deep countercurrent beneath the kuroshio in the okinawa trough

Deep countercurrent beneath the Kuroshio in the Okinawa Trough

Hirohiko Nakamura,1 Ayako Nishina,1 Hiroshi Ichikawa,2 Masami Nonaka,3

and Hideharu Sasaki4

Received 2 October 2007; revised 27 January 2008; accepted 3 March 2008; published 28 June 2008.

[1] The Okinawa Trough is geographically separated into the northern and southern partsby the Kerama Gap, and the northern part is composed of the northern, central andsouthern subbasins formed by bights of the continental slope. Previous observations haveindicated that a deep countercurrent is present beneath the Kuroshio on the continentalslope in the northern Okinawa Trough. However, its persistence over time and itsspatial structure over the entire basin have not been clarified to date. The present studyexamines moored current meter data on the continental slope in the southern andcentral subbasins of the northern Okinawa Trough for November 2004 to November 2006.Deep flows on the continental slope have a clear seasonality in the southern subbasin;eddy motions due to Kuroshio meanders are organized into a persistent countercurrentbelow the Kuroshio in the winter-spring period. During this time, high-frequencyKuroshio meanders with periods near 10 d are likely to diminish in the southern subbasinwhile low-frequency Kuroshio meanders with periods of 1–3 months tend to dominate inthe northern subbasin. In addition, a high-resolution ocean general circulationmodel is used to explore the deep flow field over the entire Okinawa Trough. The modelindicates that the deep flow field is stable in the southern Okinawa Trough over the year,whereas it is unstable in the northern Okinawa Trough particularly in the winter-springperiod. This results in a persistent countercurrent driven by cyclonic eddies on thecontinental slope in the northern Okinawa Trough.

Citation: Nakamura, H., A. Nishina, H. Ichikawa, M. Nonaka, and H. Sasaki (2008), Deep countercurrent beneath the Kuroshio in

the Okinawa Trough, J. Geophys. Res., 113, C06030, doi:10.1029/2007JC004574.

1. Introduction

[2] The Kuroshio, which enters the Okinawa Trough inthe East China Sea through the passage east of Taiwan,flows northeastward on the continental slope along thewestern boundary of the Okinawa Trough, turns clockwiseupon leaving the continental slope, and finally goes out theOkinawa Trough through the Tokara Strait (Figure 1a). Thispaper refers to some regions of the Okinawa Trough basedon geographical features as shown in Figure 1a. Thenorthern and southern Okinawa Troughs are separated bythe Kerama Gap south of Okinawa Island, which is thedeepest channel connecting the Okinawa Trough with theNorth Pacific Ocean. The southern Okinawa Trough haswater depths exceeding 2000 m in its central area. Thenorthern Okinawa Trough is composed of three subbasins,each of which is formed by a bight of the continental slope;the southern and central subbasins have water depths over

1000 m, while the northern subbasin is shallower than1000 m. The mean climatological position where theKuroshio separates from the continental slope almost cor-responds to the boundary between the southern and centralsubbasins (Figure 1b).[3] The deep countercurrent beneath the Kuroshio has

been often observed on the continental slope in the northernOkinawa Trough [Lie et al., 1998; James et al., 1999;Nakamura et al., 2003; Andres et al., 2008]. Lie et al.[1998] first studied this countercurrent in detail usinghistorical moored current meter data, which consisted of2–3 month records in the 1980s for the northern and centralsubbasins and a 9-month record in the early 1990s for thesouthern subbasin. The authors showed that the countercur-rent, which they called the slope countercurrent, was aquasi-permanent feature in the northern and central sub-basins. In the southern subbasin, the countercurrent hadwave-like characteristics to vary in direction and speed.Nakamura et al. [2003] showed a synoptic feature of thedeep flow field in the northern and central subbasins(Figure 1b) using 1-year-long records of moored currentmeters from the late 1990s. Averaged over 1 year, thesouthwestward deep countercurrent opposing the surfaceKuroshio was stronger in the northern subbasin (WCM3)than in the central subbasin (WCM1, WCM2) (see Figure 10by Nakamura et al. [2003], together with Figure 1b of thepresent study). This feature is consistent with the results

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C06030, doi:10.1029/2007JC004574, 2008

1Faculty of Fisheries, Kagoshima University, Kagoshima, Japan.2Institute of Observational Research for Global Change, Japan Agency

for Marine-Earth Science and Technology, Yokosuka, Japan.3Frontier Research Center for Global Change, Japan Agency for

Marine-Earth Science and Technology, Yokohama, Japan.4Earth Simulator Center, Japan Agency for Marine-Earth Science and

Technology, Yokohama, Japan.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JC004574

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reported by Lie et al. [1998], but the variability of thesouthwestward countercurrent by Nakamura et al. [2003] islarger than its mean even in the northern subbasin (WCM3).This fact suggests that the emergence of the persistentsouthwestward countercurrent is modulated in time in thenorthern Okinawa Trough. Following Lie et al. [1998], werefer to the southwestward deep countercurrent as the slopecountercurrent in this paper.[4] Lie et al. [1998] and Nakamura [2005] speculated

about the formation mechanisms of the slope countercurrentin the northern Okinawa Trough, but these mechanismshave not been confirmed to date. Lie et al. [1998] revealedthat several hydrographic sections across the slope counter-current have dome-shaped isotherms in the northern and

central subbasins in a pattern typically associated withupwelling. They therefore inferred that the slope counter-current in the northern and central subbasins is part of thecyclonic circulation induced by upwelling, which is closelyassociated with the surface divergence caused when theKuroshio branches into the Tsushima Warm Current. Onthe other hand, they concluded that the countercurrent in thesouthern subbasin is associated with frontal cyclonic eddiesforced by local upwelling. Nakamura [2005] examined theformation mechanism of the slope countercurrent andanalyzed the numerically modeled countercurrent, whichexhibited significant intensification near the sea bottom.The numerical results indicated that the countercurrenthad two distinct formation mechanisms. The countercur-rent in the northern and central subbasins may be a partof the quasi-steady cyclonic eddy that dominates thenorthern Okinawa Trough, or it may be a part of the eddy’ssouthward tail attributable to the topographic wave. On theother hand, the countercurrent in the southern subbasin maybe forced locally by cyclonic eddies generated by theKuroshio front meanders, although the mechanism of bot-tom intensification was uncertain in the Nakamura [2005]study.[5] Apart from the slope countercurrent beneath the

