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Analysis of quasi‐synoptic eddyobservations in the New ZealandsubantarcticMichael J. M. Williams aa National Institute of Water and Atmospheric Research Limited ,Private Bag 14 901, Wellington, New Zealand E-mail:Published online: 30 Mar 2010.

To cite this article: Michael J. M. Williams (2004) Analysis of quasi‐synoptic eddy observations in theNew Zealand subantarctic, New Zealand Journal of Marine and Freshwater Research, 38:1, 183-194,DOI: 10.1080/00288330.2004.9517227

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New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38Williams—Quasi-synopticeddyobservations: 183-1940028-8330/04/3801-0183 © The Royal Society of New Zealand 2004

183

Analysis of quasi-synoptic eddy observationsin the New Zealand subantarctic

MICHAEL J. M. WILLIAMSNational Institute of Water and Atmospheric

Research LimitedPrivate Bag 14 901Wellington, New Zealandemail: m.williams@niwa.co.nz

Abstract In June 2001 a warm core eddy wasobserved simultaneously by satellite and shipboardmeasurements to the south of the Subtropical Front,at c. 50°S 172°W. The simultaneous acquisition ofsatellite altimeter data, together with shipboard ve-locity, and temperature and salinity observations hasallowed a 3-dimensional picture of this eddy to bedeveloped. Analysis of the water mass compositionshowed it consisted of a mixture of between 45% and70% Subtropical Water; the remainder beingSubantarctic Water. The centre of the eddy was1000 m deep. To find the radius of the eddy arankinevortex was fitted to the Acoustic Doppler CurrentProfiler velocity field, this gave a best fit of 65 kmwith a range from 30 to 80 km. For an eddy of thissize the available potential energy was estimated at2.4 × 1013 J and the eddy's kinetic energy at 6.9 ×1014 J.

Keywords eddy; subantarctic; Acoustic DopplerCurrent Profiler; rankine vortex

M03006; Online publication date 15 March 2004Received 3 February 2003; accepted 24 December 2003

INTRODUCTION

In June 2001 observations were collected aboard theRV Tangaroa to the south-east of New Zealandbetween 178°W and 170°W along 50°S (Fig. 1) aspart of the fourth cruise in a study of the NewZealand subantarctic (Morris et al. 2001; Stanton &Morris 2004). Initial inspection of data collected onthis voyage indicated that the ship track had eithercrossed a warm core eddy, or a significant southwardextension of the Subtropical Front (STF). Here theanalysis and results from this data are presented.They lead to the conclusion that an eddy with aradius of c. 65 km was observed.

The broad-scale oceanography around NewZealand is generally well known and was describedin a review by Heath (1985) and summarisedpictorially by Carter et al. (1998). For thesubantarctic this was updated by Morris et al. (2001).The ocean to the east of New Zealand can be dividedin three broad oceanographic regions: north of theSTF, south of the Subantarctic Front (SAF), andbetween these two fronts. In the upper part of thewater column the dominant water mass north of theSTF is Subtropical Water (STW), south of the SAFit is Circumpolar Surface Water, and between thetwo fronts Subantarctic Water (SAW).

Both the STF and SAF have strong linkages tobathymetric features. East of New Zealand the STFappears to be topographically trapped along the topof the Chatham Rise and narrow in width (Sutton2001), and shifts south from the eastern end of therise (Uddstrom & Oien 1999). The SAF follows thesouthern and south-eastern flanks of the Campbelland Bounty Plateaux before separating into twopaths as it joins the basin-scale circulation. Thesouthern path separates from the Campbell Plateauat c. 51°S and the northern path from the BountyPlateau at c. 47°S. Immediately south of the ChathamIslands (c. 177°W), the STF and SAF are estimatedto be only a degree of latitude apart (Carter et al.1998).

