An Arctic Ocean cold core eddy

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. C10, PAGES 23,997 24,006, OCTOBER 15, 2000 An Arctic Ocean cold core eddy Robin D. Muench and John T. Gunn Earth and Space Research, Seattle, Washington Terry E. Whitledge School of Fisheries and Ocean Sciences, University of Alaska at Fairbanks Peter Schlosser and William Smethie Jr. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York Abstract. Detailed physical and chemical observations were obtained during September 1997 of a cold core eddy situated in the southern central Canada Basin of the Arctic Ocean. The eddy was about 20 km in diameter, had maximum current speeds exceeding 20 cm s 1, and extended from the base of the upper mixed layer, near 40 m, down to 400 m. Excess salt in the eddy core was consistent with addition of brine from 1 mof ice formation. Core tracer distributions indicated a lifetime exceeding 1 year and were consistent with an origin as near-surface Pacific water with some admixed terrestrial runoff. The eddy was probably formed in association with a polynya along the Alaskan Chukchi Sea coast through local water densification from surface ice formation followed by development of frontal instabilities. Formation and subsequent migration of such eddies, which may have lifetimes of several years, provide a mechanism for transporting water from the Beaufort and Chukchi Sea coastal and shelf regions into the interior Canada Basin and for contributing to maintenance of the permanent halocline. Present light ice conditions throughout the Arctic Ocean favor the formation of such eddies and may contribute to enhanced ventilation of the halocline waters. 1. Introduction This paper discusses physical and chemical observations that were made during September 1997 of a cold core eddy in the Canada Basin of the Arctic Ocean. Presence of this feature has implications with respect to two regional issues: freezinginduced water mass modification within the Arctic Basin and transport of the modified waters from the Chukchi and Beaufort coastal regions into the Canada Basin interior. Arctic waters communicate with the world ocean via Fram and Bering Straits, over the Barents Sea shelf, and through channels in the Canadian Arctic Archipelago. These waters are modified within the Arctic through admixture of shelf water that has been conditioned by terrestrial freshwater input, cooling, freezing, and tidal and wind mixing. Lateral material and heat transports below the permanent halocline are dominated by advection via currents that coincide with the steep peripheral continental slopes and the cross-basin ridge systems. This circulation scheme was first conceptualized by Aagaard [1989]. Details were later added by Rudels et al. [1994], and much of the scheme has been validated using field data from the Eurasian Basin [Schauer et al., 1997; Frank et al., 1998] and from the Canada Basin [McLaughlin et al., 1996; Smethie et al., 2000]. A numerical model by Nazarenko et al. [1998] has reproduced the essential parts of this circulation, suggesting that we understand the basic mechanisms. While the mean peripheral circulation is reasonably well understood, transports of water, heat, and dissolved material Copyright 2000 by the American Geophysical Union. Paper number 2000JC /00/2000JC000212$09.00 from the Arctic Ocean periphery into the interior basins have remained uncertain as to both mechanism and rate. Results from recent tracer studies have provided rate estimates, subject to assumptions concerning mixing, for this transport [Frank et al., 1998; Smethie et al., 2000; Smith et al., 1999]. The issue is of interest because lateral salt transport into the central basins is generally assumed to be necessary for maintenance of the permanent halocline [Aagaard et al., 1981]. One possible mechanism for this transport is by way of eddies that form on the surrounding shallow shelves or within the relatively energetic peripheral currents and subsequently migrate seaward into the central basins. Eddies, primarily anticyclonic and having both warm and cold cores, have been documented previously in the Canada Basin [Newton et al., 1974; Manley and Hunkins, 1985; D Asaro, 1988a]. Such eddies may form through instability in the Alaskan Coastal Current where it rounds Point Barrow [D Asaro, 1988b] and then migrate north into the Canada Basin. It has also been speculated, on the basis of tracer analyses, that eddies might form where the peripheral currents round the sharply curving topography of the northeastern Chukchi Cap and subsequently migrate into the interior Canada Basin, carrying with them the materials dissolved in their core waters [Smethie et al., 2000; Smith et al., 1999]. It is difficult to assess the significance of these sources because we can only speculate, in the absence of data-based evidence, on the numbers and trajectories of such eddies. Mechanisms involving densification of shelf water through brine rejection by freezing ice can also lead to eddy formation. Numerical model results have suggested that cold core eddies can be generated concurrently with downslope plume flow following extensive ice formation and subsequent convection over a shallow shelf region [Nof, 1990] and in association with 23,997

2 23,998 MUENCH ET AL.: ARCTIC COLD CORE EDDY polynyas [Gawarkiewicz and Chapman, 1995; Chapman and Gawarkiewicz, 1995; Chapman, 1999]. Shelf ice distributions have been used to infer the generation of brine adequate for plume generation in the Siberian shelf seas [Martin and Cavalieri, 1989; Cavalieri and Martin, 1994]. Finally, numerical models have been used to suggest that cold core eddies can form in situ within the central basins as a consequence of internal adjustment following freezing-induced convection [Chao and Shaw, 1996, 1998]. Such features would not necessarily contribute to horizontal transport but would locally condition the waters to the depth of penetration of the convective event. Detailed observations of eddies in the Arctic Ocean are unusual. Because the available evidence suggests that a significant eddy density is present throughout at least the Canada Basin [Manley and Hunkins, 1985; A. Plueddemann, personal communication, 