The South Pacific Subtropical Mode Water in the Tasman Sea

Journal of Ocean University of China (Oceanic and Coastal Sea Research)ISSN 1672-5182, April 30, 2007, Vol.6, No.2, pp.107-116http://www.ouc. edu. cn/xbywb/E-mail:xbywb@ouc. edu.cnThe South Pacific Subtropical Mode Water in the Tasman SeaHU Haibo', LIU Qinyu')*, LIN Xiaopeil), and LIU Wei21) Physical Oceanography Laboratory & Ocean-Atmosphere Interaction and Climate Laboratory,Ocean University of China, Qingdao 266003, P. R. China2) Center, for Climatic Research, University of Wisconsin-Madison, Madison, Wisconsin, USA(Received September 28, 2006; accepted December 20, 2006)Abstract From the synopical CTD sections in the WOCE PR11 repeated cruises, the South Pacific Subtropical Mode Water(SPSTMW) has been identified in the region of the Tasman Front Extension (TFE) around 29 S to the east of Australia. In the depthrange of 150-250m, the SPSTMW appears as a thermostad with vertical temperature gradient lower than 1.6C (100m)~ and a tem-perature range of 16.5-19.5"C and as a pycnostad with PV lower than 2X 10~10 m' s' and a potential density range of 25.4-26.0kgm3. Like the subtropical mode waters in the North Atlantic and North Pacific, the formation of the SPSTMW is associated with theconvective mixing during the austral wintertime as manifested from the time series of the Argo floats. And cold water entrains intothe mixed layer with the deepening mixed layer from September to the middle of October. During the wintertime formation process,mesoscale eddies prevailing in the TFE region play an important role in the SPSTMW formation, and have a great effect on theSPSTMW distribution in the next year. The deeper (shallower) mixed layer in wintertime, consistent with the depressed (uplifted)permanent thermocline, is formed by the anticyclonic (cyclonic) eddies, and the substantial mode water thicker than 50 m is mainlyfound in the region of the anticyclonic eddies where the permanent thermocline is deeper than 450m.Key words SPSTMW; PR11 repeated cruises; Argo floats; eddyDOI 10.1007/s11802-007-0107-5Cornuelle pointed out that there were two persistent fron-tal regions containing the separated EAC. The former1 Introductionexisted along the northern coast of New Zealand and theThe Subtropical Mode Water (STMW), known as the latter was located near 29°S, which was likely to be an18C water, was firstly found in the North Atlantic extension of the Tasman Front (TF) (Roemmich and(Worthington, 1959; McCartney, 1982; Talley and Ray- Cornuelle, 1990). The two frontal regions are interpretedmer, 1982). In the North Pacific, the STMW was reported as the sites of STMW formation, because the wintertimeto be south of the Kuroshio and the Kuroshio Extention surface temperature in these two regions were 15-17°C(KE) in the western part of the gyre (Masuzawa, 1969; and 17-19 C，respectively, being consistent with theHanawa, 1987; Suga and Hanawa, 1990; Bingham, 1992; thermostad of the SPSTMW having a temperature ofSuga and Hanawa, 1995a, b; Suga et al, 2004).15-17°C at latitudes between 30°S and 35'S and 17-19°CIn the South Pacific, the common features of the sub- at latitudes between 25"S and 30°S.tropical gyres are associated with the separated westernFig.la cited from the paper of Roemmich and Cor-boundary currents and anticyclonic recirculation, fewer nuelle (1992) displays the 17-19C SPSTMW areas inresearches have been made on the STMW. There are two terms of the thickness of the 17-19C intervals with thepossible reasons for this. First, the weakness and instabil- maximum thickness areas indicating the STMW forma-ity of the East Australian Current (EAC) and the subse- tion sites. The 17-19C SPSTMW can be seen in the fig-quently separated filaments make the STMW thermostad ure (Fig.1b) drawn from the Levitus-98 data in the sameless pronounced than that in the North Atlantic and North way. Seen from Fig.1b, the most remarkable feature isPacific. Second, the South Pacific has always been a that an area of 17-19C water thicker than 100 m ap-poorly sampled ocean in history. However, Roemmich peared as a zonal band near 29°S extending from the eastand Cornuelle (1992) found an SPSTMW to the north of of Australia to the International Dateline, correspondingNew Zealand to be a thermostad with a temperature beto the formationf 中国煤化工emperauretween 15C and 19°C. In their study, Roemmich and of 17-19C.However, RoMHCNMHGdy(1992)of* Corresponding author. Tel: 0086-532- 66782556the SPSTMW was primarily based on the data of XBTE-mail: liuqy@ouc .edu.cntransects collected from M/V Southland Star. These XBT.108Journal of Ocean University of ChinaVol.6, No.2, 2007transects generally had a meridional coverage north off especially paid to the role of eddies in the formation area.New Zealand and could not describe the SPSTMW of The last section is a summary of the paper and presents a17-19C formed in the region of the Tasman Front Ex- perspective of future work.tension (TFE) near 29°S. Thus, the object of the presentpaper is to study the SPSTMW in the TFE by using a Va-2 Data and Methodsriety of recently available data sources, including the pro-filing floats deployed in the South Pacific as part of the 2.1 DataArgo program.2.1.1 CTD dataSynopical CTD sections provide accurate measure-ments of temperature and salinity with high spatial reso-20°Slutions. Hence we begin the description of the SPSTMW80SS7100V100 , 80了o80工by examining the high-quality CTD sections in the South①2080)580 (i2g n60ooPacific. As part of the WOCE Hydrographic Project, CTD0° t60data were collected by R/V Franklin along PR1I repeatedj60 o0cruises from 1989 to 1994. Along each section, CTD0° L150°E80°150"wmeasurements were made from the sea surface down tothe bottom. Three parameters were measured during wa-a)ter sampling: temperature, salinity and dissolved oxygen.0-In all of the sections in situ bottle measurements were20°一6(- 6060---60.made for calibration and the data were thought to be ac-0一80 |curate to 0.002C in temperature, 0.003 PSS-78 (UNESCO0o80--Practical Scale of 1978) in salinity and 0.1% in pressure.100 。