Eddies are an important feature of the globalocean as they play a significant role in the transfer

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184 New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38

lens 7&Sea mount

Fig. 1 Map showing the June2001 Conductivity-Temperature-Depth stations (numbered open cir-cles), end-points for the mixingratio calculation (squares labelledNMS, SMS), Bryden & Heath(1985) mooring location (black cir-cle labelled BH85), and approxi-mate location of the SubtropicalFront (dashed line), based onCarter et al. (1998).

of energy and water properties across differentspatial and temporal scales. In particular they canhave a significant role in the transfer of heat, salt,and nutrients across ocean fronts. Interactionbetween eddies and the mean flow is greatest in areaswith strong current systems. The presence of theAntarctic Circumpolar Current (ACC) and thecomplex bathymetry south and east of New Zealandmake this a significant region for eddy activity, aview supported by Stammer (1997) in hiscalculations of eddy kinetic energy from Topex/Poseidon (T/P) data, and by Jayne & Marotzke(2002) in their analysis of an ocean generalcirculation model. Indeed, inspection of monthly seasurface temperature (SST) composites (NIWA SSTArchives; Uddstrom & Oien 1999) suggests eddieswith warm cores regularly form along the STF to thesouth-east of New Zealand, with their frequency ofoccurrence appearing to increase in late autumn andearly winter.

Stanton & Morris (2004) focused on analysingcurrent meter records on the Campbell Plateau andon the Subantarctic Slope. Combining the currentmeter data with satellite altimeter and temperatureand salinity data allowed estimates of the transportin this part of the ACC to be improved. They alsofound a significant role for eddies in the region. Bycombining altimeter and mooring records theymodelled the behaviour of a single eddy passingthrough a mooring array on the Subantarctic Slope.There was also an earlier mooring array in the New

Zealand subantarctic at 49°30'S 170°W (Fig. 1)which was reported on by Bryden & Heath (1985).This array, deployed between April 1978 and May1980, consisted of up to six moorings each with threecurrent meters.

Details of the observations, followed by theiranalysis are given in the next two sections. The eddyis then modelled using a rankine vortex to giveestimates of its radius and velocity. Calculations ofthe available potential energy and the eddy's kineticenergy are presented, followed by conclusions.

OBSERVATIONS

Three observational data sets were collected onboard the RV Tangaroa from 10 to 14 June 2001: aseries of Conductivity-Temperature-Depth (CTD),continuous Acoustic Doppler Current Profiler(ADCP), and surface temperature recordings. TheCTD stations and cruise track are shown in Fig. 1.In addition, satellite observations of sea surfaceheight anomalies (SSH) from T/P and EuropeanSpace Agency Remote-Sensing Satellite (ERS), andSST observations from the NIWA SST archive(Uddstrom & Oien 1999) were examined.

In the area of interest, eight hydrographic stationswere occupied. At all of these stations temperature,salinity, and dissolved oxygen were sampled to theseafloor (Fig. 2). All of the fields show distinctlydifferent properties at stations 72 (49°58.63'S

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Williams—Quasi-synoptic eddy observations

BB 6S 70 71

185

72 74 75

10

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e 1000

| 2000

300C

4DQC

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3^.2 24 2 id 4 34.5 G 34.7 24 3

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Fig. 2 A, Temperature (contour interval (CI) = 0.5°C); B, salinity (CI = 0.05 psu); and C, (overleaf) dissolvedoxygen (CI = 10 µmol kg–1) along the CTD transect shown in Fig. 1.

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New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38

70 71 72 73 1A 75

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160 170 290

. ivo= -^J

173°1.49'E) and 73 (49°59.26'S 172°1.80'E). Atthese two stations the isotherms (Fig. 2A) aredepressed with temperature changes along isobars ofup to 2.5°C near the surface decreasing to 0.5°C at1000 dbar. Fig. 2B shows a homogenous mixed layerabove 120 db, with the salinity at stations 72 and 73up to 0.3 psu above the surrounding waters. Below400 dbar, stations 72 and 73 are c. 0.05 psu saltierthan their neighbours. Oxygen levels decrease insidethe eddy. Along pressure surfaces above 180 dbar,oxygen decreases from c. 270 µmol kg–1 outside, to260 µmol kg–1 inside the eddy. Between 180 and 350dbar the lateral variation is from 260 to 240 µmolkg-1. Below 350 dbar the lateral gradient in oxygenis small.