1999], this may reflect the operational difficulties inherent in making ocean observations through pack ice. Horizontally closely spaced or continuous observations are needed to detect features that might be eddies, and detailed subsequent surveys are then needed in order to determine whether the features are, in fact, eddies and to assess their structures. Such operations are prohibitively difficult when attempted from surface vessels operating in pack ice. The recent use of submarines for collecting Arctic Ocean data has, however, allowed detailed horizontal mapping that is well suited for detection and study of mesoscale features such as eddies. This paper describes, and assesses some regional implications of, a cold core mesoscale eddy for which detailed observations were obtained in the Canada Basin of the Arctic Ocean during Observational Program Oceanographic investigations were made in the Arctic Ocean from the U.S. Navy submarine Archerfish (SSN 678) during August 1997 as part of the Scientific Ice Expedition (SCICEX) program. Instrumentation carried aboard Archerfish allowed continuous horizontal profiling and discrete sampling of selected parameters along the cruise track and vertical profiling of some of the same parameters at selected sites along the track. Unlike oceanographic data obtained as vertical profiles that are typically spaced kilometers apart in the horizontal, the instrumentation used on Archerfish collected upper ocean data that were at various times closely spaced or continuously recorded in both the vertical and the horizontal. These data allowed us to construct a detailed threedimensional representation of the eddy described below Instrumentation Temperature T, salinity S, and dissolved oxygen concentration DO data were recorded continuously while the vessel was underway. These data were measured using a pair of SeaBird SeaCat conductivity, temperature, and depth recorders (CTDs) that were mounted in the topmost portion of the vessel s sail. It was possible in this way to obtain detailed horizontal profiles of T, S, and DO at fixed depths that were chosen primarily for operational reasons to be either 118 m or 218 m. The CTDs were factory calibrated prior to and following the cruise. The S and DO sensors were calibrated at frequent intervals during the cruise against values derived aboard ship from water samples that were obtained using a throughhull manifold. Vertical water sample profiles were obtained of the upper ocean, at selected sites, by way of the through-hull manifold while the vessel dove in a tight spiral pattern. Samples were analyzed for S using a Guildline Portasal salinometer that was calibrated using standard seawater at the start and finish of each set of sample runs. Samples were analyzed for DO using an automated titration apparatus that was calibrated against a known standard at the start and finish of each set of sample runs. Use of the two independent CTDs allowed for continuous real-time cross calibration. After correcting for ambient vertical gradient offsets resulting from a 15 m vertical displacement between the CTDs and the sampling ports, agreement between the CTD and calibration values was generally within 0.02 in S and 0.1 ml/l in DO. Temperature was judged, on the basis of cross calibration of the two recording CTDs and on the basis of precruise and postcruise calibrations, to be accurate to 0.02 C. Additional water samples were obtained from the throughhull manifold for later analyses. Samples for inorganic nutrient analyses were immediately frozen and kept frozen until they were analyzed ashore following the cruise. Accuracy of the reported values is estimated, on the basis of replicate analyses of a subset of samples [Whitledge et al., 1981], to be 0.25 M for phosphate and nitrate and 0.5 M for silicate and ammonium and did not appear to have been significantly compromised by deterioration of the frozen samples. The 18 O samples obtained from a vertical spiral cast taken in the eddy core were analyzed at the Institute for Environmental Physics at the University of Heidelberg, Germany, using standard mass spectromatic procedures [e.g., Roether, 1970; Fairbanks, 1982]. Precision of the 18 O data is, allowing for all foreseeable analytical errors, about %. Samples for tritium and helium isotope analysis were collected in copper tubes sealed by stainless steel pinch clamps and were measured at the Lamont- Doherty Earth Observatory noble gas laboratory. Tritium data are reported in tritium units (TU, where 1 TU denotes a tritium to hydrogen ratio of ). The 3 He data are reported in delta notation defined by 3 He (R s R a )/R a 100%, where R a [Clarke et al., 1976]. Precision of the tritium data is about 2% or 0.03 TU, and that of the 3 He values is about %. Precision of tritium/ 3 He ages calculated using the tritium and 3 He data is 0.5 year. Vertical profiles of T and S were obtained between 20 and 1000 m depths at selected locations (Figure 1) using Sippican model Under-Ice Submarine-Launched Expendable Conductivity, Temperature, and Depth (UISSXCTD, abbreviated herein as xctd) probes. These probes are launched from the submerged submarine, rise under their own buoyancy to a depth of 15 m, then release from their buoyant components and free-fall. Profiles are measured during the free-fall periods when the probes remain connected to the shipboard data logger via signal cables. Probe depths are computed on the basis of a known fall rate, T and conductivity values are measured by probe sensors and returned as functions of depth, and S is subsequently computed from these values. The design accuracy of the probes is 0.02 C int, 0.05 in S, and5mindepth except for the uppermost 5 m of the record (15 20 m depths), when the probe sensors are unreliable during equilibration and the data are typically discarded. Comparison among probe results, CTD measurements, and the bottle-derived values for S has suggested that probe T and S measurements fell within the design specifications. Some of the probes were outfitted with pressure-activated switches that inserted a spike into the datastream at a depth known to within 1 m, and these verified accuracy of the depths returned using the descent rate computations.

