temperature and salinity mean and variability within the subtropical front over the chatham rise,...

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Temperature and salinity mean andvariability within the subtropical frontover the Chatham rise, 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 (2002) Temperature and salinity mean and variabilitywithin the subtropical front over the Chatham rise, New Zealand, New Zealand Journal of Marineand Freshwater Research, 36:2, 281-298, DOI: 10.1080/00288330.2002.9517086

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New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36: 281-2980028-8330/02/3602-0281 $7.00 © The Royal Society of New Zealand 2002

281

Temperature and salinity mean and variabilitywithin the Subtropical Front over the Chatham Rise, New Zealand

STEPHEN M. CHISWELLNational Institute of Water and Atmospheric

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

Abstract Estimates are made of the mean and vari-ability in temperature and salinity across the Sub-tropical Front along 178°30'E, east of New Zealand.These estimates are made from six sections takenover 5 years, but biased towards spring and autumnsampling. Monte-Carlo simulations of the potentialerrors introduced by this sampling suggest that themean fields can be reasonably well determined, butthe annual cycles have large relative error. April1999 residuals were anomalously warm and salinecompared with the other five sections. These anoma-lies may be linked to an increased strength of theWairarapa Eddy at that time.

Keywords Subtropical Front; climatology

INTRODUCTION

In the Southern Hemisphere, the Subtropical Front(STF) separates Subtropical Water (STW) to itsnorth from Subantarctic Water (SAW) to its south.East of New Zealand, the STF appears to betopographically locked to the Chatham Rise (Fig. 1),which rises to a depth of c. 350 m (Heath 1976).Over this rise, the STF is a region of relatively highprimary productivity, and as a result of this highprimary productivity and the consequent food-web,the Chatham Rise supports major deep waterfisheries for orange roughy, Hoplostethus atlanticus,

M0 1061Received 6 July 2001; accepted 1 October 2001

and oreo, Allocyttus niger and Pseudocyttusmaculates (Annala 1992), and is an important regionfor juvenile hoki, Macruronus novaezelandiae(Hatanaka et al. 1989).

The STF is likely to be a major Southern Hemi-sphere sink for atmospheric carbon dioxide into theocean because of physical solubility and biologicalpump processes and has an important, but as yetunmeasured role in the global carbon cycle (Enting& Mansbridge 1991; Siegenthaler & Sarmiento1993). The STF may also have a role in climatechange: a slowing in the growth rate of atmosphericcarbon dioxide between 1992 and 1994 has beenassociated with larger oceanic sink operating in thoseyears (Sarmiento 1993; Robertson & Watson 1995).

Although the importance of the STF to New Zea-land's fisheries and its potential role in climatechange have long been recognised, the details of thebiogeochemical processes that occur in the frontalregion are unknown. For example, we now know thatwaters south of the STF are high in macronutrients(nitrate, dissolved reactive phosphorus, and silica),but are low in micronutrients (iron). As a result, it isthought (Boyd 2001 pers. comm.) that high produc-tivity within the front is because the front providesa regime that has sufficient macronutrients andmicronutrients to drive primary production. How-ever, what triggers and limits phytoplankton growthis not well understood. Existing data show that thereis considerable variability in the timing and levelsof primary productivity. It is most likely that thisvariability in the primary production occurs becauseof variability in the physical mechanisms within thefront. Chiswell (2001b) shows that the Reynoldsstresses cannot be computed well unless one aver-ages over periods much longer than the biologicaltimescales, and as a result, entire phytoplanktonblooms can take place during regimes with consid-erably different energetics than the long-term means.

As a result of this uncertainty in the STF system,multidisciplinary programmes designed to studyboth the biology and the presumed physical drivingmechanisms have been in place since 1992. Duringthese programmes, several research cruises have

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282 New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36

40°S

42°S

44°S

172°E 174°E 176°E 178°E 180°W 178°W

Fig. 1 Station locations from the April 1999 cruiseshown as circles. The 1000 m contour shown as a dashedline indicates the location of the Chatham Rise. Surfaceexpression of the Subtropical Front (STF) lies approxi-mately along the crest of the rise. Also shown are vectorsof sea surface circulation derived from TOPEX/Poseidonaltimeter data for the mid point of the April 1999 cruise.Lines superimposed on the fields show the lines used tocalculate the Wairarapa Eddy indices (see text).

been made to the region. However, these infrequentand sporadic measurements provide little more thanan insight to the processes, and a more completeunderstanding of the system will rely heavily onnumerical modelling.

