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, UK

    New Zealand Journal of Marine andFreshwater ResearchPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tnzm20

    Circulation 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:10.1080/00288330.2003.9517199

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  • New 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 2003

    691

    Circulation within the Wairarapa Eddy, New Zealand

    STEPHEN M. CHISWELLNational Institute of Water and Atmospheric

    Research LimitedP.O. Box 14 901Wellington, New Zealandemail: s.chiswell@niwa.cri.nz

    Abstract 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; eddy

    M02052; Online publication date 31 October 2003Received 27 June 2002; accepted 27 June 2003

    INTRODUCTION

    There 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 in

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  • 692 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37

    the 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 components

    are 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).

    DATA

    HydrographyFigure 1 shows the locations of stations occupiedduring a 9-day survey o