the wairarapa coastal current

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The Wairarapa Coastal CurrentStephen M. Chiswell aa National Institute of Water & Atmospheric Research Ltd , P. O.Box 14 901, Wellington, New Zealand E-mail:Published online: 29 Mar 2010.

To cite this article: Stephen M. Chiswell (2000) The Wairarapa Coastal Current, New ZealandJournal of Marine and Freshwater Research, 34:2, 303-315, DOI: 10.1080/00288330.2000.9516934

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New Zealand Journal of Marine and Freshwater Research, 2000, Vol. 34: 303-3150028-8330/00/3402-0303 $7.00 © Tho Royal Society of New Zealand 2000

303

The Wairarapa Coastal Current

STEPHEN M. CHISWELLNational Institute of Water & Atmospheric

Research LtdP. O. Box 14 901Wellington, New Zealandemail: [email protected]

Abstract A new name is proposed for the rela-tively cool, fresh, northwards-directed flow along theWairarapa coast of New Zealand. Data from tworesearch cruises plus historical hydrocast dam areused to show that the water within this current ismodified subtropical water with sources likely to bethe Southland and D'Urville currents. This currenthas previously been known as the Canterbury Cur-rent or as an extension of the Southland Currer t, butbecause of its source and location, a better name isthe Wairarapa Coastal Current. One-month-longcurrent meter records made off the Wairarapa coastshow flow continuously to the north during Febru-ary 1998. Volume transports within the WairarapaCoastal Current in February 1998 were c. 1.6 Sv offCape Palliser, diminishing northwards as the currentbecomes entrained into the East Cape Curren:.

Keywords Wairarapa; coastal current; water massanalysis

INTRODUCTION

It is well known that New Zealand intersecis theeastward-flowing southern arm of the South PacificSubtropical gyre (e.g., Stanton et al. 1997). Flowsalong the east coast of the North Island are thusgenerally southward to about the latitude cf theChatham Rise (Fig. 1) and can be regarded as a re-attachment of the South Pacific westward boundarycurrent. Locally, however, the currents east of the

M99037Received 23 June 1999; accepted 17 January 2009

North Island are regarded as two separate currents,with that north of East Cape termed the East Auck-land Current, and that south of the cape as the EastCape Current. Inshore of the East Cape Current,waters have lower salinity and temperature thanoffshore, and the current has often been observed toflow to the north (i.e., in the opposite direction to theEast Cape Current). This inshore current has not beenstudied extensively, and many references regard itas an extension to the Southland Current, whichflows north along the east coast of the South Island.This notation stems from Heath (1972b), who pre-sented evidence for the continuity of the SouthlandCurrent northwards past Banks Peninsula and pro-posed withdrawing the earlier name CanterburyCurrent. Heath states that "characteristic low tem-perature and salinity water of this current apparentlyresults from the lower salinity subsurface water ofthe Southland Current, found over the continentalslope south of Banks Peninsula being forced up-wards in its passage through Mernoo Gap."

In this note, I use available hydrographic data toshow that the temperature-salinity (T-S) structurewithin the inshore current is inconsistent withHeath's explanation, and show that the waters areunlikely to be purely those from the SouthlandCurrent. Instead, it appears that the source waters forthe current are a mix of subtropical waters from theSouthland Current and the D'Urville Current, whichflows into Cook Strait from the west. This mixingtakes place east of Cook Strait and south of theWairarapa Coast. For this reason, I propose a newname for the current—the Wairarapa Coastal Current(WCC).

This note proceeds as follows. After a brief re-view of the historical observations relating to theWCC, and a review of the data sets used here, Ipresent hydrographic and Acoustic Doppler CurrentProfiler (ADCP) data from February 1998 that shownorthwards flow inshore of about the 1000-2000 misobath. This flow extends at least as far up the coastas Mahia Peninsula, and possibly as far north as EastCape. Assuming geostrophy, the current has avolume transport of c. 1.6 Sv, which decreases