3 MUENCH ET AL.: ARCTIC COLD CORE EDDY 23,999 Figure 1. The eddy survey track, with 118 and 218 m sail-mounted instrument depths shown as solid and dashed lines, respectively. The line A-A indicates the transect from which data were used to construct the vertical sections shown in Figures 2 and 3. Gray-filled circles show sites for xctd probe drops, solid triangles show locations of along-track water-sampling sites, and the encircled dense cluster of points near the eddy center marks the location of the eight-depth spiral cast. The inset map shows the location of the eddy site, with bottom contours at 1000 m intervals derived from the ETOPO5 database, and locations of two 1993 Pargo stations that are referred to in the text Sampling Scenario The eddy was identified in real time when anomalously low T and S values were logged by the sail-mounted CTDs during a southerly transit across the Canada Basin (Figure 1). The vessel then carried out a survey of the eddy during which continuous data monitoring ensured that the feature was being encompassed by the survey (Figure 1). Most of the survey was carried out at a sail-mounted CTD depth of 218 m, with only two of the 10 transects being run at shallower depths. Expendable probes (xctds) were deployed at the indicated locations, and a spiral cast at the indicated site obtained water samples at eight depths from 55 to 235 m. Eight other samples were obtained along legs 8 and 10 from 131 and 43 m depths, respectively. The eddy survey required 27 hours to complete. 3. Observed Features The cold core eddy was observed in the southern central Canada Basin of the Arctic Ocean 150 km north of the shelf break off Alaska (Figure 1). The regional currents have not been documented quantitatively but have been characterized qualitatively on the basis of historical water mass analyses [Aagaard, 1989]. Mean basin-wide circulation below the halocline is cyclonic and concentrated over the steep continental slopes that surround the basin. There is no evidence that significant mean currents occur in the interior Canada Basin at these depths remote from the continental slope boundaries. The upper mixed layer circulates anticyclonically, following the ice motion of the Beaufort Gyre. Currents in the halocline presumably reflect a balance between the deep and upper mixed layer circulations. No T or S anomalies that might have indicated the presence of other, similar eddies were found elsewhere along the cruise track. The vertical T and S structure of the subsurface, lenticular eddy is illustrated along a transect that passed near its center (Figure 2) and as a horizontal mapping of T at 218 m (Figure 3). Eddy radius, defined as distance from the approximate center to the point of highest tangential geostrophic current speed, was 10 km. Core T was lower than in the surrounding ambient water from near the base of the upper mixed layer ( 40 m) down to 400 m. Minimum core T occurred between about 150 and 250 m and ranged from about 1.75 C tothe freezing point, which was about 1.82 C. Salinity within the eddy was less than ambient below 185 m and greater than ambient at shallower depths. The permanent thermocline and halocline were deeper by 40 m and stronger within the eddy than outside. The T and S signatures of the eddy were not discernable shallower than 40 m or deeper than 400 m. Vertical profiles of current speed were computed using the xctd data and assuming geostrophy. A reference level of 450

4 24,000 MUENCH ET AL.: ARCTIC COLD CORE EDDY Figure 2. (top) Vertical distributions of T and S along transect A-A, indicated in Figure 1, and (bottom) vertical T and S profiles obtained within and outside the eddy using expendable probes. The vessel transited this section at a sail-mounted CTD depth of 218 m. The T and S contours above 50 m have been omitted from the contour plots for clarity of presentation. dbar was selected for these computations because the vertical gradient in geostrophic shear had approached zero by this level. Circulation was everywhere anticyclonic, and tangential speeds were greatest and exceeded 22 cm s 1 near 185 m depth. A horizontal map of the dynamic topography at 185 m relative to the 450 dbar level implies closed circulation contours and shows a concentration of the highest current speeds in a band about 5 km in radial dimension and situated 8 10 km outward from the eddy center (Figure 4). The maximum vertical shear that was associated with the eddy occurred below the current maximum between about 225 and 250 m depths. Because of the strongly curving streamlines, the speeds cited here are minimum estimates, and Manley and Hunkins [1985] note