~8CCalculations of potential density in this paper are refer-enced to zero pressure.R/V Franklin did not follow a constant routine during180°150"Wthe 5-year period and the PR11 repeated cruises changeda great deal from year to year. At the same time, due toFig.1 Thickness of the 17-19C layer in the subtropi-some unavoidable circumstances such as bad weather, thecal South Pacific. (a) Adapted from Roemmich andcourses of ship cruise appeared complicated and wereCornuelle (1992). (b) Calculated based on Levitus-98often divided into several legs. As shown in Table 1, datadata at intervals of 20m. The 100m contour is shownin bold to denote the SPSTMW formation area in thisfrom cruises Jul 1992 and Mar 1994 are not adopted inlayer.our study for the reason of their sparse samples or limitedspatial coverage. The routes of the remaining four cruisesThe remainder of the paper is organized as follows. are shown in Fig.2, and from the figure we can observeSection 2 describes the data sets used in the study and the that each cruise is generally composed of several merid-sampling procedures of CTD and Argo floats data. ional and zonal sections. Aiming at the SPSTMW in theAnalysis of the CTD data is made in Sections 3 and 4，region of the TFE near 29°S, we select four zonal sectionsmainly focusing on the characteristics of the SPSTMW. as follows: the sections along 28°S during cruises OctIn the following section, about the formation and distri- 1989 and Feb 1990 and the sections along 30°S duringbution of the mode water are discussed, attention beingcruises Oct 1991 and Jul 1993 (Fig.2).Table 1 Summary of CTD observations of the R/V Franklin PRII cruises between 1989 and 1994Section (selected)Longitudinal range ofTemporal span ofCruise No.DateReason (not selected)the sectionFr 10/89Aug. 15-Sep. 27 198928S172.13*-153.53'EAug. 27-Sep. 03 1989Fr 02/90Feb. 26-Apr. 07 199028'S172.91*-153.53'EMar. 11-Mar. 18 1990Fr 10/91Nov. 15-Dec. 15 1991153.29*-169.97°ENov.17-Mar. 251991Fr 07/92Sep. 19-Oct. 06 1992Limited coverageFr 07/93Sep. 11-Oct. 05 199330°S171.99*-153.48'ESep. 24-Oct. 011993Fr 03/94Mar. 10-Apr. 03 1994Sparse samplesshows that three floats were deploved in 2001 and the2.1.2 ARGO float dataothers in 2004.中国煤化工val increasedIn the region of the TFE, six profiling floats have beenwith depth; forYHCNMHG of float IDdeployed since 2000. Their trajectories are drawn in Fig.2 5900109, the interval was 8m wthin 0-200m, 24m with-and corresponding information is listed in Table2. Table2 in 200-440m and 56m for the rest. The longest span of.HU H. B. et al: The South Pacific Subtropical Mode Water in the Tasman Sea109the drifting floats was about 23 months (float ID track correlated errors (Le Traon et al, 1998; Ducet et al,5900105), and float ID 5900105 also operated for about 2000), with several improvements by Ducet et al. (2000).11 months after deployment.Data used in the present work is provided on aMERCATOR 1/3° grid.2.1.3 Acoustic doppler current profiler (ADCP) data一Cruise Oct 1989ADCP data were also collected by R/V Franklin from20°S一Cruise Feb 19901989 to 1994 as a part of the WOCE projects. The current-。Cruise Jul 1993velocity was collected at intervals of one hour and each59001055900094velocity profile was linearly interpolated to 10m in the5900605 ? 5900606vertical. The maximum depth for measurement is 300m30°in the cruises of 1989, 1990 and 1991, but only 140m in5900632 Y5900109the cruise of 1993.2.1.4 Data for sea level anomaly maps40°Maps of Sea Level Anomaly (MSLA) are obtainedevery 7 d from a complete reprocessing of TOPEX/50°160170POSEIDON (10-day repeated orbit periods) and ERS-1/2data (35-day repeated orbit periods) for a period ofalmnost Fig.2 The map of WOCE PR1I repeated cruises by R/V10 years (October 1992 through February 2002). T/PFranklin (selected from Table 1) and Argo float trajectories.Ship cruises are differently denoted: cruise Oct 1989 ismaps were available fothere were no T/P+ERS combined maps from January ofshown in solid line marked by dots, cruise Feb 1990 indashed line marked by crosses, cruise Oct 1991 in solid line1994 to March of 1995 (ERS-1 geodetic phase). The marked by pluses and cruise Jul 1993 in dashed line markedmapping method for MSLA is a global sub-optimalby asterisks. The Argo floats ID are marked at the site ofspace/time objective analysis taking into account along- deployment.Table 2 Summary of Argo float observations after deployment in 2000 in the region of25* -30'S 155 -180°E(updated until Sep. 27, 2004)Longitudinal range ofLatitudinal range ofVertical samplingFloat IDDateStation No.the trajectoryintervals (m)Jul.