Before competent models of the system can bedeveloped, however, we need information on themean state of the STF and its variability. Thus, theaim of this article is to present estimates of the meantemperature and salinity fields together with theirvariability along 178°30'E. This line was chosenearly on in the programme as a repeat line, and isshown in Fig. 1. Since 1996, six cruises have beenmade along this meridian in May 1996, May 1997,September 1997, April 1999, October 1999, andOctober 2000. Full water column temperature andsalinity sections were made during each cruisebetween c. 46°S and 41°S, except during October1999 when profiles were made only to 1000 dbar.

A cursory analysis of these data show that deeptemperature and salinity during April 1999 werehigher than during the other cruises. This was alsoat a time when sea surface temperature (SST) wasanomalously high (see later). As a result, this articlealso describes the April 1999 event, and makes someestimates as to its cause.

The largest temperature signal in the upper watercolumn is the result of seasonal heating, thus it isassumed that the temperature can be described interms of mean seasonal and residual fields:

T(y, p, t) = T(y, p) + T(y, p, t) + T'(y, p, t) (1)

where overbar indicates the mean, tilde indicates theseasonal cycle, prime indicates the residual field, yis latitude, p is pressure, and t is time. Althoughsalinity is not directly affected by seasonal processes,it is treated the same way.

In this note, I make the first estimates of all threeterms on the right hand side of Equation 1. Becausethe April 1999 section turned out to be so anomalous(see later), the climatology is computed without theApril 1999 data. The five remaining cruises wereclustered during either spring or autumn (Table 1).With so few samples, and such non-uniform samplespacing, an error analysis of the climatological termsbecomes important because poor sampling canintroduce significant aliasing and biases in estimatesof both mean and periodic components (e.g.,McPhaden 1982). Several analytical techniques existfor such error analysis, but here more direct Monte-Carlo simulations (e.g., Firing & Lukas 1985) areused because they require fewer assumptions. Withonly five sections taken over 5 years, one cannothope to compile a long-term climatology, and to alarge extent, this analysis must necessarily be acomparison of the April 1999 section against theother sections.

At this point, it is worth commenting on thenotation used in this article. The term "residual" isused to indicate the departure from climatology,

Table 1 Times of first and last stations made along178°30'E.

Cruise Dates

May 1996May 1997Sep 1997Apr 1999Oct 1999Oct 2000

26-29 May 199623-26 May 199727 Sep-1 Oct 199710-15 Apr 199929-31 Oct 199911-16 Oct 2000

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Chiswell—Temperature and salinity within the Subtropical Front 283

where "climatology" indicates the mean andseasonal cycle. Somewhat arbitrarily, the term"anomaly" is used to indicate features that occur<5% of the time. For normally-distributed processesthis occurs when the feature is 2 standard deviationsaway from the mean value.

DATA

HydrographyThe dates of the cruises are listed in Table 1. Cruisesare denoted here by the month and year of the midpoint of the cruise.

During each cruise, stations were occupied along178°30'E. A Seabird CTD (conductivity-temperature-depth) profiler in a 12-place rosette with1.2 litre Niskin bottles was used to make continuousvertical profiles of temperature and salinity at eachstation. Water samples were collected to calibrate theconductivity sensor. CTD data collection andprocessing methods were the same as those detailedin Chiswell et al. (1993) and Walkington & Chiswell(1993). Temperature and salinity were processed to2 dbar bins. Temperature is estimated to be accurateto 3 x 10-3oC, salinity to 5 x 10~3. The spatial extentand spacing of the hydrographic stations varied fromcruise to cruise.

Temperature and salinity from each cruise weremapped onto a common latitude and pressure grid.The latitude grid ranged from 45° 18'S to41°6'S witha spacing of 0.1°. The pressure spacing was 2 dbar.

Following previous work in this region (Warren1970; Heath 1972; Chiswell & Booth 1999),geostrophic velocity was calculated with a depth ofno motion at 2000 dbar. Before calculation, tempera-ture and salinity were interpolated across theChatham Rise. The velocity field was then maskedout in the region of the rise. This technique is sim-plistic, but a comparison with ADCP-derived veloci-ties (see later) shows reasonable agreement over theChatham Rise.

