Cold-core anomalies at the subantarctic front, south of Tasmania

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Deep-Sea Research I 51 (2004) 1417–1440 www.elsevier.com/locate/dsr

Cold-core anomalies at the subantarctic front, south of Tasmania Rosemary Morrowa,, Jean-Rene Donguyb, Alexis Chaigneaua, Stephen R. Rintoulc a

LEGOS, 18, av. Edouard Belin, 31401 Toulouse Cedex 9, France b Le Bas Bout, 22490, Plouer-sur-Rance, France c CSIRO Marine Research and Antarctic Climate and Ecosystems Cooperative Research Centre, GPO Box 1538, Hobart, TAS 7001, Australia Received 29 July 2003; received in revised form 27 April 2004; accepted 8 July 2004 Available online 27 September 2004

Abstract Eight years of altimetry and hydrographic data are used to examine the formation, propagation and vertical characteristics of cold-core rings formed north of the Subantarctic Front, SAF, in the region south of Tasmania. Altimetry allows us to follow the spatial–temporal evolution of these cold, low sea level anomalies. Most of these coldcore rings are formed from unstable meanders of the SAF, and the interaction of the meanders with bathymetry appears to influence the eddy spawning. The interaction of a westward propagating cold-core eddy with an eastward propagating equatorward meander can also trigger a cold-core eddy spawning event. Hydrographic sections show that these eddies have cool, low-salinity cores reaching to at least 1500 m depth. During summer, their surface temperature signal is eliminated only 2–3 weeks after spawning, however, their surface salinity signature is maintained for 2–3 months. We estimate that these cold-core eddies could contribute an annual heat deficit of
3.8  1019 J, and an annual salt deficit of
1.6  1012 kg, and contribute to cooling and freshening the region north of the SAF where mode waters form. The salt deficit is equivalent to that introduced by the northward Ekman transport, suggesting that eddies may play an important role in transporting low-salinity water into the Subantarctic Zone. r 2004 Elsevier Ltd. All rights reserved. Keywords: Mesoscale eddies; Antarctic fronts; Eddy flux; Satellite altimetry; Xbts; Southern ocean; South of Tasmania

1. Introduction Corresponding author. Tel.:+33-5-61-33-29-44; fax: +33-

5-61-25-32-05 E-mail address: [email protected] (R. Morrow).

The existence of a circumpolar channel, unblocked by land, in the latitude band of Drake Passage has a profound impact on the nature of

0967-0637/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2004.07.005

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the global ocean circulation and climate (Rintoul et al., 2001). The channel permits a vigorous exchange of mass, heat, salt and other tracers to be carried between the ocean basins by the Antarctic Circumpolar Current (ACC). The ACC links the circulation of the ocean basins and allows a globalscale overturning circulation to exist. The lack of continental barriers also has implications for the meridional circulation. With no land to support zonal pressure gradients, there can be no mean meridional geostrophic flow across the channel at depths shallower than the sea floor topography. The meridional heat flux required to balance the 0.3–0.7 PW (1 PW=1015 W) of heat lost by the ocean to the atmosphere at high southern latitudes (Gordon and Owens, 1987; Keffer and Holloway, 1988) must be accomplished by some mechanism other than mean advection. de Szoeke and Levine (1981) concluded that poleward heat flux by eddies was the most likely candidate. Subsequent studies have confirmed this suggestion. Direct measurements of eddy heat flux from a limited number of current meter sites consistently show poleward eddy heat flux across the ACC, with values of 15.4 kWm
2 at a site near Drake Passage (Johnson and Bryden, 1989) and 11.3 kWm
2 south of Tasmania (Philipps and Rintoul, 2000). These current meter results are sufficient to explain the missing meridional heat transport, if their values are integrated around the circumpolar belt. (For example, if the point measurements of 11–15 kWm
2 are multiplied by the circumpolar path-length of 20,000 km, and by a typical depth of 4 km, this results in a net poleward heat flux of 0.9–1.2 PW). Recent numerical model estimates from a 0.251 resolution eddy permitting model confirms these observational results, showing a net poleward eddy heat transport of 0.6 PW across the ACC (Jayne and Marotzke, 2002). These poleward eddy heat fluxes can occur in a number of ways, via standing topographic eddies or meanders, or time-varying transient eddies in the form of small meanders or isolated rings. If we consider a time-varying rotating local mean current direction which follows the mean axis of the meandering circumpolar current (as in Hall, 1986; Morrow et al., 1994; Philipps and Rintoul, 2000), we can eliminate the component due to

standing topographic eddies. The net eddy heat transport is then entirely established by transient eddies or rings traversing the ACC jets. A poleward eddy heat flux can occur when warmcore eddies detach from the meandering jet and move warm water poleward, or when cold-core eddies detach from the ACC and move cold water equatorward. These transient eddies have a second role in transferring salt or freshwater across the polar fronts. Cold-core eddies transport cool, lowsalinity polar water across the polar and Subantarctic Fronts (SAFs) into the Subantarctic Zone where mode and intermediate waters form. Once they are within the Subantarctic Zone, mixing of their core waters can contribute to cooling and lowering the salinity of the ambient subantarctic water. Mixing is most efficient in the surface layer where the property gradients are largest; the surface mixed layer can reach 500–600 m depth in winter (Rintoul and Trull, 2001). Lateral mixing from these deep-reaching eddies can potentially modify the water mass characteristics down to a few thousand metres (Philipps and Rintoul, 2000). The strong northward Ekman transport also injects a large volume of cool, low-salinity polar water into the Subantarctic Zone (Rintoul and England, 2002), but this is limited to the surface layer (50–100 m depth) where the temperature is quickly modified by air–sea interactions north of the fronts. The freshwater signal is not very strongly eroded by the air–sea interactions. In this study, we track the detachment and evolution of a number of transient cold-core eddies in the Subantarctic Zone south of Tasmania, using a combination of altimetry data and hydrographic data from expendable bathythermograph (XBT) and conductivity–temperature–depth (CTD) measurements, over the period 1993–2000. We will consider whether these anomalies are meanders or detached rings, follow their pathways and where possible, evaluate their thermal and salt content. The aim is to gain insight into how long these cold-core anomalies remain north of the SAF, where they can mix in with the warmer and saltier subtropical water masses. The impact of these eddies on net heat flux depends on whether

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they are re-absorbed or decay, and here we use altimeter data to determine their fate. The few XBT and CTD transects which intersect a coldcore eddy will be used to calculate the available heat and salt content. This technique has previously been used for tracking warm-core eddies in the Agulhas Retroflection (see de Ruitjer et al., 1999 for a complete review of Agulhas eddy studies). The heat content and evolution of coldcore rings has also been studied in Drake Passage (Joyce et al., 1981). With this data set, we endeavour to shed some light on the role of these eddies in transporting cool, low-salinity water into the Subantarctic Zone.

