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Eddies in the Labrador Sea as Observed by Pro®ling RAFOS Floats and Remote Sensing

MARK D. PRATER Graduate School of , University of Rhode Island, Narragansett, Rhode Island

(Manuscript received 5 June 2000, in ®nal form 5 June 2001)

ABSTRACT Data from pro®ling RAFOS ¯oats, TOPEX/Poseidon altimetry, and the alongtrack scanning radiometer (ATSR) aboard ERS-1 have been used to describe the spatial and seasonal patterns of variability in the Labrador Sea. Peaks in sea surface height (SSH) variability appear in two regions: off the west Greenland shelf near 61.5ЊN, 52ЊW where the 3000-m isobath separates from the shelf, and in the center of the basin at 58ЊN, 52ЊW. Both locations show seasonal ranges in SSH variability of up to 40 mm, with the Greenland site, having largest variability in January±March, leading the central site by 50 days. A image from the ATSR at the Greenland site shows numerous eddies, both cyclonic and anticyclonic, being formed by injection of West Greenland Current water into the Labrador Sea interior. Data from pro®ling RAFOS ¯oats launched in 1997 as part of the Labrador Sea Deep Convection Experiment are used to describe three of the West Greenland Current eddies in detail. One of the sampled eddies was anticyclonic, while the other two were cyclonic. The eddies contained various mixtures of Irminger Sea Water. Peak azimuthal velocities ranged from 22 to 42 cm sϪ1, and diameters from 20 to 50 km. Although the ¯oats were at a depth of 375 m, the surface elevations derived from cyclogeostrophy agreed with those obtained from TOPEX/Poseidon. The temporal and spatial patterns in SSH variability are thought to be caused primarily by seasonal variations in the strength and stability of the West Greenland Current and, less likely, by eddy formation following deep convection in the basin interior.

1. Introduction ever, of the data recovered, the most striking features observed were of three eddies found near the west The Labrador Sea is one of the few locations in the Greenland shelf. The appearance of these eddies with world's that experiences deep convection, with supporting data from satellite thermal images and altim- complete mixing of the over the top 1500± etry indicated that eddies were common in the Labrador 2000 m. The mixed layer begins cooling in the fall, and Sea. These eddies could act as the primary mechanism by February the seasonal has been eroded for homogenizing the intermediate waters of the Lab- away over large regions. The maximum mixed layer depths are typically observed in March. However, the rador Sea and for restratifying the upper waters. In ad- restrati®cation of the surface waters happens quickly; dition, eddies can play a role in convection, by locally by May much of the upper water column (the top 200± modifying the near-surface density structure and by 500 m) has been restored to its preconvection state. To trapping waters undergoing modi®cation by atmospher- better understand the convection processes and water ic cooling (Legg et al. 1998). mass transformations in the Labrador Sea, an intensive Labrador eddies have been described before, but not observational program was sponsored by the Of®ce of always while in the Labrador Sea. Elliott and Sanford Navel Research's Accelerated Research Initiative of (1986) describe a lens of predominantly Labrador Sea Oceanic Deep Convection (Lab Sea Group 1998). As origin, with core properties of 34.96 psu and 3.90ЊC. part of this experiment, pro®ling RAFOS ¯oats were The vertical extent of the eddy was from 1000 to 3000 deployed in the Labrador Sea from four cruises, span- dbar, with the core at 1500 dbar. The eddy was anti- ning the time from fall 1996 to summer 1998. Unfor- cyclonic, with a velocity maximum of 28.6 cm sϪ1 at tunately, a series of hardware and software problems a radius of 14.4 km. The eddy was found far from the diminished the anticipated amount of data to be recov- Labrador Sea near 31ЊN, 70ЊW. Closer to the Labrador ered [details can be found in Prater et al. (1999)]. How- Sea, Pickart et al. (1996) found an extraordinarily weak eddy (maximum velocity of 0.5 cm sϪ1 ) of upper Lab- rador Sea Water. This feature is perhaps better desig- Corresponding author address: Dr. Mark D. Prater, Graduate School of Oceanography, University of Rhode Island, South Ferry nated as a ``blob'' or pulse of distinct water property Rd., Narragansett, RI 02882. advected by the Labrador Current along the 2000-m E-mail: [email protected] isobath near Flemish Cap (49ЊN, 44ЊW). In the Lab-

