Structure and Seasonal Variability of the East Sakhalin

2430 VOLUME 33J O U R N A L O F P H Y S I C A L O C E A N O G R A P H Y

q 2003 American Meteorological Society

Structure and Seasonal Variability of the East Sakhalin Current

GENTA MIZUTA

Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan

YASUSHI FUKAMACHI AND KAY I. OHSHIMA

Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan

MASAAKI WAKATSUCHI

Institute of Low Temperature Science, Hokkaido University, Sapporo, and Japan Science and Technology Corporation, Kawaguchi, Japan

(Manuscript received 26 July 2002, in final form 28 April 2003)

ABSTRACT

In order to clarify the structure and seasonal variability of the flow field near the western boundary of theSea of Okhotsk, long-term mooring measurements were carried out from 1998 to 2000 in this region. In mostof the mooring period a persistent southward flow (the East Sakhalin Current) was observed, which extendsfrom the surface to a depth around 1000 m. The speed of this southward flow clearly changed seasonally. Thepeak monthly mean speed along 538N at a depth of 200 m attained a maximum of 37 6 9 cm s21 in Januaryand a minimum of 10 6 8 cm s21 in July. Three different cores of intense flow were identified in the southwardflow. The first core was centered over the continental slope and had rather large vertical extent, reaching thebottom on the slope. The second core was trapped over the shelf near the surface and was observed from Octoberto November. This core was associated with less saline surface water affected by the Amur River discharge.The third core was intensified toward the bottom on the slope. The spatial and temporal distribution of thisbottom-intensified core coincided with that of dense shelf water, which is formed over the broad shelf in thenorth. The intensity of this core damped within a few hundred kilometers from the northern end of Sakhalinprobably because of strong mixing of dense shelf water with surrounding waters. The total transport of thesouthward flow at 538N was 6.7 3 106 m3 s21 in the annual average, varying from a maximum of 12.3 3 106

m3 s21 in February and a minimum of 1.2 3 106 m3 s21 in October. Most of the transport was maintained bythe first core of the southward flow.

1. Introduction

The Sea of Okhotsk (Fig. 1), which is a marginal seaadjacent to the North Pacific, is a possible region whereNorth Pacific Intermediate Water (NPIW) is ventilatedto the atmosphere (Talley 1991; Yasuda 1997; Warneret al. 1996). Dense shelf water (DSW) formed in thenorthern shelf region in the Okhotsk Sea is believed tobe a source water that ventilates NPIW. Since the west-ern boundary of the Okhotsk Sea is a pathway of DSW(Kitani 1973; Talley 1991) and sea ice (Watanabe 1963b;Parkinson and Gratz 1983), the flow field in this regionis important as a basic quantity that determines theirtransports. The schematics of the surface circulation byLeonov (1960) and Moroshkin (1966) show that thereis a northward flow along the eastern boundary and a

Corresponding author address: Dr. Genta Mizuta, Graduate Schoolof Environmental Earth Science, Hokkaido University, Kita 10 Nishi5, Sapporo 060-0810, Japan.E-mail: [email protected]

southward flow, called the East Sakhalin Current (ESC),along the western boundary to form a general cyclonicgyre in the Okhotsk Sea. The distribution of East Sa-khalin Current Water (ESCW), which is less saline sur-face water affected by the Amur River discharge, sup-ports the presence of the ESC and suggests its seasonalvariability. Watanabe (1963a) showed that ESCWspreads to the southern end of the Okhotsk Sea alongthe east coast of Sakhalin Island from November toDecember, indicating the presence of a southward flow,namely the ESC, there. The advance of sea ice indicatesthat the ESC is present in winter (Watanabe 1963b; Par-kinson and Gratz 1983). The obscure southward extentof ESCW in summer suggests that the ESC weakens orturns eastward near Cape Terpenia (Kajiura 1949; Wa-tanabe 1963a).

The distribution of dynamic topography in theOkhotsk Sea consists of a general cyclonic circulationin the north and an anticyclonic circulation in the deepKuril Basin in the south (Wakatsuchi and Martin 1991;Watanabe and Wakatsuchi 1998). The flow field in the

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NOVEMBER 2003 2431M I Z U T A E T A L .

FIG. 1. A map showing locations of moorings. The rectangularregion in the lower left panel indicates the location of the observationregion. Circles indicate mooring sites. Dashed line denotes a typicalpath of ESCW assumed for the calculation of an advection length inFig. 10. Shaded region indicates the northwest shelf, which is themajor production area of DSW. A cross to the south of M7 is thelocation where the mooring M7 was recovered.

western boundary region, where the water depth issmall, is omitted in these analyses. The dynamic to-pography is consistent with the schematic surface cir-culation by Leonov (1960), Moroshkin (1966), and Wa-tanabe (1963a).

In the Okhotsk Sea a new water mass, namely, DSW,is formed by severe atmospheric cooling and associatedsea ice production in the northern shelf (Kitani 1973;Alfultis and Martin 1987). This water mass has near-freezing temperature and maximum density around 27.0su and spreads southward as a cold and oxygen-richtongue along the western boundary at depths between200 and 500 m (Kitani 1973; Talley 1991). Thus it issuggested that a southward flow is present not only atthe surface but at these depths. Mixed with surroundingwaters, DSW becomes an intermediate water in theOkhotsk Sea. This Okhotsk Sea intermediate water isbelieved to enter the North Pacific through deep pas-sages between Kuril Islands and to influence the char-acteristics of NPIW (Talley 1991; Yasuda 1997; Warneret al. 1996).

Recently, Ohshima et al. (2002) examined the sur-face-flow field in this region in September–December,using surface-drifter-buoy trajectories. They showedthat there is a southward flow along the east coast ofSakhalin. A part of this southward flow turns eastwardnear Cape Terpenia and joins with an anticyclonic meancirculation in the Kuril Basin. A part of the flow con-tinues southward. These features of the flow field areconsistent with the 1960s schematics (Leonov 1960;Watanabe 1963a; Moroshkin 1966). They also showedthat the southward flow along Sakhalin consists of twodifferent cores of intense flow along the 50–100-m iso-baths and 300–900-m isobaths.

