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

Classification of water masses It sounds old-fashioned, but… Have we fully utilized high-quality WOCE data for meaningful classification of water masses? P14 179E WHP Pacific Atlas (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.

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

Central Waters are classical good examples. Meaningful classification of water masses 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 Waters Awareness 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 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 hypotheses 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 Pacific 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 research Thickening and cooling of CMW associated with mid-1970s regime shift 1966/75 winter 1976/85 winter76/85-66/75 winter Yasuda and Hanawa (1997) Heavy shade: dT/dz < 1.5°C/100m Light shade: dT/dz < 2.0°C/100m Temperature section along 39°N

Mode water formation in OGCM 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 Isopycnal PV Outcrop Low PV results from large lateral induction.

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

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

Mode 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 waters Suga et al. (submitted/poster) …defined as winter mixed layer with properties same as those of mode waters CMW STMW CM W STM W ESTM 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: climatology 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 data Repeat section (temperature) along 165°E in spring by JMA, Oka & Suga (submitted/poster) Shade: PV < 1.5x m -1 s -1 Kuroshio Extension Front Kuroshio Bifurcation Front Subarctic Front likely representing spatial structure of formation region

Mode waters captured by high-quality data Oka & Suga (submitted/poster) Theta-S relation of mode waters: 165°E in spring, KEF KBF SAF STMW Lighter CMW Denser 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 section Watanabe (personal communication ) Potential density Potential vorticity Lighter CMW Denser CMW Jul/Aug 2001

CMWs in fine-mesh OGCM detrainment entrainment CMW( ) North branch of KE DCMW?( ) South of SAF STMW( ) South of KE Tsujino & Yasuda (poster) MLD in late winter Annual subduction rate

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

Vertical structure of STMW and CMW STMW: 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 density Density gradient Theta gradient 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…

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.

Outlook: mode waters in the turbulent ocean (Uehara et al., submitted/poster) Pycnostad detected by Argo float, summer & autumn, 2001 Core PVThickness “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.”