᭧ 2002 American Meteorological Society

Unauthenticated | Downloaded 09/29/21 12:31 PM UTC 412 JOURNAL OF VOLUME 32 rador Sea, Gascard and Clarke (1983) used instruments similar to Swallow ¯oats to observe an anticyclonic eddy with a radius of 10 km and maximum velocities of 24 cm sϪ1 . Hydrographic data supported the sug- gestion that the eddy was formed by the baroclinic instability of a rim current bordering a convectively mixed patch. Peterson (1987) used ice-¯oe motion in satellite imagery to observe nine cyclonic eddies along the 3000-m isobath in the northwest Labrador Sea. The radii of the eddies varied from 10 to 21 km, and max- imum azimuthal velocities from 3 to 35 cm sϪ1 . Lilly and Rhines (2002) and J. M. Lilly (2001, personal com- munication) used moored current meter and hydro- graphic data from 56.75ЊN, 52.5ЊW to develop very detailed descriptions of Labrador Sea eddies. Lilly and Rhines presented six eddies that propagated through the : four middepth, cold-core, anticyclonic eddies, and two upper-, cyclonic warm-core ed- dies. The cold-core eddies had properties consistent with convectively mixed Labrador Sea Water, and are similar to the Elliott and Sanford (1986) feature. The shallower warm-core eddies had characteristics of Ir- minger Sea Water. The purpose of this paper is to examine eddy vari- FIG. 1. The Labrador Sea Basin. The ground tracks of the TOPEX/ ability in the Labrador Sea, initially motivated to un- Poseidon passes in the Labrador Sea are shown where the water depth derstand the rapid restrati®cation of the upper water exceeds 1000 m. The two shaded circles are the domains used in com- column of the Labrador Sea after convection. The Lab- puting sea surface height variability from TOPEX/Poseidon data. Bathy- rador Sea, with TOPEX/Poseidon ground tracks, is metric contours are given at 200, 1000, 2000, 3000, and 4000 m. shown in Fig. 1. A particularly striking sea surface ther- mal image of the Labrador Sea is presented and dis- cussed in section 2, while sea surface height variability from the coast. Numerous mushroom vortices (see Fe- (as a surrogate for eddy activity) obtained from TOPEX/ dorov and Ginsburg 1986) consisting of paired cyclonic Poseidon altimetry is discussed in section 3. In section and anticyclonic eddies can be seen, along with indi- 4, three eddies observed by pro®ling RAFOS ¯oats are vidual eddies and other smaller scale temperature ®la- described in detail. Finally, the paper is summarized and ments, patches, and structures. One dipole eddy was a few speculative hypotheses are given in section 5. observed that contained cold surface waters in its an- ticyclonic half, warmer waters in the cyclonic partner, and trailed a 50±100 km ®lament. The dipole is clearly 2. Alongtrack scanning radiometer seen as having been ejected from the frontal region in A unique view of the surface of the Labrador Sea the center of the image. A subjectively derived inven- was obtained from the alongtrack scanning radiometer tory counted ®ve dipole pairs and four other eddies with (ATSR) on the European Space Agency's remote sens- no discernible partner. The diameters of the eddies based ing satellite ERS-1 (Mutlow et al. 1994). This satellite on the sea surface temperature fronts ranged from 25 system views the sea surface in four infrared wave- to 65 km. The polarities of the eddies were estimated lengths, and as such can only derive SST for areas that by whether the small-scale temperature ®laments as- are cloud-free [refer to Mutlow et al. (1994) for instru- sociated with the eddies spiraled cyclonically or anti- ment description and processing algorithms]. An un- cyclonically out from the center. The polarities appeared usually clear view of the Labrador Sea occurred on 10 to be evenly distributed, with six cyclones, seven an- July 1992 and the resulting brightness temperature im- ticyclones, and one of undetermined rotation. The eddies age for the 11-␮m wavelength is shown in Fig. 2. The were most prominent along and on either side of the cold waters over the west Greenland shelf inshore of 3000-m isobath after that contour separated from the the 2000±3000 m isobath and in the northern reaches shelf and turned to the southwest. of the Labrador Sea are shown in purple, while the This same isobath has been observed in the analysis warmer waters of the central Labrador Sea are in red. of both surface drifters (Cuny et al. 2002) and Pro®ling The most striking features are the numerous swirls and Autonomous Lagrangian Circulation Explorer (PAL- eddies throughout the image. The eddies appear to be ACE) ¯oats at 700 m (Lavender et al. 2000) as a tran- ejected from the west Greenland shelf at 61ЊN, 52ЊW, sition region between the strong cyclonic circulation near the location where the 3000-m isobath separates crossing the Davis Strait along the 1000±2000 m isobath

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FIG. 2. Sea surface brightness temperature image of the the northern Labrador Sea from the ATSR on the ERS-1 remote sensing satellite (from C. T. Mutlow 1999, personal communication). The image is from the 11-␮m wavelength infrared sensor and shows numerous eddies near where the 3000-m isobath separates from the shelf. and the somewhat weaker anticyclonic ¯ow in the basin 3. Sea surface variability from TOPEX/Poseidon interior. It is not known what role the eddies play in altimetry this transition. Are the eddies created and maintained by the counterrotating currents? Or is the anticyclonic To quantify the sea surface variability (as a proxy for current in the interior maintained by an eddy ¯ux of eddy activity) in the Labrador Sea, the Center for Me- vorticity? The very narrow temperature gradient along teorology and Physical Oceanography/Massachusetts the west Greenland shelf shown in Fig. 2 has broadened Institute of Technology (CMPO/MIT) TOPEX/Poseidon to 300 km in the center of the image. This could have Altimetric Data Set (King et al. 1994) was used. This been the result of eddies stirring the surface waters or dataset provided by CMPO/MIT has been pre-processed simply the spreading of surface properties due to the with all pertinent engineering, atmospheric, environ- barotropic components of the reacting mental, and tidal corrections incorporated. The data to the divergence of the isobaths. have been interpolated onto uniform 6.2-km alongtrack