These studies suggest that there is a southward flow,namely, the ESC, along the east coast of Sakhalin, thatthe ESC exists in a wide depth range, and that the in-tensity of the ESC changes seasonally, at least near thesurface. However, quantitative features of the flow fieldhave not been explored in detail. The geostrophic cal-culation contains a difficulty in the assumption of thelevel of no motion, especially in the western boundaryregion where DSW spreads over the bottom. Becauseof sea ice coverage, hydrographic observations and sur-face-drifter measurements are difficult in winter. Directmeasurements of velocity have not been reported exceptfor short-term or coastal measurements (Luchin 1995).

From 1998 to 2000 we conducted long-term currentmooring measurements off the east coast of Sakhalin aspart of a joint Japanese–Russian–U.S. study of theOkhotsk Sea. The moorings consisted of ADCPs, cur-rent meters, conductivity and temperature sensors, andthermistors at depths from 100 to 870 m. In this studywe analyze velocity data obtained by ADCPs and cur-rent meters. Our purpose is to clarify, by direct mea-surement, the spatial structure and the seasonal vari-ability of the ESC quantitatively. The organization ofthis paper is as follows. In the next section we describe

the data used in this study. The results of the currentmeasurement are presented in section 3. These resultsare summarized with additional discussion in section 4.

2. Data

Locations of our moorings are shown in Fig. 1. InJuly 1998 we deployed two moorings along 49.58N andfour moorings along 538N off the east coast of Sakhalin.These moorings were recovered in September 1999, andfour of these moorings were redeployed. We also de-ployed three moorings along a line extending north-eastward from the northern end of Sakhalin (hereinafter558N line) in September 1999. All moorings were re-covered in June 2000. The mooring array covered con-tinental shelves, which are typically the regions shal-lower than 200 m, continental slopes, and the offshoreregion (Fig. 1). Mooring sites M1, M5, M8, and M9were located in the shelf region, M2, M3, M6, and M7were in the slope region, and M4 was in the offshoreregion. A shaded region in Fig. 1 indicates the northwestshelf, where a major part of DSW seems to be formedby intense ice production in polynyas and exported



TABLE 1. Mooring site, location, instrument type, bottom depth, nominal instrument depth, mooring period, and length of good data.Acronym CM indicates a current meter. Acronyms ADCP/CM and CM/ADCP indicate an ADCP for Jul 1998–Sep 1999 and a current meterfor Sep 1999 and a current meter for Sep 1999–June 2000 and vice versa.

Station Lat (8N) Lon (8E) Type Depth (m) Dnom (m) Period Length (days)

M1M2

M3

53.053.0

53.0

144.0144.4

144.8

ADCP/CMCMCMADCPCMCM

100480

970

100200430190460870

Jul 1998–Jun 2000Jul 1998–Jun 2000

Jul 1998–Sep 1999

349689689406406233

M4

M5M6

53.0

49.549.5

145.5

144.5146.5

CM/ADCPCMADCPADCPCMCM

1720

130790

180470130180480740

Jul 1998–Jun 2000

Aug 1998–Sep 1999Aug 1998–Jun 2000

689686348663548587

M7

M8M9

54.9

54.754.5

143.9

143.5143.0

ADCPCMADCPADCP

480

11090

200430110

90

Sep 1999–Jun 2000

Sep 1999–Jun 2000Sep 1999–Jun 2000

291291101179

FIG. 2. The vertical distribution of velocity errors caused by theinterpolation. Labels near the top of the lines indicate the mooringsite at which velocity data were obtained. See text for details.

southward (Martin et al. 1998; Gladyshev et al. 2000,2003). All our moorings were located in regions southof this shelf. The moorings consisted of ADCPs (RDInstruments BroadBand Self-Contained ADCP 150 kHzand Workhorse Sentinel ADCP 300 kHz), current meters(Union Engineering RU-1), conductivity and tempera-ture sensors, and thermistors. The accuracy of the flowspeed is less than 1 cm s21 for ADCPs and 2% of actualspeed for current meters.

Nominal depths of current meters and ADCPs andtheir mooring periods are shown in Table 1. In the sloperegion, cold and dense water that is largely affected byDSW exists at a depth range of 200–500 m. Some ofthe current meters were placed within this depth range.Most data were collected successfully. For the current

meter moored at a depth of 870 m at M3, velocity dataindicated that there was no motion until 16 January1999. After that the velocity data started to indicate tidaland other motions similar to those at other depths at thesame site. So we used the data only in this period forthis current meter. Depths of the instruments moored atM7 changed by about 240 m in early December 1999and 180 m in late March 2000, because the mooringwas advected about 20 km southward by a strong flow.We calibrated depths of these instruments by using depthto the surface measured by the ADCP at the top.

We filtered out tidal and other high-frequency motions(Thompson 1983) and obtained hourly tide-free velocitydata. The current-meter records had no gaps so that thefilter was applied directly. The shallower part of theADCP records was sometimes bad for 6–9 hours perday, presumably due to weak echoes caused by verticalmigration of scatterers. Such bad data were mostly lim-ited within 100 m from the surface over the slope andoffshore regions and within 60 m or less in the shelfregion. We interpolated velocity data at these depths,by assuming that tidal and other high-frequency motionsare significantly correlated in the vertical direction. Thefraction of bad velocity data at a depth of 20 m was 3%for M1, 14% for M3, 34% for M5, 21% for M6, 25%for M7, and ,1% for M8 and M9. Details of this in-terpolation and the associated error are described in theappendix. Figure 2 shows the vertical distribution of therms of interpolation error yerr. Long-dashed, solid, anddash–dotted lines indicate yerr calculated for the wholemooring period at mooring sites M1, M3, and M5, re-spectively. Dashed and dotted lines indicate yerr at M7calculated for the two periods before and after the depthchange in December 1999, respectively. Mooring sitesM7 and M3 were in the slope region, and M1 and M5were in the shelf region. Relatively large errors occurrednear the surface at M7 from September to December.For ADCPs in the slope region other than M7, errors



FIG. 3. Stick plots of velocity observed at mooring site M3 at depths of (a) 170, (b) 460, and (c) 870 m.Velocity vectors are drawn in every two days. Velocity data at a depth of 870 m were bad until Jan 1999.

are less than 3 cm s21. For ADCPs in the shelf regionthere are rather large interpolation errors near the sur-face at M5. Errors at M1 are small because of the smallfraction of bad data. Errors at M8 and M9 are smallerthan those at M1 (not shown).

Temperature and salinity around the mooring arrayswere also measured with CTDs (SeaBird SBE 911 Plusand Neil Brown Mark III-B) and XBTs (Tsurumi Seiki)during three cruises in July 1998, September 1999, andJune 2000. For the CTD sensors, pre- and postcruisecalibration were performed. Salinity data obtained bythe CTD sensor were calibrated with the water samplemeasurements.