2. The Data Sets 2.1. Altimetric sea level anomaly data The altimeter data used in this study are the combined Topex/Poseidon (T/P) and ERS1-2 data provided by CLS-Space Oceanography Division. The combined data set provides the best available spatial–temporal resolution for observing mesoscale features, particularly in the high-latitude Southern Ocean where the first internal Rossby radius is of order 10 km. The data set spans 9.25 years from October 1992 to December 2001 and corresponds to sea level anomaly (SLA) relative to a 3 year mean (January 1993 to December 1995). Details of the mapping technique used to derive the 0.251 by 0.251 gridded data are given by Le Traon et al. (1997), and a discussion of the aliased high-frequency errors is given by Morrow et al. (2003). Globally, this data set resolves wavelengths greater than 150 km, with a temporal resolution of 20 days (Ducet et al., 2000). In the Southern Ocean where the groundtracks converge, we can resolve 100 km wavelengths, and variations at 50 km wavelength are present but reduced by 50% in energy. During the 15 month ERS1 ice-monitoring and geodetic mission (December 26, 1993–March 31, 1995), there were no ERS data available for ocean mesoscale studies. For this period, T/P data alone were mapped using the same spatial and temporal decorrelation scales. This leads to a reduction in

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eddy kinetic energy levels of around 30% globally (Ducet et al., 2000), which corresponds to smallscale features which are not observed between the T/P groundtracks. Since the groundtrack coverage converges in our study region, we should be able to track moving cold-core eddies with a typical diameter of 100 km, although we may ‘‘lose the signal’’ temporarily as the smaller eddies pass between T/P groundtracks in 1994/95. 2.2. Climatological data To better distinguish between meanders and transient eddies we have sometimes added a climatological mean steric height to the altimetric sea level anomalies. The steric heights (in m) relative to 2500 m depth are calculated from the Southern Ocean Climatological Atlas (Olbers et al., 1992). 2.3. CTD, XBT and thermosalinograph data Seven full depth repeat CTD sections from the WOCE (World Ocean Circulation Experiment) SR3 line were collected between Tasmania and Terre Ade´lie (Antarctica) during voyages of the research vessel R/V Aurora Australis between October 1991 and November 2001, roughly covering each season of the year (Rintoul and Sokolov, 2001; Rintoul and Trull, 2001; Rintoul et al., 2001). The station spacing for these CTD sections is around 55 km with more tightly grouped measurements in the frontal regions. The data processing for the January 1994 section used in this paper is described in Rosenberg et al. (1995). As part of the SURVOSTRAL program (SURVeillance de l’Oce´an auSTRAL), XBT and thermosalinograph measurements are obtained between Tasmania and Terre Ade´lie in Antarctica every austral summer. The French Antarctic supply ship ‘‘l’Astrolabe’’ was used to obtain 6 XBT and thermosalinograph (TSG) sections per year along the nearly repeating line from Hobart, Tasmania (431S, 1471E) to the French Antarctic base Dumont D’Urville (661S, 1401E) (Fig. 1). The first XBT and TSG section for each austral summer occurs at the end of October, the final

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Fig. 1. Eddy kinetic energy levels from 7 years of Topex/Poseidon and ERS data in the Southern Ocean south of Tasmania. The mean position of the SURVOSTRAL transects is marked as the thick solid line between Tasmania and Antarctica. The thin contour represents the 3000 m depth contour. The mean position of the SAF is marked, after Sokolov and Rintoul (2002).

section is in mid March, and our time series spans from October 1992 to March 2000. Continuous surface temperature and surface salinity measurements are available from the thermosalinograph, with an average measurement every 5 min (at least one per nautical mile). We are able to maintain high-density XBT sampling due to the presence of an onboard observer: weather and faulty probes permitting, XBT measurements are made every 35 km, and every 18 km in the frontal zone from 481S to 541S (see Fig. 1). Most observations attained 800 m depth, but nearly all reached at least 500 m. All XBT data are carefully quality controlled by the CSIRO Marine Research Laboratory, Hobart, Australia, using the techniques outlined in Bailey et al. (1994).

3. The study region Within our study region just south of Tasmania, the ACC is composed of a series of fronts. These fronts are identified in the vertical sections of temperature, salinity and potential density (sigma0) for the SR3 CTD section of January 1994 (Fig.

2) and described in detail by Rintoul et al. (2001), Sokolov and Rintoul (2002) and Chaigneau and Morrow (2002). South of Tasmania, the strongest of these fronts is the SAF which is found between 501S and 531S, with large horizontal gradients extending from the surface to the bottom creating a large eastward mean current (Rintoul et al., 2001). The SAF carries the majority of the ACC transport. Estimates from 6 repeat CTD sections south of Tasmania show that the ACC transport is on average 147710 Sv; of this, 105 Sv is associated with the SAF (Rintoul and Sokolov, 2001). The Polar Front (PF) is located further south between 531S and 541S, and marks the northward limit of the low-salinity polar water masses. In the Antarctic zone south of the PF, the upper layer is characterized by the cold, fresh, dense waters, including Antarctic Surface Water and Winter Water, which are both characterized by salinities of o33.9 in the surface layer (Orsi et al., 1995; Chaigneau and Morrow, 2002). North of the SAF lies the warmer, saltier subantarctic waters where subantarctic mode water (SAMW) is formed (Rintoul and Trull, 2001). Below the SAMW lies the salinity minimum marking the core layer of the

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Fig. 2. Vertical sections from a CTD transect from Hobart (431S) to Antarctica (661S) along the WOCE SR3 line in January 1994, showing (a) potential temperature, (b) salinity, and (c) potential density from the surface to 1500 m depth. The main frontal positions for the STF, SAF and PF are indicated.

Antarctic Intermediate Water (AAIW) at about 1000–1200 m depth. The 27.2 isopycnal, which roughly coincides with the AAIW core layer, rises sharply towards the surface layer from 511S to 531S between the PF and SAF indicating that fresh water subducted from the surface layer may help maintain the AAIW tongue. However, the AAIW in this sector of the Southern Ocean is relatively low in oxygen and CFCs (Rintoul and Bullister,

1999), suggesting that direct ventilation by subduction along the outcropping isopycnal is relatively weak in this region. In the SURVOSTRAL region between 1451E and 1501E, the SAF and PF are constrained to pass on the equatorward side of the Southeast Indian Ridge. To the north and downstream of the ridge, the SAF is subject to strong mesoscale variability in the form of meanders and transient