Sea surface temperatureSea surface temperature data from AVHRR satelliteshave been collected and archived at NIWA since1992. Details of the processing are summarised byUddstrom & Oien (1999). Here, monthly-mean SSTaveraged over the c. 50 x 50 km box shown in Fig.1 are used to provide a time series of SST at the STF.Following Equation 1, the residual SST is computedby removal of a mean and annual cycle.

Acoustic Doppler Current ProfilerOn five of six cruises, Acoustic Doppler CurrentProfiler (ADCP) data were collected from a ship-mounted installation. Data acquisition andprocessing methods are described in Oien (1997,unpubl. data).

AltimetryTOPEX/Poseidon (T/P) data are collected andprocessed by Li-Li Fu (Jet Propulsion Laboratory).Gridded surface height anomalies are provided byD. Roemmich (Scripps Institution of Oceanography)are combined with the mean circulation fromRoemmich & Sutton (1998) to provide absolute seasurface height fields. These are then used to computesurface geostrophic currents. Details can be foundin Chiswell & Booth (1999).

RESULTS

Temperature and salinityTemperature sections from all cruises are shown inFig. 2. (Note that in this and following figures, thepanels have been grouped with the autumn sectionsto the left, and the spring sections to the right.) Asexpected, all sections show characteristics typical ofthe STF. Near the surface, warmer STW lies to thenorth of the rise, and cooler SAW lies to the south.The STF can be seen as the steeply sloped isothermsnear the crest of the rise. In the sections that extendfar enough to the north, one can see isotherms slop-ing upwards to the north. This bowling of the iso-therms is the signature of the Wairarapa Eddy, whichis typically centred near 41 °S (Roemmich & Sutton1998). (This eddy appears to be topographically in-duced by the presence of the Chatham Rise actingas a barrier to southward flow.) The thermocline atthe base of the mixed layer is much sharper south ofthe rise but this is more pronounced in autumn thanin spring.

There is evidence of what is taken here to be theseasonal signal in temperature, with surface watersgenerally warmer by a degree or two during autumn(April, May) than in the spring (September,October). However, temperature in April 1999 wasclearly warmer over much of the section than in anyof the other cruises. Surface temperature was as highas 20°C as far south as 43°S, whereas it did notexceed 16°C anywhere in any of the other sections.Deep temperature also appears warmer during April1999 north of the rise.

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Sep 1997

1000

120

10

'0

120

10

120

10

44 42Latitude (°S)

40 46 44 42Latitude (°S)

40

Fig. 2 Temperature (°C) sections from the six cruises made along 178°30T3. Locations of the stations for each cruiseare shown as diamonds.

The spatial structure of salinity is similar to that temperature is plotted as a function of salinity (T-S)of temperature, and is not shown, but the salinity for all cruises (Fig. 3). To the north STW is morefeatures in the hydrography can be seen when saline than SAW to the south. The presence of

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Chiswell—Temperature and salinity within the Subtropical Front

May 1996 Sep1997

285

34.6 35S

35.4

Fig. 3 Temperature plotted as a function of salinity (T-S) for each of the six cruises. Potential densities of 26 and 27are shown as guidelines.

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Antarctic Intermediate Water (AAIW) leads to asalinity minimum centred near 1000 dbar north ofthe Chatham Rise and near 600 dbar south of the rise.Northern AAIW has salinities of 34.46, whereassouthern AAIW has salinity near 34.3. Generally,SAW south of the rise has little salinity variationwith depth, the pycnocline there is controlled bytemperature not salinity. As in temperature, salinityappears anomalous in April 1999, with highersalinities north in the STW, and slightly lowersalinity in the SAW.

Compared with the other spring sections, T-Sfrom April 1999 shows an envelope of warmer, lessdense water which extends into a region of muchhigher surface salinity in the STW. Maximumsalinity in April 1999 was 0.2 higher than in any ofthe other sections.

The low temperature (and low salinity) feature inMay 1997 seen at 41°S at depths of c. 200 m (Fig.2) is an intrusion of SAW that has become imbeddedin the STW to the north. Such features are relativelycommon (e.g., Heath 1976), but usually occur onlyabove the depth of the sill of the Chatham Rise. Moreunusually, a deep intrusion of southern AAIW wasfound north of the rise during May 1996 (Chiswell& Sutton 1998). This deep eddy extended between1000 and 2000 dbar and so does not appear in thefigures. Data from stations affected by either of theseintrusions were not used to compute the climatology.

