermore, no Gulf Stream eddies were nearby to the Middle Atlantic Bight for May 26, 1982. complicate the interpretation, the nearest having Small-scale eddies are present all along the front been eddy 84-7 well to the west of the study area from the New York Bight apex eastward past Cape off Hudson Canyon. We separated the observations Cod and onto the margin of Georges Bank. Eddies into three 5-day-long parts, A, B, and C, during with both apparent cyclonic and anticyclonic each of which we conducted intensive buoy tracking relative vorticittes appear. The typical eddy and shipboard hydrography. Part A was from June diameter is only 40 km, much smaller than the 12 to 16, part B from June 18 to 22, and part C diameter of Gulf Stream warm-core eddies (200 km). from June 23 to 27. What generates these eddies is not clear. The In Figure 2 we show the bathymetry of the study nearest Gulf Stream eddy in Figure 1 is well to area, the locations of the moorings we used, and the southwest off Delaware Bay (only partly our principal hydrographic lines. We concentrated visible because of fog), too far away, especially our observations near the shelfbreak between 700 from Georges Bank, to be a plausible source of and 71øW. Here the shelfbreak, characterized by eddy energy. the 200-m isobath, is nearly zonal, so we selected

Houghton et al. [1986] studied an anticyclonic horizontal coordinates, as shown, with x positive eddy in slope water near the shelfbreak south of westward and y southward. For convenience in New England, probably similar to one of those in referring to our hydrographic stations based on Figure 1. They found a shelf water plume shipboard navigation, we use units of nautical extending well offshore, strong swirling currents, miles (minutes of latitude) for these distances and a clear hydrographic signature to the eddy. (1 n. mi • 1.852 km). In the study area, two

There is a good possibility that these submarine canyons impose alongshore variations on shelfbreak eddies were, in fact, closely linked to the bathymetry of the upper slope, Atlantis Canyon the detached shelf water parcels so often near 70015•W and an unnamed canyon near 70o30•W, reported. The latter may be only manifestations while Veatch Canyon lies east of the study area of shelfbreak eddy structure misinterpreted as (Figure 2). fully detached parcels only because sufficient The moorings were set along three across-shelf alongshelf resolution in hydrography was lacking. lines. Farthest east, along x = -21 n. mi, the None of our observations showed fully detached Geological Survey had set two moorings, G1 and G2, parcels, only coherent eddy structure. However, while at x • 0 the University of Delaware set one given the limited scope in space and time of our mooring, UD, and along x • 21 n. mi, observations, we cannot exclude the possibility Lamont-Doherty had set moorings S1 through S6. that "calving" and detached parcels exist The alongshelf spacing of these three lines, 21 n. independently of shelfbreak eddies. mi, was the same as the scale of the shelfbreak

Garvine et al.' The Morphology of Shelfbreak Eddies 15,595

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E&8. •. •0• 7 tbe=•a• &•a•e o• the •&ddte At•a•t&c •&•t •o= 0756• o• •ay 26, Z982. •e• B•sta•d &s •&s&b•e to •be •o=tb (top), •e• 5e=sey a•d De•a•a=e •ay uo uhe west.

•e• •o• •&• apex eastward to georges •a• a•d &s b&SbZy co•to•ed by sbe•b•eak eddies •&t• typ•ca• d&a•ete=s o• 40 •a. Eo8 obscu=es •ucb o• •be •=o• sout• o• the •ew

eddies we wished to study (Figure 1). The system. About five fixes per day were available coverage on the center line of these three was with an error of under 0.3 km. Three of these sparse, mooring UD only, but we deployed our buoys buoys were used in both parts B and C, two drogued along or near to this line. We give details of at 10 m and one at 35 m. the instrument settings in Table 1. All current We obtained the bulk of our hydrographic data meters gave good data throughout the study period. from about 400 CTD casts using a Nell Brown

Our drogued buoys were of two types. We Instrument Systems Mark IIIB unit, newly deployed a total of about 90 Coastal Ocean calibrated. We limited our casts to the upper 200 Dynamics Experiment (CODE) buoys [Davis, 1985] m in deep water. Our four principal hydrographic drogued at one of four depths: the surface, 20 m, lines appear in Figure 2. We chose the 35 m, and 65 m. These buoys have exceptional across-shelf lines 2 and 4 to extend from about current-following ability, even in wind waves. We the 100-m isobath on the shelf seaward across the deployed and tracked them in groups of about 30 front and into slope water; we separated them as during each of the three parts of the experiment. shown by 21 n. mi alongshelf to resolve the We obtained most fixes from the National Center expected scale of shelfbreak eddies. Line 2 for Atmospheric Research (NCAR) Queen Air research passed through moorings S2 to S4, while line 4 aircraft and the rest from the R/V Cape Henlopen passed through mooring UD. We established CTD using radio direction-finding (RDF) methods from profiling stations at 3 n. mi intervals along both platforms. Usually, we obtained two fixes these lines of 30 n. mi total length. Line 3 we per day for each buoy from the aircraft, one in used to sample alongfront structure with CTD early morning and one in late afternoon. The profiles, again at 3 n. mi intervals. We located standard fix error we estimate at a radius of 2 km it 21 n. mi seaward of the onshore ends of lines 2

for aircraft fixes and 4 km for shipboard fixes, and 4 in part A, 18 n. mi (as shown in Figure 2) the former comparable to the unresolved tidal in part B, and 27 n. mi in part C. Along line 1 displacements in the data. we performed only XBT profiling, again at 3 n. mi

Our second type of buoy was supplied by the intervals. Usually we occupied these lines U.S. Coast Guard Research and Deployment Center, sequentially, completing a circuit in somewhat who tracked them by satellite using the Argos less than 1 day, sufficiently fast to minimize

15,596 Garvine et al.' The Morphology of Shelfbreak Eddies

72 ø 71 ø 70 ø 69 ø 42 ø 42 ø

41 ø 41 ø

D 8o

40 ø 2 40 ø

Fig. 2. Map of the study area, with north to the top. Isobaths are shown with depth in meters. Current meter moorings are noted by solid circles and identifying symbols (Table 1). Environmental buoy EB8 is shown to the east. Solid lines denote the four principal hydrographic lines, dashed lines the x,y coordinate axes. The large rectangle coincides with the field of view of the satellite images used.

distortions produced by eddy translation vertical transects along the four principal alongshelf. Supplementary temperature data were hydrographic lines (Figure 2) using standard available from thermistor chains along line 2 at contouring software. We employed no horizontal moorings S1, S3, and S4. smoothing for these; instead the data were

