Circulation within the Wairarapa Eddy, New Zealand

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This article was downloaded by: [University of Illinois Chicago]On: 22 October 2014, At: 18:56Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UKNew Zealand Journal of Marine andFreshwater ResearchPublication details, including instructions for authors andsubscription information: within the Wairarapa Eddy,New ZealandStephen M. Chiswell aa National Institute of Water and Atmospheric Research Limited ,P.O. Box 14 901, Wellington, New Zealand E-mail:Published online: 30 Mar 2010.To cite this article: Stephen M. Chiswell (2003) Circulation within the Wairarapa Eddy, NewZealand, New Zealand Journal of Marine and Freshwater Research, 37:4, 691-704, DOI:10.1080/00288330.2003.9517199To link to this article: SCROLL DOWN FOR ARTICLETaylor & Francis makes every effort to ensure the accuracy of all the information (theContent) contained in the publications on our platform. 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Terms &Conditions of access and use can be found at Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37: 6 9 1 - 7 0 40028-8330 /03 /3704-0691 $7.00 The Royal Society of N e w Zealand 2003691Circulation within the Wairarapa Eddy, New ZealandSTEPHEN M. CHISWELLNational Institute of Water and AtmosphericResearch LimitedP.O. Box 14 901Wellington, New Zealandemail: s.chiswell@niwa.cri.nzAbstract The Wairarapa Eddy appears as a per-manent anticyclonic eddy situated off the east coastof the North Island, New Zealand. In April 2001 aspatial survey of the eddy was made on a shipequipped with an Acoustic Doppler Current Profiler(ADCP). The absolute circulation at 100 m was es-timated by objective mapping of the ADCP-derivedvelocities to produce a velocity field that has en-forced non-divergence. Assuming that enforcingnon-divergence produces the best estimate of thegeostrophic flow, this velocity field can be used asa "level of known motion" to reference geostrophicvelocities at other depths. In particular, it can be usedto estimate the velocity at 2000 dbar, which is oth-erwise used in this region as a level of no motion.The resulting flow at 2000 dbar has a mean speedof 0.07 m s-1, and appears to be well correlated withthe surface flow.Keywords geostrophic circulation; level of nomotion; eddyM02052; Online publication date 31 October 2003Received 27 June 2002; accepted 27 June 2003INTRODUCTIONThere appear to be several permanent or semi-per-manent anticyclonic and cyclonic eddies embeddedin the flow around the east coast of the North Islandof New Zealand, some of which may have impor-tant implications for biological processes (e.g., Brad-ford et al. 1982). The exact number and locations ofthese permanent features has been difficult to pindown because historical hydrographic surveys haveusually been at too coarse a spatial resolution to un-ambiguously identify the smaller-scale features inthe flow.However, recent work (Roemmich & Sutton1998) has pointed to the existence of three large(100 km diam.) anticyclonic eddies which appear tobe permanent features of the circulation. In particu-lar, one of these eddies, centred near 41S, 17830'Eis generally referred to as the Wairarapa Eddy. Nodetailed dynamical analysis of the eddy has yet beenmade, but it is probably formed by retroflection ofthe East Cape Current by the presence of theChatham Rise.Although the Wairarapa Eddy has only recentlybeen considered important enough for it to be named,its presence has been inferred for some time, withearly hydrographical analyses showing anticyclonicflow in the region (e.g., Heath 1975). The eddy wasalso inferred from biological considerations byLesser (1978) who speculated that "recirculationbetween the East Cape Current and an anticycloniceddy formed where it turns offshore" (i.e., theWairarapa Eddy) could retain rock lobster larvaelong enough for them to develop to the post-larvalpuerulus stage. This hypothesis is supported byChiswell & Roemmich (1998) who simulated larvaltrajectories using geostrophic surface velocities de-termined from Topex/Poseidon (T/P) altimeter datacombined with a hydrographic climatology. LaterChiswell & Booth (1999) added to the corroborationwhen they showed higher levels of larvae within theeddy than outside.It is not yet clear whether interannual variabilityin lobster larval survival is related to variability inDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 692 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37the strength or location of the Wairarapa Eddy, orwhether the larval survival is controlled by biologi-cal processes. To answer questions