Geostrophic velocityGeostrophic velocity referenced to a level of nomotion at 2000 dbar was computed for each section,and is shown in Fig. 4. Although each section isdifferent, there are features that commonlycharacterise the flow. In particular there are oftentwo eastward flowing jets, one each side of the rise.

The northern jet is the southern arm of theWairarapa Eddy. In three of six sections, currents inthis jet exceed 0.3 m s-1 near the surface, although inthe remaining sections, it is relatively weak andunstructured, but only in October 1999 does this armof the Wairarapa Eddy not reach 0.1 m s~l. Just southof this jet, there is a narrow band of westward flow,right up against the flanks of the rise. This reverse flowappears to some extent in all sections except in April1999. In October 2000, it was particularly strong. Overthe rise itself, currents are usually eastward. Thesouthern jet is the part of the Southland Current thatflows on the south side of the rise. It appears generallyweaker than its northern counterpart.

How well the geostrophic currents measure thecirculation, particularly over the rise where the ocean

is shallower than the assumed depth of no motion,can be gauged by comparing the surface geostrophicvelocity with surface velocity measured by theADCP (Fig. 5), although one would not expect exactagreement between these two measures (because ofinadequacies in the assumption of a level of nomotion, and because of ageostrophic circulation).The agreement is remarkably good, with the ADCPdata showing all the major features seen in thegeostrophy. In particular, the strong northern jetsseen in May 1997, April 1999, and October 2000 arewell matched in both data sets. Where the reverseflow just south of the northern jet intersects thesurface, it appears also in the ADCP data. RMSdifferences between the geostrophy and ADCPcurrents where the depth is greater than 2000 dbarare shown for each cruise, and vary between 7 and17 cm s"1.

Climatology and residualsThe error analysis (see Appendix 1) shows that themain effect of the sampling is that one cannotestimate the annual cycle with any confidence if oneallows both amplitude and phase to vary in the fittingprocedures. However, the phase is the best known apriori value, and by setting it to correspond to peaktemperature on 27 February, one can make estimatesof the standard errors that have some significance.

Mean temperature, salinity, and velocity fields areshown in Fig. 6, along with the estimated standarddeviations of the errors. The mean temperatureshows precisely what one would expect, withrelatively warm STW to the north, and relativelycooler SAW to the south. Compared with theindividual sections, the mean temperature issmoother because of the averaging, although someof the structure seen in the STF does show through.The standard deviation in To is generally less than0.5°C, except in the frontal zone, where the potentialerror could be as high as 1.5°C. Mean salinity alsoreflects the individual sections. There is a weaksubsurface tongue extending to the south. Meansalinity in the tongue at 45°S is 34.39. The standarddeviation in So mimics that in To being highest overthe north slope of the Chatham Rise. The meanvelocity field shows the general features seen in eachsection, with both the strong eastward flows on eitherside of the rise and the reverse flow on the northernflank all appearing in the mean.

The amplitudes of the seasonal cycle intemperature and salinity are shown in Fig. 7, alongwith their respective estimated standard deviations.The standard deviations are lowest where the

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May 19960

200

Pre

ssur

e (d

bar

o

o

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-innn

I ° ~A A A AAA^WWSA

\j\c> o o cHO—|

May 1997

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1000

287

i0.5

' -0 .5

i0.5

' -0 .5

i0.5

44 42Latitude (°S)

40 46 44 42Latitude (°S)

Fig. 4 Geostrophic velocity (m s J) sections from the six cruises made along 178°30T3. Locations of theeach cruise are shown as diamonds. Positive values denote eastwards velocity.

' -0 .540

stations for

amplitudes are highest (because of increased signal The amplitude, by definition, is positive, so it has ato noise). Temperature shows two lobes of high skewed distribution (which tends to Gaussian foramplitude in the upper water column, with an small values of a (xjx{). Thus, a test of significanceapparent low in amplitude over the Chatham Rise, of the amplitude estimate is not straightforward, and

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New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36

May 1996 Sep 1997

\

\ / x - s

May 1997 Oct1999

''/""X 7/

0.07

^ A /A/Apr 1999 Oct 2000

0.13 0.1146 44 42

Latitude (°S)40 46 44 42

Latitude (°S)40

Fig. 5 Surface geostrophic velocity (m s"1, dashed lines) plotted as a function of latitude for all six sections. Whereavailable (five cruises), zonal current from the Acoustic Doppler Current Profiler (ADCP) is superimposed (solidline). Standard deviations between geostrophy and ADCP values are shown in lower left of each panel.

here we limit our comments to noting that except inthe two lobes, the standard deviation in the amplitudeis as large as, or larger, than the amplitude, so thatfor much of the water column, the amplitude ispoorly determined. Within the STF itself, one cannotmake any statistically significant estimate of theamplitude.