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33°S

New Zealand Journal of Marine and Freshwater Research, 2000, Vol. 34

36°S

39°S

168°E 172°E 176°E 180°W

Fig. 1 Sea surface temperature from 1-month composite of images taken in February 1998. Superimposed on seasurface temperature are the CTD (conductivity-temperature-depth) station locations from the cruises in 1994 and1998 (dots). Nine transects radiating out from New Zealand (labelled 1-9) are discussed in the text. Also shown areschematic indications of oceanic circulation around New Zealand (in italic): the East Auckland Current (EAUC); EastCape Current (ECC); Southland Current (SC); D'Urville Current (DC); Wairarapa Eddy (WE); and the WairarapaCoastal Current (WCC). The Subtropical Front (STF) can be seen as the region of higher temperature gradients lyingalong the Chatham Rise. The 1000 and 250 m isobaths are shown as dashed lines.

progressing north, presumably because of mixingand entrainment with the East Cape Current. Ananalysis of historic temperature-salinity structureshows that water within the WCC is too warm andtoo saline to have originated purely from theSouthland Current, but instead that it contains a sig-nificant contribution from the D'Urville Current.Finally, I use available current meter data to makesome estimates of the mean and variability over 1month in the along-shore flow.

The main water masses that are important in thisregion have distinct temperature-salinity character-istics, which allows one to estimate mixing levelsbetween them. Surface water to the north-east isSubtropical Water (STW), below this lies SouthPacific Central Water (SPCW) and below that Ant-arctic Intermediate Water (AAIW) which containsthe well-known salinity minimum. South of theChatham Rise and offshore of the South Island, sur-face waters are Subantarctic Waters (SAW), which

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Chiswell—Wairarapa Coastal Current 305

also overlie AAIW. AAIW north of the ChathamRise is older than AAIW south of the rise, c onse-quently its salinity minimum has been eroded bydiffusive processes. Thus in this region, AAIW hasa northern and southern branch, which can he dif-ferentiated by the strength of their salinity minima.STW and SAW water are separated by the Subtropi-cal Front, which lies along the Chatham Rise Thisfront also runs along the shelf break along ths eastcoast of the South Island, where it is known as theSouthland Front. Southland Current water inshore ofthe Southland Front is mainly subtropical water,mixed with some Australasian subantarctic water(Houtman 1966), and it has generally been suggestedthat the source of the current is water from the Sub-tropical Front west of New Zealand (e.g., Heath1975). A full discussion of the masses is beyond thescope of this article and they are well treated else-where (e.g., Tomczak & Godfrey 1994).

HISTORICAL OBSERVATIONS

Sea surface temperature often shows a cool tcnguelying adjacent to the east coast of the North Island,such as can be seen in a 1-month composite of seasurface temperature derived from satellite (Fig. 1).This tongue is variable in strength and spatial extent,but its persistence can be judged by the fact that it isoften seen in monthly composite images. Althoughsea surface temperature is not necessarily a goodindicator of circulation, since it can be quite affectedby local heating and or vertical mixing, the presenceof such a tongue has often been taken as evic enceof a northwards flowing current bringing relativelycool water up the coast.

Perhaps the earliest documentation of the WCCwas deduced from drift card observations. Brodie(1960) reports drift card movements from BanksPeninsula north past Cook Strait and along the eastcoast of the North Island. Cards dropped offSouthland all flowed north within the current, someas far as Gisborne. Brodie comments that "it i;> no-table that... the north easterly winds common overnorth Canterbury do not appear to influence the driftswithin [the current]", and also reports this current ashaving cool sub-Antarctic water (my bold foremphasis). Brodie believed that the current origi-nated off Canterbury, and called it the CanterburyCurrent (this is the name "withdrawn" by Heath).

Heath (1972b), as already stated, thought theWCC stemmed from the Southland Current, but

interestingly, his earlier work (1969) shows drift cardpaths flowing south through Cook Strait and aroundinto the WCC, and his later work (1976) shows aschematic (his fig. 1) which shows current patternscloser to those derived in this note.

METHODOLOGYData used here come from research cruises made inMay 1994 and February 1998. The 1994 cruise wasdesigned as a survey of the Southland Current, andthe 1998 cruise was designed as a survey of the EastCape Current. A series of transects radiating out fromNew Zealand was occupied during the cruises (Fig.1). For the purposes of this note, nine of thesetransects have been labelled, with the northern mosttransect labelled Transect 1. Transects 1-5 weremade during the 1998 cruise, and Transects 6-9 weremade during the 1994 cruise.