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eddies (Fig. 1). Philipps and Rintoul (2000) and Morrow et al. (1994, 2003) have shown that the region around 140–1451E is one of growing eddy energy, with maximum eddy kinetic energy exceeding 400 cm2s
2 occurring in the deep basin downstream. The eddy kinetic energy is very small over the Ridge, and the presence of this topographic barrier blocks the southward movement of eddies along the SURVOSTRAL line (Fig. 1). These mesoscale sea level anomalies (and their associated sea surface temperature anomalies) propagate to the east within the main axis of the ACC, and generally propagate westward either side of the mean current (Hughes, 1995). Hovmuller diagrams of altimetric sea level anomalies in our study region along different latitude lines exhibit this propagation pattern (not shown). Poleward heat fluxes were measured across the SAF at the AUSSAF current meter array at 50.51S, 1431E (Philipps and Rintoul, 2000) and at 51–521S at the SURVOSTRAL line (Morrow et al., 2003). Growing meanders of the mean jet and detached rings can both contribute to a poleward heat flux, v0 T0 o0. This occurs in baroclinic instability, for example, if northward flows are on average cooler than southward flows. The transient eddy contribution to the poleward heat flux occurs in two ways: from warm-core eddies moving southward from the SAF or cold-core eddies moving northward. In this region south of Tasmania, the deep-reaching warm-core eddies will be quickly blocked by the Southeast Indian Ridge and either dissipated or reabsorbed back into the ACC. This is accentuated by the southward deviation of the fronts to the east of the SURVOSTRAL line, following the Southeast Indian Ridge (Fig. 1). The warm-core eddy contribution to the meridional heat flux is potentially quite important, if the eddies fully mix with waters south of the ACC as they dissipate against the Southeast Indian Ridge. However, for this observational study it was extremely difficult to separate and track the warm-core eddies and meanders in the turbulent region against the ridge, so we have not been able to quantify their contribution. In contrast, cold-core eddies moving equatorward of the SAF can traverse 2–31 latitude before they are blocked by the south Tasman Rise

at 481S. Since they are easier to track and quantify, and since they also contribute in bringing freshwater into the Subantartcic Zone, we concentrate for the rest of this study on the role of cold-core eddies in transferring heat and salt north of the SAF.

4. Cold-core eddies detected by altimetry The 10-day maps of TP+ERS altimetry data are used to track the evolution of cold-core eddies and meanders detected in the region 140–1501E and north of the SAF to 451S. The technique is to track cold-core anomalies which maintain a negative sea level anomaly of less than –10 cm; ‘‘long-lived’’ anomalies last for more than 6 months, ‘‘short-lived’’ anomalies have a shorter duration. The southern limit is defined as the timevarying SAF position, chosen as the 1.9 m steric height contour (calculated from the Olbers 0/ 2500 m mean steric height plus sea level anomalies from the TP+ERS altimetry data). This is the mean SAF steric height contour as defined by Sokolov and Rintoul (2002). Fig. 3 shows annual maps of the position of the ‘‘long-lived’’ cold-core anomalies for the period 1993–2000. The statistics for our 27 ‘‘long-lived’’ cyclonic eddies are given in Table 1. During the 1993–2000 period, 17 ‘‘short-lived’’ eddies were also spawned at the SAF. The statistics of these short-lived eddies are given in Table 2, but for clarity they are not presented in Fig. 3. We define three different types of negative SLA: (1) cold-core eddies formed along the South Tasman rise, which will essentially transport subantarctic waters in their core (upper case letters, Fig. 3); (2) northward (cold) meanders of the SAF (lower case gray letters); and (3) cold-core eddies which pinch off the SAF, which can transport Polar Frontal Zone (PFZ) water in their core (bold upper case letters; the PFZ is defined to lie between the Subantarctic SAF and PFs Polar Fronts). In some cases, the same anomaly is followed from its meander phase (represented by a lower case letter) till it separates off as a cold-core eddy (same letter but in bold upper case). Although we are mainly interested in the third type of eddy, monitoring the other

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are not included in our study. Eddies D, H, L, U, W and Y are spawned around 44–461S at 1451E on the western edge of the STR. These eddies propagate to the WSW and after 10–12 months arrive close to the SAF at 139–1411E, where they are either advected eastward just north of the SAF (eddies D, L, Y) or are absorbed into a cold meander of the SAF (H, W). These eddies can help trigger large northward meanders of the SAF, and may also play a role in transporting subantarctic water south of the SAF. Two other eddies are generated on the southern flank of the STR near 481S, 1501E (eddies P and S). These eddies drift westward along the southern flank over a 6 month period, and decay around the SURVOSTRAL line at 1451E. These cold-core eddies are likely to carry subantarctic water from 1501E back across the SURVOSTRAL line. 4.2. Cold meanders of the SAF

Fig. 3. Monthly mean positions of the 27 ‘‘long-lived’’ coldcore anomalies described in the text, separated into yearly plots. The different letters indicate cold-core rings spawned at the STR (black upper case), SAF meanders (gray lower case), and cold-core rings spawned at the SAF (bold upper case).

cold-core eddies and meanders is important for such events as the spawning of SAF cold-core eddies, or in the merging of two cold-core eddies.

Cold meanders are defined as having negative SLA (o
10 cm) just north and adjacent to the 1.9 m steric height contour, with distinctive elongated SLA patterns (lower case gray letters, Fig. 3). These meanders propagate eastward with the mean current, taking 4–6 months to propagate from 1401E to 1451E. Within our study region, many large meanders develop around 1401E, in the deep basin downstream of where the Southeast Indian Ridge turns abruptly to the SE (meanders a, g, m, n, o, t, v, x). Other large meanders develop upstream and downstream of the SURVOSTRAL line (meanders c, i, k, q, qu, z). Although these meanders contribute to poleward eddy heat flux, and to diapycnal mixing, they are not the subject of this present study, except in their role in spawning cold-core eddies.

4.1. Cold-core eddies generated at the south Tasman rise (STR)

4.3. Cold-core eddies generated at the SAF

Each year, one or two long-lived, cold-core eddies are generated on the western or southern flank of the STR, and propagate southwestward across our study zone to the SAF (upper case letters, Fig. 3). Many more ‘‘short-lived’’ cyclonic eddies are generated each year at the STR, but rapidly decay within the Subantarctic Zone, and

A large number of cyclonic eddies pinch off from the SAF and drift northwards, transporting water from the PFZ into the Subantarctic Zone (bold upper case letters, Fig. 3). Most of the ‘‘long-lived’’ eddies we monitored are spawned from SAF meanders, which are amplified by the topographic change around 1451E near the

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Table 1 Characteristics of ‘‘long-lived’’ cold-core eddies detected by altimetry north of the SAF Eddy

A B C D E F G H I J K L M N O P Q QU R S T U V W X Y Z

Formation

Duration

Region

Position(lat, lon)

Date

Detached SAF eddy (months)

Total (months)

Fate

SAFa SAF SAF STRiseb SAF SAF SAF STRise SAF SAF SAF STRise SAF SAF SAF STRise SAF SAF SAF STRise SAF STRise SAF STRise SAF STRise SAF

48S 48S 50S 46S 51S 51S 48S 45S 51S 51S 52S 44S 51S 50S 49S 48S 50S 50S 50S 48S 50S 44S 51S 45S 50S 45S 51S

Jan 93 Jan 93 Jan 93 Jan 93 Jan 93 Jul 93 Apr 94 Mar 94 Jul 94 Jul 94 Oct 94 Jan 95 Jan 95 Oct 95 Jan 96 Jun 96 Sep 96 Jan 97 Jul 97 Jul 97 Mar 98 Mar 98 Jan 99 Feb 99 Apr 99 Oct 99 Aug 00