such as theserequires development of numerical models of theregion. And to this end, some effort has been madein determining how well the deep geostrophic circu-lation can be determined from altimetric measure-ments of sea surface height (Chiswell 2001).However, computing the absolute geostrophic cir-culation from either hydrography or altimeter meas-urements usually requires making an assumption thatthe flow becomes zero at some reference level, or"level of no motion".Previous workers (e.g., Stanton et al. 1997) havecommonly assumed a level of no motion of 2000dbar for east of New Zealand. This level of no mo-tion has been chosen principally to be consistent withHeath (1972) who suggested a level deeper than1500 dbar for the Hikurangi Trench and 2000-2500dbar in the south-western Pacific Basin. There issome other evidence to suggest that 2000 dbar maybe a reasonable choice for the Wairarapa EddyWarren (1970) used the same level east of NewZealand, arguing that it lies in the middle of the deepoxygen-minimum layer which he suggests is a re-gion of slow horizontal velocity.Testing the level of no motion assumption in thisregion by direct observation has not yet been done.However, in April 2001, a cruise was conducted inthe Wairarapa Eddy, during which a spatial surveyof the eddy was made on a ship equipped with anhull-mounted Acoustic Doppler Current Profiler(ADCP). In addition, a mooring from the centre ofthe eddy returned currents from 70, 1500, and3100 m. These data provide a means to evaluate thelevel of no motion assumption, and it is the aim ofthe work described in this paper to perform such anevaluation.ADCP data can be used to reference hydrographicdata by providing an estimate of the absolutegeostrophic currents at some reference level. In prin-ciple, once these currents are known, the hydro-graphic currents can be referenced to themasChereskin & Trunnell (1996) put itone computesa level of "known motion" rather than a level of nomotion.However, ADCP measurements rarely penetratebeyond the top 200 m of the water column, and theseupper layers have significant ageostrophic circula-tion caused by tides, wind-driven flows, and near-inertial oscillations. This ageostrophic circulation isusually impossible to remove because it is difficultto model, and the spatial and temporal componentsare highly aliased by the ship's track. If the ADCPobservations extended into the geostrophic interiorof the ocean then one could reference the geostrophicvelocities by using a least-squares fit to the shear.Instead, one is forced to choose a relatively shallowreference level and use the best means possible toremove the ageostrophic motion from the ADCPdata. The technique used here does so by using thefact that geostrophic flow is non-divergent, and socomputes the "known motion" by fitting a streamfunction to the ADCP measurements (by definitiona stream function provides a non-divergent velocityfield). Effectively, one makes the assumption that theageostrophic terms are divergent, and do not contrib-ute to the stream function.The technique closely follows the method usedby Sutton & Chereskin (2002) in their analysis of theEast Auckland Current, and by Chereskin &Trunnell (1996) in their analysis of the CaliforniaCurrent. It is based on a derivation by Bretherton etal. (1976), and further details are given by Walstadet al. (1991) and Denman et al. (1985).DATAHydrographyFigure 1 shows the locations of stations occupiedduring a 9-day survey of the Wairarapa Eddy. ASeabird CTD (conductivity-temperature-depth)profiler in a 12-place rosette with 1.2 litre Niskinbottles was used to make continuous vertical profilesof temperature and salinity at each station. Watersamples were collected to calibrate the conductiv-ity sensor. CTD data collection and processing meth-ods were the same as those detailed in Chiswell etal. (1993) and Walkington & Chiswell (1998). Tem-perature (T) and salinity (S) were processed to 2 dbarbins. Temperature is estimated to be accurate to3 mC, salinity to 5 x 103.CTD casts were made to within 20 m of the seafloor in a grid with nominal spacing of 0.5 in lati-tude and longitude. The survey started near the north-ern end of the grid on 3 April 2001 and finished 9days later at the southern end.ADCPA 150 kHz RDI broad-band ship-mounted ADCPwas used to collect vertical profiles of current whileunderway. The ADCP was set up to produce profilesconsisting of 10-min ensembles of once-per-secondpings. The nominal vertical resolution was set to 8 m.Navigation was derived from an Ashtech globalDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 ChiswellCirculation within the Wairarapa Eddy 693-, ~s174DE 1-C-L 17fl"E 1B0"W i 7SUWFig. 