Kuroshio in the Okinawa Trough, the deep countercurrenthas also been observed beneath other western boundarycurrents in the world’s oceans. In the Shikoku Basin of thewestern North Pacific, abyssal currents below about 3000 mdepth were observed to flow equatorward along the north-western slope, opposing to the surface Kuroshio [Fukasawaet al., 1987, 1995]. Imawaki et al. [1997a] also observed acountercurrent beneath the Kuroshio on the northwesternslope of the Shikoku Basin in November 1993, whichexperienced intensification near the sea bottom at waterdepths of 300–800 m; the moored current meter recordindicated that this countercurrent lasted from October 1993to September 1994 [Imawaki et al., 1997b]. In the NorthAtlantic, an equatorward flow is permanently present indeep water along the western boundary. This flow is knownas the Deep Western Boundary Current (DWBC) in theglobal thermohaline circulation scheme of Stommel andAarons [1960a, 1960b]. On the other hand, there have beenno observational studies that have showed the existence of adeep countercurrent separate from the DWBC. Holloway[1992], however, suggested that the deep equatorward flowin the North Atlantic Ocean is caused not only by globalthermohaline circulation but also the Neptune effect, whichrefers to the statistical dynamical tendency of eddy-topog-raphy interaction to induce mean circulation. It is actuallydifficult to distinguish between DWBCs related to globalthermohaline circulation and deep countercurrents due toother effects. For example, we may have this difficulty inthe velocity section of the Gulf Stream shown in Figure 6aof Pierce and Joyce [1988]; a deep southward flow on theslope in this section seems to be different from the DWBC,because its location is separate from the core of the highoxygen water that characterizes the DWBC. As mentionedabove, the deep equatorward flow may be a ubiquitousfeature on the western slopes of oceans in the NorthernHemisphere.[6] A final goal of the present study is to establish the

dynamics that govern the deep flow field of the entire

Figure 1. (a) Bathymetry of the Okinawa Trough, with aschematic drawing of the typical Kuroshio path (gray curve)and the place names used in the present study. (b) Meancurrent vectors (black sticks) and magnitudes of the detidedcurrent fluctuations along principal axes (red sticks: theirlength denotes twice the standard deviation), both whichwere estimated from moored current meter records fromdepths of 500–560 m over the period November 1997 toNovember 1998 for WCM1–3 and November 1997 toMarch 1998 for ECM1 [after Nakamura et al., 2003]. Theclimatological mean position of the Kuroshio path (thicksolid blue line) and standard deviations from its meanposition (thin solid blue lines) are taken from Yamashiroand Kawabe [2002].

C06030 NAKAMURA ET AL.: DEEP COUNTERCURRENT BENEATH THE KUROSHIO

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Okinawa Trough, particularly focusing on the deep coun-tercurrent beneath the Kuroshio. We first clarify observedfeatures of the slope countercurrent’s temporal and spatialvariations in the northern Okinawa Trough. Section 3 dealswith observed time series data from moored current metermeasurements in the southern and central subbasins forNovember 2004 to November 2006. In section 4, thepersistence of the slope countercurrent over time in thesouthern and central subbasins is examined in relation toKuroshio path meander activity in the northern subbasin. Insection 5, we discuss several velocity and temperaturesections across the slope countercurrent. Moreover, a real-istic high-resolution ocean general circulation model(OGCM) is used to explore the deep current system overthe entire Okinawa Trough. In section 6, we analyze modeldata to present typical deep flow patterns over the entireOkinawa Trough. Section 7 gives conclusive remarks andproposes further work.

2. Observational Methods

[7] Four current meter moorings (SCC1, SCC2, SCC3,and SCC4) were deployed in deep water on the continental

slope in the northern Okinawa Trough (Figure 2). SCC1 andSCC2 were located on the southern flank of the southernsubbasin, and SCC3 and SCC4 were placed in the southernpart of the central subbasin. All moorings were deployedfrom November 2004 to November 2006. Each mooringwas equipped with two 3-D acoustic current meters(Falmouth Scientific, Inc.); the top and bottom instrumentswere positioned at about 520 m and 625 m deep beneaththe Kuroshio, respectively, on the continental slope whosewater depth was about 700 m. Details on the deploymentperiods, mooring positions, water depths and instrumentdepths are presented in Table 1. The current-meter depth ateach mooring was calculated from pressure sensor data onthe instrument. The current meters recorded temperature(T) and eastward, northward and upward velocity compo-nents (u, v, w). Sampling intervals were 1 h for allmoorings. Raw u, v, and T records were low-pass filteredto remove both semidiurnal and diurnal tidal components,using a 50-h, second-order, forward-backward Butterworthfilter, and daily means were calculated from ensembleaverages for each day.[8] Temperature and velocity distributions were observed

during three cruises in November 2004, 2005, and 2006,with expendable bathythermographs (XBTs), expendableconductivity-temperature depth profilers (XCTDs) and ashipboard acoustic Doppler current profiler (ADCP).Twelve sectional observations using these instruments werecarried out along lines across the Kuroshio and the slopecountercurrent near each mooring site. Only three sections(Figure 2) are examined in the present study. Currentvelocities were measured continuously along the ship’strack with a shipboard ADCP (RD Instruments, 75-kHznarrowband mode). The depth range of ADCP was set tobe greater than 30 m with a vertical bin length of 16 m anda sampling interval of 15 s. The ship velocity was calcu-lated from position fixes obtained by differential GlobalPositioning System (DGPS). Raw ADCP data were pro-cessed using CODAS3 software.[9] A time series of the Kuroshio Position Index (KPI)

[Kawabe, 1995] in Tokara Strait was analyzed to examinethe relations between the deep slope countercurrent and thesurface Kuroshio. Daily records of KPI from 1984 to 2006were derived from sea level data gathered at the Tanega-shima, Nakanoshima, and Amamioshima Islands (eachisland is denoted by an initial in Figure 2). Thirty-ninetidal components were removed from the sea level data,which were also corrected for barometric pressure effects.

Figure 2. Mooring sites (closed circles), and locations ofshipboard ADCP cross-sections used in the present paper(bold solid lines). Open circles mark the mooring sites byNakamura et al. [2003]. T, Tanegashima Island; N,Nakanoshima Island; A, Amamioshima Island.

Table 1. Mooring Identification, Positions, Water Depths, Instrument Depths and Deployment Periods

Mooring ID Latitude, �N Longitude, �E Water Depth, m Instrument Depth, db Period,d/m/year UTC

First Deployment (Nov 2004 to Nov 2005)SCC1 27�02.90 125�56.60 721 520 625 26/Nov/2004–23/Nov/2005SCC2 27�19.20 126�08.10 697 514 619 27/Nov/2004–23/Nov/2005SCC3 28�21.60 127�02.30 710 522 627 26/Nov/2004–25/ Nov/2005SCC4 28�46.50 127�09.70 720 522 627a 25/Nov/2004–25/Nov/2005

Second Deployment (Nov 2005 to Nov 2006)SCC1 27�03.10 125�56.60 711 529 629 25/Nov/2005–19/Nov/2006SCC2 27�19.10 126�08.10 701 513 618 25/Nov/2005–18/Nov/2006SCC3 28�22.30 127�02.10 712 523 628 27/Nov/2005–17/Nov/2006SCC4 28�46.20 127�09.40 712 529b 634 28/Nov/2005–16/Nov/2006

aShort record due to battery trouble: 25/November/2004–10/January/2005.bShort record due to battery trouble: 28/November/2005–30/June/2006.


