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Ocean Water Masses-T-S Diagramsand Upper Ocean Waters

C. Chen

General Physical OceanographyMAR 555

School for Marine Sciences and TechnologyUmass-Dartmouth

1. Definition: Water bodies with particular characteristics of temperature and salinity;

2. Method: The temperature (T)-salinity (S) diagram (relative conservative)

3. Applications: a) identify the water masses and its sources and b) water mixing

T

S0

=26.

0

=26.5

=27.0

=27.

5

=28

.0

=28.

5

Basic Concepts

Single point: A water type

A smooth curve: a water mass

Propertion of I

Propertion of II=b

a

S

T

II (T2, S2)

I (T1, S1)

R

a

b

S2 S1

T2

T1

Propertion of type I =b

a + b

Propertion of type II =a

a + b

I : II : III =b

a + b:d

c + d:

f

e+ f

S

T

II (T2, S2)

I (T1, S1)

R

ab

S2 S1

T2

T1

III (T3, S3)

c

d

e

f

S3

T3

Propertion of type I =b

a + b!100%

Propertion of type II =d

c + d!100%

Propertion of type III =f

e+ f!100%

Three Water Types

Temperature (T)

I

II

III

I

II

III

200-600

600-1000

1000-1400

I

II

III

2o

200

600

1000

1400

6o 10oC

2o

200

600

1000

1400

6o 10oC

2o

200

600

1000

1400

6o 10oC

1000

34.0 34.4 34.8

200

600

1400

34.0 34.4 34.8

200

600

1400

34.0 34.4 34.8

200

600

1400

34.0 34.4 34.8

10o

6o

2o

34.0 34.4 34.8

10o

6o

2o

34.0 34.4 34.8

10o

6o

2o

800

200-400

1200-1400

800

200-400

1200-1400

Depth (m

)D

epth (m)

Depth (m

)

Salinity (S) S

T

In general, mixing of two water mass shows a straight line on a T-Sdiagram, but it is not always true.

Example: The North Atlantic Central Water in the Subtropic GyreObservational site

200

400

600

800

1000

Depth (m

)

Source region

36.6 20o

36.2 16o

35.8 12o

35.6 36.0 36.4 36.8

20

16

12

18

14

Tem

pera

ture

(oC

)

Salinity

Formation of the water mass by thesinking of surface water alongisopycnic surfaces. This type of thewater mass shows a straight line onthe T-S diagram with no relevance tomixing of two water masses.

QS. Could we use the in-situ water temperature and salinity to draw aT-S diagram and compare the different water masses?

Salinity

24.0

24.5

25.0

25.5

26.5

26.0

27.0

27.5

28.0

28.5

29.0

1.5

2

2.5

34

6 5

7 8

9 10 12 1614 18

203540

4550

20

15

5

10

0

33.5 34.0 34.5 35.0 35.5 36.0 36.5

Tem

pera

ture

(oC

)

??

The T-S curve for Meteor station 200 at 9o S in the Atlantic Ocean.

Potential Temperature and Potential Density

Depth (m)

100

1000

Parcel 1

Parcel 2

T1, P1

T2, P2

P: water pressureT: water temperature

If T2>T1, does it means that the water parcel 2 is warmer ?

Answer: NO! The water is slightly compressible and these two waterparcels have different pressures

How could we compared two water parcels with different pressures?

T(P): in-situ water temperature

T(Po) = : potential temperature

Adiabatically (no thermal contact withthe surrounding water)

Reference pressure level

Then,the waters can compare each other based on their potential temperature.

In-situ Water Density (t) and Potential Density ()

The water density:

! = !(T ,S,P)

where T and S are the in-situ water temperature and salinity.

The sigma-t (t) is defined as

!t

= "(T ,S,P) #1000

Replacing T by the potential density, we can define the potential density(sigma-) as

!" = #(",S,Po) \$1000

In some applications, you can find

!"o

; !"1; !

" 2!

defined using the different reference pressure levels.

East Pacific Ocean East Atlantic Ocean

2

6

107

12

13

15

203040

1

2

3

4

5

10

2045

1

Surface or Upper Water(SW)

Central Water(CW)

IntermediateWater(IW)

Deep or BottomWater(DW or BW)

SW

CW

CIW

IW

DW

CIW-California Intermediate Water: low salinity MIW-Mediterranean Intermediate Water: high salinity

West N. PacificCentral Water

East N. PacificCentral Water

Pacific Equatorial Water

West S. PacificCentral Water East S. PacificCentral Water

SubantarcticSurface Water

Subantarctic Surface Water

Pacific Subarctic Upper Water

West N.AtlanticCentralWater

East N.AtlanticCentralWater

Atlantic Equatorial Water

S AtlanticCentralWater

Subantarctic Sur

face Water

Subantarctic S

urface

Water

Atlantic

Subar

ctic

Upper

Water

Indian EquatorialWater

S. Indian Central Water

Subantarctic Surf

ace Water

Subantarctic Surface Water

Arab SeaWater

Bengal

Bay Water

Indonesia Upper Water

Upper Ocean Water Masses

Strongly influenced by the pattern of surface currents in the mixed layer and the upper part ofthe permanent thermoclines

Central Waters are characteristics of subtropic gyres.

South Indian Central Water(SICW)T: 8.0-25.0oCS: 34.6-35.8 PSU

West North Pacific CentralWater (WNPCW)T: 10.0-22.0oCS: 34.2-35.2 PSU

East North Pacific Central Water(ENPCW)T: 12.0-20.0oCS: 34.2-35.2 PSU

West South Pacific CentralWater (WSPCW)T: 6.0-22.0oCS: 34.5-35.8 PSU

East South Pacific CentralWater (ESPCW)T: 8.0-24.0oCS: 34.4-36.4 PSU

West North Atlantic CentralWater (WNACW)T: 7.0-20.0oCS: 35.0-36.7 PSU

East North Atlantic CentralWater (ENACW)T: 8.0-18.0oCS: 35.2-36.7 PSU

South Atlantic Central Water(SACW):T: 5.0-18.0oCS: 34.3-35.8 PSU

Indian OceanPacific OceanAtlantic Ocean

Sargasso SeaSubtropic Gyre

CapeHatteras

GuyanaCurrent

North Eq

uatorial C

urrent

North AtlanticDrift

Subpolar Gyre

90o 30o 0o

20o

40o

40o

20o

18 degree water

18o degree water is central water. It is also called mode water. It ischaracterized by the minimum vertical density gradient.

Ekm antransport

Surface w indstress

Ekm anpum ping

Subtropic gyreW esternBoundaryC urrent

Formation of the Central Water: the convergence and subduction of the subtropicgyre and also vertical convection caused by the wintertime cooling

Type 1: Isopycnal mixing mixing along the density surface

Type 2: Diapycnal mixing mixing across the density surface

A T-S diagram also can help us determine the characteristics of watermixing in the water column

=25.0

= 25

.5

= 26

.0

= 26

.5

= 27

.0

= 27

.5

S

Pot

entia

l tem

pera

ture

Salinity

=25.0

= 25

.5

= 26

.0

= 26

.5

= 27

.0

= 27

.5

S

Pot

entia

l tem

pera

ture

Salinity

Type 1 Type 2

East Pacific Ocean East Atlantic Ocean

2

6

107

12

13

15

203040

1

2

3

4

5

10

2045

1

Central Waters show the diapycnal mixing, so it does not form only bythe along-density surface subduction

T-S diagram also can be used to track the mixed water back to theirupstream source.

An example in Kuroshio southwest of Japan

From Chen et al. (1994)