We obtained satellite imagery of SST from the interpolated between the 3 n. mi interval NOAA 7 Advanced Very High Resolution Radiometer profiles. As Burrage and Garvine [1987] have (AVHRR) scanner during nearly cloud free periods shown, highly energetic supertidal frequency in the study area for June 1984 from the National internal waves are common in the study area during Marine Fisheries Service at Narragansett, Rhode the stratified season that are unavoidably aliased Island, who corrected them for both geometric add into standard hydrographic data, rendering them atmospheric effects. We also obtained sea level unreliable for quantitative use, as in computing and wind data, the latter primarily from a moored the field of vertical current shear from thermal buoy, EBB (Figure 2). Wind stress was moderate or wind balance. Our needs, however, require only light throughout the study period, never exceeding qualitative interpretations to deduce eddy 0.6 dyn/cm 2 , with the mean directed northeastward. structure. Because eddy production here is likely produced by We make extensive use of satellite SST maps. flow instability, we ignore the light wind forcing We produced these by smoothing the standard and associated subtidal sea level variations in processed data whose resolution is 0.6 n. mi (1.11 our account below of eddy morphology. km) using a 3x3 cosine filter and subsampling to

alternate points; thus our resolution is 1.2 n. 3. Data Analysis mi. We in turn used these data to produce the

color contour plots in Plates 1-18. (Plates 1-18 We analyzed our data using both conventional can be found in the separate color section in this

and unconventional methods. We used conventional issue.) Where anomalously low temperatures were methods for initial processing of the CTD and XBT present, we deleted all color. Thus our profiles, averaging values into 0.5-m vertical interpretation of cloud-covered areas will show as bins and smoothing the profiles with a 6.5-m-long white spaces. The NCAR aircraft was fitted with a cosine filter to remove fine-scale features not of radiometer. This permitted us to compare the interest. We arranged these profiles to form surface temperature field along the low-altitude

Garvine et al.: The Morphology of Shelfbreak Eddies 15,597

TABLE 1. Details of Current Meter Moorings

North West

Mooring Depth, m Latitude Longitude Ins trument Ins trument

Type* Depth, m

s1 82

s2 124

s3 148

s4 500

40028.0' 70055.0'

40014.5' 70o55.0'

40006.0' 70o55.0' 39o55.1' 70o55.0'

s6 2300 39o36.0' 70o55.0'

UD 97 40o21.5 70ø27.5'

G1 100

G2 204

40o10.9' 69o58.3'

39o57.7' 70o01.1'

VMCM 10

VMCM 10

VMCM 10

VMCM 10

AAND 120

VMCM 20

GO 20

GO 35

END 65

VACM 56

VACM 93

VACM 10

VACM 54

VACM 129

*VMCM, vector measuring current meter' AAND, AAnderaa RCM-5' GO, General Oceanics 6011; END, Endeco 105; VACM, vector averaging current meter.

aircraft track with the corresponding position of Unlike the current meter record, however, these a satellite image obtained within a few hours of a buoy current estimates apply to moving positions, given flight. The spatial correlation not fixed ones. Nevertheless, the resulting coefficients averaged 0.82, and submesoscale current estimates, subsampled hourly, provided us features were quite similar between the two with a pseudo-Eulerian current field that we could [McCarthy, 1986]. superpose, together with the current meter

Our current meter records overlapped the study low-passed currents, directly on satellite SST period, extending from May 15 to July 15, 1984. maps. A typical map contains about 40 superposed After analyzing them for tidal properties to current vectors, not merely the 14 from the assure data reliability, we subjected each record moorings. to a 33-hour half power point low-pass filter and As our results in section 5 make clear, resampled it hourly. Thus for each of the 14 shelfbreak eddies produce high horizontal current meters listed in Table 1 we had an gradients, both in the alongshelf and across-shelf Eulerian record that ran throughout the study directions. While we could easily resolve this period. spatial variability along our four hydrographic

Our buoy data, in contrast, were Lagrangian lines with their 3 n. mi station intervals, we records. To permit use of their inherent velocity were unable by simple spatial interpolation to information in the same format as the current estimate the horizontal structure within the

meter data, especially for superposition on the rectangle bounded by the lines, except at the satellite SST maps, we transformed these into surface using satellite SST maps. Such equivalent Eulerian records. This process, used interpolation smeared important eddy features successfully by Garvine [1977], is unconventional nearly beyond recognition. Instead, we used a but straightforward, and we illustrate it in method of interpolation based on the dominance of Figure 3. We plotted the raw data from the time alongshelf subtidal advection of alongshelf series of buoy fixes in x,y. We then computed a smoothed continuous trajectory using a polynomial fit in x,y of degree n and least squares minimization for all fixes but the first, the deployment point. We forced the trajectory through that point because its associated uncertainty was quite small, unlike the RDF fixes. We selected n subjectively for each buoy trajectory, favoring smaller values to avoid artifacts associated with large trajectory curvature. The solid curve in Figure 3 gives the smoothed trajectory for n i 3 and is typical. One could apply the same technique to the subsampled raw data progressive vector plot from a current meter record. In both cases the fitted

t I t 20 10 0

n mi

"trajectory" would represent an estimate of the Fig. 3. Sample buoy trajectory. Open circles corresponding low-pass-filtered "trajectory," time denote radio direction finding fixes, the letter derivatives of its coordinates x,y providing "D" the deployment site, and the solid line the estimates of low-passed current at a given time. fitted polynomial trajectory for n- 3.

15,598 Garvine et al.: The Morphology of Shelfbreak Eddies

hydrographic spatial variability [Wong and We concluded that the outcomes of these three Garvine, 1985]. Burrage and Garvine [1988] tests were sufficiently good for us to use the recently showed strong evidence of this dominance advective interpolation method of (1) to interpret for the shelfbreak front in summer farther south eddy structure within the hydrographic rectangle. off Maryland. We were led to attempt use of this method for our study area because there the eddies 4. Frontal Properties appeared to have reached a mature state, subject mainly to advection, while to the east, in We present in this section some properties of contrast, perturbations showed rapid growth, as we the shelfbreak front we observed to provide a show in section 5. context for the shelfbreak eddy morphology of the

In our application we interpolate at alongshelf next section. The reader may obtain a more distance x for fixed across-shelf distance y, complete observational account of the front in depth z, and time t for a hydrographic field this region from Beardsley and Flagg [1976], property P from Voorhis et al. [1976], and Houghton and Marra

[•g83].

P(x,y z t) - P(x ,y,z t -At) (1) In Figure 4 we show a vector plot of the mean ' ' o ' currents during the study period from the 14

current meters listed in Table 1. The view here, where Xo is an observation point, such as one on as in all subsequent maps, has south to the top line 4, and At t (X-Xo)/U, where U is a constant and west to the right, so that the positive x and alongshelf advection velocity. Thus (1) states y directions have conventional orientation. These the simple alongshelf advection at fixed currents show clear frontal character, those over alongshelf velocity U. We used it to interpolate the shelf (shoreward of the 200-m isobath) having properties downstream (in the direction of U) or been mainly westward, alongfront at speeds of 10 upstream, as desired. A similar interpolation has cm/s and greater, while those over the slope were been widely used for interpreting turbulence variable and indistinguishable from zero. measurements and has been called the frozen-field Although this current structure is comparable to approximation [Tenneckes and Lumley, 1972]. longer time averages from other mooring arrays in

