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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. 39Chiswell—WairarapaandHikurangiEddies: 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: [email protected]
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 near178.3°E, 41.2°S and 176.2°E, 42.4°S. 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 178.5°E, 41°S,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
36°S
37°S
38°S
39°S
40°S
176°E 178°E 180°W 178°W 176°W
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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Chiswell—Wairarapa 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 field—where 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 176°E, 42°S. Barnes termed this eddy "E", andrecognised it as different from an eddy centred at179°E, 41°S (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 near176°E, 42°S, in their analysis, and they only see alarge eddy centred at 178.5°E, 41°S. 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 38°S, andwest of 179°W 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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Chiswell—Wairarapa 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 the three surveycruises, and illustrates temporal variability in theWairarapa Eddy. These fields have been objectivelymapped using a Gaussian decorrelation scale of 2°in latitude and longitude—chosen to be consistentwith Roemmich & Sutton. Where the depth is lessthan 2000 m, dynamic height was computed byextending temperature and salinity fields shorewardsfrom the nearest deep station. For example, if thewater depth for the first station less than 2000 m deepwas 1800 m, its vertical profiles would be extendedby appending the 1800-2000 m water propertiesfrom the adjacent deep station. For comparison, thefigure also shows mean dynamic height fromRoemmich & Sutton. Note there is a change incolour scale from cruise to cruise.
During February 1998, two regions of highdynamic height indicate the Wairarapa Eddy was notin its mean position as given by Roemmich & Sutton.Instead there was a small eddy centred near 177°E,42° S, and a larger lobe of anticyclonic circulationcentred near 180°E, 40°S. Although the two northerntransects extended beyond the maximum in dynamicheight, they were further apart at their offshore endsthan the decorrelation scale of the objective analysis,so that the structure of the dynamic height field is
uncertain. During April 2001, the Wairarapa Eddyappears more nearly like the Roemmich & Suttonmean in that there is only one eddy present.However, dynamic height is higher than Roemmich& Sutton (max. values of 2.35 dyn m in 2001compared with 2.23 dyn m in the Roemmich &Sutton mean). During March 2003, dynamic heightagain shows two lobes rather than a single eddy, butthe southern eddy was not as far to the west as in1998.
Using D to represent dynamic height from 0 to2000 dbar, D= AD0/2000, one can write the surfacedynamic height in terms of a mean and time-varyingcomponent:
D(x,y,t) = D(x,y) + D'(x,y,t) (1)
where at any location, the long-term mean of thetime-varying component is zero.
Here, the CTD stations provide 206 discreteobservations of D, and the AVISO data provide 206estimates of D' by choosing the AVISO datum at thenearest grid point and time to the CTD observation.(The AVISO data are so smoothed in space and timethat they did not warrant interpolation to the CTDlocations.) The AVISO sea surface height measure-ments were converted to dynamic height bymultiplying by g/10.
The long-term mean of the time-varying com-ponent is zero. However, the mean of a discretenumber of observations may not be zero because ofnon-uniform sampling in time or space. Thus the bestestimate of the true mean, —D, is given by removingthe time-varying terms and computing the averagefrom an objective analysis:
E(D)= i = 1 to 206 (2)
where the angle brackets indicate an objectiveanalysis.
Before computing the objective analysis, it isinstructive to consider the distribution of £>',. Fig. 3shows the histogram of the 206 estimates of £>'.Their mean value was 0.06 dyn m, and their standarddeviation was 0.09 dyn m. When plotted against time(Fig. 4), D'j shows a statistically significant (at 95%confidence) trend, having a slope of 0.021 dyn m peryear, and a correlation squared of 34%. Dynamicheight from the CTD profiles, Di, shows astatistically indistinguishable trend (slope = 0.02 dynm per year, correlation squared of 27%). As aconsequence, the differences show no trend (r2= 2%,Fig. 4C).
Thus over the 11 years from 1993 to 2003, therewas a trend in dynamic height that was captured in
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126 New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39
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1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
Fig. 4 Time series of: A, dynamic height anomaly, £>', determined from the Topex/Poseidon altimeter; B, dynamicheight, D, determined from 206 conductivity-temperature-depth (CTD) profiles; and iC, the difference, D. - D'.. Lineson plots show least-squares best fits, with correlations-squared as indicated.
both the CTD and satellite data. The equivalent trendof 2 cm per year of sea level rise within the eddysystem is interesting, and is returned to in the nextsection. For now, Fig. 4 is merely used to illustratethat the trend seen in D' is real, and is removed fromthe estimates of the mean. This is encouraging, sinceone does not expect the mean to have a trend.However, it is worth noting that although the
£>, - D'j differences show no trend, their absolutevalue depends on the time period used to make theanalyses, since the long-term mean was explicitlyremoved from the AVISO data.
