Bottom Currents in Nankai Trough and Sagami Trough*

Journal of the Oceanographical Society of Japan

Vol.41, pp.388 to 398, 1985

Bottom Currents in Nankai Trough and Sagami Trough*

Keisuke Taira•õ and Toshihiko Teramoto•õ

Abstract: Mean flows and velocity fluctuations are described from direct measurements ofbottom currents made at three stations across Nankai Trough and two stations in SagamiTrough from May 1982 to May 1984. Aanderaa current meters were moored 7m above thebottom. The observed mean flows indicate a counter-clockwise circulation in Nankai Troughwith current speeds of 0.9-2.1 cm sec-1. The mean flows were larger on the slopes than onthe flat bottom of the trough. The mean flows observed in Sagami Trough show an inflowinto Sagami Bay which is considered to be a part of the Oyashio undercurrent from the norththat flows along the eastern coast of Honshu. Velocity fluctuations with periods greater than100 hr were less energetic in the troughs than those at a station west of Hachijo-jima Island.A highly energetic fluctuation with a period of 66.7 hr was observed on the northern slopeof Sagami Trough in the velocity component parallel to the trough axis. A maximum currentspeed of 49 cm sec-1 was observed in Sagami Trough.

1. IntroductionWe made direct measurements of bottom cur-

rents from May 1982 to May 1984 by deployingmoorings at three stations in Nankai Trough,and at two stations in Sagami Trough. NankaiTrough is located south of Honshu, the mainIsland of Japan. The deep extends northwardand enters Suruga Bay. Sagami Trough stemsfrom the Izu-Ogasawara Trench which lies eastof Honshu. The deep extends northwestwardbetween the continental shelf off Boso Peninsulaand the Izu Ridge, and enters Sagami Bay.

Direct measurements of deep currents alongthe western boundary of the North AtlanticOcean revealed an equatorward flow with amaximum speed of several tens of cm sec-1

(Jenkins and Rhines, 1980; Weatherly, 1984).Deep western boundary currents have not beenobserved in the Pacific Ocean and in the Philip-

pine Sea (Warren, 1981). Nankai Trough islocated at the northern boundary of the PhilippineSea, and Sagami Trough at the northwesternflank of the deep North Pacific. Deep boundary

currents may be expected in these areas.The Oyashio water from the north is suggested

to be flowing into Sagami Bay on the basis ofwater ct aracteristics in the deeper lay-rs (Uda,

1937). Horikoshi (1957) showed that biologicalspecies originating from the Oyashio are foundin Sagami Bay. Marumo (1966) found that aspecies of plankton (a chaetsgnath Sagitta elegans)appears in Sagami Bay at depths greater than350m in winter. The plankton reproduces insummer in the surface layers of the Oyashio.He inferred that the plankton is carried by theOyashio undercurrent along the eastern coast ofHonshu and estimated the mean speed of theOyashio undercurrent to be 4.3cm sec-1 basedon the distance between Sagami Bay and thesurface Oyashio current. Omori (1967) showsthat a species of zooplankton (a copepod Calanuscristatus) from the Oyashio water is distributedin Sagami Bay at depths from 600m to 1,200m.Thus, a deep flow from the north is expectedto be observed at the stations in Sagami Trough.

The Kuroshio axis detected by GeomagneticElectro-Kinetograph (GEK) is reported to liefrequently over Nankai Trough and SagamiTrough. The velocity fields in the surface layersover the troughs are subject to the fluctuationsof the Kuroshio. Taira and Teramoto (1981)showed that the velocity fluctuations of low-frequency are energetic under the Kuroshio westof Hachijo-jima and in the Kuroshio west ofOshima. Dominant periods are suggested to be33 days and 100 days. Examination of energylevels at other stations is important for investi-

gating the nature of the velocity fluctuations.

*Received 18 February 1985;in revised form 18

July 1985;accepted 2 Augnst 1985.

†Ocean Research Institute, Univensity of Tokyo,

Nakano-ku,Tokyo 164,Japan.

Bottom Currents in Nankai Trough and Sagami Trough 389

The current measurements of the present studywere planned primarily to study the above-mentioned problems, and partly to collect currentdata for the KAIKO project (Japan-France Co-operative Project to study subduction), for which

dives of a manned submersible were made in1985. Knowledge of the magnitude of bottom.

currents in the troughs was required for evalu-ation of environmental conditions for the dives.