1 Locations of hydrographic stations (circles) made during a survey of the Wairarapa Eddy in March-April2001. Location of the mooring is shown as a diamond. Mean dynamic height , as determined by Roemmich& Sutton (1998) is shown as contours. The 2000 and 3000 m isobaths are also shown.positioning system (GPS). The latitude and longitudewere recorded at the end of every ensemble, and usedto remove ship motion from the measured currents.ADCP data were processed using the CODASsystem, provided by Eric Firing at the University ofHawaii. Final data used here consist of 10-min en-sembles. The estimated error in velocity of eachensemble is less than 0.01 m s1 (Sutton & Chereskin2002).The maximum depth range of an ADCP is con-trolled by several factors including the number ofscattering particles in the water column, and weather(heavy weather introduces air bubbles into the upperwater column, which reduces the ADCP range). Fig.2 shows a histogram of the number of 10-min en-sembles as a function of depth. The highest returnrate was just below 50 m, and fell off rapidly below100 m. Very few profiles extended as deep as 200 m.Current metersA current meter mooring has been maintained nearthe nominal centre of the Wairarapa Eddy (41S,17830'E) since October 2000. Initially, this moor-ing had current meters at 70 and 1500 m depth but,in March 2001, an additional meter was deployed18 m above the sea floor depth of 3120 m.Downloaded by [University of Illinois Chicago] at 18:56 22 October 2014 694 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 3750E 100f200250xADCP return050S 100I 150o2000 200 400 600 800 1000 1200Number of ensembles250B| Temperature |10 15 20Temperature (K)25Fig. 2 A, Histogram of number of 10-min data ensembles from the Acoustic Doppler Current Profiler (ADCP)during the survey of the Wairarapa Eddy. Return rate was highest at 60 m. Nominal 100 m reference level currentswere calculated as the mean currents between 82 and 114 m as indicated by horizontal lines. B, Temperature profilesfrom the survey indicate the depth of the mixed layer was typically between 50 and 100 m.TidesRemoving tides from the ship-board ADCP data isproblematic because of the spatially varying track.Here we use the output of a finite-element tidalmodel of Walters & Goring (2001) as an estimateof the barotropic tide and remove it from the ADCPdata on a sample by sample basis.Two aspects of the tide removal need comment.First, we have little robust analysis of how well theWalters & Goring model predicts the barotropic tidalcurrents. Second, strong baroclinic tides are gener-ated at both the Kermadec Ridge and Chatham Rise(Chiswell 2000; Chiswell & Moore 1999), and wesuspect that the tides within the Wairarapa couldhave significant baroclinic content that is not re-moved by the tidal model. The baroclinic tide has awavelength of the order of 150 km, and may con-tain a coherent structured component as well as in-coherent energy.100 200Distance (km)300Fig. 3 Normalised spatial covariance function for sealevel, derived from regridded AVISO TOPEX/Poseidondata (dots). Best-fit function (see text) is shown as a solidline.METHODThe geostrophic flow at a given depth (p) can be con-sidered in terms of a stream function (\\fp) where:and vp = -p dy and '* dx(e.g., Gill 1982). On anf-plane, the stream functionand dynamic height are related so that:ref+V prefwherefis the Coriolis parameter, and ADp/pref is thedynamic height between thep and pref levels.Thus the geostrophic referencing problem reducesto deriving the stream function at some level of"known motion" (pref) and using dynamic heightrelative to that level to compute the absolute currentselsewhere.The technique used here enforces non-divergenceby computing \\fpref from the ADCP-measured cur-rents using an objective-mapping technique that re-quires five covariance functions (shown later), whichDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 ChiswellCirculation within the Wairarapa Eddy 695are derived from the spatially-lagged dynamic heightcovariance function, Cdd (r), where r is the spatiallag (km).Cdd can be estimated from satellite observationsof sea level from T/P altimeter. Fig. 3 shows Cddcomputed at the centre of our hydrographic arrayover the bounds of the array using time series ofgridded sea level. That is, we show normalisedcovariance between sea level at the centre of thearray and sea level elsewhere plotted as a functionof distance. Data for depths less than 2000 m havebeen excluded.Chereskin & Trunnell (1996) and Sutton &Chereskin (2002) use a 2-parameter function fornormalised Cdd based on Walstad (1991):(1)where a and b are chosen to fit observations. Here,we use the same functional form for Cdd and com-pute a least-squares best fit which results in a =130 km and b = 180 km. The main advantage ofusing this functional form for Cddis that we can usethe same five covariances that go into the schemeas given by Chereskin & Trunnell (1996). There issome scatter in Cdd, but the fit to the correlationpasses reasonably well through the cluster, and thereis little in the figure to suggest that Cdd should bemapped using a different function. Our values of aand b are higher than used by Sutton & Chereskin(2002) (they obtained a = 112, b = 119 km fromexpendable bathymetric thermistor data), which sug-gests that the spatial correlation length scale withinthe Wairarapa Eddy is slightly longer than in the EastAuckland Current.With Cdd defined by Equation 1, the fivecovariances as a function of location (x,y) that go intothe scheme are:Cvv=\(R(r)-S(r)) + S(r)rv = - ( )dF-^R(r) = -\^r-Ideally, one would reference geostrophic currentsto ADCP measurements using a reference level asdeep as possible to get out of the region dominatedby wind-driven and other ageostrophic currents.Here, we chose our reference level to be nominallyat 100 m, as a compromise between being in thewind-driven layer, and losing too much data becauseof the poor rate of return (Fig. 2). Currents at thenominal depth of 100 m were computed as the av-erage current in the 81-114 m centred depth bins.Tides were removed from the ADCP data using theoutput of the finite-element tidal model of Walters& Goring (2001), although in practice, this makeslittle impact on the calculation oi\yprep probably be-cause much of the energy in the tidal band is becauseof baroclinic tides.Thus, the process can be described as using theADCP-measured currents to derive a non-divergentvelocity field and its corresponding stream functionat 100 m (i.e., U100, V100, \)/100). One assumes thatthese fields are the best estimate of the geostrophicflow at 100 m, and the velocity at any other depthcan be calculated from dynamic height. (Note: pres-sure and depth are used interchangeably here todescribe the vertical ordinate. Thus we assume thatthe differences in velocity between 100 m and 100dbar (for example) are far less than the errors asso-ciated with this analysis.)The objective mapping returns a normalised er-ror estimate. The normalised error tends to be lowwithin the area encompassed by the cruise track, butclimbs rapidly with distance away for the edges ofthe track. Determining the acceptable error is some-what subjective, but is generally not critical becauseof the rapidity with which the error function in-creases outside of the cruise track. Here we choseto accept estimates with an error estimate of 0.145.RESULTSFigure 4 shows the 10 min ensemble ADCP currentsnominally at 100 m, denoted as (ua, va), in vectorform. These currents are superimposed on surfacedynamic height relative to 2000 dbar, ADo/2Ooo> de-rived from an objective analysis of the CTD data. Al-though there is scatter in the directions and strengthsof the currents, there is a general agreement betweenthe direction of the currents and the directions im-plied by the dynamic height field. By and large,currents on the west side of the eddy tend to be southor south-west directed, whereas those on the east sideof the eddy tend to be directed to the north. There isDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 3741 S42S43S174UE 176E 178E 180W 178WFig. 4 Reference level (nominally 100 m) currents derived from the Acoustic Doppler Current Profiler. Every sec-ond 10-min ensemble current vector is shown superimposed on dynamic height, ADopm, derived from an objectiveanalysis of the Conductivity Temperature Depth data.variability superimposed on this general trend, forexample rotation of the vectors caused by near-in-ertial oscillations and/or tides during the times theship was on station.Figure 5 shows the zonal component of the 100 mvelocity shown in Fig. 4 as a time series. The figurealso shows the zonal component of the tide derivedfrom the tide model. Because the tide model does notremove the baroclinic tide, and because there maybe significant energy at inertial frequencies (here theinertial period is c. 18.7 h), the detided referencevelocity was smoothed with a running-mean filterthat removes most energy at periods of 1 day or less(shown in the figure as a thick line). This smoothedreference velocity, which is denoted as (us, vs), wasthe input into the objective mapping. The figure alsoshows the difference between the smoothed refer-ence velocity and the 100 m velocity. This