Figure 5 shows the objectively analysed fit,< Di - D'j >. This analysis was made with adecorrelation scale of 1° in latitude and longitude.Compared to the Roemmich & Sutton mean (Fig. 1),
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Chiswell—Wairarapa and Hikurangi Eddies
36°S
37°S
38°S
127
1.9
174UE 176°E 178UE 180uW 178UW
Fig. 5 Estimate of the mean field off the east coast of the North Island, New Zealand, obtained from objectiveanalysis of D - £>'. Dashed line is the 2000 m isobath. W and H indicate the eddies referred to in the text.
our estimate has two eddies. The largest, labelled"W", is centred just south-west from the Roemmich& Sutton centre. In addition, there is a weaker eddy,labelled "H", centred near 176.2°E, 42.4°S. Thiseddy is close to the eddy documented by Barnes(1985).
It is also of interest to compute the transportassociated with the mean dynamic height field. Here,the transport relative to 2000 dbar is calculated by atransport stream function, which on an f-plane canbe defined by vertically integrating dynamic height:
I Q 2000
where f is the Coriolis parameter, and Wref is anarbitrary constant.
As with surface dynamic height, the best estimatethe mean stream function requires removal of thetime-varying component. By analogy to Equation 2,this estimate can be written as:
1 0 2000= 7 f Jo
where AD0/p is dynamic height as measured fromCTD profiles, AD'0/p is the time-varying componentas measured by the satellite altimeter.
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128 New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39
First vertical EOF
20000.2 0.4 0.6 0.8
Dynamic Height (dyn m)
Fig. 6 First vertical empirical orthogonal function (EOF)of dynamic height computed from the 206 conductivity-temperature-depth profiles. This EOF explains 97% of the
However, since the satellite measurement is onlyof the surface height, one needs some method ofestimating the vertical profiles of AD '0/p. If oneassumes that the variability seen in the ensemble ofCTD casts owing to sampling in both space and timereflects the variability in the time-varying component(e.g., Chiswell 2001), one can use an empiricalorthogonal function (EOF) analysis to compute theprincipal mode of variability. The first EOF, whichexplains 97% of the dynamic height variance, isapproximately linear with depth (Fig. 6). The timeseries of this EOF shows a 56% correlation-squaredwhen regressed against £>', from the AVISO data.This suggests that some of the variability seen in theEOF may be a result of spatial variability across theeddy rather than temporal variability. Nevertheless,our best estimate of AD'0/p is derived from the EOF,thus at each CTD station, one can now compute thecomplete dynamic height profile and proceed withthe integration shown in Equation 3.
Figure 7 shows the resulting stream function. Thearbitrary coefficient of integration, W^, was chosenso that the stream function has a value of zeroapproximately over the 2000 m isobath. In the mean,there is 15 Sv of circulation within the main bodyof the Wairarapa Eddy, and between 5 and 10 Sv inthe southernmost eddy. It should be noted that thisis the transport relative to 2000 dbar, and Roemmich& Sutton suggest that the Wairarapa Eddy issignificantly deeper than 2000 dbar. Chiswell (2003)also found the Wairarapa Eddy to be deeper than2000 dbar and suggests that at 2000 dbar the meancurrents could be as high as 7 cm s-1.
Trend in dynamic heightFigure 4 shows that between 1993 and 2003 therewas a 20 cm rise in sea level within the WairarapaEddy region, which is seen in both the CTD andsatellite data. This agreement suggests that localtrends in sea level can be computed accurately fromthe AVISO data set, and that one can use AVISO tolook at the spatial structure of the trend.
Figure 8 shows the local slope and correlation-squared derived from trend in the AVISO product—i.e., the slope and r2 obtained by regressing seasurface height (SSH) against time at each AVISOgrid point. The figure also shows a mean dynamicheight compiled from combining the mean fieldshown in Fig. 5 with the Roemmich & Sutton mean.
The slope shows five features: two lobes east ofthe North Island (labelled "H") where it reaches2 cm/year, an additional lobe of high slope north-eastof the North Island where it reaches 1 cm/year, andtwo lows (labelled "L") where the slope is near zero.The correlation-squared reaches 30-40% in the lobesof high slope; elsewhere it is less than 20%.