-Dec. 2001.156.13*-159.94'E25.47°. -28.07°S10, 20, 50Oct. 2001-Sep. 200369170.77*-179.84'E .25.03*-29.58S8,24, 565900109Sep.2001-Aug. 200232168.09*-172.93*E27.99°- 31.02°S8, 24, 565900605Jul.- Sep. 2004157.06*-158.16E25.04*-27.07"S6, 18, 30, 45, 605900606164.89*-166.17°E26.15*-27.37.S5900632Aug. -Sep. 2004164.88*-167.77°E29.96*-30.83'S6.18, 30,45,60the sea water properties from the profiles were considered2.1.5 SST data sourceto be smooth enough for calculation with no need for anyThe AVHRR MCSST data set contains weekly aver-smoothing procedures.aged Multi-Channel Sea Surface Temperature (MCSST)Since the SPSTMW is also characterized by a pycnos-data between 11 November 1981 and 7 February 2001,tad, a minimum in the vertical density gradient or the po-derived from the NOAA Advanced Very High Resoltion tential vorticity (PV) minimum. Smoothed data fromRadioneter (AVHRR). The data are given on an CTD were firstly interpolated linearly to 0.05kgm2 pcequal-angle grids of 2048 pixels longitude by 1024 pixelstential density intervals, and then the PV,latitude; therefore, the spatial resolution in pixels per de-PV =(f Iρ)(△σg/Oz), .gree of longitude and latitude is 2048/360, so the resolu-tion of the SST is practicable for eddies.was calculated for each standard potential density level,where f is the Coriolis parameter, ρ is in situ density,2.2 Analytical Tool△σg and△z are respectively the differences of potentialSections of temperature and vertical temperature gra- density and depth between the isopycnals immediatelydient were prepared to highlight the SPSTMW thermo-above and below a given level. The water mass with thestad. The CTD data was firstly interpolated linearlyto 10 low PV associated with the wintertime deep convection ism intervals and then smoothed vertically using a Hanning usually the sourc中国煤化工is conservedfilter; then the vertical temperature gradient was calcu- during the advecn the STMWlated by the finite-difference method. As for the Argo formation area, :YHCNMHG...... ;心a reasonablefloats’profiles sampled at intervals of 6m, 8m or 10m, tracer of STMW..110Journal of Ocean University of ChinaVol.6, No.2, 2007tween 14C and 20°C. Along the same section about half3 The Temperature and Potential Densitya year later, a thermostad indicated by the shaded areawith a temperature between 18C and 19°C was foundSections Along PR11 Repeated Cruisesbetween longitudesI 54°-170°E at a depth of 100-250 mThe temperature and potential density sections along(Fig.4a). The mode water was also indicated by the28"S in October 1989 (Figs.3a and b) show a distinct shaded area between 25.4kgm^ and 25.7kg m3 isopy-character of the deep mixed layer in late wintertime. The cnals (the pycnostad ofPV<2x10~10 m's') in Fig.4b.SST along the section generally ranges from 19C to 20"Cand the sea surface potential density from 25.4kgm to25.6 kg m3. If the mixed layer depth (MLD) is defined as192021 22100 1434the depth at which the temperature is 1°C lower than that132018of the surface, the MLD was generally deeper than 150m宜200- 19/7-and reached more than 250m in some longitudes. How-300ever, it was yet much shallower than that in the STMWformation area in the North Pacific where the MLD400少13112would exceed 400 m during the wintertime (Suga and1112Hanawa, 1990). This is probably because the EAC is a155°160°165°170°Eweaker western boundary current compared to the Ku-a)roshio in terms of heat transport. Meanwhile, the low PV:24.224-24.224(PV<2x10l'9m's') source was embedded in the deepmixed layer during the course of strong wintertime con-10033225.6~125.425.25.8.vection and it would serve as a tracer of the mode water官2005.655L 25.625.. 25.826.2in the succeeding seasons.营30026.4 了26.2~26.2，26.4~26.4-个26.426.650026 26.626.81020官200(b)5 300Fig.4 As in Fig.3, but for the 28°S section in cruise Feb1990. The bar on top of the figure shows the positions ofthe floats.500160*Figs.5a and b show the temperature and potential den-(asity sections along 30°S in October 1990. Early in theaustral spring, the mixed layer retreated rapidly and the| 26.4seasonal thermocline began to form, so the STMW wasclearly observed at the depth of 150-200 m between lon-巨200gitudes 157°E and 166* E, the thermostad being between复300262.the 17°C and 19C isotherms and the pycnostad beingbetween 25.6kgm3 and 26.0 kg m3. The water masses40near 155°E and 169E appeared characteristic of weak50r 268stratification, with vertical temperature gradient lower170°E .than 1.6 °C (100m)~' from the surface down to 300m(Fig.5a). Weak stratification was not so obvious nearFig.3 Plots of seasonal cycles from the 28°S section in155 °E in the potential density section, for the shaded areathe Cruise Oct 1989. (a) Temperature along the 28°Sdenoting the pycnostad had largely shrunk (Fig.5b). In thesection, marked on contours with 1°C intervals. The 12Carea near 169°E, the pycnostad was separated from thecontours are shown in bold. Areas with vertical tem-perature gradients less than 1.6C (100m)-' are shaded.mixed layer and an STMW layer was seen at the depth ofThe mixed layer depth (MLD) is defined as the depth at100-300 m, with potential densities of 25.6-26kgm3. Inwhich the temperature is 1C lower than the surface andaddition, a common feature can be found in Figs.4 and 5:indicated by the base of the upper shaded