Water massesWater masses––classification, formation classification, formation

and modificationand modificationToshio Suga

Tohoku University, Japan

WOCE and Beyond18-22 November 2002

San Antonio, Texas, USA

Classification of water massesClassification of water massesIt sounds old-fashioned, but…

Have we fully utilized high-quality WOCE data for meaningful classification of water masses?

P14 179EWHP PacificAtlas (Talley)

Theta

Salinity

My answer is…

No, we haven’t. We need to utilize high-quality data such as WHP data for meaningful classification and description of water masses more eagerly.

The aim of this talk is to show reasons of the above answer with using the North Pacific mode waters as examples.

OutlineOutline• How useful is meaningful classification of

water masses to understand the ocean? • Brief overview of the North Pacific mode

waters• Mode water formation:

“OGCM” vs. “observational climatology”• New features in the Central Mode Water

formation area revealed by high-quality data

• Mode waters: pycnostad vs. thermostad

Central Waters are classical good examples.

Meaningful classification of water Meaningful classification of water massesmasses

But it should be something leading to better understanding of important processes in the ocean.

We don’t know what it is in advance generally.

Central WatersCentral WatersAwareness of Central Waters led to recognition of subduction process in the subtropical permanent pycnocline

Iselin (1939)

Vertical T-S profiles:

Sargasso Sea

Eastern North Atlantic

Surface T-S relation in winter along sections:

East

West

How can we define a water How can we define a water mass?mass?

“A body of water with a common formation history, having its origin in a particular region of the ocean” by Tomczak (1999)We usually define a water mass before we fully understand its formation history.“working hypothesis”

Water masses as working Water masses as working hypotheseshypotheses

Definition/classification of water masses

Understanding of oceanic processes

iteration

Better classification of water masses will lead to better understanding of the ocean

Mode waters in the North Mode waters in the North PacificPacific

These mode waters are particular parts of Central Waters: thermostad/pycnostad.

(Hanawa & Talley, 2001)

Subtropical Mode Water (STMW)

Central Mode Water (CMW)

Eastern STMW (ESTMW)

“Further classification of Central Waters”

Significance of mode waters in climate Significance of mode waters in climate researchresearch

Thickening and cooling of CMW associated with mid-1970s regime shift

1966/75 winter 1976/85 winter 76/85-66/75 winter

Yasuda and Hanawa (1997)

Heavy shade: dT/dz < 1.5°C/100mLight shade: dT/dz < 2.0°C/100m

Temperature section along 39°N

Mode water formation in Mode water formation in OGCMOGCM

Mode waters are subducted from the cross points of the outcropping line and MLD front.

Xie et al. (2000)

Winter surface density (thick dashed)MLD (thin)

MLD front

8.25

5.25

0.25

7.24

Isopycnal PVOutcrop

Low PV results from large lateral induction.

PV (PV (QQmm)of the water subducted )of the water subducted from the mixed layerfrom the mixed layer

huw

ufQ mm

0

Cross-isopycnal

flow

Lateral inductio

nVertical

pumpingAccording to Williams (1989; 1991)

:MLD

Mixed layer climatologyMixed layer climatology

Suga et al. (submitted/poster)

• Late winter (Feb/Mar)• Small smoothing scale, typically a few degrees

Mode water climatologyMode water climatology

Suga et al. (submitted/poster)

• North Pacific HydroBase: isopycnal climatology• Mode water properties are identified as those of isopycnal low PV core

CMW

STMW

ESTMW

Example of isopycnal PV Theta-S relation of mode waters

Darker shade: lower PV

ESTMW

Probable formation sites of mode Probable formation sites of mode waterswaters

Suga et al. (submitted/poster)

…defined as winter mixed layer with properties same as those of mode watersCMW

STMW

CMWSTM

WESTM

W

MLD front

New mixed layer climatology and HydroBase climatology suggest that—•STMW formation is due to large lateral induction as suggested by the OGCM result.•CMW and ESTMW formation is primarily due to small cross-isopycnal flow.

We definitely need more work with high-quality data including Argo data.

Formation area of CMW: Formation area of CMW: climatologyclimatology

Nakamura (1996): “north of the 9°C Front”

Different descriptions based on the different climatologies…

Suga et al. (1997):“south of the Kuroshio bifurcation front”Temp. at 300m

MLD

Because of their low resolution, both may be insufficient.

Mode waters captured by high-quality Mode waters captured by high-quality datadata

Repeat section (temperature) along 165°E in spring by JMA,

Oka & Suga (submitted/poster)

Shade: PV < 1.5x10-12m-1s-1

Kuroshio Extension Front

Kuroshio Bifurcation Front

Subarctic Front

likely representing spatial structure of formation region

Mode waters captured by high-quality Mode waters captured by high-quality datadata

Oka & Suga (submitted/poster)

Theta-S relation of mode waters: 165°E in spring, 1996-2000

KEF

KBF

SAF

STMW

Lighter CMWDense

r CMW

“Subarctic Mode Water”?

Is the distinction between lighter and denser CMWs meaningful classification or too much detail?

There are a few observational and model results supporting its significance.

High-density XCTD sectionHigh-density XCTD section

Watanabe (personal communication)

Potential density

Potential vorticity

Lighter CMW

Denser CMW

Jul/Aug 2001

CMWs in fine-mesh OGCMCMWs in fine-mesh OGCM

detrainment

entrainment

CMW(25.9-26.2) North branch of KE

DCMW?(26.4-26.5) South of SAF

STMW(25.2-25.5) South of KE

Tsujino & Yasuda (poster)

MLD in late winter

Annual subduction rate

Mode waters: thermostad vs. Mode waters: thermostad vs. pycnostadpycnostad

(Suga et al., 1997)

STMW: thermostad = pycnostad

15°-17°C layer thickness

10°-12°C layer thickness

PV

PV

STMW

CMW

CMW: thermostad < pycnostad

(Suga et al., submitted/poster)

Vertical structure of STMW and CMWVertical structure of STMW and CMWSTMW: 30.1°N, 137°E (WHP P10)

CMW: 40°N, 179°E (WHP P14N)

Both T and S are homogeneous.

Both T and S are less homogeneous but compensating each other.

Vertical gradients of temperature and Vertical gradients of temperature and densitydensity

Density gradient

Th

eta

gra

die

nt

CTD date within the pycnostads corresponding to

STMW (P10)CMW (P14N)

Difference in the vertical structures is possibly associated with difference in the formation and modification processes…

ConclusionsConclusions•Formation processes of mode waters are not fully understood; there are still fundamental discrepancies among observations and models.•Meaningful further classification of mode waters is possible based on high-quality data such as those from WHP.

•Detailed structures of mode waters are not even described very well but will be useful to understand their formation histories.

Outlook: mode waters in the turbulent Outlook: mode waters in the turbulent oceanocean

(Uehara et al., submitted/poster)

Pycnostad detected by Argo float, summer & autumn, 2001

Core PV Thickness

“New challenge”, which requires collaboration among high-density surveys, Argo, numerical models, satellite altimeters…

I hope this talk has conveyed some general ideas about what we need now to utilize water masses sufficiently as “working hypotheses” for understanding oceanic processes, such as“It is still true that better classification leads to better understanding.”