There is an almost one-to-one correspondencebetween features in the slope and dynamic height,although south of 38°S, highs in the slope co-occurwith highs in dynamic height, whereas north of 38°S,they co-occur with lows in dynamic height. Inparticular, the Wairarapa Eddy is associated with apositive slope, whereas the North Cape and EastCape Eddies are associated with local minima in theslope (and low correlation-squared). Two otherpermanent features in the flow, an anticyclonic eddyeast of the Wairarapa Eddy over the RekohuEmbayment, and a cyclonic eddy over the KupeAbyssal Plain are associated with highs in the slope.
Thus Fig. 8 suggests that over the 11 years from1993 to 2003, the anticyclonic Wairarapa andRekohu Eddies strengthened. There was no statis-tically significant trend in the other eddies.
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36°S
37°S
38°S
39°S
129
15
- 1 0
176UE 178°E 180°W 178°W
Fig. 7 Transport stream function (in Sverdrups; 1 Sv = 106 m3 s-1) derived from vertically integrating the meandynamic height field (see text). Dashed line is the 2000 m isobath.
VariabilityHeath (1973) stated, "… Small eddies, which areprobably periodically shed off from the permanenteddy are guided by the bottom topography towardsKaikoura ….. The 50-70 day periodicity in theseeddies is probably linked to a similar periodicity inthe East Australia Current...". Roemmich & Suttonstated," although variable in strength (the WairarapaEddy's) position is the least variable of the threenamed eddies". Although these statements are notmutually exclusive (the East Cape and North CapeEddies could be highly variable), they hint at quitedifferent behaviour for the Wairarapa Eddy.
With 11 years of AVISO data, we can now beginto make more quantitative estimates of the variabilityin the Wairarapa Eddy. A movie of the eddy systemfrom the AVISO data reveals in a qualitative mannerthe complexity of the East Cape Current-WairarapaEddy system.
Perhaps the most noticeable feature of the systemis an irregular shedding of eddies from near EastCape into the Wairarapa Eddy, followed by mi-gration of these eddies south-west along theHikurangi Trough, much as discussed by Garner(1969) (although the subsequent eastward migrationalong the Chatham Rise he suggested does not
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Slope of trend (dyn m year 1) of trend
172 174 176 178 180 178 176 172 174 176 178 180 178 176
Mean dynamic height
172 174 176 178 180 178 176
Fig. 8 Slope and correlation-squared of trend fromTopex/Poseidon altimeter estimates of dynamic height,D'. (see text). Also shown is the mean dynamic height toillustrate co-locations of trend and mean dynamic height.Eddies named by Roemmich & Sutton are: NCE, NorthCape Eddy; ECE, East Cape Eddy; WE, Wairarapa Eddy.Two further eddies are: KE, Kupe Eddy; RE, RekohuEddy.
occur). Fig. 9 shows a time sequence of one sucheddy-shedding event that took place between July1997 and January 1998. The plots show dynamicheight anomaly from AVISO added to a mergedmean field that incorporates the mean shown in Fig.5 where it exists, combined with the Roemmich &Sutton mean elsewhere. The plots are shown in analong-stream coordinate system, which has the x-axisrotated 125° from east so that it runs more-or-lessalong the coast south of East Cape. The coordinateorigin is East Cape.
At the start of the time series (11 July 1997)dynamic height shows three anticyclonic eddies,labelled A, B, and C. Eddy A was 200 km upstreamof East Cape, eddies B and C were centred 50 and400 km downstream of East Cape, respectively. As
the time sequence progresses, all three eddies movedownstream along the Hikurangi Trough. At the endof October 1997, the eddies are very close to themean locations shown in Fig. 5. As the sequencecontinues, eddy C is progressively squeezed into thehead of the Hikurangi Trough until it merges witheddy B in January 1998. By the end of the sequence,eddy A is in approximately the location occupied byeddy B 6 months earlier.
A Hovmuller (time-alongshore distance) plotconstructed for the band between 45 and 245 kmoffshore (i.e., 200 km offshore from the 2000 misobath) is shown in Fig. 10. The long-term trend andannual cycle have been removed to emphasise themesoscale variability. Individual eddies can easilybe seen in the figure. In particular, eddies A, B, and
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Chiswell—Wairarapa and Hikurangi Eddies
11 Jul 1997
131
15Aug 1997
19Sep1997
-350-200 0 200 400 600
(km)
31 Oct1997
05 Dec 1997
09 Jan 1998
-200 200 400 600(km)
Fig. 9 Sequence of dynamic height, ADopm, (from Topex/Poseidon altimeter estimates plus the mean shown in Fig.8). Data are plotted in an alongshore coordinate system with the origin centred at East Cape. The 2000 isobath isshown as a dashed line. Horizontal lines at 45 and 200 km offshore indicate averaging used in construction of space-time plot (Fig. 10).
