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Exchange of Water Masses between the East China Sea and the Kuroshio Off Northeastern Taiwan

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Pergamon

Continental Shelf Research, Vol. 15, No. 1, pp. 19-39, 1995 Copyright (~) 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0278-4343/95 $7.00 + 0.00

0278--4343(93)E0001--O

E x c h a n g e o f w a t e r m a s s e s b e t w e e n t h e E a s t C h i n a S e a a n d t h e

K u r o s h i o o f f n o r t h e a s t e r n T a i w a n

C. T. A.

CHEN,*

R. Ruo,* S. C.

P A I , t

C. T. LIVt and G. T. F. WON6$

(Received 5 April 1993; accepted 27 June 1993)

Abstract--At least six water masses take part in the mixing processes between the East China Sea and the Kuroshio off northeastern Taiwan: the Kuroshio Surface Water (SW), Kuroshio Tropical Water (TW), Kuroshio Intermediate Water (IW), East China Sea Water (ECSW), Coastal Water (CW) and the Taiwan Strait Water (TSW). SW is depleted in nutrients and normalized alkalinity but has the highest temperature and pH of all these waters. TW has relatively high temperature, and the highest salinity of all waters. The salinity maximum in the Kuroshio is usually between 100 and 300 m deep, with large interannual and seasonal variability. IW is characterized by a salinity minimum, high nutrient content and alkalinity, but low pH and oxygen. ECSW is low in salinity, temperature and nutrients, but high in oxygen and normalized calcium and alkalinity. CW has low salinity and nutrient content but is high in normalized alkalinity. TSW is generally depleted in nutrients. The characteristics of the above mentioned waters are discussed. The mixing percent- ages of SW, TW, IW, and the composite Shelf Surface Water (composed of ECSW, CW and TSW)

off

the northeast corner of Taiwan in September 1988 and December 1989 are calculated.

I N T R O D U C T I O N

Ia ~ has been known for 50 years, or longer, that blocking of the Kuroshio (Black Stream) by

the East China Sea continental shelf break off the northeast corner of Taiwan is

responsible for the generation of a shelf-slope circulation pattern that triggers a major

exchange between the East China Sea and the western North Pacific Ocean. This

temporally and spatially variable exchange engages waters of deep ocean origin and of

strong fluvial influence. The interaction of these two water types is similar to that found

inshore of the Gulf Stream off the southeast coast of the United States

(BLANTON

e t

al.,

1981; ATKINSON, 1985). The circulation that promotes and sustains this exchange consists

of a complicated system of Kuroshio branch currents, meanders, upwelling and frontal

eddies (UDA, 1941; LIO et al., 1992).

The Kuroshio water is characterized by high salinity, and is relatively easy to identify in

the East China Sea. It is, however, difficult to quantify the mixing percentages of water

masses near the East China Sea shelf break. Early attempts to study the mixing processes

involving four or more water masses were mostly qualitative

(MILLER,

1950; YANG, 1984;

* Institute of Marine Geology, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China. tlnstitute of Oceanography, National Taiwan University, Taipei, Taiwan, Republic of China.

:~Department of Oceanography, Old Dominion University, Norfolk, Virginia, U.S.A. 19

(2)

20

C.T.A. CHEN

et al. N 27

26 °

25 °

24 ° .

2 3 ° I l I I I I I I 2 0 ° 1 2 1 ° 1 2 2 ° 1 2 3 ° 1 2 4 ° 1 2 5 ° E

Fig. 1. Station locations. Depth contours are in m.

LI and Lu, 1987). We report in this paper two sets of data collected off northeastern

Taiwan, one collected in September 1988 and one in December 1989. We also make an

attempt to quantify the mixing percentages.

EXPERIMENTAL

Fourteen stations were occupied by R.V.

O c e a n R e s e a r c h e r I

on 11-14 September 1988

(cruise 179) along a transect normal to the 200-m isobath at the shelf break of the East

China Sea (Fig. 1). Eight of these stations were reoccupied during 16-18 December 1989

(cruise 237). Stations 1-7 were located on the shelf in the East China Sea with depths of

approximately 100 m. Stations 8-11 were on the continental slope. Stations 12-14 were

located in the Okinawa Trough. The water depth at the deepest stations exceeded 1400 m

(Ruo, 1989; CrIEN

et al.,

1991).

The profiles of temperature, salinity and total fluorescence were measured by a Seabird

SBE 9/11 CTD. The CTD was pumped and the sampling rate was 24 Hz. The sensors were

calibrated just before the 179 cruise by the Northwest Regional Calibration Center.