Determination of the maximum speed and mean

Fig. 1. Mooring stations of current meters in Nankai Trough (SR1, SR2 and SR3) andin Sagami Trough (SG1 and SG2). Bottom contours are shown for depths of 1,500m,2,000m and 3,500m. An arrow shows the mean flow at each station (see, Figs. 4and 5). Two arrows from SG1 are from Record 5 and Record 9.

Table 1. Data regarding current meter moorings in Nankai Trough and Sagami

Trough. Data quality of each record is noted in the right column.

390 Taira and Teramoto

flow is important for examining erosion of sea-

bed and processes of sedimentation, which wereinvestigated through observations and collection

of samples by the submersible.This study is the first long-term measurement

of bottom currents in Nankai Trough and SagamiTrough. Histograms of current speeds, mean

currents in the troughs, and velocity fluctuationsare examined using the current records.

2. Current measurements and data analysis

The current measurements were made in the

northern portion of Nankai Trough as far as

the Zenisu Ridge (SR1, SR2 and SR3, see Fig.

1 and Table 1). Station SR2 was set on a flat

bottom of the trough. The distance between

the 3,500m isobaths on either side of the trough

is about 13 km along 34•‹N. Station SR1 was

set on the northwestern slope of the trough,

and Station SR3 on the southeastern slope.

Current measurements were made on the north-

eastern slope of Sagami Trough at Stations SG1

and SG2 (Fig. 1). The distance between the

1,500m isobaths on either side of Sagami Trough

is about 8km at the northeast of Oshima Island.

We used an Aanderaa current meter (RCM-6)

linked to an acoustic release (L-type, Nichiyu-

Giken, Co.) with a short rope in order to set

the meter near the bottom. Each current meter

was set 7m above the bottom at the mooring

stations in the Nankai Trough and Sagami

Trough. An extra current meter was set 27m

above the bottom at Station SR2. Mooring

operations were made from the R/V Tansei

Maru (the cruises KT-82-4, KT-83-1, KT-83-8,

KT-84-4) and from the R/V Hakuho Maru

(the cruise KH-83-1). Data regarding the moor-

ings are tabulated in Table 1. Each mooring

was successfully recovered.

We obtained nine records of current meters

with a sampling time interval of one hour.

Quality of the current meter records was not

always good: Record 4 (SR3, see Table 1) had

spiky noise in the current speeds, indicating that

the rotor counter had not been cleared after

some recordings. Records 6 and 8 gave almost

meaningless data, indicating malfunction of the

electric circuits due to the temperature decrease

from the air to the sea.

Three kinds of data sets were prepared from

each of seven records: Nos. 1, 2, 3, 4, 5, 7 and

9. One is a time series of east and north

components of hourly velocity. The second is

daily velocity calculated by averaging the hourly

velocities over 24 hr from Oh to 23h on each

day. The third is hourly velocity in directional

components normal and tangential to the axes

of the troughs. We selected 320•‹T (true) as the

normal and 50•‹T as the tangential direction for

stations in Nankai Trough, while the normal

direction is 50•‹T and the tangential direction

140•‹T for the stations in Sagami Trough.

Histograms of current speeds were estimated

directly from the records of the current meters.

Power spectra were computed for the hourly

time series of velocity components in directions

normal and tangential to the trough axes.

3. Observational results3.1. Frequency histograms of current speedsFigure 2 shows frequency distributions of cur-

rent speeds observed 7 m above the bottom in

Fig. 2. Histograms of current speeds 7 m above the bottom in Nankai Trough

(SR1 and SR2). Cumulative distributions are shown with solid lines.


the Nankai Trough. The threshold speed forthe Aanderaa rotor is about 1.5 cm sec-1, andzeros of rotor revolution are converted to aspeed of 1.5 cm sec-1. Occurrences of velocitiesin the range from zero to 1.5 cm sec-1 werefew. The currents were strong enough to bemeasured accurately by the current meters. Themaximum current speed was 24 cm sec-1. Thedistributions indicate that larger values of velo-city appear with low probability. Figure 2 alsoshows cumulative distributions of current velo-city. More than 50 % of the total were smallerthan 7 cm sec-1.