During each transect, hydrographic measure-ments were made with a CTD (conductivity-tem-perature-depth) profiler mounted in a 12-placerosette with 1.2-litre Niskin bottles attached to ob-tain water samples. Continuous vertical profiles oftemperature and salinity were made at each station.Water samples were used 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 & Sutton (1997). Profileswere processed to produce data every 2 dbar. Tem-perature is estimated to be accurate to ±0.002°C,salinity ±0.005.

Dynamic height, AD0/2000, was computed relativeto 2000 dbar, principally to be consistent with previ-ous calculations (e.g., Heath 1972a), where it was usedas a level of no motion. There is other evidence tosuggest that this may be a reasonable choice: Warren(1970) used the same level east of New Zealand, ar-guing that it lies above the deep tracer features (whichare indicative of meridional flow) and in the middleof the deep oxygen-minimum layer which he suggestsis a region of slow horizontal velocity. Calculatingcurrents where the sea was shallower than 2000 dbarwas made using methods similar to those described byFiadeiro & Veronis (1982). However, it should benoted that calculating currents over the shelf is alwayssubjective, and the values derived here should be takenas an indication of the flow, rather than a good deter-mination of it. The vertical shear will be well deter-mined, but the absolute values may be in some error.

The 1998 cruise was made using R/V Tangaroa,and this ship has a wide-band 150 kHz ship-board

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306 New Zealand Journal of Marine and Freshwater Research, 2000, Vol. 34

ADCP mounted in a pod under the hull. An AshtekGPS provides navigation, although bottom-trackingwas used where possible. Data are collected in 8 mvertical bins, and are processed as documented byOien (1997). Final data used here are verticallyaveraged over the top 50 m.

Historical hydrocast data from the NIWA (Na-tional Institute of Water & Atmospheric Research)archives are also used. Temperature is from revers-ing thermometers and salinity from water samples.Vertical spacing of the bottles varied, but was typi-cally 5-20 m in the upper 100 m, and higher atdeeper depths. Data used here were collected be-tween 1969 and 1980. They are mostly distributedover the summer months.

In 1998, two current meter moorings weredeployed within the Wairarapa Coastal Current forabout 1 month. These moorings were deployed at theinshore ends of Transects 3 and 4 (Fig. 1), in c. 250m water depth. Each mooring had one current meterplaced at a nominal depth of 100 m.

RESULTS

Hydrographic survey 1994-98Figure 2 shows averaged (0-50 m) currents derivedfrom the shipboard ADCP during the 1998 cruise(there are no ADCP data from the 1994 cruise). Tideshave been removed using model results of Walters& Goring (unpubl. data). The vectors are super-imposed on dynamic height ADo/2ooo> objectivelymapped using a decorrelation distance of 1 ° latitudeor longitude. Two regions of high dynamic heightindicate a large lobe of anticyclonic circulationcentred c. 180°E 40°S (this is the Wairarapa Eddy(Roemmich & Sutton 1998)), and a smaller eddycentred c. 177°E 42°S. Although there is somevariability in the ADCP measurements, presumablycaused by processes such as inertial oscillations,there is good agreement between the current vectorsand the dynamic height field. One can see, forexample, the East Cape Current flowing south alongthe inshore slope in dynamic height, and anticycloniccirculation around both eddies.

The WCC can be clearly seen in the ADCP meas-urements, where almost all vectors inshore of the1000 m isobath show a current running north-eastparallel to the isobaths. Vectors in Fig. 2 from cur-rents having a north-east component have been plot-ted in black for emphasis. The mean speed of flowsshown in black (most are in the WCC) is 0.2 lms"1 .Strongest currents were at the inshore end of

Transect 4, and were c. 0.3 m s '. It is also interest-ing to note the flow around Cape Palliser is sugges-tive of flow through Cook Strait. There is someuncertainty about the presence of the WCC betweenMahia Peninsula and East Cape, in part this is be-cause of lack of sampling within that area, but wherewe do have data on the shelf, the currents are small.Just south of East Cape, there is a suggestion ofnorthward flow, which is part of a small (-20 kmradius) cyclonic eddy; this structure is also seen incurrent meter data from 1995 (Chiswell &Roemmich 1998). There is no evidence of the WCCnorth of East Cape where the inshore flow is to thesouth.