Similarly, the seasonal cycle in salinity is poorlydetermined over most of the region. The amplitudeof the seasonal cycle in velocity has no significance,and is not shown.

Figure 8 shows residuals T from each of the sixcruises. Except during April 1999 and October 1999,T was everywhere less than 2°. By far the largesttemperature residuals occurred during April 1999,when T' was high in the mixed layer across the entiresection, and there was a deep warm anomalyextending to between 400 and 600 dbar depth near42° 15'S. Values of T in this lobe were as high as 5°near the surface, 3° at 400 dbar, and 1° at 600 dbar.There is a suggestion that T was also high at theextreme northern edge of the section.

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T,

CD

en 600CD

800

100044 42Longitude (°S)

40 46 44 42Longitude (°S)

40

Fig. 6 Mean temperature (°C), salinity and zonal geostrophic velocity (m sJ) across the Subtropical Front (STF)(left hand panels). Values are calculated using all sections except April 1999. Also shown are the standard deviationsof these estimates (right hand panels).

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-1000

0.15

0.05

44 42Latitude (°S)

44 42Latitude (°S)

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Fig. 7 Seasonal amplitudes for temperature (°C) and salinity. Also shown are the standard deviations of the ex-pected errors in these coefficients (right hand panels).

Under the definition offered earlier, a residual isregarded as anomalous if it is more than 2 standarddeviations from climatology. Although not shownhere, the standard deviation in the residuals averaged0.4°C, with highest values of about 1°C in the upperwater column. The absolute value of T normalisedby the standard deviation was <2 throughout all thesections, except for April 1999, where it reached ashigh as 10 in the deep lobe.

April 1999 event frequency and originThere are two questions of interest about the April1999 deep event. One is whether the warming inApril 1999 was anomalous in the long term and thesecond is, if so, what caused the anomaly?

April 1999 was clearly significantly warmer thanany of the other sections, but with only six sections

spanning 5 years, we cannot tell how frequently thislevel of warming occurs. Nor can we deduce muchabout its history. However, satellite derived SSTcollected since 1993 gives some insight into its pos-sible frequency. The probability that SST residualsreach such high values can be estimated from the 8years of available data, and Fig. 9 shows the timeseries of SST residuals for the 50 x 50 km box shownin Fig. 1. For this location, the standard deviation inSST residuals is 0.84°C, and April 1999 SST residualwas >2 standard deviations from the mean value. Itappears that, by chance, the April 1999 cruise tookplace right at the peak of the warmest SST residualduring the entire 8-year record. Although SST andthe deep warming could in principle be disconnected,we presume that the deep warming, too, was anoma-lous at these timescales.

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May 1996 Sep19970

291

100044 42

Latitude (°S)40 46 44 42

Latitude (°S)

Fig. 8 Residual temperature (°C) for each of the six sections (see text).

- 5

- 5

40

There are two potential mechanisms for the deep the STF can safely be discounted, because thewarming within the STF. One is a meridional horizontal gradients in temperature were not strongsouthward shift of the STF, the other is advection of enough to allow a feature of this magnitude towarmer water into the region. A meridional shift in develop, and because the April 1999 water mass

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LU

o

CO

Fig. 9 Sea surface temperature(SST) residual for the 50 X 50 kmbox shown in Fig. 1, derived frommonthly mean satellite images(lower panel). Vertical lines showmid points of the six cruises. Alsoshown (upper panel) is WairarapaEddy (WE) index (see text). WEindex has been de-meaned and nor-malised. Horizontal lines in bothpanels are the +2 standard devia-tion levels.

1993 1994 1995 1996 1997 1998 1999 2000

properties were quite different (as shown by the T-S structure; Fig. 3). Thus, we suggest that thewarming was due to an acceleration of the East CapeCurrent and/or Wairarapa Eddy. If this were so, theheightened temperature residual along the north sideof the section would be due to the northern branchof the Wairarapa Eddy.