1 6 4 — 9+7c 22 Md — 2 6 11 — 25 7 20 — 8 M 5 — 7 — M — M — 44

11 6 6 22 16 22 12 14 4 6 22 11 30 14 27 6 9 6 11 5 14 10 9 15 9 14 45

Absorbed by eddy E Absobed by eddy A Decays Decays Decays Absorbed by G Reabsorbed SAF Reabsorbed SAF Decays Exits E Decays Exits W Reabsorbed SAF Exits E Reabsorbed SAF Absorbed by eddy O Exits E Reabsorbed SAF Decays Decays Reabsorbed SAF Exits W Reabsorbed SAF Decays Reabsorbed SAF Reabsorbed SAF Remains in region

139E 142E 145E 147E 151E 151E 139E 146E 143E 147E 144E 145E 142E 142E 141E 150E 146E 144E 140E 148E 141E 145E 142E 145E 140E 145E 145E

a

SAF denotes formation at the SAF. STRise denotes and eddy formed at the STR. c Eddy E lasted 9 months alone before it absorbed eddy A; eddy AE lasted 7 months. d M denotes a SAF meander, not a detachded eddy. b

SURVOSTRAL line. Eddies A, M, O, R, T are spawned just upstream of the SURVOSTRAL line, eddies C, I, J, N, Q, Z just downstream, and eddies E, F, K pinch off from a second meander near 1501E. Some of the eddies generated upstream of the SURVOSTRAL line move northward, then WSW, before they decay or are reabsorbed back into the SAF. Eddies M and O follow this circular pathway, and have a lifetime of 20–25 months before they are reabsorbed at the SAF. Eddy A merges with eddy E near 1451E, and eddies R and T drift to the east downstream of the SURVOSTRAL line

where they decay or are reabsorbed into the SAF after 5–7 months. The eddies formed downstream of the SURVOSTRAL line either quickly decay after 2–4 months (eddies C, I) or drift eastward out of our study region with a lifetime46–8 months (eddies J, N, Q, Z). Some of the eddies generated at 1501E also follow a circular pathway (eddies E, F, K), moving north to the STR, then WSW towards the SURVOSTRAL line, before merging with other eddies or meanders of the SAF. These eddies have lifetimes of 1–2 years. In summary, most cyclonic eddies split off from the SAF are generated around 1441E, 1471E and

ARTICLE IN PRESS R. Morrow et al. / Deep-Sea Research I 51 (2004) 1417–1440 Table 2 Characteristics of ‘‘short-lived’’ cold-core eddies detached from the SAF Eddy

Formation Date position (lat, lon)

Total Fate (months)

A0 B0 C0 D0 E0 F0 G0 H0 I0 J0 K0 L0 M0 N0 O0 P0 Q0

49S 51S 51S 50S 51S 50S 51S 51S 49S 50S 50S 53S 52S 52S 49S 48S 50S

2 4 6 1 6 5 4 6 2 5 6 6 3 5 15 6 3

140E 151E 149E 140E 150E 140E 148E 148E 140E 152E 152E 150E 152E 152E 152E 139E 144E

Sep 93 Nov 93 Feb 94 Sep 95 Nov 95 Jul 96 Mar 97 Jul 97 Jul 97 Jul 97 Oct 97 Jun 98 Nov 98 Jun 99 Sep 99 Apr 00 Oct 00

Absorbed by eddy E Absobed by eddy A Decays Decays Decays Absorbed by G Reabsorbed SAF Reabsorbed SAF Decays Exits E Decays Exits W Reabsorbed SAF Exits E Remains at end Decays Remains at end

1501E, and have a mean lifetime of 9 months. Of the 19 ‘‘long-lived’’ eddies formed at the SAF (Table 1), 8 are reabsorbed at the SAF, 7 decay within the Subantarctic zone, and 4 exit the region or remain at the end of our study. More of the ‘‘short-lived’’ eddies decay within the Subantarctic zone (9), whilst only 5 are reabsorbed at the SAF and again 4 exit the region or remain drifting within the region in December 2000. 4.4. Interannual variations of the cold-core SAF eddies The near-permanent presence of cold-core anomalies north of the SAF has been previously described by Rintoul et al. (1997) based on the first few years of SURVOSTRAL measurements. Our analysis confirms that many individual eddies are spawned in the early period (6 in 1993; 2 in 1994; 2 in 1995; 3 in 1996) but relatively few in the later period (0 in 1997; 2 in 1998; 0 in 1999; 1 in 2000), and meanders are more predominant in the later years. 1997 was a particularly warm year along the SURVOSTRAL line, and the region north of the

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SAF remained warmer until 2000 (Chaigneau et al., 2003; Sokolov and Rintoul, 2003). This largescale warming may reduce the amplitude of the detached cyclonic eddies, so that we no longer track them. To test this, we have filtered the altimetric data to remove the seasonal and interannual signals. When our tracking technique is applied to the filtered data, there is only a very small change in the position and amplitude of the propagating eddies and meanders, although some ‘‘stationary’’ eddies which become blocked in one position for a few months are removed. The filtered series also exhibits more cyclonic eddies in the early 1990s, and more meandering in the late 1990s. Our conclusion is that the large-scale warming may alter their amplitude, but not the position or propagation of these anomalies. The regime change after 1997 is also marked by a change in the surface forcing, with the ECMWF meteorological data showing anomalously low (high) wind stress anomalies north (south) of the SAF during 1998/1999 (Chaigneau et al., 2003). This change in the meridional wind stress gradient could influence the transport and stability of the ACC, and as a consequence the number of rings vs meanders. This point merits further investigation in a separate study.

5. Hydrographic Structure of the Cold-core Eddies 5.1. Relation between altimetry and hydrography We assume that the positive sea level anomalies observed with altimetry at the SAF are associated with warm, salty subantarctic water, and the negative SLAs with cool, fresher polar frontal zone water. This characteristic is demonstrated in Fig. 4 which shows a time series of altimetric SLAs at one point along the SURVOSTRAL line just north of the SAF (501S, 1451E) for the period 1993–2000. The altimetric SLAs are Lanczos filtered to remove periods longer than 90 days— thus removing the seasonal and interannual variability. Subsurface XBT temperature anomalies at 300 m depth (XBT; symbol ) are also marked, again calculated relative to a monthly mean value at each latitude.

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Temperature Anomaly (deg.C)

5

50

0

0

-5

O 2002

J

A

J

O

J

A

2003

J

O

2004

J

A

J 2005

O

J

A

J

Sea Level Anomaly (cm)

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-50

2006

Fig. 4. Time series of altimetric sea level anomalies (solid line) from 1993 to 1996 at 501S, 1451E, the location at which the SURVOSTRAL line crosses the mean position of the SAF. The SURVOSTRAL summertime subsurface temperature anomalies at 300 m depth (asterisks) are superimposed.