Unauthenticated | Downloaded 09/29/21 12:31 PM UTC 414 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 32 spacing. Only those data collected over ``deep'' water ability region is centered on the location where the 3000- (greater than 1000 m) has been used so to minimize any m isobath diverges from the shelf. It is presumed that effect of tidal model inaccuracies. Data from October the high levels of SSH variability were due to enhanced 1992 to December 1998 was used in this analysis and eddy formation from the West Greenland Current. No consisted of 228 9.91-day repeat cycles. Since the pri- elevated SSH variability was visible elsewhere along mary interest was with the eddy activity in the Labrador the pathway of the West Greenland Current or the Lab- Sea, there was no attempt to determine the absolute SSH rador Current seaward of the 1000-m isobath. This in- by combining the altimeter data with an estimate of the dicates that those currents (other than in the vicinity of earth's geoid or with dynamic height estimates from the SSH variability maximum) are very stable. The sta- . Instead, the perturbation SSH was ob- bility may be due, in part, to topographic effects (see tained with a two-step process. First, the SSH from each Kearns and Paldor 2000). geographical point for all passes and for all cycles in A second maximum in SSH variability forms in the the Labrador Sea was demeaned in an iterative manner interior of the Labrador Sea at 58ЊN, 52ЊW, but is only whereby the time mean at each point was computed and evident during AMJ. This region's variability peaks at then all data falling outside of two standard deviations over 70 mm but is not a noticeable center of variability from the mean were removed. This process was repeated the rest of the year. The source of this variability is not three times to obtain a ®nal demeaned SSH. Second, a immediately apparent. One hypothesis is that the eddies linear ®t was removed from the demeaned SSH of each are an end product of deep convection. An idealized pass through the Labrador Sea to act as a low-wave- scenario is that localized cooling creates a spatially con- number ®lter. ®ned mixed layer. As cooling continues, a horizontal The TOPEX/Poseidon footprint is nominally 2 km in density gradient is formed between the mixed patch and calm sea conditions, but is a function of the the surrounding waters. A cyclonic rim current develops and increases to 10 km at the more extreme signi®cant at this density gradient, and the current eventually gen- of 10 m (Chelton et al. 1989). The sea state erates meanders, via baroclinic instabilities, that grow also affects the area of the ocean illuminated at any one and pinch off to form eddies (Visbeck et al. 1996; Jones time by the altimeter, described best as an annulus. The and Marshall 1997; Marshall and Schott 1999). Al- increasing sea state increases the radius of the annulus, though deep mixed layers can occur throughout the cen- but decreases the thickness of the band. Thus, the TO- tral Labrador Sea, the central region of the basin is not PEX/Poseidon altimeter measurement is not a ``point'' typically where the most intense convection occurs. measurement, but is an average over a sea state (and Most evidence indicates that the deepest convection and hence somewhat seasonally) dependent area. The var- the greatest heat ¯ux occurs near the Labrador Shelf, iation in area can cause the magnitude of the SSH es- where the continental winds are at their driest and cold- timate and hence the computed SSH variability to be est (Clarke and Gascard 1983; Lab Sea Group 1998; biased downward. This is most noticeable as the eddy Pickart et al. 2002). scale decreases; for example, measurements from a 10- To examine the temporal structure of the SSH vari- km footprint can better reproduce the SSH structure of ability features in greater detail, two circular spatial do- a 100-km diameter eddy than they can for a 10-km mains were de®ned centered on the West Greenland and diameter eddy. If one knew both the sea state and the the Labrador Sea Basin variability maximums (see Fig. wavenumber spectrum of the eddy ®eld, a correction 1). Each domain was approximately 300 km in diameter. could be made to estimates of altimeter-derived SSH Unlike the analysis presented above, where spatial maps variability and eddy kinetic energy. This was not done of temporal variability were presented, here a time series in this paper, but remains an area of further research. of spatial variability was considered. The assumption is The TOPEX/Poseidon data was segmented by season: that over a limited spatial domain the eddy ®eld is ho- January±March (JFM), April±June (AMJ), July±Sep- mogeneous and is slowly varying in time. The data from tember (JAS), and October±December (OND). The stan- within each 9.91-day cycle were processed as if it oc- dard deviation of the SSH values was computed at every curred simultaneously, and the standard deviation from grid point that had at least 25 good values in a season all the TOPEX/Poseidon passes falling within each rep- (approximately half of the total possible number of good resentative domain was computed. The results, spanning data values). The resulting values were interpolated onto nearly six years, are shown in Figs. 4a and 4b. An annual a uniform grid using a Gaussian smoother with a 50- variability cycle dominates, albeit with variability at km e-folding scale, and are shown in Fig. 3 (the seasons both shorter and longer timescales. The annual ranges progress clockwise from the upper left). The maximum can often exceed 40 mm at each site. Large values oc- variability occurred off the west coast of Greenland near curred at the west Greenland site in early 1997 and 1998, 61ЊN, 52ЊW, in the identical location as the eddy activity while large values were found at the Labrador Sea Basin observed by the ATSR and shown in Fig. 2. The var- site in 1993, 1994, and 1997. A lagged correlation anal- iability in this region (shown in yellows and reds) peak- ysis showed that the peak near Greenland led the Lab- ed in JMF near 80 mm and was reduced to 55 mm in rador Sea peak by 50 days. The magnitudes of the annual JAS (the season of the ATSR image). This high vari- maximums were compared to the winter (November±

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FIG. 3. Standard deviation of sea surface height anomaly as computed seasonally from six years of TOPEX/Poseidon altimetry data for the Labrador Sea. The seasons progress clockwise from the upper left.

March) North Atlantic Oscillation (NAO) index [e.g., positive and negative NAO index. However, lower var- Hurrell (1995) and shown in Fig. 4c]. High values of iability in the central Labrador Sea occurred during the the NAO index are indications of strong westerly winds, highest NAO of this period (1995), while high vari- cold air, and increased Labrador Sea Water production. ability occurred during a lower NAO (1997), although The higher SSH variability of 1993 and 1994 and the that year had a cold winter with convection in the Lab- lower SSH variability in 1996 correspond to years of rador Sea reaching to 1500 m (Pickart et al. 2002). Since

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the wind stress curl peaks in mid-January. This timing is nearly identical to the time lag between variability maxima found in the west Greenland shelf and central Labrador Sea locations. White and Heywood postulate that large seasonal EKE signals can be attributed to eddy generation from wind stress, while seasonal cycles of smaller magnitude are found in regions dominated by baroclinic instability. White and Heywood also suggest that time lag between wind stress and eddy variability may be due to eddy energy being gained from the wind at a greater rate than it is lost after the wind stress peak has occurred.