3. Results

As a typical example of the flow field, Fig. 3 showsstick plots of velocity observed at mooring site M3 inthe slope region. A southward flow was evident at alldepths for most of the period with the flow speed de-creasing with depth. At depths of 170 and 460 m theflow speed clearly changed seasonally. The maximumsouthward speed was about 30 cm s21 at 170 m in Jan-uary–February. In October the southward flow was quitesmall or even northward. Although velocity data werebad from July 1998 to January 1999 at the deepest cur-rent meter at 870 m, seasonal variability was indicatedat a depth of 740 m at mooring site M6 (not shown).Thus seasonal variability of the southward flow existedfrom the surface to a depth near 1000 m.

The horizontal distribution of monthly mean velocityvectors and the 95% confidence error ellipses at a deptharound 200 m or near the bottom in shallow sites inOctober, January, April, and July are shown in Fig. 4.The confidence error ellipses were calculated accordingto Emery and Thompson (2001), where the integral

timescale estimated from velocity data was 3 days forthe shelf region and 4.5 days for the slope and offshoreregions. There was a statistically significant southwardflow, except for a rather variable flow in the offshoreregion in October. The southward flow was almost par-allel to isobaths at most of the mooring sites in thesemonths. A relatively strong southward flow was ob-served over the continental shelf and steep slope. Theflow speed changed seasonally, being largest in Januaryand smallest in July among these four months. Along538N the southward flow was maximum over the slopein January. In July, there were separate maxima overthe shelf and over the slope. The flow speed and the95% confidence interval at the largest maximum was37 6 9 cm s21 in January and 10 6 8 cm s21 in July.

To show the seasonal variability of the vertical struc-ture of the flow field, stick plots of monthly mean ve-locity at two different depths observed at M3, M1, andM7 are displayed in Fig. 5. Here mooring site M3 islocated above the slope at 538N, M1 is above the shelfat the same latitude, and M7 is above the slope alongthe 558N line. A southward flow that changes seasonallywas evident at all these sites. The vertical shear of thissouthward flow was small over the slope at 538N (M3)in the depth range 20–200 m (Fig. 5a). On the otherhand, the southward flow near the surface was strongerthan that near the bottom over the shelf (M1) from Oc-tober to December (Fig. 5b). Velocity vectors near thebottom over the shelf were directed offshore as com-pared with those near the surface in most of the mooringperiod, probably because of a frictional bottom Ekmanlayer. Over the slope along the 558N line (M7) the south-ward flow at a depth of 200 m was stronger than thatnear the surface in most of the displayed period (Fig.5c). The vertical shear was particularly significant from



FIG. 4. Monthly mean velocity vectors and the 95% confidence ellipses at a depth around 200 m or near the bottom in shallow sites in(a) Oct, (b) Jan, (c) Apr, and (d) Jul. Solid and dashed arrows indicate velocity observed in 1998–99 and 1999–2000, respectively. Dashedlines denote 200- and 1000-m isobaths.

September to November in 1999. In this period themonthly mean flow speed was less than 10 cm s21 at adepth of 20 m, whereas the flow speed reached nearly20 cm s21 at a depth of 200 m. Therefore, the verticalstructures of the flow field at these three sites and theirseasonal variability were quite different from each other.

Figure 6 shows the distribution of the meridionalcomponent of monthly mean velocity at a vertical crosssection at 538N. In October there was an intense south-ward flow associated with the vertical shear at mooringsite M1 (Fig. 6a). The intense flow was trapped nearthe surface over the shelf. The surface-trapped flow dis-appeared in January and April (Figs. 6b,c). In thesemonths a southward flow was distributed over the wholesection. In January the southward flow speed attaineda maximum of 37 cm s21 over the slope (M2), decreas-ing gradually in the horizontal and vertical directions.The southward flow exceeded 20 cm s21 at the currentmeter 50 m above the bottom (430 m) at M2, seemingto extend to the bottom on the slope. The southwardflow was weak in April as compared with that in Jan-uary. The maximum of southward flow speed was shift-ed to M3 in this month. In the next two sections weexamine the structure of the flow field, focusing on anintense flow trapped near the surface over the shelf anda flow that increased with depth along the 558N line.Then we estimate the transport of the southward flowin the last section.

a. Surface flows on the shelfThe monthly mean velocity component normal to sec-

tions along the 558N line and 49.58N in October (Fig.

7) shows intense southward or southeastward flowtrapped near the surface over the shelf, as in Fig. 6a.Hence the velocity difference between a depth of 20 mand the bottom became large above the shelf. This ve-locity difference is statistically significant at the 558Nline and 538N since the confidence coefficient of thevelocity difference is larger than 99% there for the de-correlation timescale of 2.5 days. Though velocity dataat 49.58N contain interpolation errors of ;5 cm s21 nearthe surface (Fig. 2), the surface-trapped flow was similarto that at the other two sections. Therefore, this featureappears to be common in the shelf region off the eastcoast of Sakhalin in October.

The strong vertical shear near the surface is also iden-tified in the CTD sections along the mooring arrays (Fig.8). Figure 8a shows the vertical profile of water prop-erties and geostrophic velocity in late September 1999at mooring site M9. The ADCP velocities are also su-perimposed by crosses. Velocity data at depths less than20 m were not obtained because they are contaminatedby echos from the surface. As shown in Fig. 7a, a sig-nificant surface-trapped flow was observed in Octoberat this site. A less saline, well-mixed layer extendedfrom the surface to a depth of 15 m with a relativelysaline and nearly homogeneous layer from 35-m depthto the bottom. The water in the mixed layer is regardedas ESCW, which is affected by the Amur River dis-charge. Salinity, temperature, and, accordingly, densitychanged abruptly at the bottom of the mixed layer. Alarge geostrophic shear was located at the interface be-tween the mixed layer and homogeneous layer below.



FIG. 5. Stick plots of monthly-mean velocity observed at (a) M3, (b) M1, and (c) M7. Solidsticks headed to a cross indicate velocity vector at a depth of 20 m, and dashed sticks headed toa square indicate velocity vector at a depth of 200 m (near the bottom) for M3 and M7 (M1).Solid lines denote the trajectory of heads of velocity vectors between the two depths. Velocitydata at M1 and M3 were obtained in 1998–99 and those at M7 were in 1999–2000.