residualgives some estimate of the high frequency variabil-ity caused by tides and inertial oscillations, althoughit does not necessarily contain all the wind-drivenmotion. The standard deviation of the smoothedzonal reference velocity was 0.13 m s1 comparedwith 0.015 and 0.08 m s1 for the model tide andDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 ChiswellCirculation within the Wairarapa Eddy 6972 3 4 5 6 7 8 9 10 11 12 13April2001Fig. 5 Zonal velocity at 100 m, ua (thin line), from the Acoustic Doppler Current Profiler plotted as a time series.Also shown are the detided, smoothed velocity, aus (thick line), used as input for objective mapping. Tides from the tidemodel and residual velocity are shown with 0.4 and 0.8 m s J offsets, respectively.residual terms, respectively. Values for the meridi-onal components are slightly higher (0.185, 0.027,and 0.109 m s1, respectively). Thus the residualmotion has about one-third the variance of thesmoothed reference motion.The objectively mapped stream function, \|i100, isshown in Fig. 6 superimposed on the surface dy-namic height, ADo/2Ooo> derived from the hydro-graphic survey. Only values where the normalisederror is less than 0.145 are shown. The correspond-ing velocity vectors (u100, v100) and the smoothedreference velocity are shown in Fig. 7. TheWairarapa Eddy appears as a depression in \|i100 cen-tred very nearly on the Roemmich & Suttonclimatological mean. Compared to the climatology,the East Cape Current is not as well developed, butinstead, there are two lobes of cyclonic circulation(in the south-west and north-west of the domain)which affect flow over the shelf. These lobes placestrong onshore flow just north of Mahia Peninsula(MP in Fig. 7) and an offshore flow in the northernWairarapa Coast. Both flows are unreasonable ifthey extend all the way to the coast, and in part stemfrom the fact that the objective mapping is not con-strained to produce normal flows at the coast line.The analysis also does not have a spatially-varyingspatial scale, so that the coastal area is treated ex-actly as the oceanic regime. In reality Cdd is likelyDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 69837S38S39S40S41 S42S43SNew Zealand Journal of Marine and Freshwater Research, 2003, Vol. 3712000010000-100001-20000174E 176E 178E 180W 178WFig. 6 Objectively-mapped stream function at 100 m, \|/100, derived from the Acoustic Doppler Current Profiler-derived velocity field shown in Fig. 4. For comparison, the dynamic height, ADopm, is superimposed as green be heterotropic and have smaller scales over theshelf than within the ocean. To the north of the sur-vey, one can just see the southern extent of a sec-ond eddythis is the East Cape Eddy, which iscentred off East Cape.Figure 8 shows the dynamic height field at 100dbar with respect to 2000 dbar. To be consistent withFig. 6, it has been rescaled into stream function units,and denoted as ipioocooo- This field has been calcu-lated from an objective mapping of the dynamicheight computed at each CTD station, using aGaussian function for Cdd having a spatial e-fold-ing scale of 1 degree in latitude and longitude. (Thise-folding scale was chosen as the minimum that pro-vided dynamic height "filled-in" between the CTDstations.)Thus Fig. 6 shows the circulation at 100 m de-rived solely from the ADCP data, whereas Fig. 8shows the circulation derived solely from the CTDdata. The main difference between the fields shownin the figures is that the ADCP-derived estimateextends on to the shelf, whereas the CTD-derivedestimate is not defined in water less than 2000 dbardepth. However, where both fields are calculable,there is encouraging agreement between the two,including the lobes of cyclonic flow on the westernends of the survey. The main differences are rela-tively small differences in the shape and strength ofthe Wairarapa Eddy, and in the extent of the cycloniclobes.The difference between the stream functionsshown in Fig. 6 and 8 is the implied circulation at2000 m derived from referencing geostrophy usingthe ADCP measurements, and is shown in Fig. 9 as\|2OOO- It shows the Wairarapa Eddy penetrates to atleast this depth. Velocities over most of the regionare 0-0.1 m s1. However, along the extreme east-ern end of the domain, they reach 0.16 m s1 in aDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 ChiswellCirculation within the Wairarapa Eddy 69937S38S39S40S41 S42S43S174E 176E 178E 180W 178WFig. 7 Velocity (u100, v100), derived from the stream function shown in Fig. 6 shown as a grid of vectors. For compari-son, the smoothed reference level currents derived from the Acoustic Doppler Current Profiler