4. Eddy observations with pro®ling RAFOS ¯oats The pro®ling RAFOS ¯oat is based on the standard RAFOS ¯oat (Rossby et al. 1986), which is an acous- tically tracked, neutrally buoyant, subsurface Lagrang- ian drifter. In the pro®ling version of the ¯oat, the elec- tronics are contained within a glass pressure housing approximately 2.2 m in length and 0.095 m in diameter. The entire ¯oat weighs 17 kg in air. The ¯oat measures temperature and pressure at ®xed time intervals and uses a to record the times-of-arrival of acoustic signals transmitted from moored sound sources. The ¯oat is ballasted for a speci®ed pressure surface and will remain on (or near) that surface collecting data until the preprogrammed mission terminates. At that time, the ¯oat releases a weight, rises to the surface, and FIG. 4. Time series of spatial variability in sea surface height anom- aly for the two regions showing local maxima: (a) the west Greenland transmits the data back to the user via the ARGOS sat- shelf and (b) the central Labrador Sea. (c) The NAO index (from ellite system. The ¯oat continues transmitting at the Joint Institute for the Study of the Atmosphere and Ocean at the surface until its batteries are exhausted. The ¯oat's tra- University of Washington). jectory is computed in postprocessing using the loca- tions of the sound sources, the ¯oat-recorded times-of- arrival, and values for the speed of sound appropriate both the NAO and SSH signals show considerable var- for the study area. iability and are governed by dynamics acting on dra- The pro®ling version contains an internal volume matically different temporal and spatial scales (decadal changing (vocha) mechanism (Rossby et al. 1994) that and basinwide for NAO, annual and mesoscale for SSH consists of an internal motorized piston and - variability), a more detailed analysis will be needed to ®lled cylinder connected to the outside of the ¯oat by con®rm their relationship. a small tube. As the motor drives the piston forward Satellite altimetry has been used by several previous into the cylinder, seawater is expelled, effectively de- investigators to study the Labrador Sea. Han and Ikeda creasing the ¯oat's mass (or, equivalently, increasing its (1996) used complex empirical orthogonal function volume) and thus causing the ¯oat to become more analysis to extract temporal and spatial patterns of SSH buoyant. Likewise, when the motor draws the piston anomalies. They found a 50-mm annual signal in the back out of the cylinder, seawater is drawn in, and the mean, and attributed 80% of the variation to thermal ¯oat gains mass (loses volume) and becomes less buoy- expansion of the upper 500 dbar of the water column ant. These changes in buoyancy allow the ¯oat to move and only 20% to wind-driven effects. However, the wind vertically through the water column, enabling it to make effect on SSH was derived from a 1Њ by 1Њ barotropic vertical temperature pro®les. The vocha has 16 discrete numerical model, and thus the west Greenland and Lab- positions, including full in or out, thus permitting the rador Currents were not resolved, nor were any baro- ¯oat to also become neutrally buoyant at intermediate clinic processes included. White and Heywood (1995) depths. The ¯oats deployed in the Labrador Sea and examined seasonal changes in the Labrador Sea as part described here had equilibrium depths of 375 m and of a larger study of the North Atlantic subpolar gyre. returned to these levels after pro®ling the top 800±1000 They found local maxima at the same two sites as de- m of the water column. A typical preprogrammed mis- scribed above, and noted that the eddy kinetic energy sion had the ¯oat pro®le once every seven days. Be- (EKE) in the Labrador Sea peaked in early March, while tween pro®les (which take about six hours to complete),

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FIG. 5. Trajectory of ¯oat 404. Float pro®les occurred at 7-day intervals and are labeled along the trajectory. Small dots denote each navigational ®x and are given three times per day. The * marks the launch position. the ¯oat remained neutrally buoyant at a predetermined the fresh and warm waters in the upper few 100 m) of depth and made measurements of temperature (with a the basin interior after convection. To that end, ¯oats resolution of almost 0.001ЊC) and pressure (to 0.5 dbar) were deployed in the interior and near the west coast at 8-h intervals. An exception to this sampling schedule of Greenland. A series of hardware and software prob- occurred between deployment and the ®rst vertical pro- lems prevented many of the ¯oats from returning data ®le at seven days, when these ¯oats sampled at 2-h (Prater et al. 1999). However, three separate eddies were intervals. observed, and characteristics of each of these eddies To provide acoustic navigation for the RAFOS ¯oats, will be described. Since each eddy was observed with four sound source moorings were deployed in the Lab- one individual and separate ¯oat, the ¯oat numbers will rador Sea from the C.S.S. Hudson in October±Novem- be used to identify the eddy. ber 1996. The sound sources transmitted an 80-s tone six times a day (every 4 h), although our ¯oats listened at 8-h intervals. We initially had concerns that, when a. Float 404 the sound speed minimum rose to the surface during the winter convection period, the sound propagation dis- RAFOS ¯oat 404 had the longest eddy record of any tances would be so reduced as to make ¯oat tracking of the pro®ling RAFOS ¯oats, with 60 days in the eddy impossible. Fortunately, that was not the case, and high and making over 20 revolutions around the center (Fig. quality 400 km ranges were achieved even during the 5). The ¯oat was deployed at 60.18ЊN, 48.68ЊWon22 convection season. May 1997 from the C.S.S. Hudson, at a pressure level The ¯oats were deployed in the Labrador Sea with of 375 dbars in 2500 m of water. (All ¯oat deployments two separate purposes. First, ¯oats were deployed in are given in Table 1.) The ¯oat traveled northwestward February and March of 1997 and 1998 in regions of following the coast for several days along the bathym- deep convection. Those ¯oats pro®led to quantify the etry until caught in a cyclonic eddy. The ¯oat remained changes in the heat content of the water column in a in the eddy until the ¯oat's mission ended on 2 August Lagrangian framework and sampled temperature and 1997. The eddy's trajectory tended to follow the ba- pressure at 2±5 min intervals to estimate vertical ve- thymetry in the Labrador Sea and always remained be- locities and (with less con®dence) the vertical heat ¯ux. tween the 2000- and 3000-m isobaths. The eddy un- The methodology and error estimates of these mea- derwent several stages of translational movement, near- surements can be found in Kearns and Rossby (1993) ly stationary at ®rst, then moving at over 20 cm sϪ1 for and Prater et al. (1999), with preliminary results given 20 days, followed by a 50-day period of steady 10 cm in Prater et al. (1999). The second purpose was to ex- sϪ1 movement [in the same range of velocities as found plore the rapid restrati®cation (the reestablishment of by Cuny et al. (2002) from surface drifters]. Finally, the

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TABLE 1. Pro®ling RAFOS ¯oats in Labrador Sea eddies. Launch Latitude Longitude Pressure Surface Latitude Longitude Float date (ЊN) (ЊW) (dbar) date (ЊN) (ЊW) 395 1 Jun 1997 61.23 50.38 375 20 Jun 1997 61.91 53.61 402 31 May 1997 60.06 52.47 375 29 Jul 1997 60.69 50.38 404 22 May 1997 60.18 48.68 375 2 Aug 1997 59.94 58.83 eddy was again nearly stationary for the last 20 days of is the angular velocity of the eddy and f is the Coriolis record. parameter, of 0.35 and a period of 3.3 days (Fig. 6b). To examine the eddy's structure, the mean motion of The radial distance of the ¯oat from the center of the the eddy was removed from the position record. This eddy varied from 10 to 20 km (Fig. 6c), while the az- was accomplished by a moving box-car ®lter of the imuthal velocity ranged from 20 to 40 cm sϪ1 (Fig. 6d).

