Mean and variability in the Wairarapa and Hikurangi Eddies, New Zealand

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  • This article was downloaded by: [Central Michigan University]On: 29 October 2014, At: 13:47Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

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    Mean and variability in the Wairarapaand Hikurangi Eddies, 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 (2005) Mean and variability in the Wairarapa and HikurangiEddies, New Zealand, New Zealand Journal of Marine and Freshwater Research, 39:1, 121-134, DOI:10.1080/00288330.2005.9517295

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  • New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39ChiswellWairarapaandHikurangiEddies: 121-1340028-8330/05/3901-0121 The Royal Society of New Zealand 2005

    121

    Mean and variability in the Wairarapa and Hikurangi Eddies,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 mean and variability of the circula-tion off the east coast of the North Island, NewZealand are investigated using shipboard conductiv-ity-temperature-depth (CTD) and satellite altimeterdata collected between 1993 and 2003. The altim-eter data are used to adjust the in situ observationsfor the mesoscale eddy variability before computingthe 11-year mean in dynamic height. Mean dynamicheight shows two anticyclonic eddies, centred near178.3E, 41.2S and 176.2E, 42.4S. These loca-tions are consistent with previous historical obser-vations of the Wairarapa and Hikurangi Eddies,respectively. A long-term trend in both in situ andsatellite data shows that dynamic height rose at anaverage rate of up to 2 dyn cm year-1 in the centreof the Wairarapa Eddy which is consistent with astrengthening of the eddy over the 11 years. Thesatellite data show periodic shedding of theWairarapa Eddy from near East Cape at a rate ofbetween two and three eddies per year. Thus, ratherthan indicating a permanent stationary eddy, themean eddy reflects a region where the eddies tendto stall out or merge with the previous eddy. Often,the eddies will continue up the Hikurangi Trough sothat the Hikurangi Eddy can be regarded as an olderWairarapa Eddy.

    Keywords Wairarapa eddy; circulation; climatol-ogy

    M04121; Online publication date 18 February 2005Received 17 June 2004; accepted 23 November 2004

    INTRODUCTION

    The circulation off the east coast of the North Island,New Zealand is dominated by a number of perma-nent or semi-permanent mesoscale eddies embeddedin the East Auckland and East Cape currents. Ofthese eddies, at least three have been consideredpermanent enough, or important enough, to warrantbeing named: the North Cape Eddy, the East CapeEddy, and the Wairarapa Eddy (Roemmich & Sutton1998).

    The Wairarapa Eddy is the southernmost of thesethree eddies, and is found over the HikurangiTrough, trapped between the Chatham Rise and thesouth-east coast of the North Island (Fig. 1). It isanticyclonic, warm-core, and is probably formed bythe retroflection of the East Cape Current (ECC)forced by the presence of the Chatham Rise(although no one has made a specific analysis of itsdynamics). Roemmich & Sutton (1998, hereafterRoemmich & Sutton) comment that the WairarapaEddy "appears to be the deepest of the three warm-core eddies, exceeding 2000 m", and although theyacknowledged their lack of temporal and spatialsampling within the eddy, speculated that it was the"least variable of the three named eddies". Theydescribe the eddy as being centred 178.5E, 41S,and suggest it is "constrained by the bathymetricwedge of deep water between the coast of (the) NorthIsland and the Chatham Rise".

    The importance of the Wairarapa Eddy arisesfrom its ability to retain lobster larvae long enoughfor them to reach metamorphosis. New Zealand rocklobster (Jasus edwardsii) have a larval life ofbetween 1 and 2 years and, for the species to survive,metamorphosis must take place within 200 km of thecoast (Jeffs et al. 2001). Without an entrainmentmechanism, larvae would be advected well outsidethis limit. Booth (1994), Chiswell & Roemmich(1998), and Chiswell & Booth (1999) used larvaltows and numerical simulations to show that larvaeare trapped in the eddy, and that were it not for thepresence of the eddy, this species would probablynot exist on the east coast of New Zealand. The

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  • 122 New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39

    36S

    37S

    38S

    39S

    40S

    176E 178E 180W 178W 176W

    Fig. 1 Map showing locations of all conductivity-temperature-depth (CTD) profiles made in water deeper than2000 m since the launch of the Topex/Poseidon satellite (October 1992). Station locations are superimposed on themean surface dynamic height as determined by Roemmich & Sutton (1998). EAUC and ECC indicate the East Auck-land and East Cape Currents, respectively. Insets show the CTD locations for the three survey cruises discussed in thetext. The 2000 m isobath is shown as a dashed line.