Figure 3 shows frequency distributions of cur-rent velocity observed 7 m above the bottom inSagami Trough. The distribution for SG1 is

calculated from Records 5 and 9. The cumu-lative distribution for SG1 shows that the 50 %-

point lies at about 7 cm sec-1, and that themaximum velocity observed was about 29 cmsec-1. Cumulative distribution for SG2 showsthat the 50 %-point lies at about 10 cm sec-1,while the maximum was 50 cm sec-1. A strong且owwithaspeedexceeding40cmsec-1was

observed during two events described later.

3.2. Mean flows in the troughs A progressive vector diagram is one convenient

way of presenting direction and steadiness ofa mean flow for a highly fluctuating current.

Figure 4 shows progressive vector diagrams from

the current records obtained in Nankai Trough.Every fiftieth day is marked with a dot. A

Fig. 3. Histograms of current speeds 7 m above the bottom in Sagami Trough

(SG1 and SG2). Cumulative distributions are shown with solid lines.

A

B

C

D

Fig. 4. Progressive vector diagrams from the current records obtained in Nankai Trough.

Duration of the measurement and mean speed are noted in each diagram.


A

C

B

Fig. 5. Progressive vector diagrams from the current records obtained in Sagami Trough.

Duration of the measurement and mean speed are noted in each diagram.

S R2-3

T

N

T

N

Fig. 6. Velocity components tangential (50•‹ T, true) and normal (320•‹ T)

to the axis of Nankai Trough (Record 3, at Station SR2).


steady south-southeast flow was observed at SR1The average speed over 289 days was 2.1 crr.

sec-1. The mean flow at SR2 was eastwarcwith a speed of 0.9 cm sec-1 averaged over 374

days. The two diagrams for Record 2 ancRecord 3 are almost identical to each other. A

height difference of 20 m between the two cur.

rent meters gave little difference in estimationof the mean flow in the bottom layer. ThE

progressive vector diagram from Record 4 aiSR3 shows a north-northwest mean flow at an

average speed of 1.6 cm sec-1 over 349 days,Noise in the records was severe from the 100t1

day to 250th day. The average current over thefirst 100 days was north-northwest at 1.9cm

sec-1. The records for the last 100 days gave

almost the same mean flow.Figure 5 shows progressive vector diagrams

from the current records obtained in SagamiTrough. The mean flow was west-southwest

from Record 5 at SG1, and averaged 1.9 cm sec-1

over 286 days. The mean flow from Record 9 was almost westward and averaged 1.5 cm sec'

over 343 days. The station for Record 9 was

1.5 km east of the station for Record 5 (seeTable 1). Local topography is considered to beresponsible for the differences in the mean flows.The mean flow at SG2 was west-southwest atan average speed of 4.4 cm sec' over 103 days.

The mean flows observed in Nankai Troughand Sagami Trough are shown with arrows inFig. 1. A counter-clockwise circulation is clearlyindicated in Nankai Trough. Speeds of themean flows are stronger on the slopes than onthe bottom of the trough. Directions of meanflow in Sagami Trough were from west to west-

southwest. An inflow to Sagami Bay was ob-served.

3.3. Time variation of current velocityFigure 6 shows hourly velocity for the first

104 days observed 7 m above the bottom atSR2 (Record 3, see Table 1). We plotted velo-city components tangential (50°T, true) andnormal (320°T to the axis of the Nankai Trough.Tidal currents were the most prominent.

Figure 7 shows hourly velocity at SG2 (Record7) in the directions tangential (140°T) andnormal (50°T) to the axis of Sagami Trough.

SG2-7

T

N

T

N

Fig. 7. Velocity components tangential (140•‹ T) and normal (50•‹ T) to the axis of

Sagami Trough (Record 7, at Station SG2). The crosses on the time scale

indicate the days of the strong fluctuation on 19 March,, and 18 May 1983.


Velocity fluctuations of the tangential component

were different from those of the normal com-

ponent. Tidal currents were prominent in the

latter. The tangential component of velocity

shows a 2-3 day period fluctuation with a large

amplitude as well as the tidal currents. Current

speeds exceeding 40 cm sec-1 were observed during

two events, one on the 23rd day (19 March

1983) and the other on the 83rd day (18 May

1983). A strong flow of 49.0 cm sec-1 to 135°T

was recorded at 11 : 00 on 19 March. The cur-

rent speed exceeded 40 cm sec-1 for three hours.

In the other event on 18 May, the current

speed exceeded 40 cm sec-1 for three hours with

a peak of 43.6 cm sec-1 to 129°T at 15: 00.