that the Canada Basin eddies should reflect a gradient wind rather than a geostrophic balance. The Rossby number Ro for the eddies is 0.1, however, which suggests that the geostrophic approximation provides a reasonable estimate of the eddy current speeds. The eddy structure, especially its baroclinicity, which is forced by S, is emphasized by the T, S, and 0 anomalies relative to the ambient surrounding waters (Figure 5). The anomaly field for each parameter was computed by subtracting a mean profile, which was constructed using the two endmost profiles on the transect, from the T, S, and 0 fields along the transect. Disregarding the upper mixed layer above 40 m, T anomaly was negative at all depths and was greatest near 230 m, whereas the S and 0 anomalies were negative below 185 m and positive at shallower depths. The depth of vanishing S and 0 anomalies fell within the lower halocline near 185 m and coincided with the depth of highest current speeds. The eddy core near 218 m depth contained markedly higher DO (nearly 7 ml/l) than surrounding water at the same depth (5.75 ml/l) (Figure 6). Vertical distributions of chemical and tracer variables were derived from the spiral cast taken near the eddy core. At depths 150 m, silicate was 40 M, nitrate was 11 M, phosphate was 1.9 M, ammonium was 0.7 M, chlorophyll a concentrations were about 7 8 ng/l, phaeopigment concentrations were 20 ng/l, and dissolved organic nitrogen (DON) concentrations were 45 M (Figure 7). The vertical gradients at depths 100 m suggest depletion of nutrients in the upper ocean by productivity processes, consistent with relatively high Figure 3. Distribution of T at 218 m depth, constructed using all available data. Dotted lines show track segments along which data were recorded, and circles show sites of expendable xctd probe launches.

5 MUENCH ET AL.: ARCTIC COLD CORE EDDY 24,001 Figure 4. (left) Dynamic height of the 185 dbar surface relative to 450 dbar, with a contour interval of 1 dynamic centimeter, and (right) vertical distribution of baroclinic geostrophic current speed referenced to 450 dbar along transect A-A, with positive current speeds directed into the page. concentrations of DO (Figure 6) and chlorophyll a (Figure 7). The 18 O values deeper than 100 m in the core varied from about 1.5 to 1.57 parts per million (ppm) (Figure 8a). Tritium values varied from 1.9 to 2.8 TU and decreased with increasing depth (Figure 8b). The 3 He values varied between 1.1 and 0.4% and showed no significant depth-related trend (Figure 8c). Corresponding tritium/ 3 He ages are 2.8 years at 152 m depth, 2.8 years at 183 m, and 1.3 years at 235 m (Figure 8d). 4. Discussion Some insight concerning a source region and possible generation mechanisms for the cold eddy are essential if we are to assess its significance to the regional oceanography. To this end, we first examine the core chemical and tracer properties Source Region We first assess the distributions of inorganic nutrients and dissolved oxygen, moving on subsequently to tracer concentrations. The silicate, nitrate, phosphate, and ammonium concentrations were typical of values observed in prior studies on the Chukchi and Beaufort Sea shelves [Cooper et al., 1999]. Maximum vertical gradients in nutrient concentration between 50 and 100 m probably reflected a decrease in biological uptake with increasing depth as light limitation became significant and are consistent with data collected during August, a time of peak primary production, of earlier years [Parrish, 1987; Harris, 1992]. The derived parameters NO and PO [Broecker, 1974] were and M in the eddy and 811 and 955 M in the surrounding water, respectively. The resulting NO/PO ratios within the eddy were These ratios compare favorably with those on the Bering shelf [Cooper et al., 1999], consistent with partial origin of eddy core waters from the Bering Sea. Chlorophyll a and DON concentrations were higher in the eddy core than outside, as were those for dissolved organic matter, including phaeopigments and lignin phenols (not shown). These concentrations are consistent with the presence in the core of older water in which remineralization of particulate organic matter had occurred and may reflect partial derivation of the eddy core water from terrestrial sources [Opsahl et al., 1999]. Figure 5. Eddy (left) T, (middle) S, and (right) density (as 0 ) anomalies (see text) relative to the ambient waters along transect A-A, with contour interval of 0.1 unit for each variable. Regions of negative anomaly are shaded, and zero contours and contours for the uppermost 70 m have been omitted for clarity of presentation.