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Following the method of Yamashiro and Kawabe [1996],KPI was calculated to represent the latitude of the Kuroshioaxis in Tokara Strait. We analyzed a KPI time seriesspanning the same period as the mooring observations.

3. Results From Moored Current MeterMeasurements

3.1. Mean Flow Structure and Principal Axes

[10] Figures 3a and 3b show mean velocity vectors for theupper layer (about 520 m depth) and the lower layer (about625 m depth) at the mooring sites, respectively. The meanvelocity vectors for the lower layer at all moorings aredirected southward or southwestward along the continentalslope, indicating the presence of the slope countercurrent.On the other hand, the slope countercurrent disappears forthe upper layer at SCC1, SCC2, and SCC3. Mooring SCC4is the exception, and its mean velocity vector is directedsouthwestward as seen for the lower layer. This indicatesthat characteristics of the slope countercurrents in thesouthern and central subbasins are different, especiallyconsidering that the upper layer velocity at SCC3, nearthe boundary between the central and southern subbasins,

leads away from the continental slope. This difference maybe related to the Kuroshio separation, because the climato-logical mean path of Kuroshio separates from the continen-tal slope near the boundary between the southern and centralsubregions.[11] Figures 3c and 3d show standard deviations of

velocity along the principal axes [Emery and Thomson,2001] for the upper and lower layers, respectively. Thestandard deviations along the major axes are considerablylarger than the amplitudes of the mean velocity vectors at allmooring sites. Such deviations can be ascribed to eddyvariability due to Kuroshio meanders, because velocityfluctuations in the records are consistent in frequency withthe Kuroshio meanders reported in previous studies (seesection 3.2). Unlike the mean velocity vectors, the standarddeviations along the major axes in the southern and centralsubbasins have the almost same amplitude.[12] It is worth discussing features of the slope counter-

current over the entire northern Okinawa Trough togetherwith information on the deep current field observed inprevious studies. We refer to Nakamura et al. [2003], whoshowed the synoptic nature of deep currents in the centraland northern subbasins for November 1997 to November

Figure 3. (a, b) Mean velocity vectors and (c, d) standard deviations of the velocity variations alongprincipal axes; the length of each stick denotes twice the standard deviation. (Figures 3a and 3c) Theupper layer (about 520 m deep) and (Figures 3b and 3d) the lower layer (about 625 m deep). Eachstatistical estimate includes the entire record of each current meter (see Table 1).


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1998 (see Figure 1, and also Figure 10 in their paper for thevertical velocity structure). We compare statistical currentstates at the moorings SCC4 in the present study and WCM1by Nakamura et al. [2003], because the two sites are closelylocated. Amplitudes of the mean vectors at SCC4 (3.9 and1.7 cm � s�1 near 525 m and 630 m, respectively) are almostequal to those at WCM1 (2.9, 3.7, and 2.9 cm � s�1 nearthe 500, 605, and 805 m depths, respectively); the standarddeviations of velocity variations along the major axes forSCC4 and WCM1 have the almost same amplitudes ofapproximately 10 cm � s�1 in both the upper and lowerlayers. The deep current states observed in the presentstudy and the earlier study are therefore consistent, so it isreasonable to compare the two observations for the entirenorthern Okinawa Trough. On the basis of this fact, weconclude that the slope countercurrent intensity over long-term averages, such as the annual mean, is large in thenorthern, southern and central subbasins.

3.2. Time Domain

[13] Figures 4a–4d exhibit current vector time series forthe upper layer, and Figures 4e–4h display them for thelower layer. Both time series for the upper and lower layersare clearly influenced by high-frequency variations withamplitudes greater than 10 cm � s�1 and dominant periods of10, 17, and 20 d (data shown in section 3.3 for SCC1 andSCC4). The dominant periods mentioned above almostcorrespond to those of Kuroshio front meanders reportedin previous studies: e.g., 7, 11, and 16 d by James et al.[1999], and high-frequency band meanders (period of 8–

16 d) and intermediate-frequency band meanders (period of16–24 d) by Nakamura et al. [2003]. The Kuroshio frontmeanders with these periods are attributable to mixedbarotropic-baroclinic instability at the Kuroshio front nearthe shelf edge [James et al., 1999; Nakamura, 2005]. Thesouthward flow tendency characterizing the slope counter-current is masked by eddy variability due to Kuroshio frontmeanders at most moorings, but persistent southward flowsare detected in the winter-spring periods of 2005 and 2006at the lower layer for SCC1 and SCC2.[14] In order to remove the influence of Kuroshio front

meanders, the current-vector time series were low-passfiltered with a cut-off period of 25 d. The low-pass filteredtime series for the upper layer (Figures 5a–5d) indicatepersistent tendencies of the southward current at SCC4while eddy motions are predominant during the entiremooring period at SCC1, SCC2, and SCC3. This featureis consistent with the flow tendency revealed by the meanvelocity vectors (Figure 3a). Dominant periods of theseeddy motions are 1–2 months; this period range corre-sponds to the low-frequency band (with a period longer than32 d) by Nakamura et al. [2003], which is related to theKuroshio path meanders that are identifiable in the northernsubbasin. In contrast to the time series for the upper layer,the low-pass filtered time series for the lower layer(Figures 5e–5h) show clear southward flow tendencies.These southward flows are associated with eddy motionswith periods of 1–2 months, as seen in the upper layer. Theeddy motions are organized in the lower layers of SCC1(Figure 5h) and SCC2 (Figure 5g). As a result, the persistent

Figure 4. (a–d) Current vector time series for the upper layer (about 520 m deep), and (e–h) for thelower layer (about 625 m deep): (Figures 4a and 4e) SCC4, (Figures 4b and 4f) SCC3, (Figures 4c and4g) SCC2 and (Figures 4d and 4h) SCC1.


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southward flows at SCC1 and SCC2 exhibit a 4–5 monthduration in the winter-spring periods of 2005 and 2006 (e.g.,for SCC1, mid-February 2005 to late June 2005, and mid-December 2005 to mid-May 2006). The amplitudes ofsouthward flows in these periods are significantly larger atSCC1 and SCC2 than at SCC3 and SCC4.[15] Figure 6 shows temperature records for the upper and

lower layers at each mooring. We can see a seasonal cycleto all time series in the upper layer, although its amplitudeand phase modulate year by year. This seasonal cycle isroughly characterized as having its highest temperature nearJanuary–June in 2005, its lowest temperature from October2005 to January 2006, and a return to its highest tempera-ture near April–August in 2006. The winter-spring persis-tent southward flow in the lower layer of SCC1 and SCC2appears during the period of highest temperature in 2005,and emerges during the periods of temperature increase andhighest temperature in 2006. Temperature records at allmooring sites clearly show variations with periods of 1–2 months, which are probably caused by low-frequencyKuroshio path meanders. These variations tend to be en-hanced during temperature increases and diminished duringtemperature decreases.