Because the advective model of (1) is central this area [Beardsley and Flagg, 1976; Beardsley et to much of our final representation of shelfbreak al., 1985], the results are sensitive to the eddy morphology, we tested it carefully using averaging interval. The vigorous eddy motion that three independent means. First, we computed the we show (section 5) in the front and slope regions cross-correlation functions for fixed y and z averages nearly to zero over the 15-day interval between temperature at lines 2 and 4 and averaged of the study. them over each line. Between 35- and 65-m depths, A typical across-shelf hydrographic section where the eddy structure is strongest, the cross that shows some of the frontal features appears in correlation was maximum for line 2, lagged 8 days Plate 2, constructed from one CTD section along behind line 4, and had a mean value of 0.70. This line 2. The view is eastward, offshelf to the latter value indicated that similar structures right. The front at this season lacks strong were passing each line successively, while the surface signature, but is nevertheless still 8-day lag gave us an objective estimate of the intense in the temperature (T) and salinity (S) bulk advection velocity U: 2.6 n. mi/day or 5.6 fields below the seasonal pycnocline that reaches cm/s westward. We used this value for all to about 25-m depth. The shelfbreak front appears applications. Second, following Burrage and as a steeply sloping boundary in the temperature Garvine [1988], we compared contoured temperature field between cool shelf water to the left between sections along lines 1 and 3 (the alongshelf 25 and 100 m in a feature called the "cold pool" lines) with their corresponding interpolated [Houghton et al., 1982] and warm slope water to sections based on (1) and temperature time series the right. The front has a similar strong salinity from corresponding y values on line 4, the horizontal gradient, with fresher water on the upstream line. Correspondence was quite good, shelf water side, but the density (at) gradient at similar to that of Burrage and Garvine [1988, the front is weaker, because the T and S Figure 6]. Third, we compared the interpolated differences are compensating in their effects on field of temperature at a fixed level, with at. satellite SST maps. Plate 1 shows one such Of the three hydrographic fields of Plate 2, comparison. Plate lb shows the SST anomaly field temperature provides the clearest image of the at 0400 LT on June 22 for the same spatial domain frontal structure. Note, however, a complicating as the interpolated field in Plate la. (In both feature in Plate 2a. Beyond the front lay a cool panels, as for all maps hereinafter, south is now feature, surrounded by slope water and stretching to the top.) The latter field is for 35-m depth between y t 21 and 30 n. mi and in depth between where the eddy structure is especially clear, but 25 and 50 m. One may discern it, less readily, in other levels show similar form. Both lines 2 and the S and •t fields. The T, S properties clearly 4 were used for interpolation to obtain the establish its shelf water origin. Such regions of greatest alongshelf field possible. (We shelf water, surrounded by slope water in deliberately used different colored filling for across-shelf hydrographic sections, have been the contours in the panels because different interpreted, as we noted above, as detached depths, and hence temperature ranges, are parcels produced by "calving" [Cresswell, 1967; represented.) The similarity in spatial pattern Wright, 1976]. As we will show, such features in is striking, the plume of cold shelf water, of our study, including the one in Plate 2, were particular interest to us, being nearly coincident connected in three-dimensional space to shelf in the two panels, running upward (offshore) and water, not detached, and were part of the to the left (east). structure of shelfbreak eddies.


i i i i i i i i i i i i i i i i

20

SLOPE

48

36

S4

10 24

20• •'•'•--- S3 (•----- 10

G1

•1/ 56 UD 65

S2 (•-- lO

x (n mi) O I I

20 cm/s

y (n mi)

Fig. 4. Study period mean currents from the moored current meter records. Note that the top of the figure is now toward south, the positive y direction, while right is toward west, the positive x direction. The solid line gives the 200 m-isobath and roughly marks the shelfbreak. The field of view is shown in Figure 2. Open circles denote mooring sites with symbols as in Table 1. The radius of a circle corresponds to a current speed of 1 cm/s (see speed scale at lower right). Numbers at the tip of current vectors denote instrument depth in meters.

Another property with contrast near the front rather than providing a narrative of the is the Eulerian time scale for hydrographic observations. properties. In Figure 5 we contrast the autocorrelation for low-pass-filtered temperature 5.1. Frontal Waves measured from the thermistor chain at mooring S1, always well inshore in shelf water, with that from Classical fluid dynamic stability theory mooring S3 in the center of the front. Both requires prescription of an appropriate autocorrelations are vertical averages of 10 undisturbed or basic state for subsequent analysis thermistor temperatures at each mooring above 120 [Pedlosky, 1987, p. 492]. Within our study area, m. The correlation time scale is clearly shorter eddy disturbances were always present, so we can near the front than onshore over the shelf, about offer no direct assessment of an undisturbed 9 days for the former against 16 for the latter, state. To the east beyond our observational site, if one uses the time to fall to a correlation of however, satellite SST maps gave us views of what 0.1. We associate this difference with different appears to be the undisturbed front and amplifying processes: the rapid change in temperature near frontal waves. the front as shelfbreak eddies sweep by in Plates 3-6 form a sequence of SST maps with contrast to the slower change over the shelf. time increasing. As for all subsequent SST maps, Because the mean alongshelf speed at the two the field of view is the same as that in Figure 4, moorings was about the same (Figure 4), we south and slope water at the top, west to the attribute the time scale contrast to different right. Current vectors have the same scale as in alongshelf space scales. If we again use 5.6 cm/s Figure 4 but now are color coded to indicate depth as the advection speed, then over the shelf the and include buoy currents in Plate 6. The space scale was about 80 km, while near the front temperature field shown is the anomaly it was only about 45 km, roughly the diameter of temperature, i.e., the sea surface temperature the shelfbreak eddies we report in the next less the mean for a given frame. We found the section. anomaly field most suitable for tracing eddy

features, as it suffered less fluctuation from 5. Eddy Morphology frame to frame due to diurnal insolation,

especially in successive nighttime and daytime In this section we present the principal sequences. The seven contours are drawn as solid

results of the paper, a description of the black lines at intervals of 0.50C from -1.50 to morphology of the shelfbreak eddies we found. As +1.5 o , the range corresponding to the contours a framework for the description, we have chosen to between dark blue and blue and between red and address particular features of the eddy structure, magenta. drawing on results throughout the study period, For all four maps (Plates 3-6) the surface

•5,600 Gatvine et al.' The Morphology of Shelfbreak Eddies

n,, o..

o 0

o i-

i

..

,oo e'.00 •2'.00 •'.00 2•.••'-•---,•00 LAG (DAYS)

; .... '00

:Do

As we noted, the wavelength was about 33 km (18 n. mi), the same as the baroclinic Rossby radius we computed from our frontal hydrography. The wave was thus not a long wave. The apparent phase speed of the trough was about 8 cm/s alongshelf, comparable to the mean currents on the shelf. The wave grew from a small disturbance (amplitude/wavelength- 0.1; Plate 4) to a large one with backward breaking crest (Plate 6) in 1 day. Such rapid growth was typical for the waves James [1984] found for roughly similar fronts, but with a flat bottom, in his numerical model studies. In contrast, Flagg and BeardsIcy [1978] computed the linear stability of a simple two- layer frontal system, but with bottom topography representative of the shelf and slope in our study area. They found unstable frontal waves, but much longer growth times, typically 80 days. Their analysis would apply directly only to the wintertime shelfbreak front, however, when it reaches from bottom to surface. Perhaps the overlying seasonal pycnocline covering both shelf and slope water to 25-m depth in our study period reopens the possibility of rapidly growing waves, despite the stabilizing tendency of the steep slope. One of us (G.G.G.) is presently analyzing the linear stability of this frontal configuration.

