TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 63 (No. 4), pp. 589-600, 2007

New Type of Pycnostad in the Western Subtropical-Subarctic Transition Region of the North Pacific: Transition Region Mode Water

Hiroko Saito1, Toshio Suga1,2*, Kimio Hanawa1 and Tomowo Watanabe3

1Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
2Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
3National Research Institute of Far Seas Fisheries, Fishery Research Agency, Shizuoka 236-8648, Japan

(Received 30 October 2006; in revised form 21 February 2007; accepted 21 February 2007)

Abstract: A new type of pycnostad has been identified in the western subtropical-subarctic transition region of the North Pacific, based on the intensive hydrographic survey carried out in July, 2002. The potential density, temperature and salinity of the pycnostad were found to be 26.5–26.7σθ, 5°–7°C and 33.5–33.9 psu respectively. The pycnostad is denser, colder and fresher than those of the North Pacific Central Mode Water and different from those of other known mode waters in the North Pacific. The thickness of the pycnostad is comparable to that of other mode waters, spreading over an area of at least 650 × 500 km around 43°N and 160°E in the western transition region. Hence, we refer to the pycnostad as Transition Region Mode Water (TRMW). Oxygen data, geostrophic current speed and climatology of mixed layer depth in the winter suggest that the TRMW is formed regularly in the deep winter mixed layer near the region where it was observed. Analysis of surface heat flux also supports the idea and suggests that there is significant interannual variability in the property of the TRMW. The TRMW is consistently distributed between the Subarctic Boundary and the Subarctic Front. It is also characterized by a wide T-S range with similar density, which is the characteristic of such a transition region between subtropical and subarctic water masses, which forms a density-compensating temperature and salinity front. The frontal nature also tends to cause isopycnal intrusions within the pycnostad of the TRMW.

*Corresponding author E-mail: suga@pol.geophys.tohoku.ac.jp

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