Figure 10 shows maps of the monthly-mean SSTresidual (i.e., after the mean and annual cycle havebeen removed) for the region for the 6 months fromNovember 1998 to April 1999. In November 1998,there was no anomalous SST, but in December 1998a warm anomaly appeared off the coast of the NorthIsland. Although the SST residual field is very noisyand there is no unambiguous interpretation of thefields, there are patches of warm SST anomaly inJanuary, February, and March 1999 which areconsistent with the advection of this patch south-westwards down the coast and then eastwards alongthe northern flank of the Chatham Rise. What causedthe anomaly to develop in the first instance is notclear, but the figure supports the idea that the April1999 anomaly was the result of increased flow in theWairarapa Eddy.

One way to test this hypothesis is to use T/Paltimeter data to map the Wairarapa Eddy. Fig. 1shows the surface geostrophic circulation derivedfrom T/P data for 14 April 1999. Indices ofWairarapa Eddy circulation were derived as the total

surface current through the lines shown in Fig. 1, andas the maximum dynamic height within the eddy.There is some variability between the indices,probably associated with spatial fluctuations of theeddy, but they tend to show similar features. Fig. 9shows the dynamic height index, i.e., after removalof its mean and annual cycle, for comparison withSST residual.

Figure 9 shows that for both of these parameters,the only time in the 8-year long records that eitherparameter exceeded the 2-standard deviation levelwas in early 1999, although the index leads SST re-sidual by a month or two. One would not expect asimple relationship between the Wairarapa Eddyindex and SST residual because the eddy index doesnot take into account temperature gradients. How-ever, that the index exceeds its 2-standard deviationlevel at essentially the same time as SST residualsuggests that the two events were correlated.

SUMMARY AND DISCUSSION

The main aim of this work was to provide estimatesof the mean and variability of temperature andsalinity sections across the STF. Since the advent ofhigh-precision CTD profilers, few high-resolutionhydrographic sections have been made across theSTF. Most of these have been made at a variety of

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Nov1998 Dec 1999

293

Feb1999

Apr 1999

174 176 178 180Longitude (°E)

178 172 174 176 178 180Longitude (°E)

178

Fig. 10 Sea surface temperature residual (°C) for November 1998-April 1999.

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longitudes, and it is only along 178°30'E that thereis more than one repeat section.

The comparison between ADCP and geostrophygives some confidence that the main features seenin the velocity sections are well described. Thepresence of the two eastward flowing jets is of nosurprise, since they have been documented before inthe literature. However, the westward-flowingsubsurface jet inshore of the Wairarapa Eddy has notso far been described. Chiswell & Sutton (1998)noted it in their description of a deep eddy in theAAIW, but did not recognise that it is apparently apersistent feature.

The six sections used here were taken over 5 yearsbiased towards spring and autumn sampling, whichimmediately raises the question of how well the meanand annual cycles can be determined. The Monte-Carlosimulations of the errors suggest that the mean fieldscan be reasonably well determined, but the annualcycles have more relative error, although to get anyconfidence in the seasonal cycle we had to assume thephase of the seasonal cycle was 1.

Out of necessity, this study has neglected anycontribution from interannual variability and it isimpossible to estimate how much interannual signalshave contaminated our estimates of the annual cycle.

Given the uncertainties in the climatology, onehas to address the question of whether one canaccurately compute residuals when the seasonalcycle is relatively unknown. Above 600 m, the April1999 residuals in the deep lobe were larger than thepotential errors in the seasonal cycle by factors of 3or more (Fig. 8), suggesting that the April residualsare robust. Repeating this analysis without removingthe seasonal cycle leads to exactly the sameconclusions about the April 1999 anomalies. Thus,we can be certain that the April 1999 conditions wereanomalous compared with the other five sections, butthe conclusion that they were anomalous comparedwith the long-term variability rests heavily on theassumption that there is a direct linkage between thesurface and deep residuals. There is no way to testthis, except circumstantially through the observationthat the residuals are surface intensified, and that theyare co-located spatially with the SST residuals.