Figure 8 shows hourly velocity at SG1 (Record

9) in the directions tangential (140•‹T) and normal

(50•‹T) to the axis of Sagami Trough. Tidal

currents were prominent in both components.

The two to three day period fluctuation was

not recorded.

3.4. Spectra of velocity fluctuations

We computed spectral densities from hourly

velocities in the directions tangential and normal

to the axes of the troughs by taking a lag

numoer or ivy. opectrai aensities were integratea

in four period-bands in order to examine the

distribution of kinematic energy (Table 2).

Observational results by Taira and Teramoto

(1981) are also included in the table for com-

parison. The record from Hachijo-jima-West

was obtained at station HW (33°02'N, 139°00'E;

water depth 1,795 m and meter depth 1,670 m),

and that of Oshima-West at station OW (34°44'N,

139•‹14'E; water depth 484 m and meter depth

250 m).

The variance in Band I shows energy with

long-period fluctuations and it is smaller than

5.4 cm2 sec-2 in both troughs except for the

tangential component of Record 7 at SG2 in

Sagami Trough. The variances observed at

Hachijo-jima-West and Oshima-West stations are

10-20 times larger than those in the troughs.

The variance in Band II shows energy of fluctu-

ations with periods from 30 h to 100 h. The

variances are large for the tangential component

of Records 5 and 7 in Sagami Trough. The

variances are small for the remaining stations.

The variance in Band III shows the energy of

tidal currents, and 42-82 % of the total variance

SG1-9

T

N

T

N

Fig. 8. Velocity components tangential (140•‹ T) and normal (50•‹ T)

to the axis of Sagami Trough (Record 9, at Station SG1).


Table 2. Mean, variance and distribution of variance in four period-bands. Normal and

tangential directions for Nankai Trough are 320•‹ (true) and 50•‹, respectively. Those for

Sagami Trough are 50•‹ and 140•‹, respectively. Position, water depth and current meter

depth for each record are shown in Table 1. The records of HW (33•‹02'N, 139•‹00'E;

water depth 1,795 m and meter depth 1,670 m) and OW (34•‹44'N, 139°14'E; water depth

484 m and meter depth 250 m) are reported in Taira and Teramoto (1981). Directions

normal and tangential to the isobath are approximately east and north for both stations.

The Band I of their analysis is for periods greater than ten days.

Fig. 9. Power spectra of the velocity components tangential (140•‹ T) and normal

(50•‹ T) to the axis of Sagami Trough at Station SG1 (Records 5 and 9), and at

SG2 (Record 7). A bar shows the 95% confidence limit for the estimates.


is included in this band. The variance in Band

IV shows energy of short-period fluctuations.

The variances in both directions are almost of

the same magnitude. The fluctuations in the

band are almost isotropic.

Figure 9 shows power spectra computed from

current records obtained in Sagami Trough

(Records 5, 7 and 9). Large spectral peaks at

0.08 cph (cycle per hour) common to the spectra

are due to the semidiurnal tidal currents. Diurnal

tidal currents have spectral peaks at 0.04 cph,

and they are larger for the tangential component

than for the normal component. The largest

spectral peak is located at 0.015 cph (66.7 hr

period) for the spectrum of he tangential com-

ponent of Record 7. The spectral densities at

this frequency are high for the spectrum of the

tangential component of Record 5. No spectral

peaks are found at frequencies lower than 0.015

cph in these spectra. The large variance in

Band I for the tangential component of Record

7 is considered to be caused by leakage from

the spectral peak at 0.015 cph.

4. Discussion

A counter-clockwise circulation is indicated by

the mean flows observed in Nankai Trough.

Ishizaki et al. (1983) measured the current 105 m

above the bottom at a station (33°45.9'N, 137•‹

35.4'E; 2,200 m deep) on the northwestern slope

of Nankai Trough. The station was located

99 km southwest of Station SR1. They observed

a southwestward (240°T) mean flow with a

speed of 2.3 cm sec-' averaged over 132 days.

The mean flow is along the isobath. Nishida

and Kuramoto (1982) measured the current at

2,450 m at a station (33'00'N, 138°10'E; 3,980 m

deep) on the eastern edge of the flat bottom of

Nankai Trough. The station was located 130 km

southwest of Station SR2. They observed a

northeastward (41°T) mean flow with a speed

of 3.0 cm sec' averaged over 186 days. The

mean flow is along the axis of the trough.