¯oat's positions, with a variable window length based The values of R␨ ranged from 0.2 to nearly 0.5 (Fig. on the orbital period of the ¯oat. Window lengths of 6e). The characteristics of this and of the other eddies 2.7±5.3 days (which correspond to 8±16 acoustic nav- described in this paper are summarized in Table 2. igation ®xes) resulted in the ``best'' (judged subjec- The ¯oat made 10 pro®les over the top 800 m of the tively) trajectory of the eddy-referenced ¯oat (Fig. 6a). water column. All the pro®les were approximately one

Azimuthal velocities (␷ ␪) were then computed using cu- Rmax from the eddy center, which is in the transition bic splines of the trajectory and then differentiating the region between the homogeneous core and the surround- splined values at the times of the measured ¯oat loca- ing ambient waters. Thus, the core properties of the eddy tions. The radial distribution of ␷ ␪ is shown in Fig. 6b. are not well measured by the ¯oat; however, for mid- The structure of the eddy is not readily apparent, but latitude eddies Rmax is typically between the radius of when the data are averaged in 2-km radial bins, the the core and the radius of the property front (Hebert et velocity is shown to increase radially to approximately al. 1990; Prater and Sanford 1994). The pro®les do show 18 km. The error estimates for the binned mean velocity, (Fig. 7) a strong thermocline at about 50 m. The eddy shown by the solid vertical line extending plus/minus was capped with cooler waters initially (below 3ЊC and one standard deviation from the mean, are typically less not discernible in Fig. 7) and transitioned to warmer than 4 cm sϪ1. A subjective ®t through the data is made, waters later (above 5ЊC). To warm the upper 50 m by assuming a solid-body rotating core 2ЊC in 35 days requires a mean heat ¯ux of over 135 WmϪ2. The Comprehensive Ocean±Atmosphere Data r Set (COADS; da Silva et al. 1994) mean climatology ␷ ϭ V (1) ␪ max for June and July in the northwestern Labrador Sea ΂΃Rmax shows an average heat ¯ux of 120 W mϪ2 in the region and a quadratic far ®eld decay of the eddy trajectory, suggesting that the surface layer 2 temperatures above the ¯oat were consistent with local Rmax ␷ ␪ ϭ Vmax , (2) modi®cation (in a Lagrangian sense) rather than ex- ΂΃r change due to advection. The in¯uence of local warming may effect the surface temperatures of the eddies shown where Vmax is the maximum value of ␷ ␪ and Rmax is the in the ATSR image (Fig. 2). However, from this record radius where Vmax is observed. Although the radial gra- it is not obvious whether the eddy motion extended to dient is discontinuous at r ϭ Rmax, this model is com- monly used to describe eddy structure (e.g., Prater and the surface and the change in surface properties was Sanford 1994). Other formulations have been used to purely due to local heat ¯ux, or if the eddy was sliding model velocity structure in eddies (rϪ1 decay, exponen- under regions of varying T/S properties. The cold plume tial decay, etc.); however, the differences in overall ``descending'' to nearly 200 m in pro®les 3±5 (Fig. 7) Ϫ1 is more easily explained by the eddy (and ¯oat) ad- structure are minor. Values of Vmax ϭ 40 cm s and vecting post by the upper ocean feature. The temperature Rmax ϭ 18 km were found to be consistent with the measured data. These values indicate an eddy with a at the level of the ¯oat remained near 3.8ЊC except during a brief period where the isotherms dipped, raising vorticity Rossby number R␨, de®ned as 2␻/ f where ␻ the ¯oat temperature to over 4ЊC. The decrease in tem- perature anomaly in the core of the eddies may not be TABLE 2. Labrador Sea eddy characteristics. entirely due to stirring as the eddies move away from Vorticity Temper- the shelf, but also to an increase in the ¯oat's distance Eddy Days Velocity Radius Rossby ature from the center of the eddy (Fig. 6). The lifting of the (¯oat) in eddy (cm sϪ1) (km) number (ЊC) isotherms at depth between pro®les 3 and 4 is coincident

395 9 Ϫ42 25 Ϫ0.27 4.75 with an increase in both ␷ ␪ and the radial distance of 402 28 22 10 0.35 3.59 the ¯oat trajectory in the eddy, and also marked the 404 65 40 18 0.35 3.86 ¯oat's movement away from the west Greenland shelf.

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FIG. 6. Summary of the eddy characteristics of ¯oat 404. (a) Trajectory of the ¯oat relative to the translating center of the eddy. (b) Azimuthal velocity as a function of radial distance from the center of the eddy. Large black dots are average velocity in 2-km radial bins. Vertical black lines are Ϯ the error of the mean. The thick gray line is a subjective model ®t through the data. (c) Low-pass estimate of the ¯oat's radial distance from the center. (d) Low-pass estimate of the ¯oat's azimuthal velocity. (e) Low-pass estimate of the eddy's vorticity Rossby number.