This geostrophic shear was associated with a south-eastward flow in the upper layer relative to the lowerlayer, and is in good agreement with the vertical shearmeasured by the ADCP. Thus the surface-trapped flowobserved by the ADCP in September was associatedwith ESCW whose density and velocity are largely dif-ferent from those of waters below.

Figure 8b is similar to Fig. 8a except for mooring siteM1 in July 1998. Salinity near the surface in the ESCWwas less than 27. Water properties were strongly strat-ified from the surface to a depth of 20 m and almostvertically uniform below. There was no clear mixedlayer in this month: the strong stratification existed ata depth shallower than that in September at M9. Thusvelocity data in this strong stratification could not bemeasured by an ADCP. Geostrophic velocities as wellas velocities measured by the ADCP were almost uni-form below the strong stratification.

The depth of the surface-trapped flow at M9 variedwith time (Fig. 9), where a running-mean filter over 3days is applied to remove rather noisy variability. Thedepth of maximum shear coincided with the bottom ofthe mixed layer in late September (Figs. 8a and 9). From

late September to early October the maximum shear wasat a depth around 20 m with a rather uniform layerextending below. The maximum deepened with time andshifted to a depth around 30–70 m in late October. Thesurface-trapped flow was most evident in this season.Then the maximum reached the bottom in late Novem-ber and disappeared in December. The velocity becamealmost vertically uniform after January.

In order to see how ESCW spreads from its origin atthe Amur River to the observation region over time, wecalculate an advection length due to the flow. We assumethat ESCW extends along the path indicated by thedashed line in Fig. 1, which mostly coincides with the100-m isobath. The runoff from the Amur River in-creases abruptly from nearly zero in May, when snowand frozen soil start to melt (Ogi et al. 2001). Thus weassume that the runoff before May is negligibly small.If waters that originate from the Amur River are ad-vected southward, the position of waters that start at themouth of the river in May becomes the southern limitof ESCW. We also assume that ESCW is advected atthe same speed as observed at M1 until it reaches themidpoint between M1 and M5 and continues southward



FIG. 6. Contours of the monthly mean velocity component normal to a vertical cross section at 538N (cm s21) in(a) Oct, (b) Jan, and (c) Apr. Positive value indicates a southward flow. Shaded region indicates velocity more than15 cm s21. Dots represent positions at which velocity data were obtained.

FIG. 7. The same as Fig. 6, but for the monthly mean velocity component (cm s21) along the(a) 558N line and (b) 49.58N in October, respectively.

at the speed observed at M5. Then we obtain the positionx of the southern limit of ESCW by integrating themonthly mean speed at M1 and M5. Here we use thevelocity component parallel to the path at a depth of 20m for the advection speed. We also calculate 95% con-

fidence limit of the position from velocity variance ob-tained for each month using the Lagrangian integraltimescale of 1.7 days derived from the surface-drifterdata (Ohshima et al. 2002). The solid line in Fig. 10shows the relation between time t and x, where we define



FIG. 8. Vertical profiles of water properties (a) at mooring site M9 on 19 Sep 1999 and (b) at M1 on 23 Jul 1998. Solid, dashed, dotted,and long-dashed lines denote, potential temperature (8C), salinity, density, and geostrophic velocity (cm s21) that is fitted to velocities observedby an ADCP. Crosses indicate the velocity component normal to the mooring array observed by an ADCP.

x 5 0 at the mouth of the Amur River. Bars indicate95% confidence limits, and dots denote the position ofthe southern limit of ESCW determined from other ob-servational data. That is, the dot in November is basedon the fact that ESCW arrives at the Hokkaido coast, x5 1600 km, in this month (Watanabe 1963a; Itoh andOhshima 2000). The dots around x 5 400, 600, and 900km indicate the southern limit of ESCW observed inJune 2000, July 1998, and September 1999 in our CTDdata, respectively. The CTD data shown in Fig. 8 sug-gest that the salinity of ESCW is changed not only byhorizontal advection but by vertical mixing. To elimi-nate the salinity change by vertical mixing, we definedthe southern limit as the position where the salinity av-eraged within 50 m from the surface is equal to 32.Though these are rough estimates, ESCW positions pre-dicted from the flow speed agree with the observed ones.The advection length indicates that ESCW reaches allour mooring sites by October so that it can drive thesurface-trapped flow at these sites in October. TheESCW disappears in winter, presumably because it ismixed with more saline waters below (Itoh and Ohshima2000), and appears again at the Amur River in May. InFig. 10, a dashed line shows the advection length ofwaters calculated from the flow speed at a depth of 100m. The arrival of these waters at Hokkaido is delayedby 50 days as compared with that at 20 m. Thus thevertical shear associated with ESCW affects its owndistribution.

b. Bottom-intensified flows

As we have demonstrated at the beginning of thissection, the southward-flow speed observed at mooringsite M7 increased with depth, especially in September–November 1999. Figure 11 shows the distribution oftemperature and potential density in September 1999 atvertical cross sections along the 558N line and 538N. Toexamine temperature structure near the slope more pre-cisely, we used XBT data in addition to CTD data.Along the 558N line the waters near the bottom on theshelf and slope had temperature of 21.58C or less anddensity of 26.8–27.0 su (Fig. 11a). These waters areregarded as DSW. The DSW was distributed near moor-ing site M7. Associated with DSW, isopycnals were sig-nificantly inclined near the shelf break. The inclinationof isopycnals indicates that there was a southeastwardflow that increases with depth near M7.

Along 538N a temperature minimum was observedover the slope at a depth around 450 m (Fig. 11b). Thisminimum existed near M2. Since density at the tem-perature minimum was around 27.0 su, the minimumseems to be affected by DSW. The DSW that retains itsoriginal features so strongly as that along the 558N linewas absent at this latitude. Kitani (1973) pointed outthat waters with 27.0 su off Sakhalin are warmer andpoorer in oxygen than the original DSW. He suggestedthat DSW is modified by mixing with warm and oxygen-poor water coming from the Pacific. Yamamoto et al.(2002) examined the distribution of DSW, by using d18Oas a tracer of brine rejected from sea ice into DSW.



FIG. 9. Time series of the vertical profile of the velocity component normal to the mooringarray at mooring site M9. A running-mean filter over 3 days is applied to velocity data. Crossesindicate the depth at which vertical shear attains the maximum. Maxima less than 30 cm s21 (100m)21 are omitted. The velocity profiles are drawn in every 3 days.