Taira and Teramoto (1981) observed a northward

(358•‹T) mean flow of 2.4 cm sec' at 1,670 m

at Station HW which was located 134 km south-

southeast of Station SR3 (see Table 2). These

results indicate a counter-clockwise circulation

in the deep layers of the Philippine Sea: a north-

ward flow along the western flank of the Izu

Ridge, a turning flow in the northern part of

Nankai Trough, and a southwestward flow alongthe continental slope of Honshu.

Deep flows in Sagami Trough were shown tooriginate from the north. If we consider thatthese flows are part of a deep western boundarycurrent in the North Pacific, the magnitude ofthe current is smaller by one order of magnitudethan that observed in the North Atlantic. Theupper layers over the trough are influenced bythe Kuroshio. The Prompt Reports of Sea Con-ditions (Hydrographic Department, MaritimeSafety Agency, 1982 and 1983) show that east-ward or northeastward currents prevail at thesurface over Sagami Trough with a speed 1-2knots (0.5-1.0 m sec-'). The southward or south-westward mean flows in the deeper layers mustbe limited vertically by the Kuroshio. Theobserved deep flows may be considered to bethe Oyashio undercurrent. The speed of themean flow observed at SG2 is 4.4 cm sec-1, whichis in good accordance with the mean speed of4.3 cm sec-1 for the Oyashio undercurrent esti-mated by Marumo (1966).

Taira and Teramoto (1981) show that a 33-day

period fluctuation of current velocity is dominantunder the Kuroshio west of Hachijo-jima. ThePrompt Reports of Sea Conditions show that a

part of the Kuroshio axis lies just over themooring station SR2 three times during thecurrent measurements: mid-May 1982, late-August 1982 and late-February 1983. Thevariances in Band I were small for Records 2and 3 at SR2. They were about 10 % as largeas that at the Hachijo-jima-West station (seeTable 2).

The velocity fluctuation of 66.7 hr-period wasfirst observed at SG2 in Sagami Trough. The

fl uctuation was entirely limited to the velocity

component tangential to the axis of the trough.This fluctuation was also recorded in Record 5at SG1, but was not recorded in Record 9 atthe same station (see Fig. 9). The station forRecord 5 was 1.5 km west of that for Record 9.We considered that the fluctuation occurred ina rather limited area and that it was veryenergetic especially on the northeastern slope ofSagami Trough.

An extremely strong flow was observed atstation SG2 during two events. The eventsaccompanied the fluctuation of 66.7 hr period.Strong currents first flowed out of Sagami Bay.


A return flow of 36.9 cm sec-1 to 298•‹T was

recorded at 6: 00 on 20 March 1983, 19 hr after

the peak of 49.0 cm sec-1 to 145•‹T at 11:00 on

19 March. The return flow lasted from 3:00

to 6:00 with a speed exceeding 35 cm sec' in

the direction 298°T-326°T. A temperature in-

crease from 2.43•Ž to 2.62•Ž was observed during

the event, and the temperature decreased after

the return flow. The return flow in the second

event was observed to be 30.3 cm sec' to 307•‹T

at 9:00 on 19 May 1983, 18 hr after the outflow

of 43.6 cm sec-1 to 129°T at 15:00 on May 18.

Temperature increased by 0.17•Ž from 2.24•Ž

during the event. Details of the events and

their relation to wind speed at Oshima Island

will be described elsewhere. Flows with speeds

of 50 cm sec' must be playing an important role

in processes of sedimentation and erosion of the

sea-bed (e.g., Richardson, Wimbush and Mayer,

1981).

Tidal currents observed in Nankai Trough are

described by Hamatani (1984), and the measure-

ments of bottom pressure in both troughs are

reported by Taira et al.(1985).

5. Summary

The results of this study can be summarized

in four main conclusions.(1) The mean flows

in the bottom layers showed a counter-clockwise

circulation in Nankai Trough. Magnitudes of

the mean flows in both troughs were 0.9-2.1 cm

seci, and they were larger on the slopes than

on the flat bottoms of the troughs. The circu-

lation in Nankai Trough was suggested to be

connected to a large counter-clockwise circulation

in the deep layers of the Philippine Sea.(2) A

steady flow into Sagami Bay was observed in

Sagami Trough. The flow was suggested to be

a part of the Oyashio undercurrent.(3) Velocity

fluctuations with periods greater than 100 hr

were less energetic in the troughs than those atHachijo-jima-West station.(4) An energeticvelocity fluctuation with a period of 66.7 hr wasobserved on the northeastern slope of SagamiTrough. The fluctuation was entirely limitedto the velocity component parallel to the axis ofSagami Trough.