6 24,002 MUENCH ET AL.: ARCTIC COLD CORE EDDY Figure 6. Dissolved oxygen concentration in ml/l derived from continuously recording sail-mounted instruments along two sequential transects at 218 m depth through the eddy. Oxygen values are indicated by the bold gray trace, and the bold black trace shows the corresponding cruise tracklines on the x-y plane. The eddy core is marked by high values that showed very little spatial variability. The 18 O values measured in the eddy (about 1.5 ppm) are consistent with those which have been observed in the Bering Strait and on the Chukchi shelf (about 1 to 1.5 ppm) [e.g., Cooper et al., 1997; Schlosser et al., 1999; B. Ekwurzel et al., River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean, submitted to Journal of Geophysical Research, 2000 (hereinafter referred to as Ekwurzel et al., submitted manuscript, 2000)]. The same holds for the tritium concentrations, as values in the eddy (about TU) were close to those reported in the Chukchi Sea by Ekwurzel et al. (submitted manuscript, 2000). The tritium/ 3 He ages in the eddy ( years) were also in agreement with those observed in the Chukchi (Ekwurzel et al., submitted manuscript, 2000). Their elevation above zero, a measure of the isolation of these waters since equilibrium of Figure 8. Vertical distributions of (a) 18 O; (b) tritium; (c) 3 He; and (d) the corresponding tritium/ 3 He ages derived from water samples collected within the eddy core (solid diamonds) and from two stations occupied in the same part of the central Canada Basin during 1993 (solid squares and circles). 3 He with the atmosphere, is probably related to the addition of some water containing high tritiogenic 3 He concentrations such as those found above the eddy (Figure 8c) as well as to aging of the eddy waters since its formation. The tritium/ 3 He ages place a firm upper limit of 2 years on the mean age of the core waters because mixing and consequent dilution of the eddy core by any other water type would have increased the 3 He concentration in the eddy and resulted in higher tritium/ 3 He ages. Figure 7. Vertical distributions of (a) T; (b) S; (c) phosphate, nitrate (as N N, or nitrate nitrite) and ammonium; (d) silicate; (e) dissolved organic nitrogen; and (f) chlorophyll a measured at the spiral cast site near the center of the eddy. Note that the x axis scaling in Figure 7c deemphasizes the vertical variability in PO 4 relative to N N and NH 4.

7 MUENCH ET AL.: ARCTIC COLD CORE EDDY 24,003 If we compare the tritium/ 3 He values from the eddy with those measured at stations 13 and 14 in the central Canada Basin during the 1993 cruise of the submarine Pargo (for locations, see Figure 1), significant differences emerge (Figure 8). Tritium values at stations 13 and 14 over the depth range m were significantly higher than those in the eddy. The tritium concentrations observed at Pargo station 13 over the same depth range as the eddy were 4.4 and 4.7 TU (Figure 8b), equivalent to 3.5 and 3.8 TU if corrected for radioactive decay between 1993 and 1997 (TU97). These values were nearly a factor of 2 greater than those measured in the lower part of the eddy. This means that the waters observed at Pargo station 13 contained a larger fraction of river runoff compared to almost pure Pacific water in the eddy. Only one tritium measurement is available within the eddy core depths from Pargo station 14. This measurement (3.2 TU, Figure 8b) is lower than the values at station 13 but still higher than the tritium values observed in the eddy (2.6 TU as compared to 1.9 TU in the lower water column of the eddy). The same features are apparent in the 3 He values. Whereas 3 He values in the eddy were 0% and close to the equilibrium solubility of 1.8%, those observed at Pargo stations 13 and 14 were about 36 and 17%, respectively (Figure 8c), indicative of a much longer isolation period for these waters. The tritium/ 3 He ages support this conclusion, as values within the eddy were 1.3 to 2.8 years, much lower than the 20 and 16-year ages observed at Pargo stations 13 and 14, respectively (Figure 8d). This is a clear indication that water within the eddy had been in equilibrium with the atmosphere recently (i.e., within the previous few years), probably on the Chukchi shelf. The high tritium/ 3 He age values at the Pargo stations suggest, conversely, that the ambient central basin waters had significantly longer mean residence times. To summarize, the chemical and tracer distributions that were associated with the eddy were consistent with source waters derived in part from near-surface Pacific Ocean waters and in part from terrestrial input. Pacific waters enter the region via Bering Strait, from where they advect northward across a broad expanse of the Chukchi shelf, while terrestrial waters can enter either admixed with Pacific water via Bering Strait or from the Siberian shelves. This combination of Pacific and terrestrial waters can be expected to occur over a broad expanse of the Chukchi and Beaufort Sea shelves, and we conclude that the eddy likely originated from these shelves. This origin from shallow shelf sea regions is consistent with relatively recent equilibration of eddy core waters with the atmosphere and hence substantiates the notion that such eddies can ventilate the Arctic pycnocline Some Possible