5.2. $he•f Water Plumes

Our observations showed that plumes of cold shelf water protruding into slope water were the most prominent features of shelfbreak eddies. They are likely, as well, the most important part

Fig. 5. Temperature autocorrelation function (a) of the eddy structure in affecting across-front at mooring Sl and (b) at mooring S3. exchange. Their role in shelfbreak eddy structure

appears to be analogous to the "squirts" found on the California shelf during the CODE program where

thermal front shows clearly for the easternmost similar methods were used to infer their presence area, near the 200-m isobath, from about x- -30 [Davis, 1985]. to the left margin. The first map (Plate 3 for We observed two successive eddy. groups during 1400 LT, June 9) shows the front nearly straight the study period that we named the m and $ groups. from the left margin to about x - -36, just west Each had clear shelf water plumes, the $ group's of Veatch Canyon (denoted by "V" on the 200-m stronger. isobath). Over this span the temperature changed Plates 3-6, when reexamined, imply the across the front by 2.5oc in about 4 n. mi. Just existence of the m plume in the form of cool shelf to the west (right) the across- front thermal water that had moved across the shelfbreak into gradient rapidly weakened, marking the domain of slope water. Plate 3 for June 9, for example, an eddy. By 0400 LT, June 12 (Plate 4) the front shows cool water centered near x - 0 extending to the east was distorted by an apparent wave with seaward beyond the field of view and marked "top". wavelength of about 18 n. mi (33 km) and amplitude The 10-m depth current meter at mooring G2 shows of about 2 n. mi, its crest (most seaward strong flow then to southwest (upper right) along extension) Just east of Veatch Canyon. (The the apparent center of the plume. The subsequent apparen t crest and troughs are marked by "C" and SST maps (Plates 4-6) indicate that the plume slid "T", respectively.) The successive map (1600 LT, slowly westward while over the upper slope. The June 12, Plate 5) shows apparent amplitude growth clearest evidence of its current field appears in and westward propagation. By the last map (0400 Plate 6. Note the vigorous offshelf flow at about LT, June 13, Plate 6) the amplitude had grown 12 cm/s of all five of the surface buoys (white still further and, more significantly, the eastern vectors) near the apparent center of the plume. trough had passed westward of the crest, giving Nevertheless, the contrasting motion of nearby the appearance of a "backward" breaking wave. buoys drogued deeper implies that the offshelf

We have no successive maps until June 16 flow was shallow. Two 20-m buoys (brown vectors) because cloudy weather set in. Furthermore, we had weak offshelf flow, while three 65-m buoys made no observations of our own in this eastern (blue vectors)had only alongshelf flow. region. Nevertheless, we infer that this Our hydrographic sections confirm this disturbance amplified still further to produce the shallowness. Plate 7 shows the temperature eddy structure we call the • group below. section along line 4, then extended to 42 n. mi Regardless of the wave's ultimate fate, though, length, looking eastward, offshore to the right. the close succession of maps in Plates 4-6 Its central time was 1600 LT, June 12, coincident provides scale information for frontal waves here. with the SST map of Plate 5. Note again the


general coherence between the SST map and the The seeming lack of depth dependence of this subsurface temperature in the vertical section, motion suggests that the plume was a deep feature, for example, at 25-m depth. We found such especially compared with the a plume. Again, our agreement throughout the study period, even though hydrography is supportive. Plates 12-14 show we lacked very near surface hydrographic temperature sections along line 4 looking eastward properties. Our lack of these is a consequence of at roughly 1 day intervals beginning at 1400 LT on the delay in instrument response of the CTD and June 18 (Plate 12). (Plate 12 shows 5-m amplitude XBT probes upon entry into the water and the 6.5-m oscillations of the pycnocline centered at 20-m vertical cosine filter we used; hence the whitened depth. These are spatially altased oscillations band in the upper 6 m of Plate 7. We conjecture produced by supertidal frequency internal waves. that the larger vertical temperature gradients Burrage and Garvtne [1987] present observations of associated with the uppermost parts of cold, these waves obtained at y - 1.5 along this line.) subsurface shelf water, as in Plate 7, leak At first, shelf water stretched along the line through to the very surface to signal the presence well seaward of the shelfbreak, nearly to y - 30 below of colder waters. n. mi, and extended to depths of about 100 m.

Plate 7 clearly shows shelf water (T • 110C) Evidently the • plume axis passed line 4 about both over the shelfbreak at y - 15 and farther then. On the 2 following days the sections offshore in a band from y - 25 to 38, but its (Plates 13-14) support two contrasting typical greatest depth is only 50 m. The interpretations. The first is that a shelf water appearance of the offshore band as a detached parcel was undergoing detachment or "calving." parcel is deceiving here, as the plume axis did The connecting link of less cool water (90-11 o , or not precisely follow line 4. Plate 8 shows a light blue) between the colder shelf water (<9 ø, related temperature section along line 3 or dark blue) onshore and that inside the closed (alongfront), completed on June 13 soon after the contour offshore appears to have thinned with time of the SST map of Plate 6. The view is now time, suggesting detachment of the latter in landward (northward), with west to the left, and progress. The second interpretation is that the shows the trace of the cold water plume, now on an offshore cold water remained simply connected to alongfront section. Again we see that it was shelf water along the • plume whose axis was then shallow, consistent with the lack of offshore flow passing west of line 4, revealing the warmer water in the 65-m buoys. Our later section here (not of the core of the • cyclone to the east from shown) established its alongshelf width at only about y - 15 to 21. Only the second about 10 n. mi (19 km), well less than the spacing interpretation is consistent with our horizontally between our hydrographic lines or moorings. two-dimensional data in the SST maps. The section

The • plume did not reach our study area until of Plate 14 was completed just before the June 21 the beginning of part B of our observations. map of Plate 10. Note the very high visual Plates 9-11 form a sequence of current vector maps coherence along line 4 between the temperature of for the newly seeded buoys of part B; as in Plates Plates 10 and 14. As Plate 10 shows, the warm 3-6, they are superposed on SST maps. The core of the cyclone •c was then just moving over currents appear at uniform time intervals of 1 day line 4 near y - 18, the intersection with line 3. beginning at 0400 LT on June 20 (Plate 9), while Temperature sections along line 3 further the SST maps are from 0400 LT on June 21 (both support the second interpretation. Plates 15 and Plates 9 and 10) and June 22 (Plate 11). (It was 16 show temperature sections in the y-z plane cloudy on June 20.) The two SST fields both show looking northward (landward) at 2-day intervals two cold features (blue and green) protruding beginning at 0200 LT on June 19 (soon after that offshelf beyond the 200-m isobath. The western of of Plate 12). Shelf water extended to a depth of these was the a plume (marked "ap") while the 100 m and alongshelf for about 15 n. mi (28 kin) larger, eastern one was the • plume (marked "•p"). and was drifting westward (left). Note the close Note the coherent motion of an array of 11 buoys correspondence in temperature along line 3 in the in Plate 9 situated near and east of line 4 and SST map of Plate 10. The • plume had a much between y - 24 and 40. As the color codes greater cross-sectional area, about 1.6 x 106m 2 indicate, buoys of all four depths were in this (for T • 11 ø) than did the a plume, only about 0.4 array, the deepest at 65 m (blue). In Plate 9 x 106m 3 (Plate 8). Unlike the a plume, the • their motion appears to have marked the western plume had a strong density signature. Plate 17 limb of a cyclonic eddy that we call the • shows the density on line 3 looking landward, cyclone, •c. The most seaward of the array had corresponding to the temperature field of Plate already turned east to flow along the offshore 16. The 26.2 isopycnal (boundary of dark blue) surface temperature front in the SST map. In deepened about 25 m under the • plume, while above Plate 10 the SST and current maps coincide in in the seasonal pycnocline the isopycnals shoaled time. Then the most shoreward buoy of this array slightly to form a dome over the plume, much as was on the east side of the plume axis and flowing the isotherms there did (Plate 16). offshelf and westward. The more seaward buoys, The .• plume density section led us to consider meanwhile, had now diverged, some continuing their the across-shelf current field it would have been drift east along the offshore front and some west. consistent with. Despite the small spatial scale Plate 11 shows this divergence to have continued, of current variation (20 km), the Rossby number but with slackened current speed. The offshore for current speeds of 10 cm/s is still only about front there seems to have served as a barrier. 0.05, so we expect that the subtidal flow was The SST field shows the colder water of the • essentially geostrophic, i.e., in thermal wind plume (blue and green) to have spread farther balance. Suppose the flow below 100 m had only a offshore and east (left). Shelf water appears weak across-shelf component, as we would expect thus to have moved beyond the shelfbreak over the here at the shelfbreak. Then along a vertical upper slope, where it spread and slowed. line through the • plume center of Plate 17 (about