FIG. 10. Plots of time vs an advection length obtained by integratingthe flow speed with time. Solid and dashed lines denote positions ofthe southern limit of ESCW calculated from the monthly mean flowspeed at a depth of 20 and 100 m, respectively. Bars indicate 95%confidence limit of the position. Dots denote the position of the south-ern limit of ESCW determined from the other observational data.The signs ‘‘HOK,’’ ‘‘M5,’’ and ‘‘M1’’ in the vertical axis indicatepositions of Hokkaido coast and mooring sites M5 and M1, respec-tively.

They showed that DSW is largely modified near thenorthern end of Sakhalin. These studies of temperature,oxygen, and d18O distributions suggest that DSW ismodified by mixing as it is advected from the 558N lineto 538N.

To compare the distribution of temperature and den-sity with that of velocity obtained in the same period,vertical profiles of monthly mean velocity in September1999 at mooring sites M7 and M2 are displayed in Fig.12. At both sites there was a southward or southeastwardflow. The speed of this flow increased monotonically

with depth to attain a maximum at the current meterabout 50 m above the bottom. The vertical profile ofthis bottom-intensified southward flow is consistent withthe inclination of isopycnals in Fig. 11. Along the 558Nline the isopycnals were inclined most significantly ata depth around 100–200 m because of DSW near theshelf break (Fig. 11a). The vertical shear observed byan ADCP was largest in this depth range at M7 (Fig.12). The mean vertical shear in this depth range wasabout 10 cm s21 (100 m)21. Since spacings between theCTD stations were not fine enough, we cannot calculategeostrophic velocity around M7 precisely. Instead, weroughly evaluate the geostrophic shear at M7. From thethermal wind equation the vertical shear associated withthe inclination of isopycnals is y z 5 (gDr)/( fr0Dx).Here g 5 9.8 m s22 is the gravity acceleration, f 5 1.23 1024 s21 is the Coriolis parameter at M7, r0 5 1.03 103 kg m23 is the characteristic density of water, andDr/Dx is the inclination of isopycnals. Letting Dr 50.1 kg m23 and Dx 5 104 m at a depth of 100–200 m,we have y z ; 10 cm s21 (100 m)21. This value agreeswith the vertical shear observed at M7. On the otherhand, DSW existed on the slope near mooring site M2,with its properties significantly modified from the orig-inal ones. At this site the bottom-intensified flow wasweaker than at M7. Thus the bottom-intensified flowassociated with DSW weakens significantly as DSW ismixed with surrounding waters. Since the distance fromM7 to M2 is about 200 km (Fig. 1), it is concluded thatsignificant weakening of the bottom-intensified flow andthe mixing of water occur within a few hundred kilo-meters from the northern end of Sakhalin.

Figure 13 shows temporal variations in the verticalprofile of the velocity component normal to the 558Nline at mooring site M7. A running-mean filter over 3days is applied to velocity data. There was a bottom-intensified flow from September to November. This bot-tom-intensified flow corresponds to that shown in Fig.12. In December and January the bottom-intensifiedflow disappeared, and the normal velocity componentbecame maximum near the surface. Then the maximumshifted to a depth around 150 m in February. We cannotdetermine the depth of the velocity maximum after the



FIG. 11. Contours of temperature (8C) and potential density at vertical cross sections along the (a) 558N line and (b) 538N. Shaded regionindicates temperature less than 08C. Ticks with and without numbers at the top are locations of CTD and XBT observations, respectively.

FIG. 12. Vertical profiles of the monthly mean velocity componentnormal to the mooring array in Sep 1999. Solid and dashed linesindicate profiles at M7 and M2, respectively. Solid horizontal barsdenote 95% confidence limit of the monthly mean velocity compo-nent. Crosses indicate positions at which velocity data are obtained.Bars with hatches indicate positions of the bottom.

shift of the mooring in March because velocity data ata depth of 0–200 m were not obtained. From May toJune the maximum appeared at a depth around 350 mor more. Since the bottom depth at the original mooringsite is 480 m, the depth of the velocity maximum was

within 130 m from that of the maximum of the bottom-intensified flow observed in the previous year.

We examine temporal variation of the bottom-inten-sified flow further by using the 2-yr velocity recordobtained at 538N (Fig. 14). A running-mean filter over3 days is applied. Dotted and thin solid lines indicatesouthward velocity observed at depths of 200 and 430m, respectively. At both depths southward velocitychanged seasonally. While the velocity at the deeperdepth was smaller than that at the shallower depth inwinter, the velocities at the two depths became closerto each other in summer. A solid line in the lower partof Fig. 14a indicates the difference of velocity betweenthese two depths. Positive value indicates that the south-ward velocity at the deeper depth is larger than that atthe shallower depth. The southward velocity at the deep-er depth was mostly greater than that at the shallowerdepth from May to October 1999, indicating that therewas a bottom-intensified flow in this period. The bot-tom-intensified flow was also observed from April toMay 2000 and intermittently from September to De-cember 1998. Thus the velocity difference changed sea-sonally in the observation period. Bars in Fig. 14a de-note the period in which waters with temperature lessthan 0.58C and density greater than 26.85 su are ob-served by conductivity and temperature sensors mooredat a depth of 430 m (Fukamachi et al. 2001). These coldand dense waters are regarded as modified DSW. Theperiod later than March 2000 is omitted because of badsalinity data. We see that the bottom-intensified flowand modified DSW were present roughly in the sameperiod. Figure 14b shows the southward velocity at two



FIG. 13. Time series of the vertical profile of the velocity component normal to the mooring array at mooring siteM7. A running-mean filter over 3 days is applied to velocity data. The velocity profiles are drawn in every 3 days.Triangles at the bottom indicate dates on which the mooring was advected by a strong flow.