The altimetric SLAs are positively correlated with the subsurface temperature anomalies at 300 m (r ¼ 0:49), and to a lesser extent with the SSS (r ¼ 0:43) and the SST (r ¼ 0:38). These positive correlations are significant at the 95% level, and are in the right sense (a cold-core eddy or northward meander will have negative SLA, a negative SST and T300 anomaly, and reduced salinity). In the following section, the positive relation between altimetric sea level anomalies and the subsurface temperature anomalies will be exploited, so that we can use altimetry to track the pathways of cold-core eddies, and hydrography to study their vertical structure and heat content.

over the upper 800 m which is the limit detected by the XBT profiles, and can be either cold meanders of the SAF or detached rings. The SURVOSTRAL observations crossed over part of the coldcore ring E during the summer of 1993/94, over the elongated meander, g, in 1994/95 and over the cold-core ring T in 1998/99. In the following analysis we describe three case studies based on summers where cold-core eddies crossed the SURVOSTRAL line (1998/1999; 1994/ 1995; 1993/1994). Using the combination of altimetry data, XBT and TSG data, we will consider their vertical hydrographic structure, and heat content.

5.2. XBT observations of cold-core eddies

5.3. Case 1. Summer 1998/1999 : cold ring detaches from SAF meander

From 1993 to 2000, we have 33 nearly repeating XBT sections (within711longitude) with good data coverage in the Subantarctic Zone and concurrent TSG measurements. Table 3 shows that 25 of these XBT sections crossed over part of a cold-core temperature anomaly, which was generally located between 491S and 511S, just north of the SAF. These cold anomalies extend

During the summer of 1998/1999, 4 XBT sections (Fig. 5) crossed over the cold-core anomaly T at 491S; their corresponding surface temperature and salinity fields are also shown in Fig. 5. At the end of December 1998, the cold-core anomaly at 491S had no SST signature but a weak (0.2) fresh SSS anomaly, and the 61 isotherm was only 100 m from the surface. The XBT transect at

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Table 3 Cold-Core eddy occurrences north of the SAF along the SURVOSTRAL XBT sections DATE

Latitude of Eddy

SST anom (1C)

SSS anom

T300 anom (1C)

Altimetric eddy

08 01 13 12 25 07 04 12 19 01 04 15 04 14 26 24 19 29 21 08 31 23 22 05 25

491S 491S 491S 48.51S 48.51S 491S 48.71S 491S 48.51S 491S 491S 481S 50.51S 50.51S 511S 511S 50.51S 50.51S 511S 511S 491S 491S 491S 491S 501S

01
0.51 01 01 01
1.11 01 01
1.11
2.81 0


0.2
0.2
0.25 0 0
0.25
0.7 0
0.2
0.7 0
0.2 —
0.1 0
0.1 0 0
0.4 —c
0.2 0
0.2
0.3
0.17


1.3
1.5
1.9a
2.4
1.1
0.4
1.7
0.7
0.6
0.7
1.7
1.2
2.4
2.7
2.3
3.4
1.4
1.86
2.1
2.57
2.5a
1.9
1.4
1.7
1.9

AE E E E E F F F F G M M N N N O OU OU R R T T T T X

Dec 93 Jan 94 Jan 94 Feb 94 Feb 94 Mar 94 Nov 94 Jan 95 Feb 95 Mar 95 Nov 95 Nov 95 Jan 96 Jan 96 Feb 96 Oct 96 Jan 97 Jan 97 Feb 98 Mar 98 Dec 98 Jan 99 Feb 99 Mar 99 Oct 99

—b 01 01
0.51 01 01 01 01 01 01 01 01 0.81

a

Calculated at 200 m: deeper XBT data is missing. TSG data missing. c SSS data missing. b

the end of January shows that the subsurface temperature anomaly had been reduced, only a narrow core of 61 isotherm water was present at 300 m depth and the surface SSS signature has been eroded in the surface mixed-layer. By late February, the cold anomaly was strengthened, and a cool, fresh surface anomaly appeared, suggesting an input of water from south of the SAF or that the section was now intersecting the core of the eddy. By March, the subsurface structure weakened, the SST signature showed no trace of the underlying cold-core eddy, whilst the SSS retained a 0.25 fresh anomaly. Altimetry allows us to chart the evolution of this series of cold-core anomalies with a more complete space–time evolution of the flow field over this period. Fig. 6 shows a time series of cold-core (negative SSH) anomalies in the region whose

amplitudes are o
10 cm, for the 12 month period from July 1998 to June 1999. For each 3 month plot, the anomalies for each month are color coded to show the progression from the first month (red) leading the second month (blue) with the third month in green. The monthly mean position of the cold anomaly T is also marked in Fig. 6, and can also be seen in Fig. 3. The mean 1.9 m steric height contour is calculated over each 3 month period, and marked in bold in Fig. 6. The large, messy squiggles from 491S–531S are associated with cold meanders of the SAF. Due to the bathymetry, there is little seasonal migration of the SAF, although the downstream propagation of SAF meanders is evident especially from July to September (Fig. 6a). The cool meander, t, formed on the northern side of the SAF around 1411E in March 1998.

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Fig. 5. XBT and TSG sections from Hobart (431S) to south of the SAF along the SURVOSTRAL line for the dates: (a) 31 December 1998, (b) 23 January 1999, (c) 22 February 1999, (d) 5 March 1999. For each of the 4 subplots, the lower panel shows the vertical temperature section from the XBTs, the upper panel shows the sea surface temperature (thin line) and sea surface salinity (bold line). The SAF is located by the strong vertical temperature gradients (501S–521S).

Over the next few months, this meander propagated downstream then moved northward at 1441E as the SAF deviated around a northward

extension of the Southeast Indian Ridge (Fig. 6). In late November, this elongated meander (in blue) appeared to spawn a more circular cold-core eddy,

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Fig. 6. Negative sea level anomalies of
10 cm amplitude in the region around the SAF, calculated from the mapped T/P+ERS sea level anomalies for the periods (a) July–September 1998, (b) October–December 1998, (c) January–March 1999, and (d) April–June 1999. The mean SURVOSTRAL position is shown near 1451E, the mean position of the SAF, calculated over the 3 month period, is also marked (bold line). Red–blue–green contours show negative SLAs in the first–second–third month of each plot; the bold letter indicated the mean monthly position of the anomaly mentioned in the text.

T, (in green) which drifted slowly eastward across the SURVOSTRAL line over the next few months (green contours in December 1998; then red

contours in January 1999). Note that the interaction with the bathymetry appears to play a role in spawning these eddies. The meandering current

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often moves northward around the Southeast Indian Ridge at 1441E; the cold meanders then appear to feel the southern boundary of the STR at 481S, 1461E and the cold-core anomaly pinches off.