3.3. Time-Frequency Domain

[16] The activity of the Kuroshio meanders and emer-gence of the persistent slope countercurrent are nonstation-ary seasonal phenomena. The time series of the velocitycomponent along the major axis for each current meter was

therefore analyzed using the Morlet wavelet transform eiw0t/s

e�t2/(2s2), where t is frequency, s is the wavelet scale, and w0

(here w0 = 6) is a nondimensional frequency. The transformand significance tests were performed according to themethod described by Torrence and Compo [1998]. Toreduce wraparound effects, the time series was padded withzeros. The Global wavelet spectrum (GWS) was calculatedby the time average of the wavelet power spectrum, whichis equivalent to the Fourier power spectrum smoothed bythe Morlet wavelet function in Fourier space [Farge, 1992].We show only two results here that were obtained at thelower layer of SCC1 and at the upper layer of SCC4;the former is regarded as involving the intrinsic featuresof the slope countercurrent in the southern subbasin, whilethe latter is considered to represent the potential character ofthe slope countercurrent in the central subbasin.[17] The wavelet power spectrum and the GWS for the

lower layer of SCC1 are shown in Figures 7b and 7c,respectively. The power of the GWS (Figure 7c) exceedsthe 95% confidence level for a red-noise process for theperiods near 10 d and 20 d. These periods correspond to theperiod range of the Kuroshio front meanders that develop inthe southern and central subbasins [James et al., 1999;Nakamura et al., 2003]. The wavelet power spectrum(Figure 7b) is clearly nonstationary around these periods.Here we examine variance near the 10-d period in order toreveal the relation of the 10-d period variance to thepersistent slope countercurrent. The 10 d-variance timeseries (Figure 7d) is given by scale-averaged wavelet power

Figure 5. (a–d) Low-pass filtered time series of current vectors with a cut-off period of 25 d for theupper layer (about 520 m deep), and (e–h) for the lower layer (about 625 m deep): (Figures 5a and 5e)SCC4, (Figures 5b and 5f) SCC3, (Figures 5c and 5g) SCC2 and (Figures 5d and 5h) SCC1. Data arefiltered with a 25-d, second-order, forward-backward Butterworth filter.


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over the 7–13 d band in Figure 7b. Comparison of timesequences between the persistent slope countercurrent(Figure 7a) and the 10 d variance (Figure 7d) shows thatthe 10 d variance is always lower than the 95% confidencelevel within periods when the slope countercurrent ispersistent, while that can exceed the 95% confidence levelwithin the periods when the slope countercurrent is absent.This tendency becomes somewhat unclear for variancearound the 20-d period (see Figure 7b).[18] Figure 8 shows the wavelet power spectrum and the

GWS for the upper layer of SCC4. The power of the GWS(Figure 8c) exceeds the 95% confidence level around a 20-dperiod, and the wavelet power spectrum (Figure 8b) clearlyshows nonstationary features around this period. We, there-fore, focus on the 20 d variance (Figure 8d) and calculatescale-averaged wavelet power over the 12–22 d band inFigure 8b. In contrast to Figure 7d, Figure 8d shows that the20 d variance tends to exceed the 95% confidence levelwithin periods when the slope countercurrent prevails atSCC4, although their correlation is unclear comparing tothat for SCC1.

[19] The results from Figures 7 and 8 may indicate thatthe period of the Kuroshio front meander tends to be longerto the state in which the persistent slope countercurrent ispresent than to the reversed state. On the basis of anassumption that the persistent slope countercurrent is ac-companied by the offshore shift of the Kuroshio, this featureis consistent with the result from the instability study of theKuroshio in the central subbasin with a spectral numericalmodel by James et al. [1999]; they showed that the longerperiod of the Kuroshio meanders appears to the modelbackground state in which the cross-shelf positioning ofthe current core is seaward.

4. Slope Countercurrent and Kuroshio-PathMeander Activity

[20] Recent studies of Nakamura et al. [2003] andNakamura [2005] revealed that the northern subbasin isdominated by Kuroshio path meanders with periods of 30–90 d, which are associated with the growth of cycloniceddies to the scale of the northern subbasin [see Nakamuraet al., 2003, Figure 8]. Nakamura et al. [2006] furthermoreshowed that such Kuroshio path meanders tend to occurwith larger amplitudes in December–July, based on the30–90 d variance time series of the Kuroshio PositionIndex (KPI) in the Tokara Strait. We examine seasonalfeatures of the slope countercurrent in the southern andcentral subbasins here in relation to the seasonally-fixednature of Kuroshio path meanders in the northern OkinawaTrough. The method of wavelet analysis used in thissection is the same as that of section 3.3.[21] The KPI time series (Figure 9a) and its wavelet

power spectrum (Figure 9b) indicate that the 30–90 dvariance of KPI is significantly large from February to June2006. The 30–90 d variance time series for KPI (Figure 9d)is given by the scale-averaged wavelet power over the 30–90 d band in Figure 9b. This time series shows that the30–90 d variance is relatively large from approximatelyFebruary to August 2005 and exceeds the 95% confidencelevel from February to April 2006. The two periods almostcorrespond to the December–July period in which theKuroshio path meander generally exhibits larger amplitudes[Nakamura et al., 2006].[22] The 30–90 d variance of KPI (Figure 9d) is exam-

ined in relation to the persistent slope countercurrent(Figure 7a) and the 10 d velocity variance (Figure 7d) forthe lower layer of SCC1, and also in relation to thepersistent slope countercurrent (Figure 8a) and the 20 dvelocity variance (Figure 8d) for the upper layer of SCC4.The results are summarized in Table 2 for two distinctstates: when the persistent slope countercurrent is present atSCC1, and vice versa. As shown in Table 2, emergence ofthe persistent slope countercurrent in the southern subbasin(SCC1) almost coincides with emergence of the large-amplitude motion of Kuroshio path meanders in the north-ern subbasin. The Kuroshio front meander with the 10-dperiod is stable in the southern subbasin for this situation,suggesting that the background state suppresses mixedbarotropic and baroclinic instability. The reversed state inwhich the persistent slope countercurrent disappears atSCC1 predisposes the Kuroshio front meanders with the10-d period to be active in the southern subbasin, and the

Figure 6. Temperature records for the upper layer (about520 m deep) and the lower layer (about 625 m deep):(a) SCC4, (b) SCC3, (c) SCC2, and (d) SCC1. Thin line:daily averaged data. Thick line: low-pass filtered valueswith a cut-off period of 25 d. The upper-layer temperature ishigher than the lower-layer temperature in each figure.


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Kuroshio path meanders with 30–90 d periods diminish inthe northern subbasin. The Kuroshio path pattern for thereversed state will, therefore, resemble the northern pathpattern shown in Figure 7 of Nakamura et al. [2003]. Onthe other hand, there is no clear relationship between theslope countercurrent in the central subbasin (SCC4) and the30–90 d variance of the KPI.[23] Table 2, which shows the relationships between the

persistent slope countercurrent and several Kuroshio me-ander states, is not obtained by statistical methods. This isbecause 2-year-long records are somewhat short to analyzeseasonal features by statistical methods such as correlationfunctions. Further observations will be needed to supportTable 2.

