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Water masses –classification, formation and modification

WOCE and Beyond 18-22 November 2002 San Antonio, Texas, USA. Water masses –classification, formation and modification. Toshio Suga Tohoku University, Japan. Classification of water masses. It sounds old-fashioned, but….

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Water masses –classification, formation and modification

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  1. WOCE and Beyond 18-22 November 2002 San Antonio, Texas, USA Water masses–classification, formation and modification Toshio Suga Tohoku University, Japan

  2. 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

  3. 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.

  4. 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

  5. Meaningful classification of water masses We don’t 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.

  6. 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)

  7. 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”

  8. 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

  9. 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”

  10. Significance of mode waters in climate research Thickening and cooling of CMW associated with mid-1970s regime shift Temperature section along 39°N 1976/85 winter 76/85-66/75 winter 1966/75 winter Heavy shade: dT/dz < 1.5°C/100m Light shade: dT/dz < 2.0°C/100m Yasuda and Hanawa (1997)

  11. 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)

  12. PV (Qm)of the water subducted from the mixed layer Cross-isopycnal flow Lateral induction :MLD Vertical pumping According to Williams (1989; 1991)

  13. Mixed layer climatology • Late winter (Feb/Mar) • Small smoothing scale, typically a few degrees Suga et al. (submitted/poster)

  14. ESTMW STMW CMW 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)

  15. CMW STMW ESTMW CMW STMW ESTMW MLD front Probable formation sites of mode waters …defined as winter mixed layer with properties same as those of mode waters Suga et al. (submitted/poster)

  16. 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.

  17. Suga et al. (1997): “south of the Kuroshio bifurcation front” Formation area of CMW: climatology Nakamura (1996): “north of the 9°C Front” Temp. at 300m MLD Different descriptions based on the different climatologies… Because of their low resolution, both may be insufficient.

  18. Kuroshio Extension Front Kuroshio Bifurcation Front Subarctic Front Mode waters captured by high-quality data Repeat section (temperature) along 165°E in spring by JMA, likely representing spatial structure of formation region Oka & Suga (submitted/poster) Shade: PV < 1.5x10-12m-1s-1

  19. STMW KEF KBF Lighter CMW SAF Denser CMW “Subarctic Mode Water”? Mode waters captured by high-quality data Theta-S relation of mode waters: 165°E in spring, 1996-2000 Oka & Suga (submitted/poster)

  20. 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.

  21. Lighter CMW Denser CMW High-density XCTD section Jul/Aug 2001 Potential density Potential vorticity Watanabe (personal communication)

  22. CMWs in fine-mesh OGCM DCMW?(26.4-26.5) South of SAF STMW(25.2-25.5) South of KE CMW(25.9-26.2) North branch of KE Annual subduction rate detrainment entrainment MLD in late winter Tsujino & Yasuda (poster)

  23. STMW CMW Mode waters: thermostad vs. pycnostad 15°-17°C layer thickness PV 10°-12°C layer thickness PV (Suga et al., 1997) (Suga et al., submitted/poster) STMW: thermostad = pycnostad CMW: thermostad < pycnostad

  24. Vertical structure of STMW and CMW STMW: 30.1°N, 137°E (WHP P10) Both T and S are homogeneous. CMW: 40°N, 179°E (WHP P14N) Both T and S are less homogeneous but compensating each other.

  25. 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

  26. 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.

  27. 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…

  28. 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.”

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