Discrete samples were obtained for the determination of dissolved oxygen, phosphate,

nitrate, silicate and pH onboard ship. Oxygen was measured by the Winkler titrimetric

method

(CARPENTER,

1965) with a precision of about __ 0.5%. Phosphate was determined

by the molybdenum blue method (MURPHY and RILEY, 1962). Silicate was measured by the

silicomolybdenum blue method (FANNING and PILSON, 1973). Nitrate was determined by

reducing nitrate to nitrite and then determining the nitrite formed by the azo dye method

(SxRICKLAND and PARSONS, 1972) with a flow injection analyzer, pH was measured on the

NBS scale with a glass electrode/calomel electrode pair at 25°C (CHEN, 1984; RUO, 1989;

(3)

Water mass exchange in the East China Sea

21

WorqG

et al.,

1989a,b). Alkalinity was measured by Gran Titration (BRADSnOW

et al.,

1981).

Water m a s s e s in the study area

At least six water masses take part in the mixing processes (CHEN, 1988a); the Kuroshio

Surface Water (SW), Kuroshio Tropical Water (TW), Kuroshio Intermediate Water

(IW), East China Sea Water (ECSW), Coastal Water (CW), and the Taiwan Strait Water

(TSW). SW is depleted in nutrient and normalized alkalinity (NTA = alkalinity × 35/S)

but has the highest temperature and pH of all these waters (Fig. 2). The salinity is also

high, albeit somewhat lower than TW mainly because of precipitation and mixing with

fresher shelf and coastal waters (Fig. 3). A prism of fresher surface water extending

offshelf is underlain by the shelfward intrusion of deeper and more saline offshore waters

(Fig. 3). The layer immediately beneath SW is TW which has relatively high temperature,

and the highest salinity of all waters. The salinity maximum in the Kuroshio is usually

between 100 and 300 m deep, shallower in winter and deeper in summer with a large

interannual variability. TW is clearly recognizable as a salinity maximum near the shelf

break. The temperature, salinity and density stratifications are stronger in summer than in

winter

(CHEN,

1988a; Figs 2--4).

Topographically induced upwelling at the shelf break is evident as indicated by the

shelfward- and upward-tilting isotherms, isohalines, and isopycnals which extend over the

shelf break (Figs 2-4). At the shelfward edge of the upwelling water, the isotherms,

isohalines and isopycnals formed a bulge. The frequently detected bulge has a dimension

of a few tens of kilometers. It may be caused by strong upwelling or else may represent

eddies, filaments, or meandering of the Kuroshio (WONG

et al.,

1991).

The above mentioned bulge of subsurface upwelling water is also evident in the

apparent oxygen utilization (AOU, calculated based on the equation of CHEN, 1981),

nitrate, phosphate, silicate and pH cross-sections (Figs 5-9). In the surface waters, the

A O U is near zero, the nutrients are all low but the pH is high. The upwelling water has

high A O U and nutrients but has low pH. Subsurface waters on the shelf show much

variability, but in general, the vertical gradients are larger on the shelf than offshore.

IW is located between 400 and 600 m beneath TW, and is characterized by a salinity

minimum (Fig. 3), high nutrient content and alkalinity, but low pH and oxygen (Figs 5-9).

The minimum salinity core does not upwell onto the shelf (Fig. 3). Only isopleths saltier

than 34.45, warmer than 14°C, contain more than 3.4 ml 1-1 02, and shallower than 450 m

in the Kuroshio (saltier than 34.50, warmer than 15°C, contain more than 3.6 ml 1-1 02,

and shallower than 350 m in summer) seem to extend onto the shelf (CrIErq, 1988a). Four of

the above mentioned major water types can be seen on the

T - S

diagrams for the December

cruise (Fig. 10).

ECSW is low in salinity, temperature and nutrients, but high in oxygen and NTA.

Coastal water has a wide temperature and salinity range. Since we are not studying the

mixing of river waters and seawater, we choose a salinity of 28 to represent CW. CW has

low salinity and nutrient contents but is high in NTA.

Taiwan Strait Water moves mainly northward year round, except that some surface

water may move southward in winter (we exclude from this study the coastal water flowing

southward along the Fukien coast). TSW is generally depleted in nutrients. The character-

istics of the above mentioned waters are listed in Table 1 (WANG, 1988; CHEN, 1988a).

(4)

22

c . T . A . CttEN et al.