The WCC is also seen clearly in geostrophic cur-rents derived from Transects 2-5 (Fig. 3). Contoursof three potential density (referenced to 0 dbar)isopycnals (Oo= 26.2, 26.55, and 26.9) have beensuperimposed in these plots. In all four southernsections, flow close to the coast is to the north, thisflow appears to be strongest in the upper 200 m orso, but extends all the way to the bottom. Strongestcurrents are in the southern sections (Transects 4 and5), and both the amplitude of the surface current andthe total northwards transports decreases as oneprogresses north. Sections 5 and 3 show secondarysubsurface northwards flows at depths of c. 600.Volume transports for the northwards flow boundedwithin 60 km of the coast (i.e., ignoring the subsur-face flows) are 1.6 Sv for Transect 5, diminishingto 1.3 and 0.6 Sv for Transects 4 and 3, respectively,and 0.01 Sv for Transect 2.

Thus from ADCP measurements and geostrophyit appears that during February 1998, the WCCappears within c. 40-50 km from the coast, andextends at least as far north as Mahia Peninsula.

Figure 4 shows temperature plotted againstsalinity (T-S) for Transects 1-9. For Transects 1-5,T-S from the CTD station within the WCC havingminimum salinity is shown in bold. For Transects6-9, T-S from the CTD having maximum salinitywithin the section is shown in bold. These stationsare referred to later.

Offshore subtropical water within the East CapeCurrent can be seen in Transects 1-3 as the warmest,most saline water, with surface temperatures near20°C, and salinities of c. 35.6. Inshore water in eachsection is fresher and cooler, and as one progressessouth, the trend for the inshore water (i.e., WCC)towards cooler temperatures and lower salinities isclear. At Transect 4, T-S follows this trend, but herethe lowest salinity is found at the most offshorestation since this station was within the Subtropical

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Front (see Fig. 1). Transect 5 shows two dstinctwater masses: inshore waters have salinities o:?34.8,offshore salinity is c. 35.3. Transects 6-9 showsalinities within the Southland Current are higherthan offshore. Highest salinities within the Sou MandCurrent are c. 34.6, and there is little evidence thatsalinity within the core of the current weakens ;is oneprogresses up the coast.

Thus Fig. 4 illustrates the different nature of thetwo currents. The WCC is a relatively cool, freshcurrent surrounded by warmer, more saline sut tropi-cal water offshore. In contrast, the Southlanc Cur-rent is a relatively warm, saline current surroundedby cooler, fresher subantarctic water offshore. How-ever, Southland Current salinities are considerablylower than those in the WCC—there is c. 0.2 differ-ence between the most saline Southland Current andleast saline WCC waters.

Water mass analysisThe summer of 1998 was known for anomalouslyhigh sea surface temperatures (Sutton unpubl. data),and Fig. 4 shows c. 5°C difference in sea surfacetemperature between Transects 5 and 6. Part of thisdifference is because the two transects are sepjiratedby the Subtropical Front, but part is because of thegenerally higher sea surface temperatures duringFebruary 1998 compared to May 1994. Thus, amixing analysis based on data from these two cmisesalone is likely to be affected by the biased temporalsampling. Hence data from all available NIWAhydrocasts are used to look at the historical T-Srelationships. These hydrocasts were divided intothree groups (Fig. 5). One group, denoted the SCgroup is all those casts made south of EanksPeninsula and within the 1000 m isobath. The secondgroup, denoted the WCC group is all casts madeadjacent to the Wairarapa Coast, and the third groupis all casts made within the South Taranaki Bight,denoted the TB group.

Figure 6 shows T-S from all these hydrocasts,with different symbols indicating each group. T-Sfrom the representative WCC and SC stations fromthe 1998 and 1994 cruises (those shown in bold inFig. 4) are included. T-S from two stations, deemedto be representative of water within the East CapeCurrent, and subantarctic water, labelled STW andSAW, respectively, are also included. Threepotential density surfaces (a = 26.2,26.55,26.9) arealso indicated.

Water with a density of 26.2 lies at c. 100 m alongthe WCC transects (Fig. 3), and is below the seasonalthermocline (Mark Hadfield, 1999 pers. comm.).

Water lying shallower than this level is affected byseasonal warming.