    Wairarapa Eddy may also be important to the localmarine climate, in that it brings relatively warmsubtropical water to the region (Roemmich &Sutton).

    The Wairarapa Eddy may not be as invariable asRoemmich & Sutton imply, and the mean circulationsuggested by them differs in detail from the bulk ofhistorical observations. Early work by Heath (1973)suggested that the eddy can spawn smaller eddiesthat travel up the Hikurangi Trough, and many earlierresearchers refer to an anticyclonic eddy at the headof the Hikurangi Trough in quite a different location

    from the Roemmich & Sutton eddy (see historicalnote below).

    Since Roemmich & Sutton compiled theirclimatology, there have been a number of researchcruises to the Wairarapa Eddy. Three in particularhave been designed to provide relatively high-spatialresolution surveys of the circulation within the eddy.In addition, there are now 11 years of Topex/Poseidon (T/P) altimeter data (compared to the fouravailable to Roemmich & Sutton). So it is perhapsappropriate to have another look at the mean andvariability of the Wairarapa Eddy.

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  • ChiswellWairarapa and Hikurangi Eddies 123

    The first aim of this article is to estimate the meancirculation off the east coast of New Zealand usingaltimeter data to adjust in situ observations for themesoscale eddy fieldwhere there is a large amountof variability one obtains a better estimate of themean field by removing the variability beforeaveraging the in situ data. The second aim is toquantify the variability by estimating how stable theWairarapa Eddy is, how frequent eddies are shed,and what happens to them.

    After a brief summary of the historical obser-vations, the altimeter and in situ data are used toshow that there was a long-term trend in the 11 yearsof data, and that this needs to be removed beforecomputing the mean dynamic height fields. Thetrend itself is then discussed as it shows anintensification of the circulation over 11 years. Thevariability is then qualitatively described by asequence of "snapshots" taken from the altimeterdata. Finally, quantitative estimates of the eddyshedding and their translation velocities are madefrom Hovmuller space-time plots of dynamic height.

    HISTORICAL NOTE

    The presence of an anticyclonic eddy or eddies offthe east coast of the North Island has been inferredfor some time (e.g., Sdubhundhit & Gilmour 1964),although most of these eddies were left nameless.Garner (1969) shows a large anticyclonic eddy eastof the North Island in his analysis of dynamic height,but thought," eddies which separate in the vicinityof East Cape .. would probably be migratory, firstmoving southwards then eastwards past theChatham Islands". Similarly, Heath (1973) stated," Where the ECC turns north, a large permanentanticyclonic eddy is formed .. Small eddies, whichare probably periodically shed off from the perma-nent eddy are guided by the bottom topographytowards Kaikoura .. The 50-70 day periodicity inthese eddies is probably linked to a similarperiodicity in the East Australia Current."

    Early researchers also pointed to two separatestable eddies. For example, Barnes (1985) usedsatellite sea-surface temperature data to conclude theexistence of a permanent anticyclonic eddy centredat 176E, 42S. Barnes termed this eddy "E", andrecognised it as different from an eddy centred at179E, 41S (i.e., the Roemmich & SuttonWairarapa Eddy). The Barnes eddy "E" was the topicof Bowman's (1985) analysis of the beta-effect as a

    mechanism for eddy formation, although Bowmancalls it the "Cook Strait Eddy".

    The first appearance of the term "WairarapaEddy" is in a review of oceanography by Bradford-Grieve et al. (1991) and they use the term to meanthe Barnes eddy "E". Greig & Gilmour (1992) alsouse the "Wairarapa Eddy" in their analysis of flowthrough the Mernoo Gap, and although they do notdefine its location, appear to be referring to eddy "E".