(a)

100 200 3 0 0 O_ © 4 0 0 500 600 700 s t o t [ o n 1 2 3 4 5 6 7 8 9 lO 11 12 13 14 I I I I [ I I I I I I r I I ~ 2 . ~ I - 2 " ~ / / / / / 7 - / / 2 \ ~ ~

~ _ ~ _ _ ~

/ / 2 " ~ 15 ~ 2 ~ - - 2 ( b ) s t a t i o n 100 2OO 300 (12_ © q:D 4 0 0 5 0 0 600 700 2 3 5 7 9 10 12 15 ' ' 7 ' ' m - - - - k . ' ' 12 / ~ 1~ ~ o / ~ 1 o ~

T,C

/

/ / 9 ~ / / /

(5)

Water mass exchange in the East China Sea

23

(a) 100 2OO

E 300

CL

© 4 0 0 500 -i 600 700

stotion

1 2 3 4 5 6 7 8 9 1 0

S

11 12 13 14 SW

34.5

"~-~

34.6 ~

I

~ ~ TW "34.7

346

~ . 545 ~

... 2

~ ~

'"""

( b )

s t o t i o n

100 200

E

i z : 3 0 0 c1_ © 7:3 4 0 0 500 600 700 2 3 5 7 9 10 12 13

J34.5 I ~

I SW

I'-L"~ 34.4 J

1 7 - J -

~ 3 4 . 5 ~ 3 4 . 5 ~ , / ~ ~ .55 - / A 34.4

Fig. 3. Cross-section of salinity in (a) September 1988 and in (b) December 1989. The broken lines show the salinity extremes. The major water types SSW, SW, TW and IW are marked.

(6)

24

C . T . A . Cn~N et al. (a)

stotion

I 2 3 4 5 6 7 8 9 10 11 12 13 14 0 I I I I I I / I L I I ~ i I I I 1 0 0 2 0 0

E

3OO _C CL © 7[3 4 0 0 5 0 0 600 700 25 2 6 2 6 . 5 ~

c; T

/ 2 7 ~ / .,~ (b)

s t o t i o n

1 O0 2OO X2 3O0 _+_, o_ © -~ 400 500 600 700 2 ,3 5 7 9 10 12 t.3 I I I I I I 1 1

_-..

(3- T

"~~26.s~

~ " - ~ - 2 7 ~

(7)

Water mass exchange in the East China Sea

25

(a)

100

2OO

E

500 C- CL © 400

500

600

700

station

1 2 3 4 5 6 7 8 9 10 11 12 15 14

,~ ~ ,~ J i ,~ J I ,l ,l i J r l

/

"---.~-~--

O~ 5 ~

• ~ ~

5_L_.~'/ • • u~

_____

: - - - ~ " 1 L ~ . ~ - 1 " ~ ~ _ . - 2 ~ / ~ ' - 2 . 5 ~ / 3

A.O.U. m~ ~-1 ~ / ~ "

__

" ~

insu(, dote

(b)

0

100

20O

zz .30o

CL

@

2:3 400

500

600

700

station

2 3

5

7

9 10

12 13

J

J

1

!

1 L

1 i - -

~

"

<

/1,.0 ~..: X,</.-,. 0..50

"

, ~

~

Fig. 5. Cross-section of apparent oxygen utilization in (a) September 1988 and in (b) D e c e m b e r 1989.

(8)

26

C . T . A . CHEr~ et al. (a) 100 200 500 O_ © 4 0 0 ~D 5 0 0 600

s t a t i o n

700 1 2 5 4 5 6 7 8 9 10 11 12 1,5 1 0 I I t I I I I I _ ~ - - ± 2 k 1 . 1 : = 9 . " ~ 7 : ~ - \ ' * - - - c ~ z . 7 - " - 7 ~ . ~ - -

i

/ / / ~ ~

1 7 ~ /./R ~ 1 9 ~

//-/~ " ~ -

2 1 ~

nitrate,/4mol kg q

~ ' - - - " - ~

27.

- / / ~ - 51 ~ (b) 100 20O £_ 500 o_ © 4 0 0 500 600 700

station

2 5 5 7 9 10 12 1.5

" / / ~ / / / Y / / k

9 ~

\ . ~ = .

1 ~ 1 9 _ ' - - - - n i t r a t e , / 2 , m o l k g - d / , ~ • ~

21.~-.~..

/

2 9 ~

(9)

Water mass exchange in the East China Sea

27

(a)

stGtion

3 0 0 CL (b

7:3 400

1 2 5 4 5 6 7 8 9 10 11 12 15 14

0

l

I

~

I

~ I

,J d

1 I

1

.

I

°

I

l

:=-~-__2~ :o. <

~

:

:

2OO

5OO

6o0 p h o s p h G t e , / ~ m o F k g ~

{b)

0 - -

stGtTon

2 5 5 7 9 10 12 13 J I I r I I I I - - 1 0 0

2OO

x - 5 o o o_ q)

o

400

500

6 0 0 7 0 0

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:

0 3 ~

~

~ . " 05

" ~ "

/ ~ 1 . 1 / .

phosphute,/~mol

k g - 1 / / ~ - 1 . 5 ~

/

x

(10)

28

C . T . A . Cnl~r~ et al.