Figure 6 shows a break into two distinct T-Scurves below c. 650 m (corresponding to a~26.9),because below this level, the Chatham Rise separatesthe two branches of AAIW. Above this level, T-Sforms a continuous envelope of water masses causedby mixing between the various source masses.Within this envelope, however, the data are clusteredaccording to geographical location. For example, allSC water falls into a region with salinity less thanc. 34.6. Similarly, South Taranaki Bight water hassalinities clustered around 35.1 (although this watermass shows a large salinity spread). Thus the fourdistinct near-surface water masses within the regioncan easily be categorised on the basis of salinity.Three of these water masses are modified subtropicalwater; in order of decreasing salinity, these are STWfrom the East Cape Current (5-35.5), STW from theD'Urville Current (5-35.1), and STW within theSouthland Current (5-34.6). The fourth water massis subantarctic water found offshore and south of theSubtropical Front (5-34.3).

The bulk of WCC water shows salinity fallingbetween that of the Southland and D'Urville Cur-rents, suggesting that WCC water could be formedby mixing between these two types. A precise esti-mate of the ratios of the water masses appearing inthe WCC is impossible because of the salinityspread, but it would appear that about half, or more,of the transport in the WCC could be because of theD'Urville Current. The sill depth in the SouthTaranaki Bight is c. 250 m, and as can be seen fromFig. 6, densest water within the bight has a potentialdensity of c. 26.55. Thus, one would expectD'Urville Current water to be present only above thislevel. This is consistent with the subsurface salinitymaximum seen in WCC water at densities ofc. 26.55. (This subsurface maximum is very clear inthe T-S from Section 5, but is also evident in the his-torical data seen along the 26.55 isopycnal.) Belowthe sill level of the South Taranaki Bight, water inthe WCC is principally water of subtropical originthat has been adverted into the region by the EastCape Current.

Relative proportions of pure subtropical andsubantarctic water at any station can be determinedfrom 7-5 plots using classical mixing arguments(e.g., Pond & Pickard 1978). For any given tempera-ture, salinity, and pressure (5, T, P) having poten-tial density Co, the mix is defined by:q = (SSSMSN-SS) (1)where SN and 55 are the salinities of the subtropical

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00 O XIC\ "*- 00

2 5

§1«2 o- c2 JÍ g-g S¿

¡1¡11l is¡IIIII111S S ÏO 00 S

III

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and subantarctic endpoints on the density level, A0;

i.e., SN and 55 are determined where the density levelOQ intersects the STW and SAW curves in Fig. 6.Because temperature and salinity can only beconsidered conservative properties below the level

of the seasonal thermocline (i.e., below the deepwinter mixed layer), q can only be relied on fordensities greater than c. 26.2.

When contoured against density and distance off-shore, q from Transects 2 to 5 shows clear evidence

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Chiswell—Wairarapa Coastal Current

Transect 2 Transect 3

1000

309

20 40 60 80Distance offshore (km)

4060 20 40 60 80Distance offshore (km)

2040

Fig. 3 Geostrophic sections from Transects 2-5 (see text, Fig. 1). Northwards flowing current is positive. Contoursof potential density at O"0= 26.55 and 26.9 are superimposed.

of low q water along the Wairarapa coast with q in-creasing as one progresses northwards (Fig. 7).Above the Taranaki Bight sill depth (0O = 26.5 5), qgets as low as 0.4. Between the Taranaki Bight silldepth and the top of the Chatham Rise q is c. 0.8,and below the crest of the rise, q is essentially 1.This is consistent with the presence of D'UrvilleCurrent water above the Taranaki Bight sill depth,below this sill depth but above level of the ChatiamRise, WCC water is mostly from the East C apeCurrent, with some input of Southland Currentwater. Below the level of the Chatham Rise, WCCwater is northern AAIW. Contours of geostrophiccurrent superimposed on q, show that the lob; oflow-mix water extends into the southward flow. Inpart, this may be real entrainment of WCC waterinto the East Cape Current, and in part it may be theresult of inadequacies in computing geostrophiccurrents over the shelf.

Above the seasonal thermocline depth, q isslightly more difficult to interpret. Seasonal heat-ing and vertical mixing changes density both beforeand after mixing so that although Equation 1 holdsbecause salinity is conservative, the values of Ss andSV cannot be determined by projecting alongisopycnals. In this region mixing can lower salinitiesin the upper layers by entrainment of lower salinitysubsurface water. Assuming the GQ = 26.2 isopycnalrepresents the base of the seasonal thermocline,mixing could lower salinity to no less than c. 35.2in subtropical waters. In subantarctic waters, salin-ity does not change with depth. Hence the net ef-fect of seasonal heating and mixing leads to apotential reduction in SV, but no change in Sg. In-spection of Fig. 6 shows that almost all WCC wa-ter has salinity less than 35.2, so that vertical mixingcan be excluded as a mechanism for its water massformation.