Source Mechanisms Focus upon a more specific probable source for the eddy requires consideration of possible generation mechanisms. Vertical S profiles derived from the xctd data were used to estimate the difference in water column salt content inside and outside the eddy core (see Figure 2). The upper depth for the computation was assumed to be 40 m, at the base of the upper mixed layer where maximum vertical S and 0 gradients were present and above which no trace of the eddy persisted. The lower depth was taken to be 400 m, where eddy T and S signatures had become undetectable. Vertical integration of S from m yields values of within the eddy and in the surrounding waters. The difference of 0.05 can be accounted for in equivalent brine rejection from formation of 1 m of new ice, where surface water S 30 and newly formed ice S 10 are assumed. This would require, for example, that an average of 10 cm of new ice form per day over a 10 day period. An ice formation rate of 10 cm d 1 is readily attainable in open water under winter conditions and was parameterized as a field-data-based brine rejection rate by Muench et al. [1995] of kg salt s 1 overa1m 2 column in a freezing open lead. In order for adequate brine rejection to occur, open water must be present throughout the 10 day period, because once even a thin layer of ice forms, the freezing rate drops precipitously. Extended open water periods occur in specific areas that are referred to as polynyas [Smith et al., 1990] and can also be expected to occur in association with active lead fields. Polynyas have been observed to remain open along the northeastern Chukchi Sea coast, and the associated ocean processes were documented during winter [Weingartner et al., 1998]. Ice concentration has decreased markedly from that time to 1997 [McPhee et al., 1998]. We posit a high likelihood that sufficient open water ice formation occurred to generate a salt surplus equivalent to that observed in the eddy. While the magnitude of the S anomaly associated with the eddy core can be accounted for by reasonable freezing scenarios, a mechanism is needed through which the associated density anomaly can resolve itself into an eddy. Several such mechanisms have been proposed on the basis of the results of model studies. These mechanisms involve some combination of boundary current instabilities and convection driven by surface freezing, and we examine them in light of currently available information. Chao and Shaw [1996] have proposed a mechanism for open ocean eddy formation (i.e., remote from lateral boundaries) based on a numerical model for which the initial conditions impose a near-surface S anomaly on a stratified water column. Their mechanism utilizes a surface brine source to produce a cyclonic upper layer eddy which leads through an internal adjustment process to formation of an underlying anticyclonic eddy. The upper, cyclonic eddy decays through frictional interaction with the ice cover, leaving only the deeper, anticyclonic eddy. While their model produces results that closely resemble the observed cold core eddy, the large S anomalies (2.0) used to force the model are unlikely to occur in a natural system. Results obtained during the winter 1992 Arctic Leads Experiment showed that S enhancement under freezing leads was typically O(0.1) or less [Muench et al., 1995; Morison and McPhee, 1998]. Upper layer advection and mixing, neglected in Chao and Shaw s [1996] model, were significant in the field and often quite large, and brine released from new ice formation was quickly mixed vertically and laterally through the upper mixed layer. Lead systems like those that occur in the region where the eddy was found often have high associated ice speeds and strong upper layer turbulence. It therefore seems unlikely that open ocean brine formation can generate a sufficiently large and localized salt excess to generate an eddy such as observed. Further, tracer results are consistent with a remote origin. We conclude that generation of the observed cold core, anticyclonic eddy through open ocean convection using the Chao and Shaw [1996] mechanism was extremely unlikely. Another possible mechanism for generating a cold core eddy would invoke frictional spinoff from a coastal or shelf break current as postulated by D Asaro [1988b] for the eddies that originate from the Alaskan Coastal Current off Point Barrow. Eddies might conceivably enter the Canada Basin, through this

8 24,004 MUENCH ET AL.: ARCTIC COLD CORE EDDY mechanism, at the point where the peripheral slope current rounds the sharp northeastern corner of the Chukchi Rise. This presupposes that the shoreward portion of the peripheral current contains winter-conditioned shelf water, with a significant admixture of Pacific water, that might be incorporated into eddies. In this region we would expect the slope current to flow adjacent to shelf waters that had been conditioned by freezing, and eddy generation might entrain such water. The mechanism presupposes the presence of a narrow, intense, frictional boundary flow. In the case of eddy generation north of Alaska, this flow was associated with a coastal current originating from the Bering Sea. There is no evidence supporting the presence of a similarly energetic and concentrated current in the vicinity of the Chukchi Rise. We would not