5. Cross-Sections Across the SlopeCountercurrent

[24] In order to clarify the spatial structure of the slopecountercurrent in cross-section, vertical snapshot sections

across both the Kuroshio and the slope countercurrent areexamined using ADCP velocity and XBT temperature data.Particular attention is paid to the long sections across theentire Okinawa Trough in the zonal direction. We thereforepresent three sections shown in Figure 2 from a total of 12.These snapshot sections may present the current structurethat involves not only the low-frequency currents but alsothe tidal currents. We first evaluate amplitudes of the tidalcurrent from moored current meter data. High-frequencycurrent variations with periods less than 50 h have a largeststandard deviation (8 cm s�1) of along-slope velocity at theupper layer of SCC3. The persistent slope countercurrenthas a typical amplitude of more than 10 cm s�1 (seeFigures 4g and 4h). As the persistent slope countercurrentis rather stronger than the tidal current on the continentalslope, the snapshot velocity section can represent the low-frequency current structure below the Kuroshio withinerrors of about 8 cm s�1.[25] Figures 10a and 10b show the vertical sections for

Lines 1 and 2 across the southern subbasin, respectively

Figure 7. (a) Time series of the velocity component along the major axis at the lower layer ofSCC1 (thin line: daily averaged data; thick line: low-pass filtered values with a cut-off period of 25 d).(b) Morlet wavelet power spectrum for the daily averaged data, normalized by the GWS shown inFigure 7c. The gray contours are at normalized levels of 1, 2, and 3. The thick black contour enclosesregions of greater than 95% confidence for a red-noise process (lag-1 autocorrelation, a = 0.72).Hatched regions at the lower corners of the plot indicate the cone-of-influence where edge effectsbecome important. (c) The GWS. The dashed line represents the 95% confidence level for a red-noiseprocess (a = 0.72). (d) The 7–13 d variance time series, given by scale-averaged wavelet power overthe 7–13 d band shown in Figure 7b; the thin dashed line is the 95% confidence level (assuming rednoise a = 0.72). Shaded areas in Figures 7a, 7b and 7d indicate the periods where the persistent slopecountercurrent emerges. Peak frequencies in the GWS are marked by horizontal dotted lines inFigures 7b and 7c.


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(see Figure 2). Figure 10a shows that the deep countercur-rent beneath the Kuroshio occupies the western area to126.6�E, near which the water depth is deepest (about1200 m) in the trough. To the eastern area of the trough,the deep flow below the Kuroshio is northward, and theflow is confined by the eastern slope of the trough near127�E. These currents are not the tidal currents, becausespeeds of both the southward and northward deep flows(more than 20 cm s�1) exceed the error range due to thetidal currents, and moored current meter data at SCC2immediately after the ADCP observation indicate a clearcountercurrent beneath the Kuroshio. The slope counter-current is probably part of the cyclonic circulation (oreddy) that occupies the deep northern Okinawa Trough,approximately bounded by the 1000 m isobath. Thisinterpretation is supported by results from a numericalmodel (see section 6.3). On the other hand, Figure 10bdoes not show the presence of the slope countercurrent. Inthis case, there are no clear motions in the deep layer below500 m with errors of about ±8 cm s�1.[26] Figure 10c shows the vertical section for Line 3

across the central subbasin (see Figure 2). This sectionshows the countercurrent beneath the Kuroshio. The struc-ture of this countercurrent seems to be different from that ofLine 1; the countercurrent for Line 3 is located below 200 mand its current core is detached from the sea bottom, but thatof Line 1 is characterized by bottom intensification. Thedeep flow field below about 400 m depth contains not onlythe slope countercurrent on the western slope but also thesouthward undercurrent on the eastern slope. These currents

are not regarded as the tidal currents, because these currentcores have speeds of more than 10 cm s�1 that exceed theerror range due to the tidal currents, and moored currentmeter data at SCC4 during the ADCP observation indicate acountercurrent at about 500 m depth on the western slope.These structures probably indicate that the cyclonic circu-lation (or eddy) develops in the western part of the troughwhile the anticyclonic circulation (or eddy) exists in theeastern part, as seen in Figure 10a.[27] It is worthwhile to summarize features of the coun-

tercurrents observed in all twelve sections. The counter-currents tend to emerge in two distinct depths. As seen inthe central subbasin (Figure 10c), the shallower countercur-rent spanning 200–500 m deep is associated with a dome-like isotherm structure. The dome-like feature is not relatedto the internal tides because of the following reason: apossible vertical isotherm-displacement due to the internaltides is about 15 m near 500 m depth at SCC4, which isgiven by combining the standard deviation of high-frequencytemperature variation with periods less than 50 h (0.14�C)and the vertical temperature profile by XBT (Figure 10c);this vertical displacement is significantly smaller than theobserved dome-height (more than 50 m at about 500 mdepth near 127.3�E in Figure 10c). According to the thermalwind relation, the pronounced dome-like feature may in-duce the detachment of the countercurrent from the seabottom. On the other hand, the deeper countercurrent isintensified on the sea bottom in deep water below 500 mwithout the dome-like isotherm structure, as seen in thesouthern subbasin (Figure 10a). The shallower and deeper

Figure 8. Same as Figure 7, but for the upper layer of SCC4 and the 12–22 d variance time series forFigure 8d.


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countercurrents were observed in both the central andsouthern subbasins. However, the number of sections istoo small to discuss the statistical countercurrent structuredifferences between the southern and central subbasins.

6. Slope Countercurrent in a High-ResolutionOGCM

6.1. Model Description and Method of Analysis

[28] We use data from the realistic high-resolutionOGCM simulation on the Earth Simulator (referred to as

OFES) in order to investigate the basin-scale deep flow fieldin the entire Okinawa Trough. The model is based on theModular Ocean Model (MOM3), which is configured for anear-global region extending from 75�S to 75�N. Thehorizontal grid spacing is 1/10� and there are 54 verticallevels which are variable in size. The model topography isderived from the 1/30�OCCAM bathymetry data set. Themodel was first spun up for 50 years from the rest stateconstructed from annual mean temperature and salinityfields (WOA98) and forced by wind stresses and surfaceheat and fresh water fluxes obtained from monthly mean

Figure 9. (a) Time series of the KPI. (b) Morlet wavelet power spectrum, normalized by the GWSshown in Figure 9c. The gray contours are at normalized levels of 1, 2, and 3. The thick black contourencloses regions of greater than 95% confidence for a red-noise process (lag-1 autocorrelation, a = 0.72).Hatched regions at the lower corners of the plot indicate the cone-of-influence where edge effects becomeimportant. (c) The GWS. The dashed line represents the 95% confidence level for a red-noise process(a = 0.72). (d) The 30–90 d variance time series, given by scale-averaged wavelet power over the30–90 d band shown in Figure 9b; the thin dashed line is the 95% confidence level (assuming rednoise a = 0.72).