(a)

station

I 2 .3 4 5 6 7 8 9 10 11 12 13 14 0 I I I I I l I I I I I I I L 100 200

E

3OO _C CL © 4 0 0 5 0 0 6 0 0 7 0 0

(b)

100 200

E

300 O_ @ q3 4 0 0 500 600 700

s t a t i o n

2 3 5 7 9 10 12 15 , ,

. ~ . ~ , ~

, '

, , " / ' k \ 8 ~

/ A

.f,-~<.ox'--....__&

-<&..

s i l i c a t e , / 4 m o l k9 -1 "~.--- so. ~ ' ' -

. ~ / ;, ~ 4 o . ~ . . ~ . _

~ ' 5

" - " " ~ - L

(11)

Water mass exchange in the East China Sea

29

(a) 1 O 0 200 ZOO ~E (D_ (1) 4 0 0 ~D 500 600 700

station

1 2 3 4 5 6 7 8 9 10 11 12 1.3 1~- , - ~ - - - ~ - 8 . 3 - - - ~ S . ~ , . - . ~ , 8.2~3 ~ " "

pH,25°C

~ " ~ - - 7.9 " < " ( b )

station

100 200

E

mz 3 0 0 £)_ © 400 500 600 700 2 3 5 7 9 10 12 15 8 1 8 ~ 1 2 i~ , i $ i ~ i / o /

pH 25 C

/

" - - - ~ 7 . g - - - - / / / / / / / " / ~ 7 , 7 - - - ~ - /

(12)

30 C . T . A . CHEN et al. o Fig. 10. 30 A,SSW 2O 15 10 5 3 3 . 0 PP4': .." ' " " " ~ ' ~ L .. ... PP1 2 \

\

\

B,IW Xse 33.5 34.0 34.5 35.0 ~X C'Tw C1 ?e

S

Diagram representing mixing of four water masses. The CTD data for Stas 2, 4, 6, 8, 10 12 and 14 of the September cruise are also plotted.

Table 1. Typical salinity, temperature, oxygen, p H, normalized alkalinity, nitrate, phosphate and silicate for SW, TW, IW, ECSW, CW and TSW

0 2 N T A N O 3 P O 4 SiO 2

S T, °C (mll -1) pH (mmol kg -1) ~mol kg -1) (~mol kg -1) (~mol kg -1)

SW 34.30-34.70 24.5-29.0 4.20-4.80 8.00-8.30 2.30-2.32 0.2-2.7 0-1.2 2-8 TW 34.85-34.90 20.0-23.4 4.00-4.34 8.10-8.20 2.30-2.32 0.6-2.5 0.5-1.1 2-14 IW 34.25-34.30 6.6-8.0 2.10-2.80 7.50-7.80 2.36-2.41 20-30 1.6-2.3 40-80 ECSW 30.50-33.50 8.6-20.0 3.00-4.80 8.10-8.30 2.37-2.57 0-10 ff4).6 15--40 CW 28 13.0-25.0 5.20--6.80 8.10-8.50 2.37-2.57 0-20 0~.5 15-40 TSW 33.70-34.60 19.0-28.0 4.00-5.20 8.00-8.30 2.28-2.32 0-2 0-1.5 0-15

T h e traditional T - S m e t h o d is useful f o r describing surface a n d shelf waters (MAo et al., 1964; CHEN, 1985 ; CHEN et al., 1985), b u t the u n c e r t a i n t y is large b e c a u s e neither T h o r S is strictly conservative. O x y g e n , % o x y g e n s a t u r a t i o n or A O U are subject to the s a m e limitations b e c a u s e of a i r - s e a e x c h a n g e , biological activities a n d t e m p e r a t u r e variations (which affect p e r c e n t a g e o x y g e n s a t u r a t i o n a n d A O U ) . T h e s e p r o p e r t i e s are less useful in s u m m e r b u t are m o r e useful in w i n t e r w h e n mixing b e c o m e s relatively m o r e i m p o r t a n t relative to biological activities. N u t r i e n t s , total c a r b o n dioxide (TCO2) o r p H are subject to similar p r o b l e m s as o x y g e n (CHEN, 1985, 1988b, 1993; CHEN et al., 1985).

(13)

Water mass exchange in the East China Sea

31

Calcium and alkalinity are more conservative on a shelf, especially when normalized to

the same salinity (Ct-IEN, 1985; WAN6, 1988). Chinese coastal waters also have higher

normalized calcium (NCa) and normalized alkalinity (NTA) than Kuroshio waters

because river waters generally have high NCa and NTA values (CHEN

et al.,

1985; SuJ,

1986).