J. M. Lilly (2001, personal communication) investi- line. Their Fig. 9 shows evidence of an eddy pair off gated eddies that were advected past a mooring in the the Greenland shelf, with a warmer (Ͼ4ЊC) anticyclone interior of the Labrador Sea, and found that very few and a cooler (Ͻ3.5ЊC) cyclone at the depth of the ¯oat. of the eddies, only 2 out of 33, had cyclonic rotation. However, their Fig. 9 also shows that the anticyclonic Those two eddies were thought to contain a mixture of (warm) feature is capped by cold west Greenland shelf Irminger Current water and Labrador Sea Water, with water (Ͻ2.9ЊC) and the cyclonic (cool) feature is capped temperature and salinities at the depth of ¯oat 404 of by somewhat warmer waters, in a manner similar to the approximately 3.0Њ±3.1ЊC and 34.83±34.84 psu. Cuny surface temperatures of the vortex dipole shown in Fig. et al. (2002) presented a detailed upper ocean temper- 2. This is consistent with a dipole extension of ature cross-section of the Labrador Sea basin from the D'Asaro's (1988) eddy generation mechanism. In this May 1997 C.S.S. Hudson cruise along the World Ocean view, we assume that the inshore portion of the shelf Circulation Experiment (WOCE) AR7W hydrographic current contains anticyclonic vorticity, with cold surface

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FIG. 7. Temperature and pressure records from ¯oat 404. (top) Contours of temperature from the ¯oat pro®les. The solid line near 375 dbar is the pressure recorded by the ¯oat between pro®les. (bottom) The temperature recorded by the ¯oat between pro®les. The pro®le numbers are given between the two panels. shelf waters over warm Irminger Sea Water. Likewise, four weeks the ¯oat made large (50±100 km) meanders the offshore portion of the current contains cyclonic in the northeastern Labrador Sea at speeds of 30±70 cm vorticity, with warmer (not pure shelf) water over cooler sϪ1 until being caught in a cyclonic eddy. The eddy was (not pure Irminger) water. As the jet or squirt leaves the nearly stationary at 60ЊN, 50ЊW for almost four weeks shelf, the dipole vortex forms with each eddy containing and had started to move northwest when the ¯oat mis- its source vorticity and water type. Thus, we might ex- sion ended on 29 July 1997. Removing the mean motion pect ¯oats caught in cyclones to contain waters slightly of the feature (in the same manner as described earlier cooler than the more pure Irminger Sea Water found for ¯oat 404) revealed an eddy approximately 10 km in along the west Greenland shelf. diameter (Fig. 9a) with a radial structure consistent with Ϫ1 a 10-km Rmax and 22 cm s Vmax (Fig. 9b), which results b. Float 402 in a mean core vorticity Rossby number of 0.35. The Float 402 was deployed on 31 May 1997 at 60.06ЊN, radial distance of the ¯oat from the eddy center varied 52.47ЊW at a nominal depth of 375 dbars (Fig. 8). For from 10 to 15 km (Fig. 9c), while the azimuthal velocity

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FIG. 8. Trajectory of ¯oat 402. Float pro®les occurred at 7-day intervals, and are labeled along the trajectory. Small dots denote each navigational ®x and are given three times per day. The * marks the launch position.

Ϫ1 ranged from 10 to 30 cm s (Fig. 9d). The value of R␨ simple uniform translation of the eddy was assumed, ranged from 0.1 to nearly 0.6 (Fig. 9e). and a speed of u ϭϪ8cmsϪ1 and ␷ ϭϪ2.5 cm sϪ1 The ¯oat pro®les (Fig. 10) show an initial warming produced the best empirical eddy motion over the ®nal at 400 dbar, and a continual warming thereafter of the two loops (Fig. 12a). Using 5-km radial bins, an eddy upper 150 dbar. The thermal strati®cation at the level structure consistent with a Rmax of 25 km and a Vmax of of the ¯oat was highest at pro®le 2 (ϳ4.2 ϫ 10Ϫ3ЊC 42 cm sϪ1 was found (Fig. 12a), resulting in a mean Ϫ1 m ), decreased by a factor of 2 by pro®le 4 (2.6 ϫ core R␨ of Ϫ0.27. Over the short time the eddy was 10Ϫ3ЊCmϪ1), then decreased by another factor of 2 by monitored, the radial distance of the ¯oat from the eddy pro®le 5 (1.0 ϫ 10Ϫ3ЊCmϪ1), marking the entrainment center varied from 15 to 30 km, its azimuthal velocity Ϫ1 of the ¯oat into the eddy. A decrease of strati®cation varied from 20 to 40 cm s , and its core R␨ varied from would be expected, if the ¯oat was entrained into the 0.15 to 0.30 (Figs. 12c±e). core of an anticyclonic eddy, but the ¯oat should ex- During the interval between the ®rst and second pro- perience an increase of strati®cation entering the core ®les (not shown), the ¯oat transitions from a regime of a cyclonic subthermocline eddy. Above the level of where cooler waters (3.1ЊC) were overlying warmer wa- the ¯oat, near 200 dbar, the thermal strati®cation does ters at the ¯oat level (4.9ЊC) along the shelf break to a increase from pro®le 2 to pro®le 5. This suggests that more homogeneous water upper layer (4.7Њ±4.8ЊC) the eddy is centered at that shallower level, and that the slightly off the shelf. The temperature signature was level of the ¯oat was coincident with the reduced strat- likely from Irminger Sea Water, and both this eddy's i®cation below the cyclonic core. The ¯oat temperature warmer temperature and negative vorticity were con- indicated that the eddy contained mixed Irminger Sea sistent with the generation mechanism discussed earlier Water. in this section. c. Float 395 d. Surface signature of Labrador Sea eddies Float 395 was deployed on 1 June 1997 at 61.23ЊN, In order to directly compare SSH (␩) perturbations 50.38ЊW at a nominal depth of 375 dbar (Fig. 11). Al- as measured by TOPEX/Poseidon with the eddies most immediately the ¯oat made an anticyclonic loop tracked by ¯oats, several assumptions were made. First, near the shelf break, moved westward and made two that there was little vertical shear between the level of more anticyclonic loops before surfacing on 20 June the ¯oat and the sea surfaceÐthat is, that the ¯ow was 1999. Since the ¯oat trajectory was relatively short, a nearly barotropic and that the velocity structure as mea-

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FIG. 9. Summary of the eddy characteristics of ¯oat 402. (a) Trajectory of the ¯oat relative to the translating center of the eddy. (b) Azimuthal velocity as a function of radial distance from the center of the eddy. Large black dots are average velocity in 2-km radial bins. Vertical black lines are Ϯ the error of the mean. The thick gray line is a subjective model ®t through the data. (c) Low-pass estimate of the ¯oat's radial distance from the center. (d) Low-pass estimate of the ¯oat's azimuthal velocity. (e) Low-pass estimate of the eddy's vorticity Rossby number.