areas. (b) Po-the shaded area denoting the pycnostad is much smallertential density along the 28 S section at 0.2 kgm° inter-vals. Areas with PV<2x10'l m " s' are shaded. The barthan that denoting the thermostad, indicating that thepycnostad of PV<2x10~"m's' in the present work ison top of the figure shows the positions of the floats.somewhat a stric中国煤化工vw than theRoemmich and Cornuelle (1992) defined the SPSTMW referenced thermYHCNMHdientlessthanthermostad as a layer where the vertical temperature gra- 1.6C (100m)dient is less than 1.6°C (100m)" , with a temperature be-Results for the section along 30S in the July of 1993.HU H. B. et al: The South Pacific Subtropical Mode Water in the Tasman Sea111are shown in Figs.6a and b. The most remarkable feature the two parts corresponding to the temperature difference.of Fig.6 is that there was a strong temperature (potential Generally speaking, the 12C-isotherm depth in the eastdensity) tilt at longitude 162.5"E. It extended from the was about 150m deeper than that in the west. All thesesurface to depths greater than that of the permanent ther- differences could be attributed to the effect of the eddiesmocline and divided the section into two parts. In the which will be discussed in detail in section 5. As the shipwestern part, the temperature (potential density) was cruise was taken at the end of September, the SST hadmuch higher (lighter); for example, the SST (sea surface risen one degree from the beginning of the month and didpotential density) was 20-21°C (25.0-25.4kgm) while not represent the exact temperature of the subducted wa-in its eastern counterpart it was 18-19C (25.6-25.8kgm). ter. However, the characteristics of the deep wintertimeBesides, the 12C-isotherm depths were also different in mixed layer are still reflected in the section along the July1993 cruise.J22、 ，A 2120/2019100二294 Water Properties of the SPS TMW仓200i M18Fig.7 shows the Theta-S diagram for the SPSTMW in昌300the sections during the cruises from February to Aprilin 1990 and from November to December in 1991, ancthe SPSTMW was denoted as the water masses with500PV<2x10^0 m's' and a potential density of 25.4-26155*160"165°170°Ekgm3. In the scatter diagram, the SPSTMW displayed aa)temperature range of 16.5-19.5°C but occupied a rela-tively small salinity range from 35.5 PPS78 to 35.7PPS78.-25.625.8300_26.29.5k26A40019.0155160*170'E8.5 t25.6Fig.5 As in Fig.3, but for the 30*S section in cruise Oct1991. The bar on top of the figure shows the positions of8.0 25.6the floats.7.561191919一258-26-17.0n1宜20018 2016.5量300-。17519, 26一，262212. 1335.435.635.7500- 813114121小221213Salinity (PSS 78)Fig.7 Theta-S diagram of the SPSTMW. Dots are drawn155°160° 165°from samples with PV<2X 10~10 m' s' between the sigma(avalues 25.4kgm3 and 26kgm° in the CTD sections of25.cruises Feb 1990 and Oct 1991.2525 25.4 26.6'100-号425.226.6( 25.8200To further study the temperature characteristics of the-26rSPSTMW, we prepared diagrams of vertical temperature3002325.826 t 26.2 26.2 - .6.2gradient versus temperature in the CTD sections during26.4、26.4.the Cruises of Feb 1990 and Oct 1991 and examined the .26.8 26.226.26.4core layer temperature (CLT) of the mode waters - the: 26.4 s55°160°temperature at the minimum of the vertical temperature()gradient. Fig.8 S中国煤化工C SPSTMWFig.6 As in Fig.3, but for the 28°S section in cruise Julin the two years.YHCN MH Ge can clearly1993. The bar on top of the figure shows the positions ofobserve that the nuun.viuivai giauili correspondedto the temperature of 19.0C (CLT). However, in the dia-.112Journal of Ocean University of ChinaVol.6, No.2, 2007gram for 1991, the dots scattered more desultorily than the seasonal thermocline, being accompanied with thethose for 1990 and the minimum of vertical temperature decreasing (increasing) of temperature (potential density).gradient was less distinct. This is probably due to con- The initially deepening mixed layer can also be observedtamination from a lamina of mixed layer retained as ru- from the synoptic shot of CTD section of cruise Feb 1990diment of the previous winter. The prominent minima of (Fig.4). However, convective processes dominated fromtemperature gradient in 1991 centered about 17.5 C May to the late austral winter or early spring, as the sur-(CLT). This lower CLT results from two reasons: it mayface temperature (potential density) dropped (rose) andbe caused by the difference in the spatial coverage. The the mixed layer eventually deepened to more than 150mcruise in 1990 was carried out along latitude 28* S, nearer in September. About one and a half months later, the sur-to the equator than the cruise in 1991 along 30°S. And face temperature (potential density) started to rise (drop)there was seasonal difference between the two cruises. and the deep winter mixed layer was soon capped by aWater was sampled in late summer for the former cruise shallow seasonal thermocline. In the succeeding months,and in early spring for the latter. It is natural that the the seasonal thermocline strengthened as the sufacemode waters in lower latitudes had a high CLT in a temperature (potential density) rose (dropped) to its valuewarmer season.in the austral summer. A seasonal cycle was thus com-pleted._28° S/Mar.- Apr.199030"S/Nov.-Dec. 199124 [1246.0r22 F5.5550220 t巨10024.518150|24.0。 