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Transect 7

Transect 8

Transect 9

34 34.5 35 35.5

34.5 35 35.5

Fig. 4 Temperature-salinity ( T-S) plots for the transectsmade in 1998 and 1994 (Fig. 1). Thick line in each is T-Sfrom the Wairarapa Coastal Current station having low-est salinity, or Southland Current station having highestsalinity (see text).

Current meter analysis

Currents from two moorings at 100 m, from 28January to 23 February 1998 were rotated into alongand cross-shore components. Fig. 8 shows thesecomponents after the tides have been removed. Atboth locations (130 km separation), along-shorecurrents were almost always northward, with onlytwo brief occasions when the currents reversed.Mean and root-mean-square along-shore velocitieswere c. 0.20 and 0.05 m s~', respectively at both sites.These speeds are consistent with the ADCP andgeostrophic measurements derived for theselocations.

Both records are fairly typical of current metermeasurements. There are oscillations having periodsranging from several hours to several days. Thelonger-period oscillations have larger amplitude, andcomputed spectra (not shown) are typically "red".One oscillation having aperiod of a few days, which

peaks on 10 February at the southern site, appearsto be an event also seen at the northern site about aday or so later, but otherwise, there does not seemto be much similarity between the fluctuations in thetwo records. With only 1 month of observations, therecords are too short to determine coherence atperiods longer than about a day or so. Whencoherence is computed between the records withband-averaging at the higher periods (to raiseconfidence), the only significant coherence found isbetween the along-shore currents at periods ofc. 4-5 h, with the southern record leading thenorthern one by c. 90°. No coherence was foundbetween the cross-shore components. Heath (1982)reports edge waves along the east coast of the NorthIsland with periods of c. 2.5 h, but these waves arenot evident in our records.

Interpretation of the 4-5-h coherence is beyondthe scope of this note, but given a separation of 130

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33°S

311

36°S

39°S

1Ö4UE 168UE 172°E 176°E 180°W 176°W

Fig. 5 Station locations of New Zealand Oceanographic Institute hydrocast data used in the water mass analysis.Stations are grouped according to location. Southland Current stations are plotted as diamonds; D'Urville Currentstations are plotted as dots; and Wairarapa Coastal Current stations are plotted as asterisks.

km, the observed phase difference suggests a phasespeed of 33 m s"1. (Higher speeds are required ifthere are nodes in the along-shore phase.)

DISCUSSION AND SUMMARY

The presence of a cool, fresh (relative to offshore)current flowing northwards along the East Coast ofthe North Island has been documented sufficientlyin the literature for there to be little doubt that the

WCC is present much of the time. Although they donot explicitly comment on it, AVHRR derived seasurface temperature gradients shown by Uddstrom& Oien (1999) show the current to be persistent overmuch of the year (such analyses of currents can beobscured because sea surface temperature signaturemay be affected by local heating of mixing events).

The ADCP and hydrographic survey presented inthis note show the current extending out to about the1000 or 2000 isobath (40-50 km from the coast). The1.6 Sv or so of transport estimated here appears to

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34.2 34.4 34.6 348 35Salinity (PSU)

35.2 35.4 35.6 35.8

Fig. 6 Temperature plotted against salinity (7V!i) for hydrocast data. Symbols denote geographic location of casts asin Fig. 5. Thick lines are T-S from Wairarapa Coastal Current stations and Southland Current stations seen in Fig. 4,labelled according to transect. Also plotted are T-S for a station showing subtropical water (labelled STW) and oneshowing Subantarctic water (SAW); these lines are taken as endpoints in the mixing calculations (see text).

be the first such calculation of transport wi iiin theWCC, although one survey and 1 month of currentmeter data are far too little information to mike anyestimate of the long-term mean values of tiansportwithin the current.

It seems likely that the WCC only extends as farnorth as Mania Peninsula. The Ritchie Bank extendsoffshore from Mahia Peninsula, and this could be aphysical barrier to the flow forcing most of it torecirculate there as seen in Fig. 2. Whether thecurrent extends only as far as Mahia Peninsula, ornot, there is entrainment with the offshore East CapeCurrent along its entire length, leading to the ncreasein T-S seen in Fig. 6 and the decrease in tnmsport.