Distinct temperature-salinity signatures forstations 72 and 73 (red lines) can be seen in thetemperature-salinity plot (Fig. 3). For lighter watersthese stations have a distinct signature and for denserwaters, ao>27.5, the temperature and salinity atstations 72 and 73 are similar to the other stations.

ADCP data were collected in 4-m bins to amaximum depth of 330 m. On many occasions themaximum depth was not achieved because of poorsignal return. The ADCP data was processed toremove bad data points, orient the velocity

components into northward and eastwardcomponents, and to place the data in 10-min bins.The velocity components were then verticallyaveraged and the barotropic tide component of thevelocity was removed using output from the tidalmodel of Walters et al. (2001). These velocities weresmoothed using a running mean with a filter widthof 3 km. Every third velocity vector (i.e., a 10-minaverage every 30 min) is shown along the cruisetrack in Fig. 4.

To generate a suitable "snap shot" of the SSH thatis concurrent with the shipboard observations, along-track data from both T/P and ERS have been used.These have been extracted for the region of interestfor the period of 9-17 June 2001, from the ColoradoCenter for Astrodynamics Research Altimeter DataArchive (http://www-ccar.colorado.edu/~altimetry).The along-track SSH is shown in Fig. 4. Althoughthere are significant gaps between tracks, a clearregion of anomously high SSH with a maximum ofc. 35 cm can be seen near 172°W.

High spatial resolution SST data is routinelyarchived by NIWA (Uddstrom & Oien 1999) for theseas surrounding New Zealand. At latitudes south ofc. 50° S there is frequently extensive cloud cover forsignificant periods, which inhibits the ability of

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Williams—Quasi-synoptic eddy observations 187

Fig. 3 Temperature versus salin-ity for stations 72 and 73 (redlines), the other stations along thecruise transect (black lines) and theend-points for the mixing ratio cal-culation (blue lines labelled NMS,SMS). Dotted lines are contours of

Fig. 4 (below) Along-track seasurface height, with Acoustic Dop-pler Current Profiler velocityvectors, Conductivity-Tempera-ture-Depth station positions(squares), solutions 1, 2, 5, and 9(Table 1) of the rankine vortexmodel (solid line), and the 2000,3000, and 4000 m isobaths (dashedline).

34.1 34.2 M.3 34.4 34.5Salinity fpsu)

34.6 34.7

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188 New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38

Advanced Very High Resolution Radiometersatellite's to measure SST. Unfortunately for theperiod 8-21 June 2001 in the area of stations 72 and73 there was almost continuous cloud cover, makingthe SST imagery unsuitable for investigating thestructure of the observed feature.

ANALYSIS

Initial inspection of the five data sources showsgenerally consistent results from the different datasets. The water is warmer and saltier, with anincrease in SSH, and an anticyclonic surface flowin the ADCP velocity field. This suggests theanomalous conditions around stations 72 and 73 arepossibly part of a warm core eddy. An alternativeinterpretation could be that they are meanders in theposition of the STF.