15,60P Garvine et al.' The Morphology of Shelfbreak Eddies

through x • 7.5) the across-shelf component would completely, reaching back onshelf on its eastern have remained weak, as the alongshelf density side, while the shelfbreak front reformed at the gradient was there zero. Consider a neighboring surface to bound •½ on the north about 15 km vertical line to the east (right), say at x-- 5. shoreward of the 200 m-isobath. Thus •½ appears Here the dominant isopycnal slope was upward to to have been cut off then from direct contact with the east, supporting a vertical shear such that shelf water onshore. Currents from the buoys on offshelf flow would have developed above about its eastern (left) limb and at mooring G2 (on the 75-m depth. Finally, consider a line to the west 200-m isobath) show evidence that the circulation (left), say at x - 12. There the slope was had nearly closed on itself. To the northwest comparable but opposite in sign of that to the onshore (lower right) of the shelfbreak, east, such that onshelf flow components would have reestablishment of the front appears to have developed there. The typical vertical shear generated strong westward currents there. consistent with thermal wind balance would have Because eddies often appear in counter-rotating been about 2 x 10-3/s, so that over the plume pairs, such as in dipole eddy pairs, we searched depth of, say, 80 m, currents of the order of 15 our data for anticyclones. Plates 9 and 10 give cm/s would have been present. The vertical weak evidence of one in the current structure for relative vorticity in the plume would have been the region centered on x • 12, y • 36. The outer anticyclonic, consistent with the depressed two moorings on line 2, S4 and S6, together with isopyncal beneath it. four buoy currents located to the east (left), may

Our qualitative deductions from the density indicate the presence of an anticyclone (marked field along line 3 thus point toward reversing "•a?•), its northern limb then adjoining the across-shelf flow components in a section southern limb of me. We lack hydrography this far transverse to the plume axis. We have already seaward, and the SST field in Plate 10 for such an shown vigorous offshelf motion on the plumesVs eddy is at best ambiguous. In contrast, the eastern side and suggested that it marked the cyclones had clear signatures in SST fields, western limb of the • cyclone •½. Now we note the hydrography, and currents. clear presence in Plates 9 and 10 of onshelf flow parallel to the local plume axis for an array of 5.4. Composite Morphology six buoys, again of all four depths, located then near the 200-m isobath and on the southwest margin We now combine the evidence presented earlier of the plume (in the blue and green coloring). to produce depictions of the composite eddy The plume vorticity was anticyclonic. • morphology in three-dimensional space. In

The geostrophic model thus seems useful for particular we will present images of me, •½, and diagnosis. Note that it suggests the presence then the • plume together. of two cyclonic eddies located alongshelf astride Our primary depictions are projections in the anticyclonic shelf water • plume. We named three-dimensional space of the surface of a these the m cyclone me and • cyclone •½, constant property, for example, the 11 ø isotherm. respectively, to the west and east of the plume. To produce these, we first employed the method we We discuss their structure next. described in section 3 to interpolate properties

within the hydrographic rectangle. We used all 5.3. The Cyclones the hydrographic data obtained at 3 n. mi

intervals along line 4 from 30-m depth (below the The structure of the m cyclone me shows best in seasonal pycnocline) to 80-m depth at 5

Plates 9 and 10. The SST maps show a yellow m-intervals. These data were acquired in 14 circular core of warm water (marked ..... ) of about complete sections along line 4 ranging in time •C

20-km diameter cut by line 2 near its offshore from 1530 LT on June 12 to 0900 LT on June 26. end. Note especially the current vectors in the The interpolated data then filled the surrounding green coloring. These currents three-dimensional domain from 30-m to 80-m depth, swirled around the core, bounded on its eastern from y • 0 to 27 n. mi (50 km) across-shelf, and (left) side by the • plume and on its western for x t-14 to 25 n. mi (72 km total)alongshelf. (right) side by the m plume. Plate 2 shows the The x-coordinates correspond to the time 0400 LT section through it on line 2 for 2300 LT, June 21, on June 21, the time of the SST map in Plate 10. nearly coincident with the map of Plate 11. The One could as well select other times and warm core of the eddy laid then between y = 16 and corresponding x values using (1), but the spatial 21 n. mi (Plate 2a) with its cool offshore limb pattern would be the same. Plate la, for example, near the offshore end, giving the deceptive corresponds to 0400 LT on June 22 and shows the appearance of a detached shelf water parcel. Note temperature field along 35-m depth. The time the consistency between cyclonic circulation and series data from line 4 allowed us to produce an the doming of the 26.2 isopycnal in Plate 2c near image of the • plume, •½, and me, but not the m the warm core center at y-- 18 n. mi. plume, as it had already passed line 4 when our

Just as the • plume was stronger than the m observations started. Once the data were properly plume, •½ was larger and had greater circulation gridded in space, we could view the surface of any tl•an did me. Plate 10 shows that it had a large selected property, T, S, or •t for any specified (40-km diameter) warm core of slope water partly level using the NCAR graphics program called enclosed then (June 21) by the shelfbreak front ISOSuRFACE. onshore to the north and the • plume to the west We show the image of the 11 ø isotherm in Figure and south. Subsequently, we can trace its 6. In Figure 6a the interior of the surface evolution to a mature state. Pl, ate 18 shows the contains values T < 11 ø so that we may use it as SST map for 0400 LT on June 27 with currents from an image of shelf water. (Note that where the 0400 on June 26 (at the end of buoy tracking for surface intersects a boundary of the data, the part C). The • plume now bad rolled up algorithm simply traces a line along the

Garvine et alo: The Morphology of Shelfbreak Eddies 15,603

(b) ,

I

Fig. 6. Projection of the interpolated three-dimensional image of the 11 o isotherm between 30- and 80-m depths. The interior of the surface has (a) T < 11Oand (b) has T > 11 ø.

boundary. ) The viewpoint is from above, offshelf, and westø The • plume is the clearest feature, running offshore and to the east (right). Its intersection with the vertical offshore boundary is visible and corresponds roughly with the 11 o isotherm along line 3 shown in Plate 16. To the east and onshore of the • plume lies a cavity, the site of the warm core of •c, while to the west (left) and somewhat farther offshore lies the warm

two warm cores of mc and •c dominate the image all the way from the upper contour (30 m) to the third lowest (70 m) and give a vivid impression of how deep the eddies areø

Figure 7 shows the corresponding morphology of the 26.2 isopycnal, the same one appearing as the contour line between dark blue and blue in Plates 2c and 17. Figure 7a shows the surface with viewpoint from onshore and above with at > 26.2 in the interior. Figure 7b shows the complementary field of the depth in meters of the isopycnal surface. One can readily see the channel running upward and offshore occupied by the • plume, colder, but less salty and lighter. To its west (right) lies the smaller, but steeper, height of mc with its warm, salty, heavier interior, while to its east (left) is the larger •c. At the base of these inshore is the surviving portion of the

(b)

¾ 12

0 i i I I I I I I i I -12 -6 0 6 12 18 24

core of mc, smaller in diameter. Still farther Fig. 7o The interpolated topography of the 26.2 west would be the m plume, but as we noted isopycnal surface in the same domain as Figure 6. previously, it is out of view. Figure 6b shows (a) Projection of three-dimensional image of the the same surface but with T > 11 o in the interior surface with at > 26.2 interior to it. (b) now, so that we may use it as an image of slope Contour plot of the depth (in meters) of the 26.2 water. The viewpoint is now from onshore. The isopycnal surface.