are superimposed asvectors along the cruise track. (MP = Mahia Peninsula.)region east of the cruise track. Although the objec-tive mapping returns an acceptable error estimate forthis part of the domain, we suspect that the highvelocities there may be unrealistic. The mean speedat 2000 dbar is 0.07 m s1 with a standard deviationof 0.03 ms1.SUMMARY AND DISCUSSIONThe absolute flow field at 100 m was estimated byobjective mapping of the ADCP-derived velocities toproduce a velocity field that has enforced non-divergence. If enforcing non-divergence produces thebest estimate of the geostrophic flow, this velocity fieldcan be used as a "level of known motion" to referencegeostrophic velocities at other depths. In particular, itcan be used to estimate the velocity field at 2000 dbar,which is otherwise used in this region as a level of nomotion. The resulting flow at 2000 dbar has a meanspeed of about 0.07 m s1, and appears to be wellcorrelated with the surface flow, which suggests theWairarapa Eddy penetrates at least this deep.This analysis implicitly assumes that theageostrophic motion is divergent, and can be sepa-rated from the non-divergent geostrophic flow. Thereare few, if any, data that can be used to test this as-sumption directly. The current meter mooring nearDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 70037S38S39S40S41 S42S43SNew Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37n2000010000-10000-20000174E 176E 178E 180W 178WFig. 8 Stream function at 100 m relative to a level of no motion at 2000 dbar, \|/loof,ooo, derived from the hydrographicsurvey. Assuming an f-plane, this is rescaled dynamic height of 100 dbar referenced to 2000 dbar:V o= y ^ I 0. A l s o s h o w nt h e derived velocity vectors, (u100/2000, u0/2000).the centre of the eddy returned data at 70 m, but acomparison between these data and our flow calcu-lations illustrates the difficulty in comparing pointmeasurements with those derived from a 100-kmlength-scale analysis. Fig. 10 shows meridional andzonal components from the meter at 70 m duringApril and May 2001, both before and after detidingand low-pass filtering. Also indicated are u100 andv100 at the current meter location derived from theobjective mapped field shown in Fig. 7. The objec-tively mapped U100 and v100 at the current meter sitewere 0.1 m s1 and-0.13 m s1, respectively. Com-parison of the current meter data with the objec-tively-mapped data is complicated by the fact that,as luck would have it, the cruise took place almostexactly during a period of intense inertial oscilla-tions. During the time of the cruise, the current meterdata certainly spanned these values but, for the mostpart, the currents from the meter were more nega-tive in the zonal direction, and more positive in themeridional direction (i.e., more to the north andwest). We could find no justifiable filtering that gavegood agreement between the current meter data andU100 and v100 in both components.Assuming that the inertial oscillations can beadequately removed by filtering, the difference be-tween the objectively-mapped and current meter datacan only be explained by noise in the objectivemapping and/or large ageostrophic currents in thecurrent meter record. Sutton & Chereskin (2002)were faced with a similar problem when they com-puted velocities at 2000 dbar of c. 17 m s1i.e., 2 -3 times our values. Their comparison with theircurrent meter data is ambiguous about validatingDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 ChiswellCirculation within the Wairarapa Eddy 70137S38S39S40S41 S42S43S174E 176E 178E 180Wn2000010000-10000'-20000178WFig. 9 Implied circulation at 2000 dbar, y7000, and associated current field. This is the circulation is derived using100 m velocity from the Acoustic Doppler Current Profiler to reference the geostrophic velocities.their velocities, and they comment that "ADCP prod-uct is either overestimating the flow field or thatthere is a noise contribution". Thus, it seems con-siderable effort has to be exerted to overcome theinherent mismatch in temporal and spatial scaleswhen comparing derived flow fields with currentmeter data.However, if the objective mapping does producean accurate estimate of the geostrophic flow, thenone can compute the ageostrophic flow by subtract-ing the mapped velocities from the ADCP-measuredvelocities. Fig. 11 shows the smoothed ADCP ref-erence velocity components and the correspondingU100 and v100 at the (time-varying) ship's location.The differences between these respective fields areour best estimates of the low-frequency ageostrophicterms, and have between 16 and 20% of the varianceof the geostrophic