Although the temperature and salinity obser-vations show warmer and saltier water inside theanomaly than for the surrounding water masses, itis not pure STW. STW would have a higher salinityand temperature over almost all of the water column.To determine the mix of STW and SAW a standardlinear mixing model (e.g., Chiswell & Sutton 1998)was used. At each station the mixing ratios forpotential temperature relative to 1000 dbar (qe),salinity (qs), and oxygen (qO) were calculated bylinearly interpolating along al surfaces between twochosen end-points. For example qs = (S - SS) /(SN -SS), where SS and SN are the salinity at the southernand northern end-points, respectively. The southernend point (SMS, Fig. 1) was chosen so the stationdata consisted of a typical subantarctic profile at thelatitude of the eddy. It was collected on 4 August1999. The northern end-point (NMS, Fig. 1) waschosen to be characteristic of STW. It lies to thenorth of the Chatham Rise, hence placing it to thenorth of the Subtropical Front. It was collected on24 May 1997. The temperature-salinity profiles forboth end-points are shown in Fig. 3. The use of Gl

surfaces for the interpolation gave results over thewidest possible depth range, but the overall resultswere insensitive to the choice of potential densitysurface.

The mixing ratios for potential temperature,salinity and dissolved oxygen are shown in Fig. 5.The gaps in the mixing ratios near the surface reflectthe lack of a common potential density surfacebetween the two end-points. Those in the interior arewhere the mixing calculations were off scale, i.e.,percentages less than -20% or greater than 120%.

A crossover point in the two oxygen profiles causedthe high dropout rate at depth in Fig. 5C.

All three plots exhibit a high level of consistencywith waters at stations 72 and 73 showing a highlevel of subtropical waters, particularly near thesurface where there is over 60% STW. Above 1000dbar the percentages of STW are very similar forboth temperature and salinity across the entiresection. Deeper in the water column the amount ofSTW decreases to close to zero below 1000 dbar.The oxygen shows STW levels closer to 70%extending from the surface to 1000 dbar. In thestations either side of 72 and 73 there are slightlyraised percentages, probably as a result of localhorizontal mixing between the anomalous watermasses and the surrounding water.

Figure 6 shows the geostrophic velocitiescalculated using the CTD station data and byassuming the bottom as the level of no motion. Thesection shows noticeable reversals in flow. OverBollons Seamount there is a weaker south-eastwardflow and a very weak northward flow to the east ofthis. The strongest flow in the section is associatedwith stations 72 and 73, giving a southward flow onthe west side of these stations ranging from 31 cms–1 at the surface to under 2 cm s–1 at 3000 dbar. Tothe east of these stations the northward flow peaksat 21 cm s–1 near the surface and decreases to under2 cm s–1 below 2000 dbar.

Comparison of the mean geostrophic velocity inthe top 200 dbar with the processed ADCP velocitiesis very favourable east of 175°W (not shown), witha mean difference of less than 5 cm s–1. West of175°W the geostrophic velocities tend to missfeatures of the flow and underestimate other featuresof the flow. Some of this underestimation issignificant, e.g., adjacent to the Bounty Plateau thegeostrophic velocities are 12 cm s–1 in comparisonto the ADCP's 58 cm s–1, and the weak northwardflow between 175°W and 176°W seen in thegeostrophic velocities is significantly stronger in theADCP records at 30 cm s–1. That the geostrophicvelocities do not resolve all of the small featuresobserved by the ADCP is expected given the spacingof the CTD stations. The poor results of thecomparison between the ADCP and geostrophicvelocities west of 175°W arise because the ADCPvectors lie along the CTD section, thus thegeostrophic method is unable to fully determine thevelocities.

Comparison between the geostrophic velocitiesfrom 2001 and the mooring data collected in the late1970s (Bryden & Heath 1985) show good agreement

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Williams—Quasi-synoptic eddy observations

6e 69 70 71

189

72 73 74

Fig. 5 Mixing ratios showing percentage of Subtropical Water calculated using: A, temperature; B, salinity; and C,dissolved oxygen. White areas indicate out of range values.

at depth, suggesting that both the barotropic andageostrophic velocity components are small. In theiranalysis of the mooring data they found statisticallysignificant velocities at 4000 m with u and v com-ponents of 2.69 ± 1.19 cm s–1 and 2.61 ± 0.91 cms–1, respectively. These velocities are similar to thosefound here assuming geostrophy and a bottom level(c. 4200 m) of no motion. The shallowest currentmeter on these moorings was at 1000 m where meanvelocity magnitudes were similar to the geostrophicvelocity magnitudes calculated here.