The water mass analysis presented hen: showsthat WCC water to be modified subtropical water.It is too saline (and also too warm) for it to b ; purelyderived from Southland Current water. It is also toofresh (and cool) for it to be D'Urville Currentwater—or for it to be East Cape Current watsr. Withfour water masses in the region, it is impossible todetermine the source of the WCC on the bas s of T-S

alone. However, subantarctic water can safely beruled out as a significant source because of itsgeographical location. Thus the WCC must containa significant amount of Southland Current water.That WCC water has low mix, q, only above the silldepth of the Taranaki Bight indicates the other watermass is likely to be D'Urville Current.

Thus the flow in the region can be quite describedsimply: the Southland Current flows north along andinshore of the shelf-break along the South Island.Because of topographical constraints, the SouthlandFront breaks away from the shelf break at theChatham Rise, to become the Subtropical Front. Thegeostrophic component of the Southland Currentassociated with the front also follows the ChathamRise bathymetry, but, because of conservation ofpotential vorticity, some of the inshore componentof the Southland Current flows north to Cook Straitregion. The flow through Cook Strait (the D'UrvilleCurrent) is driven by large-scale (sub-basin wide)pressure gradient (i.e., at the latitude of Cook Strait,dynamic height is higher to the west of New Zealand

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Transect 2

26.5

27

27.5

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Transect 3

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97 S«

•*-Mg

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• • I j 0.5

20 40 60 80Distance offshore (km)

100 0 20 40 60 80Distance offshore (km)

2040

Fig. 7 Mix, q, calculated from T-S properties for Transects 2-5, plotted against potential density and distance off-shore. Density levels <70 = 26.2, 26.55, and 26.9 are as marked.

than to the east). This water mixes with SouthlandCurrent water, and again, because of conservationof potential vorticity, this flow turns northwards andremains attached to the coast. Subtropical wate- fromthe East Cape Current may also contribute to theWCC both as a source water, and as a result ofentrainment as the current progresses north. Such anexplanation is consistent with the inferred carrentdirections from our ADCP and historical drift :ards.

The observation that the D'Urville Current maybe a significant source for the WCC has someimplications for the flow through Cook Strait. If theD'Urville Current contributes half the volume of theWCC, and the WCC transport is 1.6 Sv, then one canmake an estimate of the north to south flow th-oughCook Strait required to allow this transport. TakingCook Strait to be 23 km wide, and assuming theD'Urville Current is 250 m deep, the resulting strait-wide averaged flow is c. 14 cm s"1 (over the ipper250 m). Existing estimates of flow through CookStrait range hugely. For example, Bowman et al.(1983) recorded drogues passing southwards through

Cook Strait with mean speeds up to 26 cm s~'. Heath(1986) notes that flows are variable on 1—4-weektimescales, but made estimates of mean flows over1 month ranging from 4 cm s~' southwards to 7 cms"' northwards. Salinity seen on Transect 5 during1998 is at the high end of the envelope of historicalWCC water (Fig. 6), suggesting that at that time, theWCC had a high component of subtropical waterfrom the South Taranaki Bight. This would be linkedto a time of increased flow in the D'Urville Current,so that 14 cm s~' may be near the upper end of therange of flow through the strait.

Finally, the choice of Wairarapa Coastal Currentas a name needs some explanation. Since the currentdoes not appear to be an extension of the SouthlandCurrent, this term should clearly not be used. Theold term Canterbury Current is misleading becauseit implies a continuity of flow from Canterbury coastalong the east coast of the North Island, andprecludes an influence of the D'Urville Current.Similarly, the current should not be regarded as anextension of the D'Urville Current, because there is

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New Zealand Journal of Marine and Freshwater Research, 2000, Vol. 34

Cross-shore currents

Along-shore currents

25Jan Feb

1998

Fig. 8 Detided along-shore and cross-shore currents from the two moorings made in 1998 off the Wairarapa Coast,New Zealand. Upper panel: cross-shore currents; lower panel: long-shore currents. In each instance current from thenorthern mooring has been offset by 0.4 m s~'.

clearly mixing with Southland Current water. Sincethe current appears to form off the Wairarapa coast,it appears that Wairarapa Coastal Current is anappropriate name.