AcknowledgementsWe wish to thank to the captains and crews

of the R/V Tansei Maru, and the R/V Hakuho

Maru, and the members of the Division ofPhysical Oceanography, Ocean Research Institute,

University of Tokyo, for their help. We wishto express our thanks to Profs. Y. Tomoda, K.

Kobayashi and N. Taga for their cc-operation.The first draft of this paper was improved by

Dr. S. Nagasawa.

This study was sponsored by the Ministry ofEducation, Science and Culture, Japan.

References

Hamatani, M.(1984): Tidal currents in the seassouth and east to Honshu. Master Thesis,University of Tokyo, 60 pp.(in Japanese).

Horikoshi, M.(1957): Note on the molluscan faunaof Sagami Bay and its adjacent waters. ScienceReports of the Yokohama National University,Sec. II, No. 6, 37-64.

Ishizaki, H., O. Asaoka, S. Konaga and M. Taka-hashi (1983): A direct measurement of near-bottom current on the continental slope offOmaezaki, central Japan. Papers in Meteor.and Geophys., 33, 257-268.

Jenkins, W. J. and P.B. Rhines (1980): Tritium inthe deep North Atlantic Ocean. Nature, 286,877-880.

Marumo, M.(1966): Sagitta elegans in the Oyashioundercurrent. J. Oceanogr. Soc. Japan, 22, 129-137.

Nishida, H. and S. Kuramoto (1982): Deep currentof the Kuroshio around the Izu Ridge—Largemeander of the Kuroshio in 1975-1980 (IV)-,Report of Hydrographic Researches, No. 17, 241-255.

Omori, M.(1967): Colanus cristatus and submergenceof the Oyashio water. Deep-Sea Res., 14, 525-532.

Richardson, M. J., M. Wimbush and L. Mayer (1981):Exceptionally strong near-bottom flows on thecontinental rise of Nova Scotia. Science, 213,887-888.

Taira, K. and T. Teramoto (1981): Velocity fluctu-ations of the Kuroshio near the Izu Ridge andtheir relationship to current path. Deep-Sea Res.,28, 1187-1197.

Taira, K., T. Teramoto and S. Kitagawa (1985):Measurements of ocean bottom pressure with a

quartz sensor. J. Oceanogr. Soc. Japan, 41, 181- 192.

Uda, M.(1937): Results of hydrographical investi-

gations in the Sagami Bay in connection withthe "Buri"(Seriola quinqueradiata T. & S.)fishing. J. Imperial Fish. Exp. Sta., 8, 1-50.

Warren, B.A.(1981): Deep circulation of the world,ocean. p. 6-41. In : Evolution of Physical Oceano-


graphy, ed. by B.A. Warren and C. Wunsch,MIT Press, Cambridge, Massachusetts.

Weatherly, G. L.(1984): An estimate of bottom

frictional dissipation by Gulf Stream fluctuations.

J. Mar. Res., 42, 289-301.

南海トラフと相模トラフの底層流

平啓介*,寺本俊彦*

要旨:南海トラフを横断する3点と相模トラフの2点に

おいて,1982年5月から1984年5月まで底層流の直接測

定を行ない,平均流と変動流の特性を調べた.アーンデ

ラ流速計を海底上7rnに係留した.南海トラフの平均流

は,流速0.9-2.1cm sec-1の反時計まわりの循環がある

ことを示した.トラフ斜面上の方が,平坦なトラフ底部

より流速が大きかった.相模トラフでは相模湾に流入す

る平均流が観測され,本州東岸にそって南下する親潮潜

流の一部と考えられた.周期100時間以上の変動流の分

散は,いずれのトラフにおいても八丈島西方の測点に比

べて小さかった.相模トラフの北斜面上で,周期66.7

時間の,トラフ軸に平行に流動する強勢な流速変動が観

測された.相模トラフで最大流速49cmsec-1が観測さ

れた.

*東京大学海洋研究所

〒164東京都中野区南台1-15-1

Bottom Currents in Nankai Trough and Sagami Trough*

Documents

Transcript of Bottom Currents in Nankai Trough and Sagami Trough*