Figure 7 shows a comparison between the SSHanomalies along the ship track with the dynamicheight anomaly calculated from the CTD stations.The dynamic height was calculated relative to2000 dbar for the stations shown in Fig. 2, andthe mean removed from these eight stations tofind the anomaly. The SSH values come from thealong track observations, shown in Fig. 4, closestto 50°S, the nominal latitude of the CTD section.The good agreement between the two anomalyfields implies that the SSH anomaly provides a

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190 New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38

69 70 71 72 73 74 75

-24 -16

cm s• i

16 24

Fig. 6 Geostrophic velocity field through the transect (positive velocities are northward).

good representation of the underlying structure ofthe feature.

The initial examination of the observations pointsto two clear conclusions in the area of stations 72and 73: that there is an anomalous water mass, andthat there is both a strong shear and a rotational flowcomponent across the cruise transect. This flow islikely to be associated with either some sharp frontalfeature such as a meander, or an eddy. Differ-entiating between these two features is difficult withonly a slice through the area.

Spatial interpolation of the SSH anomalies for theperiod 9-17 June shows an approximately circularpositive anomaly centred at c. 50°S 172.5°W. TheSSH gradients imply velocities which form a closedcirculation. The c. 35 cm magnitude of this anomalyplaces it about two standard deviations on a normaldistribution from the mean SSH anomaly at c. 50°S172.5°W. The standard deviation was calculatedfrom the gridded AVISO SSH anomaly product from1 January 1993 to 30 June 2000, with the mean SSHfrom 1 January 1993 to 31 December 1995 removed.The combination of a SSH anomaly implying aclosed circulation with a clearly anomalous water

mass suggests that in the area of stations 72 and 73there is an eddy.

MODELLING THE EDDYUSING A RANKINE VORTEX

To gain further insight about the eddy at stations 72and 73 it was modelled using a rankine vortex, asimple model of an eddy. The rankine vortex modelassumes the tangential velocity (ve) inside the coreof the eddy behaves as a rotating solid body. Outsidethe eddy core ve decays exponentially at the rate of1/r, where r is the distance from the centre of theeddy. This gives the equation:

ve(r) = VrR–1 for r < R (1)ve(r) = VRr–1 for r > Rwhere R is the radius of the eddy core, and V is theazimuthal velocity at R.

To fit this eddy model to the ADCP velocities, thethree variables R, V, and the position of the eddy'scentre were needed. These were found by calculatingthe model velocities in the same locations as the

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Williams—Quasi-synoptic eddy observations 191

AnFig. 7 Intercomparsion betweensea surface height (+) and dynamic .—.height anomalies (o) along the ship Etrack. — 20

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Table 1 Solutions, ranked by root mean square (RMS) error size, from fitting the rankine vortex model to differentsubsets of the Acoustic Doppler Current Profiler (ADCP) velocity observations.

Solution

1234567891011

172172172173173173173172172173173

Longitude rangeof ADCP subset

(°

=52.13'°43.74'°35.34'°0.53'°8.93'= 17.33'°0.53'=52.14'°43.73'= 17.32'°8.93'

W)

171°53.35'171° 1.75'172=10.14'172°44.96'171°44.96'171°36.56'171°53.35'171° 1.75'172=10.15'172°36.56'171°44.96'

RMSerror

(cm s–1)

5.35.96.57.07.27.37.57.88.28.38.4

Latitudeof centre

(°S)

49°43.42'49°54.77'49°52.49'49°49.73'49°46.43'49°48.18'49=44.51'49°44.57'49=47.81'49°43.36'49°43.27'

Longitudeof centre

(°W)

172°24.53'172°24.65'172°27.38'172°22.30'172°14.59'172°25.76'172°22.63'172°22.59'172°25.49'172°23.18'172°23.14'

Eddyradius(km)

64.8061.6170.1558.9530.2865.5864.9270.0579.9569.4673.33

Rimvelocity(cm s–1)

45.271.767.740.769.447.649.954.259.054.857.5

ADCP velocities for a wide parameter range for allthree variables. The "best fit" solution was definedas the solution with the smallest root-mean-square(RMS) difference between the model and observedvelocities.