23.5|0023.04tSLC5NJMMJSNJMMJS “23 NJ M MJSNJMMJS12Month(ab)Fig.8 Scatter diagram of vertical temperature gradient36.0广35.8versus temperature in the CTD sections along 28S inMarch-April 1990and 28°S in November-December23/器3.6vMd1991, high-lightening the core layer temperature (CLT)a223 35.4with the minimum vertical temperature gradient.2035.2 MrWWlB 35.05 The Formation and Evolution of the34.85NJMMJSNJMMJS” NJMMJSNJMMJSSPSTMW(<)()5.1 The Seasonal Cycle of the Mixed LayerFig.9 Plots of seasonal cycles from Argo float IDPapers in the past studied the mixed layer mainly on5900105 (a) the MLD (represented by the pressure at thethe basis of climatological and synoptic data in the Southbase of the mixed layer), (b) near-surface potential den-Pacific and it was difficult to study the seasonal cycle ofsity, (C) near-surface potential temperature, (d) near-sur-the mixed layer without continuous observations. Thanksface correlated salinity.to the deployment of the Argo floats since 2000, the con-tinuous monitoring on basin scale is practicable and usu-From the above analysis, it is found that the mixedally lasts more than one austral year, enabling us to ob- layer is deepest in September. It starts to retreat rapidly inserve the seasonal evolution of the mixed layer in the the middle of October and reaches its shallowest depth ingyre of the South Pacific.the next January. This phenomenon, the mixed layer'sConsidering the SPSTMW formation area, float ID density and depth reaching their local maxima in late5900105 was selected from Table 2 as it drifted for nearly winter, was named the ‘Stommel demon' (Stommel,two austral years, long enough for studying the seasonal 1979). The‘ Stommel demon' occurs in March in theevolution of the mixed layer. Fig.9 shows the time series Northern Hemisphere and in September in the Southernof the MLD (a), near surface potential density (b), the Hemisphere.near-surface potential temperature (C) and the correctedHowever, no obvious seasonal cycle of the near surfacesalinity (d) from float ID 5900105. The mixed layer tem- salinity was observed from the float data and the timeperature (potential density) was highest (lightest) and the series followed a中国煤化工demonstratesmixed layer was shallowest between February and March that salinity in thCNMHGdidnotplayduring the austral summer. From March onward, the an important rold uHii Lilvective mixingmixed layer began to deepen and gradually detrained into and the formation of the SPSTMW..HU H. B. et al: The South Pacific Subtropical Mode Water in the Tasman Sea113keeping its wintertime characteristics that the vertical5.2 Evolution of the SPSTMWtemperature gradient was less than 1.6C (100 m)", andBesides demonstrating the seasonal cycling of thethe temperature was about 18 C.The mass of mode watermixed layer, the Argo float data in the SPSTMW forma-was embedded in the depth of 150-250 m, moving down-tion region also helps to view the subsequent advection ofward slightly after formation. After March 2003, the wa-the SPSTMW after its formation. Considering the route ofter mass gradually disappeared from the float trajectorythe drifting floats, we select floats ID 5900105 andand the depth of the mixed layer began to increase. Then5900109 to study the annual evolution of the SPSTMW,a new cycle started.In Fig. 10b, the time series of the potential density dis-since the others display a similar pattern.ne time seriesof temperature and potential density of float ID 5900105played a similar but less clear process because of theare respectively shown in Figs. l0a and b, and the shadedcontamination of the low PV water below. The mode wa-area in each time series denotes the thermostad andter formed with a potential density of about 25.6 kg m~and kept conservative in the following months.pycnostad of the SPSTMW respectively.The time series of float ID 5900109 started from theend of September 2001 and the wintertime mixed layercould still be observed at a depth as great as 250m in002092191Fig.lla. Strongconvective mixing had lasted by the endof November. The water entering the permanent thermo-cline had the temperature of 17-18C and potential den-sity of 25.8-26kgm'. From then on, the mode water was00covered by the seasonal thermocline and embedded at400Nov. Jan. Mar.May. Jul. Sep. Nov. Jan. Mar. May. Jul. Sep.depths 150-250 m. After March 2002, the mode waterseemed to disappear as inferred from the float trajectory.010202020202020303030303Month-Year(a2 185180o0 25.4300126.2262002013-M264V E001L00TmOct. Nov. Dec. Jan. Feb. Mar. Apr. May. Jun. Jul. Aug.NoyJanMar-MayouiSepNoyJanMatrMayJonssop.010202a)Fig.10 The time series of Argo float ID 5900105 (a)100 9temperature (contours in 1C interval). The 15C and26.8 C 25.6 25.620"C contours are shown in bold. Areas with vertical, 25.8昌20temperature gradients less than 1.6 C(100m)" are hatch-ed. (b) Potential density (contours in 0.2kg m3 interval).0o]26.2个个 26.2-The 25.2kgm3 and 26.2 kg m-3 contours are thickened.