200 m

Slope Water

Shelf Water

Fig. 8. The schematic of our conception for the combined • and • eddy groups observed during the study period.

shelfbreak front. It intersects the figure bottom configuration of a bottom to surface structure. (80 m) for most of its alongshelf span. One can As Houghton et al. [1988] have suggested, eddy again form an image of the geostrophic current formation may be possible only once the seasonal field using the topography of this isopycnal pycnocline has become established, limiting the surface and the assumption of only weak currents density front to the subsurface. Several-day below. Strong westward, alongshelf flow would be growth times were predicted for waves on roughly present inshore where the front survives. Farther similar frontal structures by James [1984] in his seaward, some of this flow would turn offshelf numerical studies that assumed a flat bottom. around the •c topography, while other flow enters We found that hydrographic features in the domain from offshore on the west side of the across-shelf transects that appeared in isolation plume channel to flow cyclonically around ec. to represent detached parcels of shelf water Between the two cyclones would be the anticyclonic adrift in slope water were instead part of eddy flow of the • plume. structure, especially of protruding shelf water

Finally, we give our conception of the entire plumes. But our determination of this structure eddy system we observed for both the e and • eddy was only possible because we had adequate groups in schematic form in Figure 8. alongshelf resolution in currents, hydrography,

and satellite SST maps. Are, then, all the shelf 6. Concluding Remarks water parcels long reported [Cresswell 1967;

Wright, 1976] near the shelfbreak front in the By using a combination of buoy tracking, Middle Atlantic Bight also simply portions of

intensive hydrography, satellite thermal imagery, shelfbreak eddy fields, otherwise undetected? We and moored current meters, we resolved the are inclined to think so, but with observations of structure of shelfbreak eddies at the front south only two eddy groups at one site, we have no of New England. We summarize our principal compelling evidence that other processes are findings here. absent that do produce truly detached parcels.

Eddy structure was always present within our Plumes of lighter shelf water protruding well study area, roughly 100 km alongshelf by 80 km over the upper slope and curling "backwards," across-shelf, throughout the 15-day study period opposite the direction of mean shelf flow, were in June 1984. Satellite SST maps, however, showed the most prominent eddy feature we found. Their that the front to the east of our area was well density structure was consistent with the current defined and free of eddies, but supported a field we detected with our buoy tracking, offshelf wavelike disturbance. Its wavelength, about 33 on the upstream side of the plume, onshelf on the km, was about the same as the internal Rossby downstream side, and having anticyclonic radius associated with the subsurface front. In a vorticity. We found strong contrast between the sequence of SST maps this disturbance grew in strength and depth of the two plumes that passed amplitude as it moved westward and deformed the through our study area, the • plume reaching to front into the shape of a "backward breaking" only about 50-m depth and the $ plume to 100-m wave. This disturbance may have marked the birth depth. Their cross-sectional areas differed by a of the $ eddy group. The time for large amplitude factor of 4. Clearly, these plumes are capable of growth was only about 1 day. Such rapid growth strong across-shelf and across-front transport. seems to contradict the stability analysis of this The $ plume we observed had a volume transport front by Flagg and Beardsley [1978], but their offshelf in its upstream side of about 0.1 x model basic state assumes the winter frontal 10emS/s or 0.1 Sv, about one fourth of the mean


alongshelf transport on the New England shelf an area bordering the Celtic Sea. A size of about seaward of the 30-m isobath [Beardsley et al., 30 km was most frequent. By applying results for 1985]. quasi-geostrophic instability to the thin surface

Thus there is the potential for locally great layer above the thermocline, he estimated the eddy exchange between shelf and slope water. One growth time scale to be I day. Paluszkiewicz and manifestation of such exchange would be enhanced Niebauer [1984] identified eddies at the diffustvity K for across-front diffusion of heat shelfbreak front of the Bering Sea near Prtbtlof and salt. Realistic values of this property are Canyon. Simpson [1981] described the shelf-sea vital for use in promising models of the mean fronts that are common about the British Isles and structure of the shelfbreak front, such as showed satellite imagery that reveals these fronts Chapman's [1986]. Garrett et al. [1985] estimated also as contorted with small-scale eddies. K - 1.2 x 10Sm2/s for an eddy field of similar Doubtless, more examples from still other areas scale on the Labrador shelf. Such a value far will be reported. exceeds those usually assigned in numerical models Griffiths and Linden [1981] conducted a of frontal circulation. laboratory simulation in a rotating tank of the

The role of shelfbreak eddies may thus be instability of a buoyancy-driven coastal current. analogous to the "squirts" found on the California They used a cylindrical source of surface buoyant shelf [Davis, 1985] as the prime agent of water that spread into ambient heavier water with across-shelf exchange. Some differences are, a front bounding the two fluids horizontally. For nonetheless, clear. On the California shelf, all cases they found the initially axisymmetric roughly 20 km wide, Davis found that the majority flow to become unstable with large-amplitude of buoys deployed on the shelf were transported oscillations growing whose shapes were highly offshelf within a few days. In contrast, the similar to those we found. Backward breaking Middle Atlantic Bight has a wide shelf, roughly waves of cyclonic vorticity were prominent with 100 km, and buoys that we deployed shoreward of wavelengths from 6 to 30 internal Rossby radii. the 100-m tsobath merely translated westward Their linear stability analysis pointed to alongshore. The shelfbreak eddy circulation, baroclinic instability as the chief agent for instead, was concentrated near the front. The growth. We expect that the same is true for the onset or relaxation of upwelling winds created eddies we observed. Chao and Kao [1987] used a markedly different flow fields on the California three-dimensional primitive-equation numerical shelf. During our observations, winds were light model to study instabilities of baroclinic and appeared unimportant to shelfbreak eddies. currents such as the Gulf Stream. Their Figure 6 Thus shelfbreak eddies also differ from marginal in particular shows the growth of an eddy group ice zone eddies, [Johannessen et al., 1987] in much like we found, with a plume of water having which wind stress divergence may play a major role anticyclonic vorticity departing to the side of [Hakkinen, 1986]. the current and partly enclosing a cyclonic eddy.