Eleven different velocity subsets were taken fromthe ADCP record for use in fitting the rankine vortex.The longitudinal range of each subset is shown inTable 1, along with the solutions for R, V, and theeddy centre's position. Using several differentsubsets allowed some quantification of the errorsinvolved in fitting the eddy model. In addition itreduced the potential of the observations to overinfluence the model fitting process. This can occurfor ADCP velocities located at significant distancesfrom the eddy core, as here the exponential decayin the rankine vortex model is not expected toaccurately represent the far-field ocean velocitystructure.

The results in Table 1 show some variation in thelocation of the eddy's centre. It ranges in latitudefrom 49°43.2'S to 49°54.8'S, a difference of c.21 km. Interestingly this difference is between thetwo scenarios with RMS differences less than 6 cms–1. The longitude ranges from 172°22.30'W to172°27.38'W, a difference of c. 6 km (for a constantlatitude). This smaller difference in longitude islikely to be because of the zonal nature of the shiptrack, which would be expected to provide betterresolution in longitude. The eddy radius circles forsolutions 1,2,5, and 9, are shown with the SSH fieldin Fig. 4. All of the solutions show reasonableagreement with the positive anomaly associated withthe eddy. This suggests that the radius and centreposition of the eddy can be calculated from theADCP velocities by using a rankine vortex model.

The variation in both velocity and radius reflects theproblem of fitting the rankine vortex to different

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192 New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38

175°W

Fig. 8 Comparison between results from solution 1 of the rankine vortex eddy model and observations for A, u andB, v velocity components; and C, the corrected RV Tangaroa sea water intake temperature. Solid, dashed, and dottedlines in A and B are respectively, the observed velocity, the modelled velocity, and the difference between the two.

sections of the ADCP record. Often a family ofsolutions were found. These showed little variation inthe position of the eddy's centre, but the velocity andradius results where linked by a linear relationship.This relationship allowed the angular velocity withinthe solution family to remain constant. Within thesefamilies of solutions the RMS difference varied littlein the solution, suggesting the minimum solution foundin the rankine vortex fitting is only marginally the bestsolution. Most situations where these families ofsolutions arose were where the portion of the ADCPrecord being used in the calculation was short and lay

inside the final eddy calculated from the results. Thissuggests that excluding velocities immediately outsidethe eddy core from the rankine vortex fittingsignificantly hampers the fitting process.

Comparisons between the model and observedvelocities for solution 1 are shown in Fig. 8. The twomodel velocity components show significantdifferences when compared with the observedvelocities. The model's eastward component (Fig.8A) does not fit the observed velocity particularlywell, with significant differences of up to 20 cm s–1.For the model's northward component (Fig. 8B) the

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Williams—Quasi-synoptic eddy observations 193

Table 2 Available Potential Energy (APE) and Kinetic Energy (KE) estimatesfor an eddy from this and other studies.

APE(1014J) KE (1014 J)

This studyCheney & Richardson (1976)Schultz Tokos & Rossby (1991)van Aken et al. (2003)

0.24140-9300.21-0.7550-510

6.99-560.12-0.7910-180

rankine vortex is significantly better at modelling theobserved velocities. Here the difference is smallerbetween the model and observed velocities,particularly in the region between the two verticallines that indicate the intersection of the eddy corewith the ship track. Over most of this region thevelocity differences are close to zero.