Fig. 6 shows that in February 1999, the cold ring T (blue contours) drifted to the southeast and became more zonally elongated. Maps of the absolute dynamic height (SLAs+mean climatology—not shown) indicate that the cold ring was

Fig. 7. As for Fig. 5, but for the dates: (a) 9 December 1994, (b) 12 January 1995, (c) 19 February 1995, (d) 1 March 1995.

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reabsorbed back into a cold meander of the SAF, which undoubtedly provoked the injection of cool, fresh water at the surface and the intensification of the cold ring. This process also appeared to trap a warm-core anomaly around 501S, and pushed the main axis of the SAF to a more southerly position. Over the following months, the cold-core eddy/ meander shifted to the southeast and was completely re-absorbed by the SAF downstream of the XBT line. 5.4. Case 2. Summer 1994/95: cold meander During the summer of 1994/95, the SURVOSTRAL XBT observations (Fig. 7) crossed a weak cold-core anomaly around 501S on the 12 January which had no evident surface signature in SSS or SST. This cold anomaly weakened further by the 19 February, still with no surface signature, but then developed a narrow and strong subsurface signal on 1 March 1995, with a very strong surface signature, being 0.7 fresher, and 2.8 1C cooler than the surrounding subantarctic waters (Fig. 7; Table 3). The strong surface anomalies during the warm March period suggest an input of recent polar frontal zone water. Fig. 8 shows the temporal evolution of meander, g, from the mapped altimetry data (July 1994 to June 1995). The cold anomaly is associated with the leading edge of a cold meander which was formed around 1401E in August 1994, propagated eastward over the next months, and was found just west of the SURVOSTRAL line in January/ February 1995. This meander crossed over the SURVOSTRAL line in early March, doubling back around a warm-core eddy situated around 50.51S, while the main branch of the SAF continued eastward at 521S. By April, the meander, g, appears to have detached and drifted slightly eastward, north of the large warm anomaly before being reabsorbed by the SAF in May. 5.5. Case 3. Summer 1993/94: cold meander hit by a westward propagating cold ring 4 XBT sections from 8 December 1993 to 25 February 1994 are shown in Fig. 9. For this

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summer season, the cold-core eddy is observed around 48.5–491S, with a weak influence in December, stronger in the January section, then weakening at the end of February. There is no signature of the cold subsurface anomalies in the SST fields, indicating that the cold ring is not recently formed : the surface fields have been warmed over the summer and are ‘‘decoupled’’ from the deeper cold anomaly. However, the SSS remains 0.2–0.25 fresher than the surrounding water in December and January, although this fresh surface layer is less evident in February, when the eddy intensity weakens. This cold anomaly in the XBT fields is actually associated with two distinct cold anomalies that we track with altimetry. Meander, a, which developped at 1421E along the SAF in April 1993, and eddy E which followed the southern flank of the South Tasman Rise (Figs. 3 and 10). The altimetry maps indicate that in October 1993, the meander a and eddy E have separate red contours, but in November these 2 cold anomalies merged (green contours). In early December, our XBT observations just kissed the eastern edge of the newly combined cold-core ring, AE (Fig. 10c; blue contours). In early January (Fig. 10d, red contours), the ring had grown in circumference, intensified and moved westward, drawing in a filament of cold water from a downstream meander. In mid February (blue contours), the cold ring splits: the main part of the ring is advected eastward across the SURVOSTRAL line, and by the end of February it has moved off the line and is once more interacting with a northern SAF meander (which becomes F). The second smaller cold-core ring (E) moves westward and decays in May. This time series clearly shows that our XBT observations were only on the eastern edge of the ring, which may also explain the weak SSS signature in the early sections, just 1 month after the spawning event. 5.6. CTD section through the 1993/94 cold-core ring Our XBT data are limited to 800 m depth, but in the same region we also have available seven fulldepth CTD sections from the WOCE SR3 line

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Fig. 8. (a–d) As for Fig. 6, but for July 1994–June 1995.

(Rintoul and Sokolov, 2001). Only one of these sections, in January 1994, crossed part of a coldcore eddy north of the SAF. By chance, this SR3 section crossed the western edge of the combined cold-core eddy AE detected with XBTs during 1993/1994. The SR3 track is also plotted in Fig.

10; the CTD data around 501S were measured on 5 January 1994. The presence of the cold-core eddy around 491S is clearly evident at depth in the temperature, salinity and potential density sections (Fig. 2). There is almost no signature of this eddy in the

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Fig. 9. As for Fig. 5, but for the dates: (a) 8 December 1993, (b) 1 January 1994, (c) 12 February 1994, (d) 25 February 1994.

SST, although there is a decrease in SSS; the same pattern is noted in the XBT and TSG data at the eastern edge of the same eddy in early 1994 (Fig. 9). Lower temperature, lower salinity, and increased density are evident in the vertical sections

at 491S reaching depths greater than 1500 m. (We note that we cross the edge of the eddy with the CTD section: the eddy center may be even deeper). The vertical T–S characteristics of the cold-core anomaly closely resemble the water mass structure

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Fig. 10. (a–d) As for Fig. 6, but for the dates April 1993–March 1994.

just south of the SAF. Furthermore, the marked SSS minimum of these cold rings is indeed a signature of deeper low-salinity anomalies reaching to at least 1500 m. This can have important consequences for how low-salinity water is introduced into the Subantarctic Zone.

6. Heat and salt content of these cold-core rings To estimate how much cold water is being moved north of the SAF by these cold-core rings, we have calculated the available heat content from the XBT data for all water warmer than 8 1C in the

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−2

6 4 8/12/93 1/1/94 12/2/94 25/2/94

2 0

Z

(a) −54

D8  C

rcp TðzÞ dz;

Heat content ¼

1993/1994

11

x 10

J.m

upper 500 m. The 8 1C limit is chosen since it separates subantarctic water from the cooler PFZ waters and marks the upper temperature range for the SAF definition. The upper ocean heat content from the surface down to the 81C isotherm can be derived from the XBTs (in J m
2) as

1435

−52

−50

−48

−46

−44

−42

1994/1995

11

x 10

0

J.m

−2

6 4 9/12/94 12/1/95 19/2/95 1/3/95

2 0

(b) −54

−52

−50

−48

−46

−44

−42

1998/1999

11

x 10

−2

6

J.m

where r ¼ 1027 kg m
3 is taken as a constant mean density for the region (in the absence of vertical salinity data we cannot determine the absolute density for each XBT profile). Specific heat, cp is taken as 3930 J kg
1 K
1, and T is the mean temperature for each layer (in K). Our XBT data are at 2 m intervals, which defines our layer thickness and mean layer temperature. The integral is calculated from the surface down to the depth of the 8 1C isotherm (D8 1C) or to 500 m— whichever is shallower. Since temperature is in K, the temperature changes across our region are small compared to the mean temperature, and the heat content calculation is dominated by the volume of water warmer than 8 1C in the upper 500 m. Fig. 11 shows that for our 3 chosen summer periods, the upper ocean heat content for waters warmer than 8 1C drops from around 5.7  1011 J m
2 between 451S and 481S to less than 1  1011 J m
2 south of 521S. Where the curves stop south of the SAF, there is no water warmer than 81C in the water column. There is a large variation in this heat content calculation for the cold-core rings located in the different years and the different sections. This is largely due to sampling, since most of our XBT sections crossed only the edge of a ring or meander. The largest drop in heat content occurs in 1998/1999 when the XBT sections consistently traverse near the center of a cold-core ring, and the upper ocean heat content for waters warmer than 8 1C dropped by 4.7  1011 Jm
2. The XBT section with the coolest and freshest surface water in 1 March 1995 was not associated with a large drop in available heat content, since the XBT section crossed over a very narrow leading edge of the meander.