terms. It is well beyond the scopeof this article, but if one had good models of thewind-driven circulation, it would be a test of themapping technique to compare these ageostrophicterms with the wind-driven circulation.The spatial scales of the circulation seen in Fig.6 and 7 are controlled by the parameters set in theobjective mapping, and although we have no robustway of performing a sensitivity analysis based onthese parameters it is useful to at least inspect thelikely impact of getting them wrong. These param-eters are the length scales a and b in Equation 1, andthe estimated noise error of the ADCP measure-ments.Our estimate of Cdd (Fig. 3) was derived usinggridded AVISO T/P data. It is possible that the func-tion shown in Fig. 3 reflects the objective mappingDownloaded by [University of Illinois Chicago] at 18:56 22 October 2014 702 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37IO-0.52 6 2 7 2 8 2 9 3 0 3 1 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 18 1 9 2 0Mar Apr2 0 0 1Fig. 10 Zonal and meridional currents from the current meter 70 m depth at 41S, 17830'E during April and May2001. Currents are shown both before and after detiding and filtering with a 5-day low-pass filter. Zonal currents areplotted without an offset, meridional currents are offset by 0.5 m s1. Vertical lines on 3 and 12 April indicate theduration of the Wairarapa Eddy survey. Solid squares indicate velocities extracted for the current meter site from theobjectively mapped 100-m field.used in the gridding the AVISO data, rather than anintrinsic covariance of the ocean. If so, the true oce-anic length scales a and b could be shorter thanimplied by the figure. Performing our analysis witha and b set to the Sutton & Chereskin (2002) valuesof 112 and 199 km, respectively, produces qualita-tively similar results to those shown here, except thatthere is slightly more structure to \|i100 and derivedfields. It does not change the conclusions, or thecomparison between derived velocities and the cur-rent meter values. Choosing different values for thenoise error of the ADCP measurements similarlychanges the results only in the details, and not in theconclusions.Referencing geostrophy to ADCP measurementshas a certain dynamic appealit seems better to usereal measurements to reference geostrophy ratherthan to assume the flow goes to zero at some depth.The main result of this approach is that we now havea one-time quantitative estimate of the flow at 2000dbar. This can be considered the error in referenc-ing geostrophic velocities if one assumes a 2000 dbarlevel of no motion, i.e., c. 0.04 m s1. This errorestimate is independent of depth. At the surface, theimpact is primarily in relatively small structuraldetails of the strength and location of the WairarapaEddy, but as one goes below 2000 dbar, the relativeimpact becomes much greater.Downloaded by [University of Illinois Chicago] at 18:56 22 October 2014 ChiswellCirculation within the Wairarapa Eddy 703Fig. 11 Lower panel: SmoothedAcoustic Doppler Current Profilerreference zonal velocity, u , andcorresponding velocity at the(time-varying) ship's locationfrom the objectively-mappedstream function, u100. Also shownwith 0.5 m s1 offset is the best es-timate of the ageostrophic current,u = u - u . Upper panel: as inag 100 s r r rlower panel, but for the meridionalvelocity.0.55 6 7April 20018 9 10 11ACKNOWLEDGMENTSI thank T. Chereskin and P. Sutton for valuable commentson this work. M. Bowen provided the objective mappingroutines; these routines contain contributions from J.Wilkin, N. Bindoff, and R. Raghu. I thank R. Singleton,B. Stanton, M. Walkington, L. Northcote, M. Grieg, W.Main, M. Oliver, P. Wiles, and D. Tindale for participat-ing in the cruise and for their work in obtaining the dataused here. The master and crew of R/V Tangaroa arethanked for their help in all aspects of the cruise. With-out the assistance of all these people, this work could nothave been accomplished. This work was carried out un-der Foundation of Research, Science and Technologycontract CO 1X0037. The AVISO altimeter productswere produced by the CLS Space Oceanography Divi-sion as part of the European Union "Environment andClimate project AGORA (ENV4-CT9560113) andDUACS (ENV4-CT96-0357)" with financial supportfrom the CEO programme (Centre for Earth Observation)and Midi-Pyrenees Regional Council.REFERENCESBradford, J. M.; Heath, R. A.; Chang, F. H.; Hay, C. H.1982: The effect of warm-core eddies on oceanicproductivity of northeastern New Zealand. DeepSea Research 29: 1501-1516.Bretherton, F. P.; Davis, R. E.; Fandry, C. 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