Figure 8C shows the surface temperature profilecollected at the vessel sea water intake minus anoffset of 0.3°C. The correction was estimated bycomparing data on a later cruise (17-21 March 2002)in similar water, when a more accurate thermo-salinograph was installed in line with the intakethermometer, and in comparisons between the CTDtemperature at the hydrographic stations and theintake temperature. Regardless of the offset it canbe seen there is a significant increase in the surfacetemperature between 171°54'Wand 173° lS'W fromc. 8.8°C to c. 11°C. This rise in the temperaturecorresponds with the range where the ship trackintersects the eddy.

EDDY ENERGY

The available potential energy (APE) of the eddy isthe difference between the total potential energy ofthe eddy and the amount of potential energy that itwould posses if adiabatically adjusted to a statewhere the density and geopotential surfaces coincide.While not easily defined, several methods have beendescribed to calculate it (see Hebert (1988) for alonger discussion). The most practical methodinvolves comparing the total potential energy insidethe eddy with that of some reference state. For anisolated feature such as an eddy, Hebert (1988) hasshown that far-field ocean properties can serve as areference state.

Here the formula presented by Schultz Tokos &Rossby (1991) that follows Hebert (1988) is used.This defines the APE as:

APE = 0.5 pr Jv Nr2 82 dV (2)

where pr and Nr are the density and Brunt-Väisäläfrequency of the reference state, and 8 is thedisplacement of isopycnals inside the eddy from thereference state. The state inside the eddy was foundby taking the mean of stations 72 and 73 on eachpressure level. The reference state was found bytaking the mean of stations 70, 71, 74, and 75. Thisgave an APE for the eddy of 2.4 x 1013 J, for anassumed radius of 64.8 km (solution 1) and an eddydepth of 1000 m. Variation in the eddy size can havea significant impact on the APE calculation, this isreflected in the error estimate of 25% calculated byHebert (1998) for this method.

To calculate kinetic energy (KE) the rankinevortex fit model was combined with the propertiesof a rotating solid body to give the equation:

KE = 0.25 n p D co2 R4 (3)where p is the mean density inside the eddy, D is thedepth of the eddy, and ro is the angular velocity. Fora radius of 64.8 km (solution 1) and an eddy depthof 1000 m, the KE is 6.9 x 1014 J.

Table 2 shows APE and KE estimates from thisand other eddy studies. The APE calculated here issimilar to that reported by Schultz Tokos & Rossby(1991) for a Mediterranean salt lens, but it is 3 ordersof magnitude lower than the APE reported byCheney & Richardson (1976) for a cyclonic, coldcore, Gulf Stream ring and 2-3 orders of magnitudelower than Agulhas rings (van Aken et al. 2003). TheKE value from this study is an order of magnitudelarger than for the Mediterranean salt lens, and anorder of magnitude smaller than the values reportedfor either the Gulf Stream ring, or the Agulhas rings.

CONCLUSION

Analyses of both shipboard and satellite observationshave shown that an eddy consisting of c. 60% STWwas observed in June 2001 centred at c. 50°S172°30'W. The location, size, and characteristics ofthe eddy core imply it formed to the south of the

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194 New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38

Chatham Rise and to the east of New Zealand'sSouth Island.

To determine the size of the eddy a rankine vortexwas fitted to the ADCP data across the eddy. Thefitting process gave a wide range of radii for the eddywith the best fit yielding an eddy radius of c. 65 km.The fitting process also provides a measure of theuncertainties associated with the radius calculation.The current meter moorings of Bryden & Heath(1985) recorded similar changes in temperature at1000 dbar to that associated with the eddy at 1000dbar suggesting that this depth for an eddy in thisarea is not uncommon. Their measurements do not,however, provide any evidence on shallower eddies.It seems unlikely that the eddies would be sub-stantially deeper that 1000 m. If they were deeper theadditional amount of heat or salt transported by theeddies would taper off significantly as thetemperature and salinity gradients between the insideand outside of the eddy taper off markedly below1000 m.