Table 2. Summary of the Relationships Between the Persistent Slope Countercurrent at SCC1 and Several Kuroshio Meander Statesa

Periodb SCC1 (Southern Subbasin) SCC4 (Central Subbasin) KPI

Year MonthsPresence of Counter C.

(Figure 7a)Variance: 7–13 d

(Figure 7d)Presence of Counter C.

(Figure 8a)Variance: 12–22 d

(Figure 8d)Variance: 30–90 d

(Figure 9d)

2005 Feb–May yes low yes very high in Apr relatively high2005 Jun–Dec no high in Jul–Oct yes in Jun–Sep very high in Jun–Aug low in Sep–Nov2006 Jan–Apr yes low no relatively high high2006 May–Aug no high yes in May low low

aRemarks for SCC1, SCC4 and KPI are derived from Figures 7, 8 and 9, respectively.bThe period is defined by the presence or absence of the persistent slope countercurrent at SCC1.


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climatologies from the NCEP/NCAR reanalysis data. It wasthen integrated from January 1950 to December 2006 usingthe daily mean NCEP/NCAR reanalysis data. Detailedmodel descriptions appear by Masumoto et al. [2004] andSasaki et al. [2007].[29] The present study uses snapshots sampled every 3 d

for 9 years (1 January 1998 through 30 December 2006).These snapshots include sea surface height and horizontalvelocities at 2.5 m and 526.9 m depths (hereafter called seasurface and 500 m depths, respectively). To adopt 526.9 mdepth, we compared current fields for 526.9 m and 604.5 mdepths, which almost correspond to the depths where the topand bottom current meters were set at each mooring site,respectively. There is no intrinsic difference between thetwo current fields, but the core of the slope countercurrenttends to be closer to 526.9 m depth than 604.5 m depth.Therefore, we here present horizontal current structures at526.9 m depth. Given the 30–90 d variance time series ofKPI (see Figure 3 by Nakamura et al. [2006] and Figure 9cof the present study), the analysis period was chosen so asto include a large-variance state in 1998, small-variance

states during 1999–2003, and a return to relatively large-variance states during 2004–2006.[30] A general overview of the Okinawa Trough current

fields is provided in Figures 11a and 11b, which show meanvelocity fields averaged for nine years at the sea surface and500 m depth, respectively. The mean Kuroshio flow at500 m (Figure 11b) separates from the western slope near26�N in the southern Okinawa Trough and goes toward theKerama Gap. A small portion of the mean Kuroshio flowexits the Okinawa Trough through the Kerama Gap while itsgreatest portion flows along the eastern slope of the north-ern Okinawa Trough, forming a cyclonic circulation with aweak return flow along the western slope.[31] The model data analysis strategy is to follow Table 2,

which summarizes the observational results. We thereforecalculate three indicators that are potentially equal to thosein Table 2: a time series of sea surface height (SSH) in thenorthern subbasin (see Figure 11a for the location), whichrepresents north-south migrations of the Kuroshio path inthe northern subbasin; a time series of the velocity compo-nent along the major axis at 500 m in the central subbasin;

Figure 10. Normal component of velocity measured by shipboard acoustic Doppler current profileralong (a) Line 1 (25 November 2004), (b) Line 2 (18 November 2006), and (c) Line 3 (21 November2005). Solid (dashed) white contours denote northward (southward) flows: contour interval is 20 cm s�1.Thick solid contours in Figures 10b and 10c indicate XBT temperature (�C). See Figure 2 for cross-section locations.


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and a time series of the same parameter in the southernsubbasin (see Figure 11b for the locations). Wavelet analysisis performed for these time series using the same method asin section 3.3. After the model validation based on Table 2,we classify flow fields into distinct states as in Table 2, andmake composite maps of the basin-scale surface and deepflow fields for each state.

6.2. Model Validation

[32] Figure 12a shows the SSH time series in the northernsubbasin. Figure 12b displays the 24–50 d variance timeseries for SSH that is derived from scale-averaged wavelet

power over the 24–50 d band where GWS exceeds the 95%confidence level for a red-noise process. The 24–50 dvariance for SSH reveals that seasonal phenomena, whichare generally restricted to winter-spring periods, are pro-nounced in the period from 1998–2004 but disappear in2005 and 2006. Figure 12a shows that SSH is unusually lowand its state persists in the winters of 2005 and 2006. Thisindicates that the Kuroshio path in the winters of 2005 and2006 is significantly different from that of the 1998–2004winters. The 24–50 d variance time series of modeled SSHis consistent with the 30–90 d variance time series ofobserved KPI from 1998–2004: the observed variance is

Figure 11. Horizontal fields of velocity vectors averaged over 1998–2006 at depths (a) 2.5 m, and(b) 526.9 m (>25 cm s�1 truncated). Open circles denote the locations where SSH is observed(Figure 11a) and current velocity data are obtained (Figure 11b).


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larger in 1998, smaller during 1999–2003, and larger in2004. However, the modeled variance time series is incon-sistent with the observed time series in 2005 and 2006: Theobserved variance is slightly larger during 2005–2006 (seeFigure 3 in Nakamura et al. [2006] and Figure 9d of thepresent study).[33] Figure 12c shows the velocity time series in the

southern subbasin, which is low-pass filtered with a cut-offperiod of 60 d. Slope countercurrents that persist beyond30 d (shaded areas in Figure 12) emerge in the 1998–2004winter-spring periods, appear in the summer of 2001, andconsequently disappear in 2002. Emergence of the persis-tent slope countercurrents coincides with large 24–50 dSSH variances in the winter–spring periods, ignoring theexceptional summer case mentioned above. Figure 12dshows the velocity time series in the central subbasin, whichis low-pass filtered with a cut-off period of 60 d. Unlike thesouthern subbasin, there are no clear persistent slope coun-tercurrents in the central subbasin from 1998–2002,although persistent slope countercurrents appear in thewinter-spring periods of 2003 and 2004. These featuresfound in the southern and central subbasins are consistent

with the observational results summarized in Table 2. Boththe velocity time series for the southern and centralsubbasins exhibit changes in frequency from the period1998–2002 to the period 2005–2006 with the transitionoccurring during 2003–2004. Unlike the observationalresults (Table 2), both wavelet power spectra for thevelocity time series in the southern and central subbasinsdo not reveal clear relationships to the persistent slopecountercurrent (data not shown).