4 31/12/98 23/1/99 22/2/99 5/3/99

2 0 −54

(c)

−52

−50

−48 −46 Latitude (°)

−44

−42

Fig. 11. Heat content for waters warmer than 8 1C in the upper 500 m calculated from XBT observations available during the summer of (a) 1993/1994, (b) 1994/1995, and (c) 1998/1999.

A more precise calculation can be made with the CTD section which crossed the edge of the coldcore ring in January 1994. Temperature and salinity observations can be used to calculate the available heat and salt content anomalies along isopycnal surfaces. Anomalies are used rather than total heat and salt content to highlight the presence of eddies; this technique has already been applied by Joyce et al. (1981) for cold core eddies at the SAF in Drake Passage, by Van Ballegooyen et al. (1994) for Agulhas warm-core eddies, and by Morrow et al. (2003) in the Leeuwin Current. Fig. 12 (upper panel) shows the SR3 temperature and salinity fields over the upper 2000 m, but now projected onto isopycnal surfaces for the region north of the polar front at 521S.

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Fig. 12. (a) Temperature (1C) and (b) salinity sections along isopycnal surfaces for the January 1994 WOCE SR3 section from 441S to 521S (crossing the SAF front). (c) Anomalies of temperature (1C) and (d) salinity along isopycnal surfaces for the SR3 section, calculated relative to the vertical reference profile at 49.21S. (e) AHA and ASA along the section, calculated by vertically integrating the temperature and salinity anomalies.

The cold-core eddy at 491S is still evident in this projection, and the cool, low-salinity structure influences the density levels down to 27.3 kgm
3,

that is, deeper than the density range of mode and intermediate water layers (Sloyan and Rintoul, 2001).

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The eddy influence is more clearly represented by calculating heat and salt anomalies, relative to a reference station profile taken in the ambient waters outside the eddy. The reference station we used was the profile at 49.21S representing subantarctic water which lies between the cold anomaly and the SAF. We have tested other reference profiles in subantarctic waters north of the eddy, and there are only minor variations in the resulting anomaly fields. Our chosen reference profile covers a larger density range, which helps for calculating the temperature and salinity anomalies. The vertical structure of the temperature and salinity anomalies [T0(r)-T0(ref); S0(r)-S0(ref)] along the SR3 section is shown in Fig. 12 (middle panels), again plotted along potential density surfaces. With respect to our subantarctic water reference profile, the temperature and salinity anomalies at the edge of this cold-core eddy clearly extend from the surface down to the 27.3 isopycnal (at around 800–1000 m depth), bringing cold, fresh water into the surface, mode and intermediate water density classes. In contrast, below the 27.3 isopycnal, there is little modification of water mass properties along the density surfaces, and the anomalies we see from 1000 to 1500 m in Fig. 2 are mainly due to heaving of the isopycnals. Again we note that our CTD section crosses the edge of the eddy, so properties in the center of the eddy may be modified to deeper levels. We treat these calculations as a conservative estimate for the cold-core eddy. We can also calculate the available heat content anomaly (AHA) in Jm
2 for each discrete potential density layer, sy, following the technique of Joyce et al. (1981):   AHAs ¼ ri cp hi ðrÞ T s ðrÞ
T s ðref Þ : Within each potential density layer, sy, ri is the vertically averaged density (in kg m
3); cp is the vertically averaged specific heat capacity (in J kg
1 K
1); hi(r) is the thickness of the density layer within the eddy (in m); Ts (r) is the vertically averaged temperature at the radial distance r from the eddy center (in K); and Ts(ref) is the vertically averaged temperature (K) in the same potential density layer at the reference station outside the

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eddy. Note that because we are considering a temperature difference, temperature can be defined either in 1C or K units. Similarly, the available salt anomaly (ASA) in kg m
2 is calculated for each potential density layer:   ASAs ¼ 0:001ri hi ðrÞ Ss ðrÞ
S s ðref Þ : The factor of 0.001 converts salinity in Practical Salinity Units to the salinity fraction (mass of salt per unit mass of seawater). The bottom panel in Fig. 12 shows the vertically integrated available heat and salt anomalies along the SR3 section, relative to the reference profile at 49.2 1S. From 44–45 1S, the AHA has values around 2–3  109 Jm
2; these available heat content anomalies are a factor of 281 K smaller than the total heat content shown for the XBTs in Fig. 11, which is based on the mean temperature in K. The AHA for the two CTD profiles within the cold-core eddy, integrated over the density range from st=26.6 to 27.6 kgm
3, have a mean value of
2.38  109 Jm
2, with respect to the reference profile. The ASA for these 2 profiles integrated over the same density range have a mean value of
104.64 kgm
2. We can then estimate the total heat and salt anomaly of the cold-core eddy by integrating over the radial surface pr2, assuming these vertical profiles represent the eddy interior, and the mean eddy radius is 50 km. The XBT profile of December 1998, which traverses the center of a cold-core eddy, has a latitudinal diameter of 220 km at the surface, 104 km at 500 m depth, tapering away at 800 m depth. Since we do not have enough information on the 3-D geometry of our January 1994 eddy, we use a mean 50 km radius over the entire depth in this firstorder calculation. The total heat anomaly for our ‘‘typical’’ cold-core eddy south of Tasmania is then
1.9  1019 J, with a total salt anomaly of
8.2  1011 kg. These values are slightly greater than the estimates by Joyce et al. (1981) for a similar cold-core eddy in Drake Passage (total heat anomaly of
1.2  1019 J, total salt anomaly of
2.5  1011 kg), although their calculation was further south and their upper ocean layers covered a different potential density range with colder and fresher water (st=27–27.7).