-26.Areas with PV<2X10'mi's' are shaded. The MLD isdenoted by the red dashed line in Fig. 10a.Oct. Nov. Dec. Jan. Feb. Mar. Apr. May, Jun. Jul. Aug.)101010202020202020202Examining an integrated annual cycle (July 2002- July2003), it is found from the time series of temperatureFig.11 As in Fig. 10 but for the time series of Argo float(Fig.10a) that the mixed layer had been deepening forID 5900109.several months before July 2002 and then reached itsmaximum depth in September. Meanwhile, the SST de-5.3 The Role of Eddies in the SPSTMW Formationcreased to 18-19°C and the cold water in the thermoclineand Distributionwas entrained into the deepening mixed layer. The new,shallower mixed layer started to form as shown in Fig.10As pointed by Uehara et al. (2003) and Oka and Sugawhen the SST began to rise at the end of September(2003), eddies have great influences on the formation and(Fig.9). Strong wintertime convection course would lasttransport of the STMW in the North Pacific, as reflectedby the mesoscal中国煤化工MLD and thefor more than one month, until the mixed layer rapidlymode water thiclCNMH Ge front and aretreated in the middle of October. At this time, a seasonalfYHthermocline layer was formed and covered the waternumber of anticycunic anulyuii Cuuics are generatedwhich subducted previously. This water was the STMW,in the region of the TF and TFE. These eddies are sup-.114Journal of Ocean University of ChinaVol.6, No.2, 2007posed to play an important role in the formation and dis-.Feb 1990 and Oct 1991. In addition, as all the four cruisestribution of the SPSTMW, like their counterparts in the served as part of the WOCE Shipboard ADCP Project, theNorth Pacific.current velocity data was used to confirm the eddies.Firstly, we examined the anticyclonic and cyclonic ed-AVHRR SST maps corresponding to the four PR11dies in the region of the TFE from the MSLA by satellite cruises were shown in Figs. 12a-d, superposed by the cur-altimeter. In the MSLA, positive SLA represents an anti- rent velocity at depth 100m. Fig.12d was drawn in com-cyclonic eddy and negative SLA represents a cyclonic bination with SLA maps, from which we could find thateddy. However, as mentioned in section 2, the MSLA data the SST map was so well consistent with MSLA that thehas been only available since October 22 1992 and no warm SST centers corresponded exactly to the high SLAmaps could be found corresponding to the CTD sections centers, indicating the anticyclonic eddies, and the coldof the cruises Oct 1989, Feb 1990 and Oct 1991. Fortu- SST centers corresponded exactly to the low SLA centers,nately, eddies can also be identified from high-resolution indicating the cyclonic eddies (e.g, an SST core colderSST maps, for an anticyclonic eddy corresponds to a than 19C and an SLA core lower than- -30 cm apwarm core of SST and a cyclonic eddy corresponds to a peared near the point ( 159.1°E, 28.99S ), both indicatingcold core. Thus we introduced the 18km wekly averaged the existence of a cyclonic eddy). Therefore it was reason-Multi-Channel Sea Surface Temperature (MCSST) data able to study anticyclonic and cyclonic eddies using the(Miami) to study the eddies during the cruises Oct 1989，high-resolution SST maps during the first three cruises.25'S252727°2929°3133158°62°166° 170 E21(C)222324252627(C)(a26° s28e地。3(303232'Imsy34166° 170* E154°1566° 170° E .16 17182021 2223(C)19 20 21 22(C)Fig.12 The 18 km weekly averaged MCSST maps during the periods of PR11 repeated cruises (Shaded contours in1 weeky0.1C interval.), supersoposed by current velocity vectors at 100m from the WOCE ADCP project. (a) 1989/08/30-09/06 MCSST map (corresponding to 28°S section in Cruise Oct 1989); (b) 1990/05/07-05/14 MCSST map (corre-sponding to 28°S section in Cruise Feb 1990); (c) 1991/11/17-11/25 MCSST map (corresponding to 30"S section inCruise Oct 1991); (d) 1993/09/22-09/29 MCSST map (corresponding to 30°S section in Cruise Jul 1993) and com-bined with 1993/09/22-09/29 MSLA (contours in 5 cm interval). Standard vector of 1ms " is denoted in Fig. 12d.Fig.12a shows the AVHRR SST map from August to using the deep (shallow) 12°C isotherm as an indicator ofSeptember in 1989, corresponding to the CTD section anticyclonic (cyclonic) eddies. This was presumably be-along 28°S in cruise Oct 1989. From the figure, we could cause the eddies lost their surface signature due to theidentify three anticyclonic eddies, centered respectively at spring warming and disappeared from satellite observa-longitudes 157.7°E, 164.5 E and 167.7"E, as well as three tions.cyclonic ones at 156.8*E, 159.0°E and 169.5°E, respec-As seen in Fig. 12c, four eddies were observed alongtively. Except the eddy at 159.0 E, anticyclonic (cyclonic) the course of cruise Oct 1991 in the SsT map of Novem-eddies were also observed in the CTD section (Fig.3) as ber 17-25 1991, two anticyclonic eddies centered atthe downward (upward) tilt of the permanent thermocline159.4°E and 163.0°E respectively and two cyclonic eddiesdepth represented by the 12C isotherm.at 160.6°E and 165.8"E. The anticyclonic eddy centered atIn the SST map of March 1990 (Fig.12b), only one an- 159.4°E was confirmed by the ADCP data, as the currentticyclonic eddy centered on 160.1°E could be clearly velocity vectors中国煤化工strong anti-identified. At the same time, four anticyclonic eddies cyclonic rotationYHCNMH Gt thermoclinewere identified in the CTD section (Fig.4), respectively was correspondingry uepresscu (upiIu at the locationcentered on 157.5°E, 160.1°E, 164.8°E and 168.8'E, when of the anticyclonic (cyclonic) eddies (Fig.5)..HU H. B. et al: The South Pacific Subtropical Mode Water in the Tasman Sea115Three sets of data are combined in Fig. 12d and the cir- substantially