6.3. Composite Analysis for Three Distinct States

[34] Composite maps of typical states for the winter-spring and summer-autumn periods are constructed for theperiod 1998–2000 when the seasonal nature is clearer thanin other periods (see shaded areas for the winter-springperiods and nonshaded areas for the summer-autumn peri-ods in Figure 12). Furthermore, a composite is generated foran unusual state with a significantly low SSH in winter2005 (27 January to 22 March 2005). Figure 13 shows themean surface current anomalies and the root mean square(RMS) field of the SSH anomaly for three distinct states; toclarify differences in spatial flow structure for each of the

Figure 12. (a) Time series of SSH in the northern sub-basin (see Figure 11a for the location). (b) The24–50 d variance time series for the SSH, given by scale-averaged wavelet power over the 24–50 dband; the thin dashed line is the 95% confidence level (assuming red noise a = 0.72). (c) Low-passfiltered time series of the velocity component along the major axis at 526.9 m deep in the southernsubbasin (the cut-off period is 60 d). (d) Low-pass filtered time series of the velocity component along themajor axis at 526.9 m depth in the central subbasin (the cut-off period is 60 d). See Figure 11 for thelocation of each station. Shaded areas indicate the periods when the slope countercurrent that persistsbeyond 30 d emerges in the southern subbasin during 1998–2004 (see Figure 12c).


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states, the mean current anomalies are shown as thedifference from the average over the period 1998–2006(Figure 11a). The RMS field of the SSH anomaly is usedas an indicator of the stability of the surface flow field.Figure 14 shows the mean velocity vectors at 500 m and theRMS field of the vorticity anomaly for three distinct states.The manner of mapping the velocity vector fields is thesame as that used for Figure 13. To indicate stability of thedeep flow fields at 500 m, the vorticity anomaly is adopted.[35] The surface flow field for the summer-autumn period

(Figure 13a) indicates a slightly stronger surface Kuroshioin the entire Okinawa Trough and intensification of theanticyclonic circulation in the northern subbasin of thenorthern Okinawa Trough. The winter-spring surface flowfield (Figure 13b) shows a slightly weaker surface Kuroshioon its offshore side, while the winter 2005 surface flow field(Figure 13c) shows a remarkably weaker surface Kuroshioon its offshore side over the southern Okinawa Trough andthe southern subbasin. For the winter 2005 surface flowfield, the inshore side of the surface Kuroshio is intensifiedin the northern Okinawa Trough, resulting in smoothseparation of the Kuroshio at the boundary between thesouthern and central subbasins, and formation of the strongcyclonic circulation in the northern subbasin. The compar-ison between Figures 13a and 13b shows that the surfaceKuroshio in the central and northern subbasins is moreunstable during the winter-spring period than the summer-autumn period. Note that the summer-autumn flow fieldoccasionally approaches the stable state around the periodswhere the 24–50 d variance for SSH (Figure 12b) reaches aminimum (data not shown). On the other hand, Figure 13creveals that the surface Kuroshio is stable in the northernOkinawa Trough in the winter of 2005. These states arebasically similar to the three distinct Kuroshio path states inthe northern Okinawa Trough that were obtained by thenumerical experiments by Nakamura [2005] (see Figure 4in his paper): the summer-autumn flow field is consistentwith a weakly stable northern path state associated with ananticyclonic Kuroshio meander that traverses the westernand eastern slopes in the northern subbasin, the flow field inthe winter 2005 corresponds to a highly stable southern pathstate maintained by a cyclonic Kuroshio meander associatedwith a cyclonic eddy blocked by the northern subbasin, andthe winter-spring flow field is likely to be an intermediate,unstable path state.[36] The deep flow field at 500 m for the summer-autumn

period (Figure 14a) has no significant differences from the9-year average flow field (Figure 11b). On the other hand,the deep flow field at 500 m for the winter-spring period(Figure 14b) shows that southwestward anomalous velocityvectors prevail over the entire Okinawa Trough. In thenorthern Okinawa Trough, such anomalous southwestwardflow is close to the western slope in the southern andnorthern subbasins while it attaches to the eastern slope inthe central subbasin. These results mean that the slopecountercurrent is stronger for the winter-spring period thanfor the summer-autumn period in the southern and northernsubbasins of the northern Okinawa Trough. The deep flowfield in the winter of 2005 (Figure 14c) shows a clearanomalous anticyclonic circulation over the area confinedby the 1000 m isobath, indicating the presence of the deepKuroshio flow along the western slope of the southern

Figure 13. Composite maps of the mean surface currentanomalies and the RMS of the SSH anomaly (shading) forthree distinct states: (a) the summer-autumn period in1998–2000, (b) the winter-spring period in 1998–2000,and (c) winter 2005. Note that the mean current anomalies(>40 cm s�1 truncated) are shown as the difference fromFigure 11a (the average over 1998–2006). Shadingrepresents areas whose RMS of the SSH anomaly is inexcess of 10 cm (contour interval is 2 cm).


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subbasin and its smooth separation from the western slopenear the boundary between the southern and central sub-basins. The RMS fields of the vorticity anomaly at 500 m(Figure 14) indicate that three states of the deep flow fieldhave stability tendencies similar to those for the surfaceflow field. The deep Kuroshio is more unstable during thewinter-spring period than the summer-autumn period overthe entire northern Okinawa Trough. On the other hand, the

deep Kuroshio is stable during winter 2005. Note that thesummer-autumn deep flow field occasionally becomes sta-ble, as the surface flow field approaches the stable state(data not shown).[37] Figure 14 shows a striking contrast between the

northern and southern Okinawa Troughs in deep flow fieldstability at 500 m. The deep Kuroshio paths are alwaysstable in the southern Okinawa Trough, although the cur-rent’s intensity changes among three states. On the otherhand, the deep Kuroshio paths are unstable in the northernOkinawa Trough, especially in the winter-spring periodsexcept during the winters of 2005 and 2006. We nextexamine these regional structures of stable and unstabledeep flows at 500 m for the winter-spring period.[38] Figure 15 shows a time series of the velocity field at

500 m from 21 February to 11 March 1998, in which the24–50 d SSH variation in the northern subbasin is signif-icantly large (see Figure 12b). Figure 15 indicates that theflow structure in the northern Okinawa Trough is quitedifferent from that of the southern Okinawa Trough. Thenorthern Okinawa Trough is filled with several eddieswhose horizontal scale is restricted by the width of thetrough. This feature is consistent with the observationalevidence obtained by shipboard ADCP measurementsshowing the presence of eddies with a basin-wide scalebelow the Kuroshio (Figures 10a and 10c). During thisperiod, cyclonic eddies tend to occupy the southern andnorthern subbasins with growing features, while an anticy-clonic eddy in the central subbasin diminishes gradually. Thisevidence, which is seen in the composite map (Figure 14b),suggests that each subbasin tends to have an intrinsicpolarity of eddy’s rotation. The number of cyclonic eddiesis always larger than that of anticyclonic eddies: observedratios were 2:1 on 21 February and 3:1 from 27 February to11 March. This numerical imbalance forms a cycloniccirculation over the entire northern Okinawa Trough forthe mean state as shown in Figure 11b. The deep flow fieldin the southern Okinawa Trough is stable, however, with apath extending toward the Kerama Gap. Both cyclonic andanticyclonic eddies are possibly generated near the KeramaGap, but only cyclonic eddies are likely to develop propa-gating northward.