FIG. 14. Time series of southward velocity at mooring site (a) M2 and (b) M3. A running-meanfilter over 3 days is applied to velocity data. Dotted and thin solid lines in (a) denote southwardvelocity at a depth of 200 and 430 m, respectively. Dotted and thin solid lines in (b) denotesouthward velocity at a depth of 170 and 460 m, respectively. A solid line in the lower part in(a) and (b) indicates the difference of velocity between these two depths. Positive value indicatesthat the southward velocity at the deeper depth is larger than that at the shallower depth. Shadedregion indicates a bottom-intensified flow. Bars at the bottom of (a) indicates the period in whichwaters with temperature less than 0.58C and density greater than 26.85 su were observed byconductivity and temperature sensors moored at a depth of 430 m.

depths and the velocity difference at M3. This site waslocated about 20 km offshore of M2 (Fig. 1). Accordingto Fukamachi et al. (2001), modified DSW was rarelyobserved at this site. The figure shows that a flow in-

tensified at a depth around 400 m was also absent, exceptfor short-term variability in October 1998. Therefore,the velocity, temperature, and salinity data at 538N in-dicate that the temporal and spatial distributions of the



FIG. 15. Plots of distance from the coast vs vertically integratedvelocity V at 538N in Oct 1998, Jan 1999, Apr 1999, and Jul 1999.Positive value indicates southward V. Dashed line indicates V ex-trapolated offshoreward. Dots denote mooring positions.

FIG. 16. Time series of the monthly mean transport (106 m3 s21)at 538N and vertically integrated velocity. Solid and dashed linesdenote the transport with the extrapolation outside of the mooringsite M4 and that without the extrapolation, respectively. Positive valueindicates southward transport. Crosses and open circles indicate ver-tically integrated velocity (m2 s21) at mooring sites M2 and M4,respectively.

bottom-intensified flow are largely correlated with thoseof DSW.

c. Transport of the ESC

In this section we estimate the volume transport ofthe southward flow off the east coast of Sakhalin. Wecalculate the monthly mean transport at 538N from Au-gust 1998 to August 1999 because we deployed moor-ings most intensively at this latitude during this period(Table 1). To calculate the transport, we interpolate andextrapolate monthly-mean velocities from the surface tobottom and from the coast to the most offshore mooringsite M4. We assume that velocities at the surface arethe same as those measured at the shallowest depth foreach mooring site because all of these depths are lessthan 200 m. We also assume that velocities at the bottomat mooring sites M1 and M2 are the same as thosemeasured at the deepest current meter or the deepestbin of the ADCP at these sites. On the other hand, atmooring site M3 it is shown that the ratio of monthly-mean velocities at a depth of 870 m to those at 460 mis almost constant: 0.62 with a standard deviation of0.13. Thus, by assuming an exponential form y }exp(lz) with l 5 (ln0.62)/(870 2 460) m21, we ex-trapolate velocities at depths greater than those of thedeepest current meter at M3 and M4. Then we obtainvelocities between M1 and M4 from the surface to bot-tom by two-dimensional linear interpolation. We set ve-locities between the coast and mooring site M1 to bethe same as those measured at M1. We use bottom depthdata measured at mooring sites M1–4 and the five CTDstations along 538N.

Figure 15 shows vertically integrated velocity V(x) as

a function of the distance x from the coast. The changeof V(x) between two mooring sites is not linear becauseof the bottom-depth change with x. Except for October,the distribution of V(x) had a single peak around moor-ing sites M2 and M3. Spacings of the moorings seemto be close enough to resolve this peak of V. Values ofV at M1 and M4 were less than half of those at M2 andM3. Accordingly, we expect that the transport betweenM1 and M4 was most of the total southward transportat 538N. The transport between the coast and mooringsite M1 includes ambiguities due to the extrapolationof velocities. However, this transport was small com-pared with the total transport. On the other hand, sinceV at mooring site M4 was positive, there was somesouthward transport in the region offshore of M4. If weextrapolate V linearly, it crosses 0 at a point slightlyoffshore of M4 (a dashed line in Fig. 15). As a measureof the total southward transport, we calculated the trans-port between the coast and this zero-crossing point.There were two zero crossing points between M2–M3and M3–M4 in October. We excluded the transport off-shore of the first zero-crossing point for the calculationof the total southward transport because the southwardflow at M4 was not statistically significant (Fig. 4). Wedid not extrapolate the transport in August 1998 becausethe monthly mean flow speed was too small to definea zero-crossing point properly.

The monthly mean transport at 538N (Fig. 16) wassouthward with its amplitude changed seasonally. Theunextrapolated transport (solid line) attained a maxi-



FIG. 17. Contours of the annual-mean transport (106 m3 s21), whichis integrated from the surface at the coast to a given point, along avertical cross section at 538N. Contour interval is 0.5 or 1 3 106 m3

s21. Areas 1 and 2 are regions of the surface-trapped flow associatedwith ESCW, and the bottom-intensified flow associated with DSW,respectively. Dots represent positions at which velocity data wereobtained.

mum of 11.7 3 106 m3 s21 in January and a minimumof 0.9 3 106 m3 s21 in October. The transport averagedfor 12 months from August 1998 to July 1999 was 6.33 106 m3 s21. The extrapolated transport (dashed line)was slightly larger than this. This transport attained amaximum of 12.3 3 106 m3 s21 in February and aminimum of 1.2 3 106 m3 s21 in October. The annualaverage was 6.7 3 106 m3 s21. Although the southwardflow in October was stronger than that in August–Sep-tember over the shelf (Fig. 5b), the transport in Octoberwas smallest because of a northward flow in the sloperegion. The transport depends weakly on how we ex-trapolate velocities below the deepest current meters atmooring sites M3 and M4. If we assume that velocitiesat the bottom vanish at these sites, the annual meansouthward transport decreases to 6.6 3 106 m3 s21. Onthe other hand, if we assume that velocities are the sameas those measured at the deepest current meter, the trans-port increases to 7.5 3 106 m3 s21.

Seasonal variability of the total transport was similarto that of the vertically integrated velocity V at M2 andM4 (Fig. 16). That is, both the vertically integrated ve-locity V at M2 and the unextrapolated transport wereminimum in September–October 1998 and maximum inJanuary 1999. While V at M2 decreased after January,V at M4 was maximum in March 1999. Thus the trans-port, especially the extrapolated transport, in February–March 1999 was as large as in January. The seasonalvariability of V in September 1999–June 2000 was sim-ilar to that in the previous year at both M2 and M4,except for short-term variability in September and Oc-tober. As shown in Fig. 15, we cannot calculate preciselythe transport at 538N in this period only from V at thesesites. However, it is suggested that the seasonal vari-ability of the transport in 1999–2000 was similar to thatobserved in 1998–99. The value of V in 1999–2000 wassomewhat smaller in January and larger in September–October when compared with 1998–99. Thus the am-plitude of the seasonal variability of the transport in1999–2000 was likely smaller than in 1998–99.