Typical salinity, temperature, oxygen, pH, normalized alkalinity, nitrate, phosphate

and silicate for SW, TW, IW, ECSW, CW and TSW are given in Table 1 based on data

from the following reports: UDA, 1941; CHU, 1963; CSK, 1966, 1968, 1969, 1970;

NITANI,

1972; HUANG

et al.,

1983; LIMEBURNER

et al.,

1983; WEN6 and WANG, 1985; SuI, 1986;

WANC, 1988; Rt;o, 1989; and WONG

et al.,

1989a,b, 1991.

Method for calculating the mixing percentages for a four component mixing

We have made an initial attempt to quantify the mixing percentages of four major water

masses based on data collected on R.V.

Ocean Researcher I

(Ruo, 1989; WON6

et al.,

1989a,b, 1991; CrlEN

et al.,

1991). Because there are insufficient data, ECSW, CW and

TSW have been grouped together as the Shelf Surface Water (SSW; S = 33.1, T = 28°C,

NO3 = 0.2ttmol kg -1, PO4 = 0ttmol kg -1 and pH = 8.3 in September 1988; S = 33.8, T =

19°C, NO 3 = 2ttmol kg -1, PO4 = 0ttmol kg -1 and pH = 8.2 in December 1989). The four

major water types, SSW, SW, TW and IW are marked on Fig. 3(a). SSW is mostly not

present near the study area in winter thus only SW, TW and IW are marked on Fig. 3(b).

Assuming that a water mass P is a mixture of four water masses A, B, C, and D (Fig. 10).

By drawing perpendicular lines from P to the four sides of the tetragonal we obtain four

intercepts, P1, P2, P3 and P4. The length between P and P1 is represented as

PP1.

Likewise

the lengths between: P and P2 =

PP:; P

and P3 =

PP3; P

and 1°4 =

PP4; P1

and

A =XAz; P1

and

B=Xm; P:

and

B=XBz; P2

and

C=Xc6 P3

and

C=Xc2; P3

and

D=Xm; P4

and

D=XD2; P4

and

A=XA2.

LetfA,fB, fCandfD

be the fractions of A, B, C, D in P:

fA + fB + fc + f o = l.

(1)

Likewise we can let

feb fez, re3

and

re4

be the fractions of P1, P2, P3 and P4 in P:

fpl +fe2 +fP3 +fp4 = 1.

In other words, T and S of P can now be calculated from the T and S of P l, P2, P3 and P4-

The T and S of P1 can be calculated from

fAt,

fro, Xa 1 and X m where)CA1 and fm are the

fractions of A and B, respectively, involved in forming

PI'

The objective now is to solve for

fAl, fA2, fBl, fB2, fclfc2, fol, fD2"

Water mass A also contributesfA2 to mix with water mass D to form water mass P4- Thus

A contributes fnl andfA2 altogether to form P. Similarly the contributions of B, C and D

can also be subdivided into two parts. Equation (1) now becomes:

fAl + fa2 + f r o -t- fB2 "+ fc1 + fc2 + fOl "+ fD2

---- 1.

(2)

Further,

PP1

is inversely proportional to

fn~ + f m

giving the following equation:

P P I " (fZl

+ f B 1 ) =

PP2 " (fB2 "sr- fc1) = PP3 " (fc2

+ f D 1 ) =

PP4 " (fo2 q- fA2),

( 3 )

(14)

32

C.T.A. CnEN

et al.

f A I " X A 1 ~- f B l " X B 1 , fB2 " X B 2 = f C l " XC1, f c 2 " X c 2 = f D l " X D 1 ,

By solving equations (2)-(4), we obtain:

fD2" XD2 = fA2" X A 2

(4)

(z41) -,

fA2 OJ: FP4

(Z4 ,) -1

-~

Xal ( 41) -

fB1

Ag]--fie 1

- ~ i

-X--c-1 ( V 1 -11

f~2

_Xp_

L

{~-' 1 ]-1

fez CD. PP~ ~# F-E/

fom-

CD • PP3

-fiff ii

(X' 1 )-1

fo:

D A . PP4 \Z-~4 - ~ i ]

'

where

~ ! - ± +

i + 1 +_3/_.

PPi

PPa

P P 2 P P 3 P P 4 4

It is now possible to calculate the mixing percentages in the study area (MAo, 1964; Ruo,

1989; CHEN

et al., 1990). Strictly speaking, however, T and S are not conservative

properties near surface, but neither are any of the parameters that we measured. We tried

to make the calculation using

T-S, NO3-S,

PO4-S

and p H - S pairs and found little

difference (Table 2). We thus decided to use

T - S as these data are the most reliable.