C in Fig. 9 are illustrated by guidelines. Somewhat the guidelines vary from 500 km year-1 to 1000 kmarbitrary, the figure shows six other instances where year-1 (1.6 cm s-1 to 3.2 cm s-1).there is an eddy progressing downstream along the The sequence shown in Fig. 9 was chosen fromHikurangi Trough. The downstream translation the 11 years of altimeter data because it showed aspeeds of the eddies calculated from the slopes of simple progression of eddy spawning and translation.
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132 New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39
600
-200
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
-0.1
-0.2
Fig. 10 Space-time diagram of dynamic height, AD0/?000, (from Topex/Poseidon altimeter estimates plus the meanshown in Fig. 8). Black lines indicate eddies A, B, and C seen in Fig. 9, plus several other arbitrarily shown features ofinterest.
Much of the time the interaction between eddies wasmore complex, with eddies meandering around,occasionally merging, and spawning smaller eddies.This is reflected in Fig. 10 where although manyindividual eddies can be tracked all the way up theHikurangi Trough, there are also long periods wherethere were few instances of eddy migration. Forexample, in 1998, eddy A was followed by a muchmore confused situation, where there was an eddycentred 100 km upstream of East Cape that did notappear to migrate, and then later in the year, anothereddy at c. 300 km also appears stagnant.
Because of variability, it is often difficult to trackindividual eddies, and so one cannot easily definean eddy spawning frequency. However, the long-term average appears to be 2-3 eddies per year,which would suggest a mean period betweensuccessive eddies of 120-180 days.
There is also some interannual variability in theeddy system. During 1993 and 1994, for example, theeddies appeared relatively weak and broad. In contrast,1995 and 2002 appeared to be years when coherentstrong eddies were spawned that maintained theiridentity for long periods of time. During 1998 and early1999, there was little evidence of eddy propagation.
DISCUSSION OR WHAT'S IN A NAME?
One reason for computing a mean dynamic heightfield is to provide a conceptual framework of thecirculation. We tend to think of the currents aroundNew Zealand in terms of the mean circulation—for
example, that the East Cape Current flows south-west down the east coast of the North Island, and thatthe Wairarapa Eddy is formed by retroflection of thiscurrent.
However, the mean field is a mathematical entity,and there is a danger that the mean flow can give anincorrect perception of reality. This is perhaps so forthe Wairarapa Eddy system. The system off the eastcoast of the North Island may be better described asa sequence of eddies shed near East Cape andpropagating down the coast. As they propagate downthe coast, they slow down offshore of Hawke's Bay(alongshore distance = 300 km in Fig. 9). At any onetime, there may be two or more identifiable eddiesin the region, including a small eddy at the head ofthe Hikurangi Trough.
In the mean, one obtains a dynamic height fieldas shown in Fig. 5. However, rather than indicatinga permanent stationary eddy, the main lobe of meandynamic height reflects a region where the eddiestend to stall out or merge with the previous eddy.Often, the eddies will continue up the HikurangiTrough and stall in the region of the southern lobeof dynamic height.
There is an obvious temptation to name thefeatures in Fig. 5, partly to follow previous re-searchers, and partly because it is convenient to nameeddies when discussing them. As noted above, theoriginal usage of "Wairarapa Eddy" was to refer tothe southern lobe seen in Fig. 5, but Roemmich &Sutton use it to refer to the central lobe. Most, if notall, recent researchers use the Roemmich & Suttonterminology, so it seems that the best convention to
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Chiswell—Wairarapa and Hikurangi Eddies 133
use is to refer to the central lobe as the WairarapaEddy, and the southern lobe the Hikurangi Eddy.
The Hikurangi Eddy is thus not so much aseparate eddy from the Wairarapa Eddy, but ratheran older Wairarapa Eddy that has progressed down-stream. It may be a year or more older, i.e., sinceshedding. Just when the Wairarapa Eddy becomesthe Hikurangi is entirely subjective.