We also examine the two-dimensional distribution ofthe transport Q(x, z) 5 y(x9, z9) dz9 dx9 along ax 0# #0 z

vertical cross section at 538N (Fig. 17). Here Q(x, z)represents the transport integrated from the surface toa depth 2z and from the coast to a distance x in thehorizontal direction. The total volume transport betweenthe surface and a depth of 500 m was more than 4 3106 m3 s21 and corresponds to two-thirds of the totaltransport. The ambiguity of the transport in this depthrange is rather small because we obtained velocity dataintensively there. This is why the total transport doesnot depend so much on the extrapolation method forvelocities near the bottom. Moreover, it is clear fromthis figure that the volume transport over the shelf andslope near the shelf break, where ESCW and DSW arefrequently observed, respectively, did not contribute sig-nificantly to the total transport. To check this precisely,we examine the transport in the two areas shown in Fig.

17. Area 1 is defined as the region between the coastand mooring site M1. In this area we observed the sur-face-trapped flow associated with ESCW. Area 2 is de-fined as the region deeper than 200 m between M1 andthe mid point between M2 and M3. In this area weobserved a bottom-intensified flow associated withDSW. We can estimate the transport at these areas fromFig. 17 by subtracting the transport at the upper-rightcorner of the area from that at the lower-right corner.The transport in area 1 was 0.36 3 106 m3 s21 and thatin area 2 was 0.76 3 106 m3 s21. The actual transportsof ESCW and DSW are smaller than these values be-cause these waters were distributed in limited seasonsand areas in these defined areas. It is concluded that thetransports of the surface-trapped and bottom-intensifiedflows over the shelf and slope do not contribute signif-icantly to the total southward transport, although theseflows are prominent features of the flow field.

4. Summary and discussion

We analyzed the flow field off the east coast of Sa-khalin using long-term current mooring measurements.In most of the mooring period there was a persistentsouthward flow, almost parallel to isobaths, over the



continental shelf and slope and in the offshore region(Fig. 4). The speed of this southward flow clearlychanged seasonally. The peak monthly mean speedalong 538N at a depth of 200 m reached a maximum of37 6 9 cm s21 in January and a minimum of 10 6 8cm s21 in July. This flow extended from the surface toa depth around 1000 m, reaching the bottom on the slopewhere bottom depth is not so large (Fig. 6).

In addition to this general southward flow, there wasa southward flow trapped near the surface over the shelf,especially from October to December (Figs. 5, 6, 7).The surface-trapped flow was observed commonly atthree mooring arrays along 49.58, 538, and the 558Nline. This surface-trapped flow was associated withESCW, which is less saline surface water affected bythe Amur River discharge (Fig. 8). The depth of themaximum shear of this flow coincided with that of thebottom of the mixed layer that contained ESCW in lateSeptember. Then the maximum-shear depth increasedwith time until December when it reached the bottomand disappeared (Fig. 9). According to the profilingfloats (unpublished data), mixed layer thickness off theeast coast of Sakhalin were similar to the maximum-shear depth observed by the ADCPs. It is suggested thatthe thickness of the surface-trapped flow is restricted tothe mixed layer. On the other hand, the horizontal dis-tribution of ESCW changes seasonally (Watanabe1963b). The seasonal variability of ESCW distributioncan be explained roughly by two factors (Fig. 10). Oneis the abrupt increase of Amur River discharge in Mayassociated with the melt of snow and frozen soil. Theother is the horizontal advection of ESCW by the south-ward flow. Since the advection length near the surfaceis larger than that near the bottom, it is shown that thevertical shear associated with ESCW also affects its owndistribution.

In the slope region there was a southward flow in-tensified toward the bottom around the shelf break (Fig.12). The vertical shear of this bottom-intensified flowwas the same order as the geostrophic shear associatedwith DSW, which is formed by brine rejection from seaice in the northwest shelf. At 538N the bottom-inten-sified flow was present from May to September at M2.The spatial and temporal distributions of this flow co-incided with those of DSW (Fig. 14). The bottom-in-tensified flow was also present along the 558N line.Along this line the maximum southward velocity ap-peared at a depth around 150 m in February and deep-ened with time by May (Fig. 13). Recently, Talley etal. (2001) carried out mooring measurements in thenorthwest shelf in the same period from September 1999to June 2000. They showed that the density of shelfwater begins to increase in late January just after theonset of sea ice formation. Thus the deepening of thevelocity maximum and the increase of shelf water den-sity started almost at the same time. The DSW is mod-ified by mixing with surrounding waters. It is shownthat the bottom-intensified flow weakens as DSW is

mixed with surrounding waters (Fig. 12). These featuresof the bottom-intensified flow strongly suggest that thisflow is driven by the atmospheric cooling that formsDSW. The modification of DSW is significant in a regionwithin a few hundred kilometers from the northern endof Sakhalin (Fig. 11), where a rather steep continentalslope is located close to the coast (Fig. 1).

We identified three different cores of intense south-ward flow off the east coast of Sakhalin. The first coreis centered over the continental slope. This flow hasrather large vertical extent, reaching the bottom on theslope. The second core is confined near the surface overthe shelf and associated with ESCW. The third core isintensified toward the bottom on the slope near the shelfbreak and associated with DSW. Not only the spatialdistribution but seasonal variability of these cores aredifferent from each other. From the surface-drifter mea-surements, Ohshima et al. (2002) showed that there aretwo cores of intense surface flow in regions with 50–100- and 300–900-m isobaths. These two cores likelycorrespond to the second and first ones identified by ourmooring measurements, respectively.

We estimated the total southward transport at 538Nfrom August 1998 to July 1999 (Figs. 15, 16). It seemsthat most of the southward transport is covered with ourmooring array, except for a small amount of the south-ward transport offshore of the array. The total transportis estimated to be 6.7 3 106 m3 s21 in the annual av-erage, with a maximum of 12.3 3 106 m3 s21 in Feb-ruary and a minimum of 1.2 3 106 m3 s21 in October.The surface drifter measurements suggest that thissouthward transport along the east coast of Sakhalin isalmost constant or slightly increases southward between54.98 and 498N and decreases at Cape Terpenia wherea part of the southward flow turns eastward (Ohshimaet al. 2002). As compared with the total southward trans-port, transports by the surface-trapped and bottom-in-tensified flows were small, although these are prominentfeatures of the flow field (Fig. 17). Thus formation pro-cesses of ESCW and DSW are not likely to contributeto the total transport of the ESC as compared with windand other forcings. Using a barotropic model, Simizuand Ohshima (2002) showed that the arrested topo-graphic wave (Csanady 1978) excited by the Ekmantransport converging to the coast could determine theflow field in the shelf region. However, the transport ofthis flow is about 1 3 106 m3 s21 in winter and smallcompared with the total transport of the ESC. On theother hand, the annual average of the Sverdrup transportin August 1998–July 1999 is 3.5 3 106 m3 s21 basedon integrating the wind stress curl from the eastern tothe western boundaries and reversing its sign. Windstress was obtained from the European Centre for Me-dium-Range Weather Forecasts wind data and a dragcoefficient by Large and Pond (1981). This annual-av-erage Sverdrup transport is about one-half of the ob-served transport. This discrepancy is rather significant.The discrepancy between the monthly average Sverdrup