RESULTS AND DISCUSSION

Figure 11(a) and (b) show the percentages of shelf water in September 1988 and

December 1989, respectively. In September, the shelf water dominates on the shelf, but

only in the surface layer. The percentage reduces to below 50% below 60 m. The shelf

water spreads out off the shelf break and the shelf water percentage reduces rapidly to 1%

before reaching Sta. 14. In December the picture is quite different [Fig. 11(b)]. The shelf

water makes up only 20% of the surface water on the shelf, reflecting reduced river flow

from the continent and stronger Kuroshio intrusion in winter.

Figure 12(a) and (b) show the percentages of Kuroshio Surface Water, which dominates

off the shelf break. The percentage is higher than 90% in September in the Kuroshio

region, slightly lower in December. The dome-like structure at the shelf break reflects

upwelling of deeper waters. The Kuroshio Surface Water contributes approximately 30%

of the water on the shelf in September, and up to 70% in December when the continental

fresh water outflow is low.

Figure 13(a) and (b) show the mixing ratio of the Kuroshio Tropical Water in September

and December, respectively. The core of the Tropical Water is near the salinity maximum

at about 150 m in September, slightly deeper in December. The Tropical Water does not

(15)

Water mass exchange in the East China Sea 33

Table 2. Mixing ratios based on T-S, NOTS, PO4-S and pH-S pairs for cruise 179

Method T-S NO~--S

PO43-S

pH-S

Sta. 10 Depth (m) 5O 75 100 150 200 400 Sta. 12 Depth(m) 50 75 100 20O 40O Sta. 13 Depth (m) 50 75 100 200 400 s s w s w TW IW SSW SWTW IW SSW S W T W l W SSW SWTWIW 13 71 15 1 11 67 13 9 6 76 17 1! 6 71 14 9 7 44 38 11 6 46 35 13 5 50 33 12 6 58 28 8 2 18 66 14 3 15 61 21 2 13 66 19 2 16 71 11 0 0 55 45 0 0 61 39 0 0 56 44 0 0 52 38 0 0 41 59 0 0 43 57 0 0 49 51 0 0 41 59 0 0 7 93 0 0 9 91 0 0 8 92 0 0 9 91 0 82 18 0 0 80 20 0 0 84 16 0 0 85 15 0 6 50 34 10 9 43 41 7 0 76 24 0 0 75 25 0 0 20 66 14 1 15 71 13 0 22 73 5 0 22 70 8 0 0 54 46 0 0 56 44 0 0 52 38 0 0 57 43 0 0 5 95 0 0 8 92 0 0 12 88 0 0 10 90 0 78 22 0 0 80 20 0 0 85 15 0 0 84 16 0 0 48 52 0 0 42 58 0 0 48 52 0 0 50 50 0 0 0 100 0 0 0 100 0 0 8 92 0 0 13 87 0 0 0 58 42 0 0 65 35 0 0 69 31 0 0 79 21 0 0 10 90 0 0 6 94 0 0 11 89 0 0 12 88

get to t h e s u r f a c e b u t shows its i n f l u e n c e at t h e shelf b r e a k a n d n e a r the b o t t o m o n t h e shelf, m a k i n g u p as m u c h as 5 0 % o n t h e b o t t o m in S e p t e m b e r , l o w e r in D e c e m b e r .

F i g u r e 14(a) a n d (b) s h o w t h e m i x i n g r a t i o of t h e K u r o s h i o I n t e r m e d i a t e W a t e r in S e p t e m b e r a n d D e c e m b e r , r e s p e c t i v e l y . T h e I n t e r m e d i a t e W a t e r c o n t r i b u t e s to the u p w e l l e d w a t e r at t h e shelf b r e a k a n d m a k e s u p 3 0 % of t h e b o t t o m w a t e r o n t h e shelf in S e p t e m b e r , less in D e c e m b e r .

O v e r a l l , t h e largest S e p t e m b e r - D e c e m b e r c o n t r a s t occurs o n t h e shelf. T h e o r i g i n a l " S h e l f W a t e r " c o m p o n e n t m a k e s u p 7 0 - 8 0 % o f t h e surface shelf w a t e r in S e p t e m b e r , o n l y 2 0 % in D e c e m b e r . T h e b o t t o m shelf w a t e r s c o n t a i n o n l y 10% of t h e o r i g i n a l " S h e l f W a t e r " in S e p t e m b e r a n d little o r n o o r i g i n a l " S h e l f W a t e r " in D e c e m b e r . T h e y are c o m p o s e d o f 10% K u r o s h i o S u r f a c e W a t e r , 5 0 % K u r o s h i o T r o p i c a l W a t e r a n d 3 0 % K u r o s h i o I n t e r m e d i a t e W a t e r in S e p t e m b e r , 3 0 - 5 0 % K u r o s h i o Surface W a t e r , 30% K u r o s h i o T r o p i c a l W a t e r a n d 1 0 - 3 0 % K u r o s h i o I n t e r m e d i a t e W a t e r in D e c e m b e r .