sured by the ¯oats was maintained without modi®cation VV3VR V to the surface. Second, that the SSH was in cyclogeos- ␩(r) ϭ maxf ϩ maxr2 Ϫ max maxf ϩ max , trophic balance with the velocity ®eld. This relation 2gRmax΂΃ R max2g ΂2R max ΃ states that r Ͻ Rmax (4) ␩ץ ␷ 2 ␷ f ϩϭ␪ g . (3) VR22 f VR (r ␩(r) ϭϪmax max Ϫ max max , r Ͼ R , (5ץ ␪ r gr΂΃4r4 max The model velocity ®eld for the eddy assumed here assuming the conditions that ␩ → 0asr → ϱ and the is the same as was used in the eddy descriptions above solutions are continuous at r ϭ Rmax. Only one TOPEX/ [Eqs. (1) and (2)]. Using those velocity relations in (3) Poseidon pass was found that intersected the core of a and solving for ␩, one gets ¯oat-tracked eddy. The eddies are small (Rmax Ͻ 25 km)

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FIG. 10. Temperature and pressure records from ¯oat 402. (top) Contours of temperature from the ¯oat pro®les. The solid line near 375 dbar is the pressure recorded by the ¯oat between pro®les. (bottom) The temperature recorded by the ¯oat between pro®les. The pro®le numbers are given between the two panels. compared with the separation distance between ground by the RAFOS ¯oats are of the same type as observed tracks (Ͼ100 km), and the TOPEX/Poseidon repetition by the ATSR and by TOPEX/Poseidon altimetry. The cycle is 9.91 days, which allows an eddy 100 km across cross-sectional pro®le of SSH has a slightly steeper traveling at 10 cm sϪ1 to easily pass among the TOPEX/ slope than the modeled ␩, which may be due to the Poseidon ground tracks between cycles. A comparison simplistic eddy structure function chosen. The locations of SSH from TOPEX/Poseidon and ␩ estimated from of the measured and modeled maximum SSH anomaly (4) and (5) and the model eddy parameters obtained for fall within 5 km of each other. ¯oat 404 is shown in Fig. 13. The mean of the TOPEX/ Poseidon data modeled ␩ ®eld was set equal to the mean 5. Discussion of the modeled ␩ ®eld. The modeled SSH agrees with the TOPEX/Poseidon elevation anomaly with magni- Data from pro®ling RAFOS ¯oats, TOPEX/Poseidon tudes over 150 mm, suggesting that the eddy did have altimetry, and the alongtrack scanning radiometer a surface velocity signature and that the eddies tracked (ATSR) aboard ERS-1 have been used to describe the

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FIG. 11. Trajectory of ¯oat 395. Float pro®les occurred at 7-day intervals, and are labeled along the trajectory. Small dots denote each navigational ®x and are given three times per day. The * marks the launch position. spatial and seasonal patterns of eddy variability in the ulate that at depth the eddies were actively transporting Labrador Sea. All the measurement methods show eddy and stirring Irminger water into the Labrador Sea in- activity where the 3000-m isobath separates from the terior. This mechanism may participate in the rapid sea- west Greenland shelf at 61.5ЊN, 52ЊW. Data from three sonal restrati®cation of the Labrador Sea following con- pro®ling RAFOS ¯oats were used to describe three dis- vection. tinct west Greenland shelf eddies in detail. One of the The SSH variability has an annual cycle and peaks sampled eddies was anticyclonic, while the other two from January through March. If the levels of variability were cyclonic. Peak azimuthal velocities ranged from are entirely dependent on eddy formation from the West 22 to 42 cm sϪ1, and radii from 10 to 25 km. One eddy Greenland Current, then the stability of the current must was found to have a SSH signature of over 150 mm as also have a strong seasonal dependence. Greatbatch et measured by TOPEX/Poseidon altimetry and as inferred al. (1990) noted that in March the wind stress anomaly from combining subsurface (375-m depth) RAFOS ¯oat is in the opposite direction of the West Greenland Cur- orbital velocities with a simple model of eddy structure. rent, while in July the stress is aligned in the direction A very sharp east±west surface temperature gradient of the current. The wind stress anomalies (although not exists along the west Greenland shelf, where the tran- the mean stress) are weak the rest of the year. The cur- sition between the cold shelf waters and and warmer rent also undergoes changes along its path; Krauss Labrador Sea surface waters (a change of at least 4ЊC) (1995) deployed surface drifters drogued to 100 m in can occur within a few kilometers. That sea surface the East Greenland Current. The drifters rounded Cape temperature gradient broadens signi®cantly in the north- Farewell and continued northward in the West Green- ern Labrador Sea, and that broadening might be caused land Current. The drifter velocities at 60.5ЊN were di- by the stirring action of the upper-ocean eddies and the rectionally steady with a maximum velocity of 73 cm transport of cold shelf waters hundreds of kilometers sϪ1.By63ЊN the velocity had dropped to 42 cm sϪ1, into the interior of the Labrador Sea. The subsurface and the trajectories showed meandering and irregular waters, at least down to 375 m and likely deeper, are motions. being stirred as well. At that depth the warm (3.5Њ± A second maxima in SSH variability occurred in the 5.5ЊC) and saline (34.9±35.0 psu) Irminger Sea Water center of the Labrador Sea at 58ЊN, 52ЊW and has an is found as part of the West Greenland Current between equally strong annual cycle. However, the central Lab- the shelf break and the 3000-m isobath (Lazier 1973). rador Sea variability peaks in April through June, and The pro®ling RAFOS ¯oats were in both mixed and a time series analysis shows the peaks to occur ap- more pure Irminger Sea Water regimes, and we spec- proximately 50 days after the west Greenland shelf site.