thicker mode water was found mainly in theculation of the eddies could be studied from the maps of anticyclonic eddies.SLA and SST as well as the current velocity during thecruise. A line from northwest to southeast through the300Winter MLDcruise track at 162.5 E separated a large anticyclonic re-Summer MLDgion from a cyclonic eddy. This division line corre-250SPSTMWsponded to a sharp temperature (potential density) tilt in athe CTD section (Fig.6) described in section 3. The mostremarkable anticyclonic eddy， centered at the point200(30.8*S, 157.3*E), with SST higher than 21°C and SLAhigher than 30cm, was reflected by the depression of the150thermocline near 157.0°E in the CTD section (Fig.6). Onthe other hand, the cruise transected the core of the anti-10cyclonic eddy centered at 30.1°S, 161.5°E, and the per-manent thermocline and mixed layer were depressed atthe core of the eddy (Fig.6). The cyclonic eddy, which5(centered at the point (29.0"S, 163.0'E), had an SST corecolder than 19°C and an SLA core less than -30cm. Asshown in the CTD section along the cruise, the thermo-4000012C- isotherm depth (m)cline was greatly uplifted to the east of 162.5°E with anaccompanied shallower mixed layer.Fig.13 Scatter diagram of 12C isotherm depth versusFrom the analysis above, the depression (uplifting) ofMLD and SPSTMW thicknesses. Wintertime MLDs arethe permanent thermocline usually acted as an indicatorcalculated from the temperature section in cruise Octof anticyclonic (cyclonic) eddies. So in order to further1989 and denoted by dots while summertime MLDs arecalculated from the temperature section in cruise Febexamine the relation between the smaller-scale variations1990 and denoted by pluses. The regression line be-of the MLD and the mesoscale eddies, the MLD wastween the 12C isotherm depth and wintertime MLDplotted (in dots for wintertime MLD and in pluses forindicates their linear relationship. The SPSTMW thick-summertime MLD) against the 12 C isotherm depth,ness is defined as the thickness of continuous layers in-which could represent the position of the main thermo-cluding PV minimum with PV<2X10^'m^'s' .cline in Fig.l3. On the one hand, the MLD was highlycorrelated with the 12°C isotherm depths in winter (coef-6 Summary and Discussionficient 0.79) and the slope of the regression line betweenthe 12°C isotherm depth and the MLD was near unity.In the region of the TFE near 29°S, the SPSTMW hasThat is to say, the wintertime MLD was approximately been found from the synoptical CTD sections. Theequal to the 12C isotherm depth minus 270m. On the SPSTMW is defined as a thermostad with vertical tem-other hand, no obvious linear relationship existed be- perature gradient lower than 1.6 C(100m)" and a tempera-tween the summertime MLD and the 12°C isotherm ture range of 16.5-19.5"C, or a pycnostad with PV lowerdepth and MLD in this season were generally shallower than 2x10*"'m^' s" and a potential density range of 25.4-than 100m. Thus we reach the conclusion that a deeper 26.0kg m 3. The pycnostad here is to some extent a stricter(shallower) winter mixed layer is formed where the per- way than the thermostad in describing the mode water.manent thermocline is deeper (shallower) in associationThe SPSTMW is formed during the course of convec-with anticyclonic (cyclonic) eddies.tive ventilation in September - the austral late winter asAfter it was capped by the seasonal thermocline, the observed from the time series of the Argo floats. Accom-SPSTMW was often detected as a separated layer of wa- panying the deepening of the wintertime mixed layer,ter with a minimum of PV and its thickness was subse- homogenous water penetrates into the permanent thermo-quently defined as the thickness of continuous layers in- cline and is embedded by the overlying seasonal thermo-cluding a PV minimum with PV<2x10"'m^'s'. In Fig.13，cline developing in the middle of October.the SPSTMW thickness was also plotted with starsMesoscale eddies are active in the region of the TFEagainst the 12°C isotherm depths to examine the rela- and play an important role in the formation of thetionship between the SPSTMW thickness and the perma- SPSTMW. In the wintertime, a deeper (shallower) mixednent thermocline depth. As clearly seen from the figure,layer is formed when the thermocline is depressed (up-the SPSTMW is generally distributed in a region where lifted) in association with the anticyclonic (cyclonic) ed-the 12C isotherm is deeper than 450m and its thickness dies. And the substantially thicker mode water is foundranges from 50m and 100m. Since the climatological 12C mainly in the anticwclonir rddiesisotherm depth was computed at about 440m on averageAs mentioned中国煤化工Ie (1992), anin the area of 29.5*-30.5'S, 160*-180'E from the Levi- XBT transect frcI YHCNMHGownamodetus-98 data, the isotherms deeper than 440m were pre- water near the Australla coast at 26 S, having a tempera-sumably located in anticyclonic eddies. Therefore, the ture range of 17-19°C. This is probably the evidence of.116Journal of Ocean University of ChinaVol.6, No.2, 2007the advection of the SPSTMW that a water mass spreads 11 177-11 189.around the weak recirculation for the EAC in the gyre Ducet, N, P.