expect, either, to find a large concentration of Pacific water associated with the slope current rounding Chukchi Rise. We conclude that this mechanism, while plausible for eddy generation off Point Barrow, is unlikely farther west. A third possible mechanism for eddy generation invokes the dynamics of dense water flow following coastal freezing events, and a sequence of models has been developed that assess these dynamics. First, Gawarkiewicz and Chapman [1995] developed a model for dense water circulation associated with a polynya adjacent to a straight coastline and overlying a gently sloping, regular bottom. This model was run until a steady state solution was attained. The model was then modified by Chapman and Gawarkiewicz [1995] to incorporate more realistic bottom topography, with an added cross-shelf canyon. Most recently, Chapman [1999] has modified the model to include nonsteady forcing, which represents reasonably well the variable meteorological conditions that typically force a polynya. Each of the models in this sequence generates cyclonic and anticyclonic eddies through growth of instabilities along a frontal structure that develops between the cold, dense water that underlies the polynya and waters outside the polynya area. These modeled eddies are baroclinic, and their cores consist of the cold, dense waters that are formed within the polynya. They are O(20) km in diameter and have associated tangential current speeds O(10) cm s 1. The density anomaly associated with the eddy core is always 1.0 sigma units. An eddy having these characteristics would bear a strong resemblance to the observed cold core eddy, particularly given that tangential speeds would presumably have been increased through vorticity conservation as the eddy migrated from the shallow coastal region into deeper water. We conclude that this is a likely mechanism for formation of the observed eddy and further posit on the basis of the above water chemistry and tracer results that this formation occurred along the Alaskan Chukchi Sea coast. Much of the Chukchi Sea shelf is ice-free during summer, especially during recent years [McPhee et al., 1998], and these open water areas allow new ice formation and consequent shelf water densification during autumn and early winter. Satellite Special Sensor Microwave Imager (SSMI) data show that the open water was unusually widespread, for example, in late summer Later in the winter, the northeastern Chukchi Sea coastal region is a polynya site, which can likewise lead to shelf water densification [Weingartner et al., 1998]. The Chukchi shelf contains a mixture of waters that includes both upper layer Pacific Ocean water, especially in the northeast, that has entered via Bering Strait and terrestrial runoff primarily from either Siberian rivers or the Yukon River (via Bering Strait) Eddy Lifetime The tritium/ 3 He ages indicate a maximum mean residence time for the eddy core waters of 1.3 and 2.8 years, probably closer to 1.3 years for the deeper part of the eddy. It is not possible to estimate a lower limit, because, in theory, it can be close to zero. This is similar to eddy lifetimes derived from physical considerations by Ou and Gordon [1986], who estimated spin-down times exceeding 1 year for Canada Basin eddies. Their estimate was based on a rapid (of the order of a few days) initial spin-down of the upper mixed layer through frictional interaction of the ice cover followed by entrainment of ambient water leading to a much longer term decay (of the order of a year or longer) of that portion of the eddy at and below the pycnocline. Their parameter values, based on stratification, vertical and lateral eddy dimensions, and eddy current speeds, were virtually identical to those for the eddy we describe here. We therefore assume their spin-down period of 1 year to be applicable to our case. Other considerations suggest that such eddies are long-lived features. The vertical mean Richardson number Ri N 2 /S 2 was estimated as 10 throughout the shear layer underlying the eddy core. Even given that this is quite a rough estimate made in the absence of high resolution current, T and S observations, the value far exceeds the critical number of 0.25, below which turbulent dissipation can become appreciable (see, for example, the discussion by Polzin [1996]). The large value for Ri therefore can be taken to indicate a lack of dissipation underlying the eddy core and is consistent with a long-lived feature. Recent theoretical studies (as summarized, for example, by Paldor [1999]) suggest that a strongly barotropic eddy may decay through dynamic instability processes. However, the Arctic Ocean eddies such as we discuss are strongly baroclinic. Their structures are reminiscent of the eddies found in the North Atlantic that are derived from Mediterranean outflow water ( Meddies ). These decay in part by interleaving along isopycnal surfaces followed by double diffusion [May and Kelley, 1997], which suggests a generally low level of turbulent decay and correspondingly long lifetimes. Several of the xctd traces, especially those from the outer periphery of the eddy, show traces of interleaving. These data were, however, of inadequate quality for identification of double diffusive features such as T-S staircases. A more detailed analysis of eddy decay is beyond the