T h e r e a r e r e g i o n s o n Fig. 10, say a p o i n t P ' at Sta. 6 w h e r e p e r p e n d i c u l a r lines c a n n o t b e d r a w n to all f o u r sides. Such p o i n t s a r e n e a r o n e of the f o u r sides thus are m a i n l y a m i x t u r e o f two w a t e r masses. F o r i n s t a n c e , p o i n t P ' is m a i n l y a m i x t u r e o f S W a n d SSW. T h e c o n t r i b u t i o n o f I W d i m i n i s h e s f r o m t h e p o i n t P to P ' u n t i l I W b e c o m e s zero. A f t e r t h a t , P ' c a n s i m p l y b e c a l c u l a t e d as a m i x t u r e o f S S W , S W a n d T W . C O N C L U S I O N W e h a v e c a l c u l a t e d t h e m i x i n g r a t i o s o f Shelf W a t e r , K u r o s h i o S u r f a c e W a t e r , K u r o s h i o T r o p i c a l W a t e r a n d K u r o s h i o I n t e r m e d i a t e W a t e r in a n a r e a n o r t h e a s t o f T a i w a n . I n

(16)

(a)

100 1 2 3 4 5 6 7 8 9 10 11 12 15 14 _ 2 0 0 i Z D_ © ]:9 3 O O 400 500 s t o t i o n

34

c . T . A . CHEN et al.

(b)

s t a t i o n

0 100 ~ 200 4 ~ cz © ~:D 300 4O0 500 2 5 5 7 9 10 12 15 ] I L ~ I I I I - ~ - - - ' - 2 o ~ ~ \ / 2 ° ~

I 0 ~ ~ I 0

~ / ~ I0

(17)

Water mass exchange in the East China Sea

35

(a)

c- O_ © ~D 100

station

1 2 3 4 5 6 7 8 9 10 11 12 1.5 14 0 I r I I L I ~ i I I I r I i OU lug--_ . 200 300 4 0 0 500

-

~

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( b )

station

2 3 5 7 9 1 0 I I I I I I 12 13 I I 100 200 Ca (D ~D 300 400 500

SW,%

70

(18)

36

C . T . A . CH~.N et al.

(a)

100

station

2O0 c - O _ 3 0 0 400 500 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 ~ I I L k i I i I I I I i I 1 J o J 7 9 o ~

TW , ?/oo

/ /

/ / / / . /

/

/

/

10

(b)

stotion

100 2OO ~Z C)_ q) 73 3 0 0 4 0 0 5 0 0 2 3 5 7 9 10 12 13 I I I I L I I I ~ 1 0 " 1

~

\

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i

(19)

Water mass exchange in the East China Sea

37

(a) 100 2 0 0 _c O_ © -0 5 0 0 400 5 0 0

station

1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 l I I I I I ] 2 ] 1 J I I I

IW,%

~ ~

~

(a)

s t a t i o n

2 3 5 7 9 10 12 13 0 I I I I I I I I 100 2 0 0 O_ (1) nD 3 0 0 4 0 0 5 0 0

" ~

90 ~ _

(20)

38

c . T . A . CHEN et al.

September 1988, surface shelf water in the study area was affected by the Kuroshio Surface

Water. The bottom waters on the shelf contained only 10% of the original "Shelf Water".

The rest came from the Kuroshio Surface Water (10%), Kuroshio Tropical Water (50%)

and the Kuroshio Intermediate Water (30%).

In December 1989, the original "Shelf Water" constituted only 20% of the surface water

on the shelf. Intrusion of the Kuroshio Surface Water dominated (>70%). The original

"Shelf Water" was almost nonexistent near bottom where Kuroshio Surface Water (30-

50%), Kuroshio Tropical Water (30%) and Kuroshio Intermediate Water (10-30%)

dominated. The mixing ratios offshore are also calculated and they do not show large

temporal variations.

Although it is not apparent on Figs 2-8, Kuroshio Intermediate Water also participated

in the upwelling and cross shelf mixing by a higher percentage [>30% near bottom; Fig.

12(a)] in summer than in winter [less than 10% ; Fig. 12(b)].

Acknowledgements--This work was supported by the National Science Council of the Republic of China (NSC 82-0209-M110-041). We thank the captains and crew of the R.V. Ocean Researcher I for their assistance during the cruises. Two anonymous reviewers provided constructive criticisms.