transport and that of the observed one is more signifi-cant. While the observed transport is southward throughthe whole year, the Sverdrup transport becomes north-ward in spring and summer because of southerlies inthese seasons. In order to understand the mechanismthat controls the transport at 538N in the Okhotsk Seaand its seasonal variability, we will have to considereffects of temporal variability, bottom topography, non-linearity, and other factors.

In this study we have clarified the structure and sea-sonal variability of the ESC. However, the mechanismthat determines velocity, the horizontal and verticalscales, and transport of the ESC quantitatively has notyet been clarified. It is also an open question as to whatcontrols the damping of the bottom-intensified flow andthe modification of DSW in this region, and whetherthe controlling mechanism is related to bottom topog-raphy around the northern end of Sakhalin or the descentof DSW along the slope. To answer these questions,further theoretical work and numerical experiments arenecessary.

Acknowledgments. We are deeply indebted to Y. N.Volkov of Far Eastern Regional HydrometeorologicalInstitute and K. Takeuchi of Frontier Observational Re-search System for Global Change for making the cruisesin 1998–2000 possible. Thanks are extended to the cap-tain and crew of R/V Khromov and all scientists, tech-nicians, and students who participated in these cruisesfor their collaboration. The CTD data obtained in thesecruises are provided by L. D. Talley of Scripps Insti-tution of Oceanography and S. C. Riser of School ofOceanography, University of Washington. This work issponsored by the Core Research for Evolutional Scienceand Technology, Japan Science and Technology Cor-polation.

APPENDIX

Velocity Interpolation for the ADCP Data

We consider a discrete time series of velocity ui (i5 1, 2, 3, . . .), which is observed at shallow depths,and that of velocity , which is observed at a larger0ui

depth. The subscript i denotes the time. For simplicity,time intervals are assumed to be 1 hour (constant). Be-cause of missing returns, ui is unknown for several hoursa day; contains no bad data and is used as a reference0ui

velocity. We also introduce the deviation of ui from thereference velocity 5 u i 2 . Then let us consider0u* ui i

a low-frequency component of ui,48

u 5 W u , (A1)Oi k i1kk5248

where Wk (248 # k # 48) are weights for Thompson’s4-day filter (Thompson 1983). Since ui contains baddata, we cannot calculate i. Instead, we interpolateuvelocity data and apply a filter to eliminate tides. Letting

the interpolated velocity by vi, we consider its deviationfrom the reference velocity 5 vi 2 . If internal0v* ui i

waves are weak, we can expect that tidal and other high-frequency motions are significantly correlated in the ver-tical direction. In this case, these motions are signifi-cantly weakened when we take a deviation from thereference velocity. Accordingly, we can neglect the tidaland other high-frequency components in velocity de-viation to the first order of approximation. On theu*iother hand, the low-frequency component of canu*ireadily be interpolated with respect to time. Thus wedefine asv*i

u*, when u is observedi i24 w d u*O k i1k i1kv* 5 k5224i u* [ , when u is not observed.i i24 w dO k i1k k5224

(A2)

where di is a mask function:

1, when u is observedid 5 (A3)i 50, when u is not observed.i

We used the weights for Thompson’s 2-day filter wk forinterpolation because wk are positive definite and do notcause erroneous oscillation even when the number ofobserved data is small. Adding the reference velocity,we obtain the interpolated velocity vi. Then, applyinga tide-elimination filter, we obtain the low-frequencycomponent of velocity i.v

Errors of vi and i caused by this interpolation arevexpressed as

2 2˜[(v 2 u ) ] 5 [(u* 2 u*) ], (A4)i i i i

48 48

2[(v 2 u ) ] 5 W W (1 2 d )(1 2 d )O Oi i k l i1k i1lk5248 l5248 ˜ ˜3 [(u* 2 u* )(u* 2 u* )], (A5)i1k i1k i1l i1l

where a square bracket denotes the expected value. Thequantity 2 on the rhs of (A4) mostly represents˜u* u*i i

the tidal and other high-frequency components of de-viation velocity . These components vanish if theyu*iare averaged over a tidal period. Then (A4) is rewrittenas

2 2 0 0 2˜[(v 2 u ) ] 5 [(u 2 u ) ] 1 [(u 2 u ) ]i i i i i i

0 02 2[(u 2 u )(u 2 u )]i i i i

25 2U (1 2 r ), (A6)i

where U 2 is a typical value of the variances of ui 2and 2 , and ri is the correlation coefficient0 0u u ui i i

between them. Equation (A6) indicates that the inter-polation errors become small when the tidal and high-frequency motions are correlated in the vertical direc-



tion. In this study we directly estimated the rhs of (A4)by calculating and for known velocities.˜u* u*i i

Then we can estimate an approximate value of therhs of (A5) as follows. We use a running-mean filterover 25 h in place of a tide-elimination filter for sim-plicity. We also use a relation

2[(u* 2 u*)(u* 2 u*)] 5 [(u* 2 u*) ]r ,k k l l i i k,l (A7)

where rk,l is the correlation coefficient between 2u*kand 2 . And we assume that rk,l 5 1 for˜ ˜u* u* u*k l l

| k 2 1 | # 24 because tidal motion is correlated withintidal period. Note that the last assumption overestimateserrors. Then we have

12 1 2 di1k2 1/2 2 1/2˜[(v 2 u ) ] ; [(u* 2 u*) ] Oi i i i 25k5212

2 1/2˜5 f [(u* 2 u*) ] , (A8)i i i

where f i 5 (1 2 d i1k)/25 represents a fraction12Sk5212

of the period when u i is not properly observed within612 h.

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Structure and Seasonal Variability of the East Sakhalin