    Roemmich & Sutton analysed all availableconductivity-temperature-depth (CTD) and expend-able bathythermograph (XBT) data in their analysisof the mean circulation around New Zealand. Theyused a 2 spatial correlation scale in their objectivemapping. There is no hint of an eddy centred near176E, 42S, in their analysis, and they only see alarge eddy centred at 178.5E, 41S. Roemmich &Sutton called this eddy the Wairarapa Eddy, despitethe fact that most previous usages of this termreferred to a more southern eddy.

    Perhaps because the Roemmich & Sutton workwas comprehensive and more specific about namingthe eddies, the term Wairarapa Eddy has stuck forthe more northern eddy, with all recent work usingthe Roemmich & Sutton terminology. For example,Tilburg (2001) and Chiswell (2003). Shaw &Vennell (2000) use the term Wairarapa Eddy for anorthern eddy and Hikurangi Eddy for a southerneddy.

    DATA

    AltimeterThe United States-French T/P satellite altimetermeasures sea level along the same path every 9.9156days (Fu et al. 1994), and is generally reckoned toprovide sea level accurate to a few cm (e.g., Mitchum1994). Similarly, ERS-1 and -2 satellites launchedby the European Space Agency carry a radaraltimeter, although the ground path characteristicsof the satellites differ from the T/P instrument.

    The data product used here is the AVISO "Mapsof Sea Level Anomaly" provided by AVISO/Altimetry, Space Oceanography Division, France.Seven-day maps of sea level anomaly on a one-thirddegree grid are derived from merged T/P and ERSsatellite data.

    Because of uncertainties in the geoid, T/P data aregenerally considered to be variations about the truemean sea level, and the product used here is forcedto have zero spatial mean by removing the 11-year(1993-2003) mean at each grid point.

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  • 124 New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39

    Feb1998 Apr 2001

    2.1

    Mar 2003 Roemmich&Sutton mean

    175 176 177 178 179 180 179 175 176 177 178 179 180 179

    Fig. 2 Objectively analysed dynamic height, AD0/?000 (dyn m), from the three survey cruises. Also shown is theRoemmich & Sutton mean. Note the changes in the colour scale between cruises. Dashed line is the 2000 m isobath.

    CTDAll CTD data between Chatham Rise and 38S, andwest of 179W since the launch of T/P are used (Fig.1). These comprise data from 206 casts made inwater deeper than 2000 m. There were a total of 14cruises to the region, three of which, comprising 124casts, were designed specifically as surveys of theWairarapa Eddy.

    CTD data collection was similar on all cruises. ASeabird CTD profiler in a 12- or 24-place rosettewith 1.2-litre Niskin bottles was used to make con-tinuous vertical profiles of temperature and salinityat each station. Water samples were collected tocalibrate the conductivity sensor. CTD data col-lection and processing methods were similar to those

    detailed in Chiswell et al. (1993) and Walkington &Chiswell (1993).

    RESULTS

    Mean dynamic heightFigure 1 shows the locations of all CTD casts madein water deeper than 2000 m in the region since thelaunch of the T/P altimeter. Of the 14 cruises madeto the region, three were designed specifically tosurvey the eddy system. There were a total of 206deep CTD casts. Of these, 124 were made during thethree survey cruises (shown in figure inset).

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  • ChiswellWairarapa and Hikurangi Eddies 125

    0.05 0.1D (dyn m)

    Fig. 3 Histogram of dynamic height anomaly, D'p de-termined at the 206 stations from the Topex/Poseidon al-timeter.

    Dynamic height of the sea surface relative to 2000dbar, ADo/2Ooo> was computed for each cruise. Thelower level was chosen to be 2000 dbar principallyto be consistent with previous calculations (e.g.,Heath 1972), where it was used as a level of nomotion. There is other evidence to suggest that thismay be a reasonable choice (e.g., Warren 1970).

    Figure 2 shows ADo/2Ooo for t...

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