7. Conclusive Remarks

[39] In order to examine temporal and spatial features ofthe slope countercurrent beneath the Kuroshio, we con-ducted moored current-meter observations at about 520 mand 625 m depths on the continental slope in the southernand northern subbasins of the northern Okinawa Trough for

Figure 14. Composite maps of mean current anomaliesand RMS of the vorticity anomaly (shading) at 526.9 m forthree distinct states: (a) the summer–autumn period in1998–2000, (b) the winter-spring period in 1998–2000,and (c) winter 2005. Note that the mean current anomalies(>10 cm s�1 truncated) are shown as the differencefrom Figure 11b (average at 529.6 m depth in the period1998–2006). Shading represents areas whose RMS of thevorticity anomaly is in excess of 5 � 10�6 s�1 (contourinterval is 2.5 � 10�6 s�1).


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November 2004 to November 2006. The principal featuresof the observed slope countercurrent are summarized asfollows.[40] 1. In both years, the slope countercurrent in the

southern subbasin is stronger and is found deeper than thatin the central subbasin.[41] 2. The deep flow fields are associated with eddy

motions due to Kuroshio meanders in both the southern andcentral subbasins, but that field only in the southernsubbasin tends to be organized into a persistent slopecountercurrent during the 4–5 months of the winter-springperiods.[42] 3. The winter-spring persistent slope countercurrents

in the southern subbasin coincide with active Kuroshio pathmeanders whose periods are 30–90 d in the northernsubbasin. In this situation, Kuroshio front meanders withperiods of about 10 d tend to diminish in the southernsubbasin.[43] 4. Vertical sections of shipboard ADCP velocities

indicate that the developed slope countercurrent is a part ofthe cyclonic eddy beneath the Kuroshio whose horizontalscale is the width of the northern Okinawa Trough.[44] To explore the basin-scale structure of the slope

countercurrent in the Okinawa Trough, we examined datafor 1998–2006 from a realistic high-resolution OGCM

simulation. The model reproduced most of the observedfeatures mentioned above. Basin-scale deep flow structuressimulated by the model at about 500 m are as follows.[45] 1. Over a 9-year average, cyclonic circulation occu-

pies the deep area mainly in the northern Okinawa Trough,with a stronger northeastward flow on the eastern slope ofthe trough and a weaker southwestward flow on the westernslope. Such a mean deep cyclonic circulation is obtained byaveraging instantaneous eddy fields, in which cycloniceddies are more common than anticyclonic eddies in thenorthern Okinawa Trough.[46] 2. In contrast to the northern Okinawa Trough, the

surface Kuroshio current and its associated deep flow arestable in the southern Okinawa Trough.[47] 3. The deep flow fields in the northern Okinawa

Trough for 1998–2004 have a seasonal nature similar to theobservational results; flow fields are more unstable and thenslope countercurrents become stronger for the winter–spring period than the summer-autumn period. However,deep current fields for the winters of 2005 and 2006 aredifferent from observed features: the surface Kuroshiocurrent and the associated deep flow are strongly stableand have vertical coherency. Also, the path separatessmoothly from the western slope near the boundary betweenthe southern and central subbasins, and has a stable cyclonic

Figure 15. Time series of velocity field at 526.9 m (>25 cm s�1 truncated): (a) 21 February, (b) 27February, (c) 5 March, and (d) 11 March 1998.


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circulation over the central and northern subbasins. Thesedistinct states are basically similar to three distinct Kuroshiopath states in the northern Okinawa Trough that wereobtained by numerical experiments by Nakamura [2005].[48] Finally, we discuss plausible formation mechanisms

for the persistent slope countercurrent, and note problems tobe examined further. The model indicates that the persistentslope countercurrent in the northern Okinawa Trough is partof the deep cyclonic circulation driven by deep cycloniceddies that are more common than deep anticyclonic eddies.The dynamics that is responsible for this flow feature maybe consistent with that proposed by Sakai and Yoshikawa[2005], who examined temporal evolution of a barocliniccurrent in a two-layer, f-plane numerical model configuredfor a channel with the bottom slope on its each side, andobtained a pair of barotropic currents with the shallowerregion on their right. They concluded the formation mech-anism of the barotropic currents as follows: barotropiceddies resulted from baroclinic instability stir fluid particlesin the channel; negative (positive) barotropic vorticity isformed on shallower (deeper) region according to potentialvorticity conservation; and a pair of positive and negativebarotropic vorticities forms a barotropic current with shal-lower region on its right. Y. Yoshikawa (personal commu-nication, 2007) interpreted this mechanism as Neptuneeffect [Holloway, 1992], or the statistical dynamicaltendency of eddy-topography interaction to induce meancirculation. The formation mechanism for the persistentslope countercurrent may be therefore related to Neptuneeffect. Further work is needed to clarify how the deepcyclonic circulation is formed from a surface-intensifiedcurrent such as the Kuroshio in the channel or the trough.The coupled system of the surface Kuroshio and the deepflow field has a clear seasonal nature in the northernOkinawa Trough. Future work is still needed to understandthe mechanism driving the seasonal cycle. The modelindicates that the deep flow field is stable in the southernOkinawa Trough while it is unstable in the northernOkinawa Trough. However, no observational evidence hasbeen collected to explain this difference. Further observa-tional studies are needed to observe simultaneous deep flowfeatures for the southern and northern Okinawa Troughs.

[49] Acknowledgments. Thanks go to the captain Sunao Masumitsuand crew of the T/V Kagoshima-maru, Kagoshima University for theirassistance at sea, Toru Yamashiro for providing the Kuroshio Position Indexrecord, and two anonymous reviewers for their important comments.Wavelet software was provided by C. Torrence and G. Compo, and isavailable at URL: http://paos.colorado.edu/research/wavelets. The OFESsimulation was conducted on the Earth Simulator under the support ofJAMSTEC. This study was supported by a Grant-in Aid for ScienceResearch (B) 16340142.

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��H. Ichikawa, Institute of Observational Research for Global Change, Japan

Agency for Marine-Earth Science and Technology, 2-15, Natsushima-cho,Yokosuka 237-0061, Japan.H. Nakamura and A. Nishina, Faculty of Fisheries, Kagoshima University,

4-50-20, Shimoarata, Kagoshima 890-0056, Japan. ([email protected])M. Nonaka, Frontier Research Center for Global Change, Japan Agency

for Marine-Earth Science and Technology, 3173-25, Showa-machi,Kanazawa-ku, Yokohama 236-0001, Japan.H. Sasaki, Earth Simulator Center, Japan Agency for Marine-Earth

Science and Technology, 3173-25, Showa-machi, Kanazawa-ku, Yokohama236-0001, Japan.


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deep countercurrent beneath the kuroshio in the okinawa trough

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