ACKNOWLEDGMENTS

I thank all those who contributed to the cruises and madethe collection of these data possible. Thanks to MattWalkington, Basil Stanton, Ed Abraham, Rob Stewart,Nils Oien, and Dick Singleton for participation in datacollection. Thanks also to the master and crew of R/VTangaroa for their help at sea. Mike Uddstrom, JohnMcGregor, and Phillip Wiles provided and processed theAVHRR data. Derek Goring provided the tidal modeloutput. 1 thank Michele Morris and two anonymous re-viewers for their comments.

REFERENCES

Bowman, M. J.; Kibblewhite, A. C ; Murtagh, R.;Chiswell, S. M.; Sanderson, B. G. 1983: Circula-tion and mixing in greater Cook Strait, New Zea-land. Oceanologica Ada 6: 383-391.

Brodie, J . W . I 960: Coastal surface currents around NewZealand. New Zealand Journal of Geology andGeophysics 3: 235-252.

Chiswell, S. M.; Roemmich, D. 1998: The East CapeCurrent and two eddies: a mechanism for larvalretention? New Zealand Journal of Marine andFreshwater Research 32: 385-397.

Chiswell, S. M.; Walkington, M.; Stanton, B. R. 1993:CTD data from Tasman-WOCE 1, NZOI cruise3008, Akademik Lavrentyev, WOCE sections PR-13N, PR-11. Wellington, NIWA. P. 85.

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Fiadeiro, M. E.; Veronis, G. 1982: On the determinationof absolute velocities in the ocean. Journal ofMarine Research 40: 159-182.

Heath, R. A. 1969: Drift card observations of cur-etns inthe central New Zealand Region. Mew ZealandJournal of Marine and Freshwater Research 3:3-12.

Heath, R. A. 1972a: Choice of a reference surface forgeostrophic currents around New Zealand. NewZealand Journal of Marine and Freshwcter Re-search 6: 148-177.

Heath, R. A. 1972b: The Southland Current. New Zea-land Journal of Marine and Freshwater Research6: 497-533.

Heath, R. A. 1975: Oceanic circulation and hydro ogy offthe southern half of South Island, New Zealand.Wellington, Memoir NZ Oceanographic institute72. 36 p.

Heath, R. A. 1976: Oceanic circulation in the head of theHikurangi Trench, east of New Zealand. NewZealand Journal of Marine and Freshwater Re-search 70:651-674.

Heath, R. A. 1982: Generation of 2-3 hour oscillations onthe east coast of New Zealand. New ZealandJournal of Marine and Freshwater Research 16:111-117.

Heath, R. A. 1986: In which direction is the mean flowthrough Cook Strait, New Zealand—evidence of1 to 4 week variability? New Zealand Journal ofMarine and Freshwater Research 20: 119-137.

Houtman, T. J. 1966: Repeat measurements of tempera-ture, salinity, and carbon-14 depletion at an oceanstation. New Zealand Journal of Marine andFreshwater Research 9: 457-471.

Oien, N. 1997: Acoustic DopplerCurrent Profiler(ADCP)Data from research voyage 3037. NIWA.

Pond, S.; Pickard, G. L. 1978: Introductory dynamicaloceanography: Oxford, Pergamon. 241 p.

Roemmich, D.; Sutton, P. J. H. 1998: The mean andvariability of ocean circulation past northern NewZealand: determining the representativeness ofhydrographic climatologies. Journal of Geophysi-cal Research C6: 13041-13054.

Stanton, B. R.; Sutton, P. J. H.; Chiswell, S. M. 1997: TheEast Auckland Current, 1994-95. New ZealandJournal of Marine and Freshwater Research 31:537-550.

Tomczak, M.; Godfrey, J. S. 1994: Regional oceanogra-phy: an introduction. Oxford, Pergamon. 422 p.

Uddstrom, M. J.; Oien, N. A. 1999: On the use of highresolution satellite data to describe the spatial andtemporal variability of sea surface temperaturesin the New Zealand Region. Journal of Geo-physical Research 104(5): 20729-20751.

Walkington, C. M.; Sutton, P. J. H. 1997: CTD datareport from East Auckland Current Cruise IV.NIWA Physics Section Report 97-3. Wellington.

Warren, B. A. 1970: General circulation of the SouthPacific. In: Wooster, W. S. ed. Scientific explora-tion of the South Pacific: Scripps Institution ofOceanography, National Academy of Sciences.Pp. 33-49.

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the wairarapa coastal current

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