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7. Discussion These case studies reveal a number of points which are consistent throughout our 9 year time series. Firstly, most of the rings observed in our zone detached when a cold SAF meander became unstable. By chance, one of the regions of cold ring spawning is just upstream of our repeat XBT line and the ring generation is partly determined by bathymetric changes. A large northward deviation of the SAF occurs around a northward extension of the Southeast Indian Ridge at 1441E, which brings meanders of the current in contact with the southern tip of the STR. This often causes eddy spawning. This type of generation creates rings with distinctive deep cold cores and a fresh surface signature, characteristic of water found in the polar frontal zone between the PF and the SAF. After detachment, the SAF moves back to a more southern position. If the spawning process pushes these rings far enough north, they have a tendency to propagate to the west-southwest until they reach the northern flank of the SAF where they are often reabsorbed creating a new cold meander. Otherwise, the SAF rings are advected eastward, just north of the mean jet. The timing of this spawning event is sometimes influenced by external forcing, such as when a westward propagating cold eddy traveling along the southern flank of the STR meets the meander. In this case the westward propagating cold ring (or negative sea level anomaly) is carrying relatively ‘‘old’’ cold water although its origins are uncertain. This ‘‘old’’ ring then interacts with the cold meander, creating a newly detached ring, which may have a water mass signature of both the old and the new. Further upstream, a similar process occurs. Large negative SLA rings are regularly generated near the fracture zones of the STR near 451S, 1451E. These anomalies then propagate to the west-southwest and one year later can interact with cold meanders of the SAF east of 1401E. In this case, the incoming ring is clearly carrying subantarctic water, but may trigger a larger instability in a cold meander of the SAF. Our hydrographic observations confirm that the cold-core rings observed from altimetry are moving cool, low-salinity water into the Subantarctic

Zone. The statistics in Section 4.3 show that, in the region from 1401E to 1501E, 2 cold-core eddies per year are detached from the SAF and decay within the Subantarctic Zone, and on average, one eddy per year exits our region with an unknown fate (decay or reabsorbed?). Our summer XBT and TSG observations show that these cold-core eddies quickly lose their surface SST signature; generally a few weeks of summer warming removes the cooler SST signature (we cannot be more precise due to the temporal resolution of our sampling). The fresher SSS is a better indicator of these subsurface cold-core eddies, and the surface salinity signature can last for one or 2 months after the ring detachment. The January 1994 CTD section through the edge of a cold-core ring has allowed us to calculate the vertical extent of one cold-core ring. The cold ring is 1–1.51C cooler and 0.2 less saline than the subantarctic surrounding waters, over an isopycnal range of st=26.6 to 27.3, which ranges from just below the summer surface mixed layer to 1000 m depth. Below this range, the isopycnals are deflected by the cold eddy, but the water mass characteristics do not change along isopycnal surfaces. Although this CTD transect was during summertime, the warm surface mixed layer is confined to the upper 50 m (see Fig. 2) with densities less than 26.6 kgm
3 so there is no real seasonal influence on either the AHA or ASA calculation. If we consider the region 140–1501E, on average 2 cold-core eddies separate from the SAF each year and transport cold fresh water into the Subantarctic Zone where they decay after a period of 2–12 months. These cold-core eddies would then contribute an annual heat anomaly over 101 longitude of
3.8  1019 J or
1.2  1012 W, and an annual salt anomaly of
1.6  1012 kg into the Subantarctic Zone. (The negative anomalies indicate heat and salt deficiencies with respect to the background water mass). If one extrapolates the annual heat anomaly from our calculation around the circumpolar band, the total heat transport from cold-core eddies would account for 0.04  1015 W, a factor of 10 smaller than the net meridional heat transport from the subtropical gyres into the Southern Ocean. Our calculation is

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not statistically significant, and only represents one part of the total eddy heat flux since our CTD section cuts through the edge of an eddy not the center, we have not included the possibly large contribution of warm-core eddies moving heat and salt south of the SAF, nor the contribution of ‘‘short-lived’’ eddies to the heat budget. Our results do indicate however, that these cold-core rings can influence the meridional heat budget. Furthermore, during their passage north of the SAF, their surface layers will be modified by air–sea interactions and convective overturning in winter, which can create deep winter mixed layers in this region down to 600 m depth, influencing the SAMW characteristics to st=26.9 (Rintoul and Trull, 2001). Clearly, these eddies can contribute to a freshening and cooling of the SAMW formed north of the front. Ekman transport is noted in the literature as the dominant mechanism for transporting cool, fresh water from the Antarctic Zone south of the PF into the Subantarctic Zone (Speer et al., 2000; Rintoul and England, 2002). Summertime TSG observations show that the SSS minimum is found on average 0.71 latitude north of the subsurface PF position, and the SSS gradient associated with the SAF is also found on average 0.141 north of the SAF subsurface position (Chaigneau and Morrow, 2002). It is the strong northward Ekman transport which drives the cooler, fresher surface water north of the SAF, as seen in Fig. 2. In contrast at the end of winter, our XBT observations show that the surface salinity is more aligned with the subsurface fronts, indicating that deep winter mixing of the surface mixed layer erodes this difference. Suppose we take a simple example that at the end of summer, we have a fresh salinity anomaly of 0.6 in a surface mixed layer box (salinity 33.9 south of the SAF and 34.5 in the Subantarctic Zone), being on average 50 m deep, extending 0.51 latitude north of the SAF (50 km), over the region 140–1501E (700 km at 501S). Then the total salt anomaly in the Ekman layer, available for deep winter mixing north of the SAF, will be
1.1  1012 kg—the same order of magnitude as our cold-core eddy fluxes. It is interesting that the number of cold-core rings spawned varies interannually which will have

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implications for the interannual variations in heat and salt exchange across the SAF. There are also large geographical variations along the ACC axis in terms of eddy numbers, intensity and pathways. So these local observational results should not be extrapolated outside the study region. However, a number of recent modeling studies have highlighted the role of eddies in constraining the upper and lower limbs of the Southern Ocean thermohaline circulation and in the meridional transport of heat and mass (e.g. Karsten and Marshall, 2002; Jayne and Marotzke, 2002). In the future, the role of eddies in transporting freshwater into the regions of mode and intermediate water formation also needs to be addressed with eddy-resolving models, and validated with this kind of observational data.

Acknowledgments The authors would like to thank the captain and crew onboard l’Astrolabe and the Aurora Australis, as well as our numerous volunteer observers, for helping us obtain these measurements in the frequently inhospitable weather conditions. We thank Ann Gronell and Mark Rosenberg for their help with quality control of the XBT and CTD data, respectively, and Dean Roemmich for his long-term support of the SURVOSTRAL program. We gratefully acknowledge our two reviewers for their constructive comments for improving the manuscript. The SURVOSTRAL program receives support from the Institut Polaire—Emile Victor (IPEV) and the Programme Nationale d’Etudes Dynamique du Climat (PNEDC) in France, the Australian Greenhouse Office and the Cooperative Research Centre Program in Australia, and the National Oceanic and Atmospheric Administration (NOAA – USA) through a cooperative agreement NA37GP0518. References Bailey, R., Gronell, A., Phillips, H., Meyers, G., Tanner, E., 1994. CSIRO Cookbook for Quality Control of Expendable Bathythermograph (XBT) Data. CSIRO Marine Laboratories Report, 221.

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