scope of the present work. We conclude on the basis of the core tracer results and these limited dynamical considerations that the life of an eddy such as we have described exceeds 1 year Significance Mechanisms have been proposed for maintenance of the Arctic halocline whereby water is conditioned at the continental margins and subsequently advected laterally into the basins [Aagaard et al., 1981] or else is conditioned in situ by vertical convection [Rudels et al., 1996]. Our observations support the former scenario, as the observed eddy contained a volume of water that we presume to have been conditioned by freezing on the shelf. Eddies have a significant local impact on the halocline, which is cooled and sharpened within the eddy core as compared with surrounding water at the same depths (Figure 9), and some volumetric estimates are interesting. Given a 20 km diameter and a 200 m depth for the conditioned core water (Figure 2), and allowing for its lenticular form, we estimate a

9 MUENCH ET AL.: ARCTIC COLD CORE EDDY 24,005 Figure 9. Sharpening of the halocline by eddy core waters in T-S space. The figure was constructed by plotting 1-m subsampled data from four xctd casts within the core (gray points) and four in ambient water completely outside the eddy (black points). Transitional data taken between the eddy core and the ambient waters filled the intervening T-S space and have been omitted for clarity. The freezing point line T f is shown for reference and shows that little heat has apparently diffused into the eddy core since its formation, presumably at or near T f. The 75 m and 200 m depths are shown for reference. core volume of about m 3. (This volume was probably larger directly following formation of the eddy, since some of the core has presumably been dissipated through mixing processes since eddy formation.) For comparison, a coastal polynya might have an offshore extent of 5 km, an alongshore length of 400 km, and a bottom depth of 20 m for a total underlying volume of about m 3. These numbers are based on length of the nearly straight, northernmost portion of the Alaskan Chukchi Sea coastline, water depths within 5 km of the coast, and offshore dimensions typical for a wind-forced, coastal polynya [Smith et al., 1990]. If the entire water column underlying the polynya becomes conditioned through brine addition, then a single 10 day polynya event produces a volume of water approximately sufficient to form a single eddy of the sort that we observed. Even given the uncertainty in these estimates, a coastal polynya event appears capable of producing a volume of freshly ventilated water comparable to that of an eddy. If the observed eddy originated along the Alaskan Chukchi coast during the preceding early winter 10 months prior to our observations, then it migrated northward at an effective speed near1cms 1 with an associated transport of about m 3 s 1. This can be compared with an annual production of shelfconditioned water estimated by Cavalieri and Martin [1994] to be m 3 s 1 in the western Arctic, with much of the production coming from the Alaskan Chukchi Sea shelf. If eddies have been the sole means by which water exits the shelves, volume considerations would dictate annual formation and migration of 250 eddies. Manley and Hunkins [1985] detected 126 different eddies during the course of an ice station drift, but an unspecified proportion of these were warm rather than cold core. It is not unreasonable on the basis of these observations to expect a few hundred cold core eddies to be present in the Canada Basin at any given time, and ongoing efforts at an eddy census will improve these estimates (A. Plueddemann, WHOI, personal communication, 1999). Given hypothetical eddy lifetimes exceeding a year, this number is not inconsistent with an annual formation rate of a few hundred such eddies. These numbers remain, however, uncertain pending ongoing work, and our intention here is to support the hypothesis that cold core eddies can contribute significantly to offshore transport of shelf-conditioned waters. The posited level of eddy contribution to halocline maintenance leads us to speculate concerning a plausible feedback mechanism. Assuming that eddies are formed primarily in open water polynyas, then numbers of eddies should increase as open water areas increase. The increased number of eddies would strengthen the halocline. Since the halocline helps maintain the Arctic pack ice cover by limiting upward heat flux from the warm, deeper ocean, strengthening of the halocline would decrease upward heat flux and contribute to an increased ice cover. The increased ice cover would, in its turn, reduce the opportunities for eddy formation. It is interesting, in this context, that we are now in a period of perhaps the lowest ice cover yet observed as to both thickness and extent of multiyear ice. We have no data from which to determine whether the formation rate or population of eddies is greater at present than in past years. Further, it is possible that even in periods of open water the shelf convection products sink to form deep downslope density flows rather than forming eddies, in which case the deep ocean would be ventilated instead of the halocline. Further speculation exceeds the bounds of our data. 5. 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