Water masses

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Water massesclassification, formation and
modification
WOCE and Beyond 18-22 November 2002 San Antonio,
Texas, USA

• Toshio Suga
• Tohoku University, Japan

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Classification of water masses
It sounds old-fashioned, but
Have we fully utilized high-quality WOCE data for
meaningful classification of water masses?
Theta
P14 179E WHP Pacific Atlas (Talley)
Salinity
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My answer is
No, we havent. 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.
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Outline

• 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

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Meaningful classification of water masses
We dont know what it is in advance generally.
But it should be something leading to better
understanding of important processes in the ocean.
Central Waters are classical good examples.
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Central Waters
Awareness of Central Waters led to recognition of
subduction process in the subtropical permanent
pycnocline
Surface T-S relation in winter along
sections East West
Vertical T-S profiles Sargasso Sea Eastern
North Atlantic
Iselin (1939)
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How can we define a water 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
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Water masses as working hypotheses
Definition/classification of water masses
iteration
Understanding of oceanic processes
Better classification of water masses will lead
to better understanding of the ocean
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Mode waters in the North Pacific
Subtropical Mode Water (STMW)
Central Mode Water (CMW)
Eastern STMW (ESTMW)
(Hanawa Talley, 2001)
These mode waters are particular parts of Central
Waters thermostad/pycnostad.
Further classification of Central Waters
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Significance of mode waters in climate research
Thickening and cooling of CMW associated with
mid-1970s regime shift
Temperature section along 39N
1976/85 winter
76/85-66/75 winter
1966/75 winter
Heavy shade dT/dz lt 1.5C/100m Light shade
dT/dz lt 2.0C/100m
Yasuda and Hanawa (1997)
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Mode water formation in OGCM
Isopycnal PV
Outcrop
MLD front
Winter surface density (thick dashed) MLD (thin)
Mode waters are subducted from the cross points
of the outcropping line and MLD front.
Low PV results from large lateral induction.
Xie et al. (2000)
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PV (Qm)of the water subducted from the mixed layer
Cross-isopycnal flow
Lateral induction
MLD
Vertical pumping
According to Williams (1989 1991)
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Mixed layer climatology

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

Suga et al. (submitted/poster)
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Mode water climatology

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

Theta-S relation of mode waters
Example of isopycnal PV
Darker shade lower PV
Suga et al. (submitted/poster)
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Probable formation sites of mode waters
defined as winter mixed layer with properties
same as
those of mode waters
Suga et al. (submitted/poster)
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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.
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Formation area of CMW climatology
Nakamura (1996) north of the 9C Front
Temp. at 300m
MLD
Different descriptions based on the different
climatologies
Because of their low resolution, both may be
insufficient.
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Mode waters captured by high-quality data
Repeat section (temperature) along 165E in
spring by JMA,
likely representing spatial structure of
formation region
Oka Suga (submitted/poster)
Shade PV lt 1.5x10-12m-1s-1
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Mode waters captured by high-quality data
Theta-S relation of mode waters 165E in spring,
1996-2000
Oka Suga (submitted/poster)
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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.
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High-density XCTD section
Jul/Aug 2001
Potential density
Potential vorticity
Watanabe (personal communication)
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CMWs in fine-mesh OGCM
DCMW?(26.4-26.5) South of SAF
CMW(25.9-26.2) North branch of KE
STMW(25.2-25.5) South of KE
Annual subduction rate
detrainment
entrainment
MLD in late winter
Tsujino Yasuda (poster)
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Mode waters thermostad vs. pycnostad
15-17C layer thickness
PV
10-12C layer thickness
PV
(Suga et al., 1997)
(Suga et al., submitted/poster)
STMW thermostad pycnostad
CMW thermostad lt pycnostad
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Vertical structure of STMW and CMW
STMW 30.1N, 137E (WHP P10)
Both T and S are homogeneous.
CMW 40N, 179E (WHP P14N)
Both T and S are less homogeneous but
compensating each other.
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Vertical gradients of temperature and density
CTD date within the pycnostads corresponding to
STMW (P10) CMW (P14N)
Theta gradient
Difference in the vertical structures is possibly
associated with difference in the formation and
modification processes
Density gradient
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Conclusions

• 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.

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Outlook mode waters in the turbulent ocean
Pycnostad detected by Argo float, summer
autumn, 2001
Core PV
Thickness
(Uehara et al., submitted/poster)
New challenge,
which requires collaboration among high-density
surveys, Argo, numerical models, satellite
altimeters
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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.