The linkage between the April 1999 anomaliesand increased strength of the Wairarapa Eddy at thattime is perhaps more speculative than robust. How-ever, there is little likelihood that the anomalies werecaused by meridional meandering of the STF. Thus,advection of warmer and saltier STW throughincreased flow of the East Cape Current into theWairarapa Eddy seems the most likely explanation,

and the fact that both the anomalies and theWairarapa Eddy index were above 2 standard devia-tions from their respective means would bolster thisargument. What caused this increased circulation,and whether it was relatively local, or was a resultof a much larger scale acceleration of the currentsaround New Zealand, is beyond the scope of thisarticle.

ACKNOWLEDGMENTS

I thank all those who contributed to the cruises and madethe collection of these data possible. Thanks to P. Boyd,K. Currie, M. Greig, P. Hill, M. Hopkins, W. Main, S.McClatchie, N. Oien, R. Singleton, P. Sutton, M.Walkington, and S. Wilcox, for participation in data col-lection. P. Sutton kindly provided the October 2000 CTDdata. Thanks to J. McGregor for processing the AVHRRdata. AVHRR data were collected and archived at NIWAby M. Uddstrom. I thank B. Stanton for stimulating dis-cussions. I thank L. -L. Fu (NASA Jet Propulsion Labo-ratory) for providing TOPEX/Poseidon data, and D.Roemmich and L. Lehmann (Scripps Institution of Ocea-nography) for assistance with the gridding of the data.Thanks also to the master and crew of RV Tangaroa fortheir help at sea. This work was carried out under NewZealand Foundation for Research, Science and Technol-ogy contract C01X0027.

REFERENCES

Annala, J. H. 1992: Report from the Fishery AssessmentPlenary, May 1992: stock assessment and yieldestimates. National Institute of Water and Atmos-pheric Research. 44 p.

Chiswell, S. M. 1994: Variability in sea surface tempera-ture around New Zealand from AVHRR images.New Zealand Journal of Marine and FreshwaterResearch 28: 179-192.

Chiswell, S. M. 2001a: Determining the internal structureof the ocean off north-east New Zealand fromsurface measurements. New Zealand Journal ofMarine and Freshwater Research 35: 289-306.

Chiswell, S. M. 2001b: Eddy energetics in the Subtropi-cal Front over the Chatham Rise. New ZealandJournal of Marine and Freshwater Research 35:1-15.

Chiswell, S. M.; Booth, J. 1999: Rock lobster Jasusedwardsii larval retention by the Wairarapa Eddyoff New Zealand. Marine Ecology Progress Se-ries 183: 227-240.

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Chiswell, S. M.; Sutton, P. 1998: A deep eddy in theAntarctic Intermediate Water north of theChatham Rise. Journal of Physical Oceanogra-phy 28: 535-540.

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. Physics SectionReport 93/4. 85 p.

Enting, I. G.; Mansbridge, J. V. 1991: Latitudinal distri-bution of sources and sinks of CO2: results of aninversion study. Tellus 43: 156-170.

Firing, E.; Lukas, R. 1985: Sampling and aliasing duringthe NORPAX Hawaii-to-Tahiti Shuttle Experi-ment. Journal of Geophysical Research 90: 11709-11 718.

Hatanaka, H.; Uozumi, Y.; Fukui, J.; Aizawa, M.;Livingston, M. 1989: Trawl survey of hoki andother slope fish on the Chatham Rise, New Zea-land, November-December 1983. Wellington,New Zealand, MAFish 17. 31 p.

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

Heath, R. A. 1976: Models of the diffusive-advectivebalance at the Subtropical Convergence. Deep-Sea Research 23: 1153-1164.

McPhaden, M. 1982: Variability in the central equatorialIndian Ocean, I, Ocean dynamics. Journal of Ma-rine Research 40: 157-176.

Robertson, J. E.; Watson, A. J. 1995: A summertime sinkfor atmospheric carbon dioxide in the SouthernOcean between 88°W and 80°E. Deep Sea Re-search II 42: 1081-1092.

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 103: 13 041-13 054.

Sarmiento, J. L. 1993: Atmospheric CO2 stalled. Nature365: 697-698.

Siegenthaler, U.; Sarmiento, J. L. 1993: Atmosphericcarbon dioxide and the ocean. Nature 365: 119-125.

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: 20 729-20 751.

Walkington, C. M.; Chiswell, S. M. 1993: CTD observa-tions in the subtropical convergence ChathamRise. Wellington, Physics Section, New ZealandOceanographic Institute, Physics Section Report93/1. 87 p.

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.

{Appendices follow)

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Appendix 1 Error analysis.