Cyclonic eddies formed on the upstream sides of We found the data products best suited for both plumes we observed. They had cores of detection and resolution of eddy structure were warmer, saltier, heavier slope water, each core our SST maps and buoy current fields. Our becoming progressively encircled by a limb of intensive hydrography then supplied confirming and shelf water as its adjacent plume wrapped around elaborating information, but had we lacked the it cyclonically. The $ cyclone was the greater of former products, we might have failed even to the two eddies we observed, with a diameter of suspect that eddies were present. Currents from about 40 km. The cyclones had deep structure, our moored arrays would have been the least their cores clearly visible in the interpolated valuable, used in isolation, for detection isotherm topography of Figure 6b between 30- and purposes, as the array was not nearly dense enough 70-m depth. We found no evidence of dipole eddy to resolve the complex eddy fields. Indeed, pairs, so often reported in laboratory studies, moored arrays have been employed at the New but did find weak evidence of an anticyclone England shelfbreak front for many years without associated with the $ eddy group. The combination any report of eddy structure because they have of plume and cyclone marked the principal been designed to resolve principally across-shelf structure we found for an eddy group. variations. Perhaps arrays similar to ours

We found that alongshelf advection dominated together with recently developed methods, such as the local temporal change in eddy structure, HF radar mapping of surface currents, will prove similar to the findings of Burrage and Garvine useful for better resolution of other complex flow [1988] for the shelfbreak front farther southwest. fields. The need for alongfront resolution for This advective dominance enabled us to use time shelfbreak studies whether using observations or series of across-shelf hydrographic sections to models is clear. best interpolate the structure inside the domain of our direct observations. Satellite SST maps Acknowledgments. B. Burman of the U.S. were especially useful in confirming the validity Geological Survey kindly provided us with current of the interpolated temperature field. meter data. We are indebted to many for help in

The small-scale frontal eddies we observed the field observations in the Shelfbreak 3

appear to have common elements with those reported experiment. Donald L. Murphy of the International by others. Houghton et al. [1986] studied an Ice Patrol arranged for our use of three anticyclonic eddy near our study area in July, satellite-tracked buoys. Arthur Allen of the U.S. 1983. Pingree [1978] found similar shelfbreak Coast Guard Research and Development Center eddies in the Ushant off the coast of Brittany, oversaw their deployment and data analysis. The France. Pingtee [1979] later obtained a frequency Research and Development Center kindly provided versus size distribution for shelfbreak eddies in working space and radio communication to the first


author during the field operations. Edward Brown Chapman, D. C., A simple model of the formation and the flight crew of the NCAR Queen Air research and maintenance of the shelf/slope front in the aircraft gave expert support to the aircraft Middle Atlantic Bight, J. Phys. Oceanogr., 16, operations. Russ E. Davis and Robert E. Yager 1273-1279, 1986. served unstintingly in the aircraft buoy tracking. Churchill, J. H., P. C. Cornilion, and G. W. Peter Celone and Kenneth Barton of the National Milkowski, A cyclonic eddy and shelf-slope Marine Fisheries Service processed the satellite water exchange associated with a Gulf Stream data. Captain John Gay and the crew of the R/V warm-core ring, J. Geophys. Res., 91, Cape Henlopen gave generously in their efforts to 9615-9623, 1986. overcome several difficulties that arose during Cresswell, G. H., Quasi-synoptic monthly the shipboard work. We also thank the shipboard hydrography of the transition region between scientific party for their tireless efforts' coastal and slope water south of Cape Cod, Derek Burrage, John Casaderail, Ann Masse, James Mass., Rep. 67-35, Woods Hole Oceanogr. Inst., O'Donnell, Timothy Pfeiffer, and Tad Yancheski. Woods Hole, Mass., 1967. Arthur Sundberg and Wadsworth Owen helped with the Davis, R. E., Drifter observations of coastal many logistical problems. For the SEEP-I field surface currents during CODE' The method and program we thank especially Pierre Biscaye, Steve descriptive view, J. Geophys. Res., 90, Hudson, Miguel Maccio, and the captains and crews 4741-4755, 1985. of the Endeavor, Oceanus, and Gyre. Suzanne Fairbanks, R. G., The origin of continental shelf O'Hara has provided excellent computer programming and slope water in the New York Bight and Gulf and data management assistance. We are grateful of Maine' Evidence from H•180/H•160 ratio for the support of the study by the National measurements, J. Geophys. Res., 87, 5796-5808, Science Foundation through grant OCE-8300011 to 1982. the University of Delaware and by the Department Flagg, C. N., and R. C. Beardsley, On the of Energy to Lamont-Doherty Geological Observatory under contracts DE-AC02- 76EV02185 GB and

DE-FG02-87-60555. Lamont-Doherty Geological Observatory contribution 4340.

References

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Burrage, D. M., and R. W. Garvine, Supertidal Houghton, R. W., D. B. Olson, and P. J. Celone, frequency internal waves on the continental Observations of an anticyclonic eddy near the shelf south of New England, J. Phys. Oceanogr., continental shelfbreak south of New England, J. 17, 808-819, 1987. Phys. Oceanogr., 16, 60-71, 1986.

Burrage, D. M, and R. W. Garvine, Summertime Houghton, R. W., F. Aikman, III, and H. W. Ou, hydrography at the shelfbreak front in the Shelf/slope frontal structure and cross-shelf Middle Atlantic Bight, J. Phys. Oceanogr., 18, exchange at the New England shelf break, Cont. 1309-1319, 1988. Shelf Res., in press, 1988.

Burman, B., Downslope Eulerian mean flow James, I. D., A three-dimensional numerical associated with high frequency current shelf-sea front model with variable eddy fluctuations observed on the outer continental viscosity and diffusivity, Cont. Shelf Res., •, shelf and upper slope along the northeast U.S. 69-98, 1984. continental margin' Implications for sediment Johannessen, J. A., O. M. Johannessen, E. transport, Cont. Shelf Res., in press, 1988. Svendsen, R. Shuchman, T. Manley, W. J.

Chao, S.-Y., and T. W. Kao, Frontal instabilities Campbell, E. G. Josberger, S. Sandyen, J. C. of baroclinic ocean currents with applications Gascard, T. Olaussen, K. Davidson, and J. Van to the Gulf Stream, J. Phys. Oceanogr., 17, Leer, Mesoscale eddies in the Fram Strait 792-807, 1987. marginal ice zone during the 1983 and 1984


Marginal Ice Zone experiments, J. Geophys. Ramp, S. R., R. C. Beardsley, and R. Legeckis, An Res., 91, 6754-6772, 1987. observation of frontal wave development on a

Libbey, W., Report upon a physical investigation shelf-slope/warm core ring front near the shelf of the waters off the southern coast of New break south of New England, J, Phys. Oceanogr., England, made during the summer of 1889 by the 13, 907-912, 1983. U.S. Fisheries schooner Grampus, U.S. Fish Simpson, J. H., The shelf-sea fronts: Wildl. Serv. Fish. Bull., •, 391-459, 1891. Implications of their existence and behavior,

McCarthy, R. K., The validity and use of NOAA-7 Philos, Trans. R. Soc. London, Set. A, 302, satellite derived sea surface temperatures to 531-543, 1981. gain a kinematical description of the surface Smith, P. C., Low-frequency fluxes of momentum, thermal shelfbreak front in the Middle Atlantic heat, salt, and nutrients at the edge of the Bight, M.S. thesis, 71 pp., Univ. of Del., continental shelf, J. Geophys. Res., 83, Newark, 1986. 4079'-4096, 1978.

Miller, A. R., A study of mixing processes over Tennekes, H., and J. L. Lumley, A First Course in the edge of the continental shelf, J, Mar, Turbulence, p. 253, MIT Press, Cambridge, ges., •(2), 145-159, 1950. Mass., 1972.