R E F E R E N C E S

ATKINSON L. P. (1985) Hydrography and nutrients of the southeastern U.S. continental shelf. In: Oceanography of the southeastern U.S. continental shelf, L. P. ATKINSON, D. W. MENZEL and K. A. BUSH, editors, American Geophysical Union, Washington, D.C., pp. 77-92.

BLANTON J. O., L. P. ATKINSON, L. J. P1ETRAFESA and T. N. LEE (1981) The intrusion of Gulf Stream Water across the continental shelf due to topographically-induced upwelling. Deep-Sea Research, 28,393-405. BRADSnOW A. L., P. G. BREWER, D. K. SHAFER and R. T. WILLIAMS (1981) Measurements of total carbon dioxide

and alkalinity by potentiometric titration in the GEOSECS program. Earth and Planetary Science Letters, 55, 99-115.

CARPENTER J. M. (1965) The Chesapeake Bay Institute technique for the Winkler dissolved oxygen method. Limnology and Oceanography, 10, 141-143.

CHEN C. T. (1981) Oxygen solubility in seawater, In: Solubility data series V7. Oxygen and ozone, R. BATTINO, editor, Pergamon Press, Oxford, pp. 41-55.

CHEN C. T (1984) Carbonate chemistry of the Weddell Sea. Department of Energy Technical Report, DOE/EV/ 10611-4, 118 pp.

CHEN C. T. (1985) Preliminary observations of oxygen and carbon dioxide of the wintertime Bering Sea marginal ice zone. Continental Shelf Research, 4,465-483.

CHEN C. T. A. (1988a) Exchange of water masses between the East China Sea and the Black Stream: a proposed descriptive chemical oceanographic study. Extended Abstract of the Workshop on Kuroshio Edge Exchange Processes, Stony Brook, N.Y., May 4-6 1988, pp. 6.1~5.6

CHEN C. T. (1988b) Summer-winter comparisons of oxygen, nutrients and carbonates in the polar seas. La Mer, 26, 1-10.

CHEN C. T. (1993) Carbonate chemistry of the wintertime Bering Sea marginal ice zone. Continental Shelf Research, 13, 67-87.

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CHEN C. T., C. L. W~I and M. R. RODMAN (1985) Carbonate chemistry of the Bering Sea. Department of Energy Technical Report, DOE/EV/10611-5, 79 pp.

CHEN C. T. A., R. RUo and Y. C. CHUN6 (1990) Marine chemistry and sedimentation rate in the Philippine Sea. Proceedings, Atomic Energy Council Conference, Dec. 1990, pp. 1-21. [in Chinese].

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Water mass exchange in the East China Sea

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CSK (1966) Oceanographic data report of CSK, No. 1. Chinese National Committee on Oceanic Research, Academia Sinica, ROC, 123 pp.

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FANNING K. A. and M. E. Q. PILSON (1973) On the spectrophotometric determination of dissolved silica in natural waters. Analytical Chemistry, 45, 136-141.

HUANC S. G., J. D. YANk, W. D. JI, X. L. YAN~ and G. X. CHEN (1983) Silicon, nitrogen and phosphorus in the Changjiang river mouth water. In: Proceedings of "Symposium on sedimentation on the Continental Shelf with special reference to the East China Sea", April 1983, Hangzhou, China, Springer-Verlag, pp. 220-228. LI K. and Z. F. Lu (1987) A preliminary analysis of water masses in the East China Sea in summer and winter.

Collected Papers of Kuroshio Research, Ocean Press, Beijiang, pp. 177-189 [in Chinese with English Abstract].

LIMEBURNER R., R. C. BEARDSLEY and J. S. ZHAO (1983) Water masses and circulation in the East China Sea. In: Proceedings of "Symposium on sedimentation on the Continental Shelf with Special Reference to the East China Sea", April, 1983, Hangzhou, China, Springer-Verlag, pp. 261-269.

LIu K. K., G. C. GONG, C. Z.

SHYU,

S. C. PAI, C. L. WEI and S. Y. CHAO (1992) Response of Kuroshio upwelling to the onset of the northeast monsoon in the sea north of Taiwan: Observations and a numerical simulation. Journal of Geophysical Research, 97, 12,511-12,526.

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(i

941 ) The results of the hydrographical surveys in the China Sea in summer 1939. Journal of the Imperial Fishery Experimental Station, 11, 39-97 [in Japanese with English Abstract].

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數據

Fig. 1.  Station locations. Depth contours are in m.
Fig.  2.  Cross-section  of temperature in (a)  September  1988  and (b) December  1989
Fig.  3.  Cross-section  of  salinity  in  (a)  September  1988  and  in  (b)  December  1989
Fig.  4.  Cross-section  of ot in (a)  September  1988  and in (b)  December  1989.
+7

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