To better understand eddies in the New Zealandsubantarctic there are three areas in need ofadditional study: an estimate of eddy frequency inthe region, a factor not considered in this study; afuller picture of the life of the eddies, so how theyform and change can be better understood; and highquality estimates of net heat, freshwater, and tracertransports through the region, so that the eddycomponent of these transports can be established. Allof these are potentially achievable and will helpclarify the important role of eddies in the NewZealand subantarctic and how they contribute to salt,heat, and mass transport between the Southern andPacific Oceans.

ACKNOWLEDGMENTS

I thank M. Bowen and P. Sutton for useful discussionsand comments on the manuscript; the anonymous review-ers for their comments; H. Neil, M. Walkington, P. Wiles,and the captain and crew of the RV Tangaroa for the sea-going data collection and initial data preparation. Thisproject was funded by the New Zealand Foundation forResearch, Science and Technology under contractC01X0037.

REFERENCES

Bryden, H.; Heath, R. 1985: Energetic eddies at the north-ern edge of the Antarctic Circumpolar Current inthe Southwest Pacific. Progress in Oceanogra-phy 14: 65-87.

Carter, L.; Garlick, R. D.; Sutton, P.; Chiswell, S.; Oien,N. A.; Stanton, B. R. 1998: Ocean CirculationNew Zealand. NIWA Chart Miscellaneous SeriesNo. 76.

Cheney, R. E.; Richardson, P. L. 1976: Observed decayof a cyclonic Gulf Stream ring. Deep-Sea Re-search 23: 143-155.

Chiswell, C.; Sutton, P. 1998: A deep eddy in the Antarc-tic Intermediate Water north of the Chatham Rise.Journal of Physical Oceanography 28: 535-540.

Heath, R. A. 1985: A review of the physical oceanogra-phy of the seas around New Zealand -1982. NewZealand Journal of Marine and Freshwater Re-search 19: 79-124.

Hebert, D. 1988: Available potential energy of an iso-lated feature. Journal of Geophysical Research93: 556-564.

Jayne, S. R.; Marotzke, J. 2002: The oceanic eddy heattransport. Journal of Physical Oceanography 32:3328-3345.

Morris, M; Stanton, B.; Neil, H. 2001: Subantarctic ocea-nography around New Zealand: preliminary re-sults from an ongoing survey. New ZealandJournal of Marine and Freshwater Research 35:499-519.

Schultz Tokos, K.; Rossby, T. 1991: Kinematics anddynamics of a Mediterranean salt lens. Journal ofPhysical Oceanography 21: 879-892.

Stammer, D. 1997: Global characteristics of ocean vari-ability estimated from regional TOPEX/POSEIDON altimeter measurements. Journal ofPhysical Oceanography 27: 1743-1769.

Stanton, B.; Morris, M. 2004: Direct velocity measure-ments in the Sub-Antarctic Front and overCampbell Plateau, south-east of New Zealand.Journal of Geophysical Research: In Press.

Sutton, P. 2001: Detailed structure of the Subtropical Frontover Chatham Rise, east of New Zealand. Journalof Geophysical Research 106: 31045-31056.

Uddstrom, M.; Oien, N. 1999: On the use of high-resolu-tion satellite data to describe the spatial and tem-poral variability of sea surface temperature in theNew Zealand region. Journal of Geophysical Re-search 104: 20729-20751.

van Aken, H. M.; van Veldhoven, A. K.; Veth, C.; deRuijter, W. P. M.; van Leeuwen, P. J.; Drijfhout,S. S.; Whittle, C. P.; Rouault, M. 2003: Observa-tions of a young Agulhas ring, Astrid, duringMARE in March 2000. Deep-Sea Research II,50: 167-195.

Walters, R.; Goring, D.; Bell, R. 2001: Ocean tides aroundNew Zealand. New Zealand Journal of Marineand Freshwater Research 35: 567-579.

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