It is difficult to put a precise value on the rate atwhich eddies are shed from the East Cape Current,but the rate estimated here (i.e., one every 120-180days) is about half or one-third the rate estimated byHeath (1973). What causes the eddy shedding isbeyond the scope of this paper, but it is probablybecause of some form of baroclinic instability.Whether it is simply because of accelerations ordecelerations in the ECC, or whether Rossby wavespropagating in from the east play some role intriggering the instability is open to speculation.
A second reason for producing a mean field is sothat it can be used to reference altimeter data. Asnoted previously, the geoid is unknown, and so it iscommonly assumed that altimeters give only sealevel anomalies about the true mean. For this use,the mean field should be dynamically consistent. Inother words, even if the ageostrophic terms areignored, transport should not cross the coast. Themean field shown in Fig. 5 has not been dynamicallyconstrained and (in common with other dynamicheight climatologies) is not defined for watersshallower than 2000 m. These inadequacies can beavoided by computing climatological mean fieldsfrom a combination of numerical models and obser-vations—using the model to produce a dynamicallyconsistent velocity field. It is beyond the scope ofthis paper to produce such a climatology, but anyclimatology produced from a numerical modelshould be consistent with the mean field shown here,at least in deep waters.
Similarly, the transport stream function (Fig. 7)has not been dynamically constrained although,perhaps fortuitously, the 0 Sv isoline approximatelyruns parallel to the coast.
The mean field shown in Fig. 5 is the 11-yearmean from 1993 to 2003. Since there is a trend indynamic height, the mean field would have beendifferent if a different averaging period had beenused. If the trend had the same slope everywhere,then differences between means calculated overdifferent averaging periods would be spatiallyuniform and simply equal to the average rise indynamic height between the two averaging periods.This would have little consequence since the most
important use of the mean field is to derivegeostrophic velocities. However, the trend is notspatially uniform (Fig. 8), so the spatial structure ofthe calculated mean is dependent on the time periodused to make the average.
It is also virtually impossible to seamlesslycombine the mean field computed here with thatfrom any other source, if, for example, we wantedto compute a mean field over a broader region thatincorporated our best estimate of the WairarapaEddy. The mean shown in Fig. 8 was computed byadjusting the average value of our mean so thatdifferences between it and the Roemmich & Suttonmean were minimised along the edges. Although thisis adequate for illustrative purposes, it introducesdiscontinuities in the dynamic height, and henceinfinities in the geostrophic currents.
Are our estimates of the mean and variabilityrobust? Only time will tell. The CTD sampling wasnot uniform in time (Fig. 4) nor, perhaps, evenlydistributed in space (Fig. 1). In particular, thepresence of the Hikurangi Eddy in the mean stemslargely from data from the 1998 cruise (Fig. 2).However, there is much historical evidence for theHikurangi Eddy, and it seems unlikely that it is notpresent in the mean. It may even be that the meanfield shows a much finer scale structure than shownhere, and there may be several "permanent" eddiesat the head of the Hikurangi Trough—Barnes (1985)shows several eddies present in his images.
Our observations of the trend and strengtheningof the Wairarapa Eddy over 1993-2003 comes fromtwo independent data sets, which gives us someconfidence in the result, even at 2 cm per year of sealevel rise! In their global analyses of sea level risefrom T/P data, Nerem et al. (pers. comm.) find thesame result, in that they show a trend of 1 cm peryear rise over a broad region east of the North Island(their result was at lower spatial resolution). It seemsthat the localised strengthening of the Wairarapa andRekohu Eddies is correlated with a much largerspatial scale trend.
The results shown here provide more insight intothe dynamics of this region. The implications forprocesses such as climate variability and larvalretention may well be bound up in the interannualvariability seen in Fig. 10. For example, 1999 wasboth a year when sea surface temperatures wereanomalously high across the Chatham Rise(Chiswell 2002) and larval recruitment was extreme-ly low (Booth pers. comm.). How exactly theseevents are related to variability in the Wairarapa andHikurangi Eddies remains to be determined.
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134 New Zealand Journal of Marine and Freshwater Research, 2005, Vol. 39
ACKNOWLEDGMENTS
Over the years many people collected the CTD data; Ithank all these for their efforts. The altimeter productswere produced by the CLS Space Oceanography Divi-sion as part of the Environment and Climate EU ENACTproject (EVK2-CT2001-00117) and with support fromCentre National d'Etudes spatiales. This paper was im-measurably improved from discussions with M. Bowen,G. Rickard, and M. Hadfield. Two anonymous review-ers are thanked for their comments. This work was sup-ported by the Foundation for Research, Science andTechnology Contract CO1 X0202.
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