The seasonal cycle in Equation 1 was defined to be a sinusoid having a 365.25-day period. Thus, at each (y, p) location,temperature is assumed to follow:T(y,p,t) = T0(y,p) + Tl(y,p)cos(2nt/365.25-<bl(y,pj) + T'(y,p,t) (2)where the amplitude (7\) and phase ((])) of the seasonal cycle were computed using a least squares fit. The mean (To)and residual (T') were computed as the mean and residual after removal of the seasonal cycle. Salinity was treatedsimilarly.

The fits were done excluding the April 1999 data, so that each fit was done with five samples irregularly spacedover 5 years. With such limited sampling of the seasonal cycle, there could potentially be large biases and standarddeviations in the estimates of the coefficients. A pathological case would occur if all cruises occurred at the same timeof the year, but even moderate amounts of clustering of t can lead to biases. The error estimates of the seasonal andmean cycles depend on both the relative levels of the residual signal, and the distribution of sampling times.

Here, the distribution of t is dictated by times of the cruises, so that it is the residual variance and phase of theseasonal cycle that are important.

Estimated errors in the coefficients were determined using Monte Carlo simulations. Artificially constructed timeseries with known seasonal coefficients and random residuals were resampled at the cruise times and used to estimatethe coefficients. By performing this repeatedly with different randomly drawn residuals, one obtains many estimatesof the coefficients which can be used to determine the coefficient errors. The model used here is:

x(t) = x0 + xl cos(2ra / 365.25 - 0) + e(0 (3)where x denotes the variable of interest.

The least squares fitting produces a fit:

x(tobs) = x0 + x{ cos(2mobs 1365.25 - <j)) + i(tobs)

where the time vector tohs was set to be the times of the five cruises used in the seasonal fits, and the modelled residualwas assumed to be randomly distributed with a pre-specified standard deviation given by o(e) = \|/o(x). Estimates ofthe coefficients (x0, x j , <j)) were computed 1000 times for each of many sets of prescribed x0, x j,$, and \\f which allowsthe probable bias and standard deviation in each estimator as a function of phase and residual level to be computed.

Appendix 2 shows the results for XQ= 10, x= 1, and \|/ = 0.75. Means and standard deviations of -?0i*b ˆ are plottedas a function of the phase. For this level of \|/ (which is within the range observed for temperature), the mean, x0, isrelatively well estimated with a bias of c. 0.2° and a standard deviation, o (x 0) which varies linearly with phase. Theamplitude has a bias and standard deviation that both vary with phase.

However, the figure illustrates the fundamental problem with this data set for estimating the seasonal cycle. Thebias and standard deviation in the mean and amplitude are sensitive to the phase of the seasonal cycle, yet the phaseestimate is meaningless—the phase estimate shows no relationship to the true phase. The high errors in the phaseresult from the poor distribution of tohs over the seasonal cycle. This suggests that such bootstrap methods of using thecomputed phase could lead to dangerously wrong estimates of the errors in the coefficients. Fortunately, however, theseasonal phase is the best known parameter a priori, since the annual cycle in sea surface temperature in this regionpeaks in late February or early March (Chiswell 1994; Uddstrom & Oien 1999), and there is generally a smallretardation of phase with depth (Chiswell 2001a). Thus, here we take the approach that §\ = 1 (peak temperature on 27February), and use this value to estimate model errors.

Performing the Monte Carlo simulations with combinations of x0, xx, (p, and \|/ allows one to obtain paramaterisedexpressions for the expected biases and standard deviations. These relationships are shown in Appendix 3, and wereused to compute the expected errors in the climatology.

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Appendix 2 Means and standard deviations from Monte Carlo simulations of seasonal fitting. Left hand panelsshow the average values of the estimated mean, amplitude, and phase (x0, jtj, and (pi) from 1000 simulations plotted asa function of the prescribed phase, 0. Right hand panels show the standard deviations of the estimated coefficients. Forthese simulations, the mean x0 was set to 10, the amplitude xl was set to 1, and the normalised residual level \|/ was 0.75(see text).

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Appendix 3 Normalised bias and standard derivation for estimated mean and amplitude (x0 and x{) plotted as afunction of normalised residual level \\f for the Monte Carlo simulations. Bias and standard deviations have beenscaled by the seasonal amplitude xx.

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temperature and salinity mean and variability within the subtropical front over the chatham rise,...

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