-Y. Le Traon, and G Reverdin, 2000. Global highinterior. On the other hand, the disappearance of theresolution mapping of ocean circulation from TOPEX/17-19C SPSTMW in the trajectories of the Argo floatsPoseidon and ERS-1/2. J. Geophys. Res, 105: 19477-19498.after March 2002 (Fig.11) and March 2003 (Fig.10) mayHanawa, K, 1987. Interannual variations in the winter-timeoutcrop area of Subtropical Mode Water in the western Northnot mean the destruction of the mode water. The waterPacific Ocean. Atmos. Ocean, 25: 358-374.mass has probably been advected to lower latitudes afterHuang, R. X, and B. Qiu, 1998. The structure of the wind-driv-March of the year. In conclusion, the SPSTMW is ad-en circulation in the subtropical South Pacific Ocean.J. Phys.vected along the path of the EAC recirculation system.Oceanogr, 8: 1173-1 186.A comparison of STMWs in different oceans may as- Hosoda, S., S. Xie, K. Taeuchi, and M, Nonaka, 2001.sist to improve our understanding of the SPSTMW. As is .North Pacific Subtropical Mode Water in a general circulationwell known, the South Pacific is quite different from themodel: Formation mechanism and salinity effects. J. Geophys.North Pacific and North Atlantic in many aspects. TheseRes, 106: 19671-19681.differences lead to a shallower wintertime mixed layer in Ladd, C, and L. Thompson, 2000. Formation mechanism forthe SPSTMW formation areas and subsequently a weakerNorth Pacific central and eastern subtropical mode waters. J.thermostad.Phys. Oceanoger, 30: 868-877.The SPSTMW is also distinguished from the modeLe Traon, P-Y, F. Nadal, and N. Ducet, 1998. An improvedmapping method of multisatellite altimeter data. J. Atmos.water found in the eastern gyre of the South Pacific- theOceanic Technol, 15: 522-534.South Pacific Eastern Subtropical Mode Water (SPES-Masuzawa, J, 1969. Subtropical Mode Water. Deep-Sea Res,TMW) (Wong and Johnson, 2003) in several aspects.16: 463-472.First, slity plays an important role in the formnation and McCartney, M. s.1982. The subrpial rculaiono of Modedistribution of the SPESTMW but has no significant ef-Waters. J. Mar Res, 40 (Suppl.): 427-464.fect on the SPSTMW. Second, like the mode water found Oka, E., and T. Suga, 2003. Formaton of North Pacifie sub-in the eastern North Pacific (Hosoda et al, 2001; Laddtropical mode water in the late winter of 2003. Geophys. Res.and Thompson, 2000), weak surface density gradient isLet, 30: 2205, doi: 1029/2003GL018581.the basis for the existence of the SPESTMW, and the Roemmich, D, and B. Cornuelle, 1990. Observing the fluctua-SPSTMW formation results from the wintertime convec-tions of gyre-scale ocean circulation: a study of the subtropi-tive mixing and cooling in association with the westerncal South Pacific. J. Phys. Oceanogr, 20: 1919-1 934.boundary current of the gyre.Roemmich, D., and B. Cornuelle, 1992. The subtropical modeAn interesting following-up question to the presentwaters of the South Pacific Ocean. J. Phys. Oceanogr, 22:1178-1 187. .work is the interannual variation of the SPSTMWby the oceanic convergence of heat and the heat loss fromStommel, H, 1979. Determination of water mass properties ofwater pumped down from the Ekman layer to the geostrphicthe ocean to the atmosphere during austral winter. Sinceflow below. Proc. Natl. Acad. Sci. USA, 76: 3051-3 055.Suga and Hanawa (1995a) found that the iterannual Suga, T, and K. HanaWa, 199.9 The mixedlayer ciatoloy invariations of the STMW in the North Pacific are closelythe northwestern part of the North Pacific subtropical gyrerelated to the meandering of the Kuroshio, and by infer-and the formation area of Subtropical Mode Water. J. Mar:ence, the interannual variations of the TF and TFE mayRes, 48: 543- -566.have some impact on the SPSTMW. Unfortunately, the Suga, T, and K. Hanawa, 1995a. The subtropical mode waterdata available at present is not sufficient and more Argocirculation in the North Pacific, J. Phys. Oceanogr， 25data and data from satellite altimeter and scatterometer958-970.Suga, T, and K. Hanawa, 1995b. Interannual variations of Northare expected to be useful for further studies.Pacific Subtropical mode water in the 137°E section,J Phys.Oceanogr, 25: 1012-1017.AcknowledgementsSuga, T, K. Motoki, and Y. Aoki, 2004. The North Pacific cli-matology of winter mixed layer and mode waters. J. Phys.This study was supported by the Chinese Natural Sci-Oceanogr, 34: 3-22.ence Foundation of China (NSFC) (Nos. 40276009 and Tlley, L. D, and M. E. Raymer, 1982. Eigheen Degree Water90411010); we also thank Li Lijuan for her rudimentalvariability. J. Mar: Res., 40 (Suppl): 752-775.processing of the AVHRR MCSST data. The CTD data Uehara, H, T Suga, and K. Hanawa, 2003. A role of eddies inwere obtained from R/V Franklin in the WOCE Hydro-formation and transport of North Pacific Subtropical Modegraphic Project, the Argo data from GCOS/ GOOS, theWater. Geophys. Res. Let, 30: 1 705, doi:1029/2003GL017542.ADCP data from the WOCE Data Assembly Center Wong, A., G Johnson, and W. Owens, 2003. Delayed-mode(DAC) for SADCP, MSLA from AVISO DVD products .calibration of autonomous CTD profiling float salinity data byand the AVHRR data from JPL.θ-s climatology. J. Atmos. Oceanic Technol, 20: 308-318.Wong, A, and G. Johnson, 2003. South Pacific Easterm Sub-tropical Mode W中国煤化工493-1 509.ReferencesWorthington, L. VSargasso Sea.Deep-Sea Res,|YHCNMHGBingham, F. M., 1992. Formation and spreading of subtropicalmode water in the North Pacific. J. Geophys. Res., 97:.