Mooers, C. N. K., R. W. Garvine, and W. W. Martin, Voorhis, A. D., D. C. Webb, and R. C. Millard, Summertime synoptic variability of the Middle Current structure and mixing in the shelf/slope Atlantic shelf water/slope water front, J. water front south of New England, J. Geophys. GeoDhvs. Res., 84, 4837-4854, 1979. Res., 81, 3695-3708, 1976.

_ _

Morgan, D. W., and J. M. Bishop, An example of Wright, W. R., The limits of shelf water south of Gulf Stream eddy-induced water exchange in the Mid-Atlantic Bight, J, Phys. Oceanogr., •, 472-479, 1977.

Paluszkiewicz, T., and H. J. Niebauer, Satellite observations of circulation in the eastern

Bering Sea, J. GeoDhys. Res., 89, 3663-3678, 1984.

Pedlosky, J., GeoDhysical Fluid Dynamics, 2nd edition, 710 pp., Springer-Verlag, New York, 1987.

Pingtee, R. D., Cyclonic eddies and cross-frontal

Cape Cod, 1941-1972, •. Mar. Res., 3_•4, 1-4, 1976.

Wong, K.-C., and R. W. Garvine, Variability in the hydrography and current field of the shelfbreak front south of New England, I, Hydrography (abstract), EOS Trans. AGU, 66(18), 281, 1985

F. Aikman and R. W. Houghton, Lamont-Doherty Geological Laboratory, Palisades, NY 10964.

R. W. Garvine, G. G. Gawarkiewicz, R. K.

mixing, J. Mar. Btol. •s•oc, U,K., 58, 955-963, McCarthy, and K.-C. Wong, College of Marine 1978. Studies, University of Delaware, Newark, DE 19716.

Pingree, R. D., Baroclinic eddies bordering the Celtic Sea in late summer, J. Mar, Biol. Assoc. (Received June 27, 1988; U,K., 59, 689-698, 1979. accepted July 18, 1988.)


i'a)

- 2

x (n mi)

0 12 24 36

24

12

y(n mi)

0

(b)

Plate 1 [Garvine et al.]. Comparison in the same space domain of (a) interpolated temperature at 35-m depth from (1) and (b) sea surface anomaly temperature less mean from satellite image for 0400 LT on June 22. South is now to the top. In Plate la, contour intervals are 2oc with dark blue coldest (T < 9 o ) and red warmest (17 ø < T < 19ø), while in Plate lb, intervals are 0.5 o with dark blue coldest and magenta warmest. The white line denotes the 200-m isobath.


oo.

-•oo

o

•00 ......

•oo

o

-48 -36 -24 -12 0 i2 24 36 x

•---'-•o

Plate 3 [Garvine et al. ]. Satellite SST anomaly map for 1400 LT, June 9. The field of view is the same as that in Figure 4' south to the top, west to the right. The 200-m isobath is shown in white. Contour interval is 0.5 ø from -1.5 ø to +

1.5 ø with dark blue coldest (T < -1.5 ø ) and magenta warmest (T > + 1.5o). Current vectors are superposed (speed scale at lower right). Radius of circle about vector origins corresponds to speed of 1 cm/s. Vector color denotes depth interval' white, 0-10 m; brown, 20 m; magenta, 35 m; blue, 54-65 m; black, 93-120 m. "V" denotes

Veatch Canyon, "top" the m plume.

• o

}

/o -

o

ß ' •. • ':' ' • 24

o

-48 -36 -24 -12 0 12 24 36' x

Plate 4 [Gatvine et al.]. As for Plate 3, but for 0400 LT, June 12. The white area denotes a cloud, "T" •e wave trout, and "C" the wave crest on the front to •e east.

Plate 2 [Garvine et al.]. Hydrographic across- shelf sections from CTD profiles along line 2 (Figure 2) at 2300 LT, June 21. The view is

eastward, offshore to the right. (a) Temperature •

T with contours as in Plate la (brown denotes the • -... sub-bottom) (b) Salinity S with contours in 0.5-g/kg units with red-orange saltiest (S > 35) and dark green freshest (32.5-33) (c) Density at with contours in 0.8-g/kg units with darkest blue heaviest (at > 27) and light blue-green lightest (23 < at < 23.8). The white band at top of each figure marks where data are unavailable because of instrument response and vertical filtering.

Garvine et al.' The Morphology of Shelfbreak Eddies 15, ?$5

x (n mi)

48

36

ß ,' 24

5O

E IO0

uJ 150

200

18 12 6 0 ..,, ............................... , .........

Plate 5 [Garvine et al. ]. As for Plate 4, but for Plate 8 [Garvine et al. ]. Alongshelf temperature 1600 LT, June 12. Hydrographic lines 3 and 4 for section along line 3 for 1900 LT, June 13. The part A are also shown. view is landward, west to the left.

• ,, , ::(• 36 "• 0

-48 -36 -24 -12 0 12 24 36 x

Plate 6 [Garvine et al.]. As for Plate 5, but for Plate 9 [Garvine et al.]. Satellite SST anomaly 0400 LT, June 13. Buoy current vectors now map for 0400 LT, June 21, with current field for appear. 0400 LT, June 20. Hydrographic lines 2, 3, and 4

for part B are shown. Inferred features of the e and • eddy groups are shown with subscripts "p", "c", and "a" denoting "plume," "cyclone," and "anticyclone," respectively.

y(n mi)

5O

100

150

200 ........

Plate 7 [Garvine et al. ]. Across-shelf temperature section along line 4 for 1600 LT, June 12. Plate 10 [Garvine et al. ]. As for Plate 9, but Contours are as in Plate 2a. The view is both SST map and currents now for 0400 LT, June eastward, offshore to the right. 21.


y(n mi)

-48 -36 -24 -12 0 12 24 36 x

?late 11 [Gar¾ine et al.]. As for ?late 9, but for 0400 LT, June 22.

5O

0 6 12 18 24 3o t ! ....... i ....... ! i ........ ! ..l. . i , Ii ! ß

ß .

lOO

150

200 •'

Plate 14 [Garvine et al. ]. As for Plate 12, but for 1500 LT, June 20.

y (nmi)

0 6 12 18 24 30 j i ! ! ! I ß I ! I .......... 11 ......... i,._

.

x(n mi)

18 12 6 ,0 I I ß I

5O

.,-- lOO

• 150

5O

E 1oo

u• 150

200 200

Plate 12 [Garvine et al. ]. Across-shelf Plate 15 [Garvine et al. ]. Alongshelf temperature temperature section along line 4 for 1400 LT, June section along line 3 for 0200 LT, June 19. The 18. view is landward, west to the left.

y(n ml) x (n mi)

0 6 12 18 24 30

_l ! I I I I I I l-- .I i• 18 12 6 0

I ' '- ' • ' ' I

50 50

,-..- lOO E lO0

m 15 0 m 150

2OO 200

Plate 13 [Garvine et al. ]. As for Plate 12, but Plate 16 [Garvine et al. ]. As for Plate 15, but for 1200 LT, June 19. for 0000 T, June 21.

Garvine et al.' The Morphology of Shelfbreak Eddies

x (n

18 12 6 0

5O

15,7U7

E lOO

o. 1•0 '1•1

200

Plate 17 [Garvine et al. ]. Alongshelf density section corresponding to Plate 16. Contours are as in Plate 2c.

tic o

8

36

Y

-48 -36 -24 12 0 12 24 36 x

20

?late 18 [Gatvine et al. ]. Satellite SST anomaly ma D for 0400 LT, •Tune 27, wœth currents from 0400 LT, •Tune 26. White areas denote clouds.

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