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FIG. 12. Summary of the eddy characteristics of ¯oat 395. (a) Trajectory of the ¯oat relative to the translating center of the eddy. (b) Azimuthal velocity as a function of radial distance from the center of the eddy. Large black dots are average velocity in 5-km radial bins. Vertical black lines are Ϯ the error of the mean. The thick gray line is a subjective model ®t through the data. (c) Low-pass estimate of the ¯oat's radial distance from the center. (d) Low-pass estimate of the ¯oat's azimuthal velocity. (e) Low-pass estimate of the eddy's vorticity Rossby number.

One possible hypothesis for the eddy variability in the i®ed Labrador Sea Water. The radius of the largest an- central Labrador Sea is that the eddies are formed as a ticyclone was 15 km, while the other three were 5 km. consequence of convection and therefore should appear The typical surface velocity signature was 10 cm sϪ1. during the later stages of the convection season, or after However, if these values are used with the cyclogeos- March. However, the most intense convection has been trophic model presented earlier, the maximum sea sur- observed nearer the Labrador shelf (Gascard and Clarke face displacement would only be 10 mm, which is well 1983; Lab Sea Group 1998; Pickart et al. 2002), where below the background SSH variability found in the Lab- the TOPEX/Poseidon data show the lowest SSH vari- rador Sea (Fig. 3). Thus the variability mapped in this ability. A counterpoint is given by Lilly and Rhines paper may not be a valid indicator of convective activity (2002), who used current meter mooring data to observe or intensity, at least in terms of middepth cold eddies. four middepth intensi®ed cold, fresh, anticyclonic ed- A more speculative hypothesis puts forth the idea that dies, with properties consistent with convectively mod- the export of Labrador Sea Water following convection

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FIG. 13. (a) TOPEX/Poseidon data locations (solid dots), Float 404 positions (open circles) and trajectory (solid lines), and location of eddy at the time of a TOPEX/Poseidon passover (large gray circle). (b) Comparison of TOPEX/Poseidon sea surface anomaly data (black line) and the modeled sea surface height from RAFOS ¯oat velocities (thick gray line). would have a seasonal dependence, and hence there vision process after a return to work following an injury. might be an increased demand for waters to replace the This work was supported by the Of®ce of Naval Re- exported mass. Perhaps the surface variability maximum search Grant N0001495110509 and the Defense Uni- in the central Labrador Sea is the result of a seasonal versity Research Instrumentation Program (DURIP) transport of eddies from the west Greenland shelf site. Grant N000149511077.

Acknowledgments. I thank John Kemp, John Bouth- illette, and Paul Bouchard of WHOI for the design and REFERENCES deployment of the sound source moorings. The chief Chelton, D. B., E. J. Walsh, and J. L. MacArthur, 1989: Pulse com- scientists of the four deployment cruises (Allyn Clarke pression and tracking in satellite altimetry. J. Atmos. and John Lazier of the Bedford Institute of Oceanog- Oceanic Technol., 6, 407±438. raphy, Robert Pickart of WHOI, and Eric D'Asaro of Clarke, R. A., and J.-C. Gascard, 1983: The formation of Labrador APL/UW) were all very helpful in allowing us ship time Sea Water. Part I: Large-scale processes. J. Phys. Oceanogr., 13, 1764±1778. and the resources to get the ¯oats in the water. I thank Cuny, J., P. B. Rhines, P. P. Niiler, and S. Bacon, 2002: Labrador Sea Rick Boyce (BIO), Mike Ohmart and Elizabeth Steffen boundary currents and the fate of the Irminger Sea Water. J. (APL/UW), and Robert Tavares (WHOI) for all their Phys. Oceanogr., 32, 627±647. assistance in the at-sea prelaunch checkout and deploy- D'Asaro, E. A., 1988: Generation of submesoscale votices: A new ment of the ¯oats. The sound source recoveries were mechanism. J. Geophys. Res., 93, 6685±6693. da Silva, A. M., A. C. Young, and S. Levitus, 1994: Algorithms and supervised by Uwe Send (Kiel) and John Lazier (BIO). Procedures. Vol. 1, Atlas of Surface Marine Data 1994, NOAA Jim Fontaine (URI) designed and built the RAFOS Atlas NESDIS 6, 84 pp. ¯oats, and Sandy Anderson-Fontana (URI) performed Elliott, B. A., and T. B. Sanford, 1986: The subthermocline lens D1. all the data recovery from ARGOS and ¯oat tracking. Part I: Description of water properties and velocity pro®les. J. Phys. Oceanogr., 16, 532±548. The ATSR image data was provided by Chris Mutlow Fedorov, K. N., and A. I. Ginsburg, 1986: ``Mushroom-like'' currents of the Rutherford Appleton Laboratory, United King- (vortex dipoles) in the ocean and in a laboratory tank. Ann. dom. The TOPEX/Poseidon data was provided by Ms. Geophys., 86, 507±516. Charmaine King of the Center for Global Change Sci- Gascard, J.-C., and R. A. Clarke, 1983: The formation of Labrador ence, MIT. The NAO index data was obtained from the Sea Water. Part I: Mesoscale and smaller-scale processes. J. Phys. Oceanogr., 13, 1779±1797. Joint Institute for the Study of the Atmosphere and Greatbatch, R. J., B. de Young, A. Goulding, and J. Craig, 1990: On Ocean at the University of Washington. I thank Robert the in¯uence of local and North Atlantic wind forcing on the Pickart and two anonymous reviewers for very helpful seasonal variation of sea level on the Newfoundland and Lab- improvements to the text, and Tom Rossby (URI) pro- rador Shelf. J. Geophys. Res., 95, 5279±5289. Han, G., and M. Ikeda, 1996: Basin-scale variability in the Labrador vided numerous comments and suggestions throughout Sea from TOPEX/POSEIDON and Geosat altimeter data. J. Geo- this study. Robert Marshall (URI, now University of phys. Res., 101, 28 325±28 334. Kentucky) helped me balance and focus the paper re- Hebert, D., N. Oakey, and B. Ruddick, 1990: Evolution of a Medi-

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