New type of pycnostad in the western subtropical-subarctic transition region of the North Pacific: Transition Region Mode Water |
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| Authors: | Hiroko Saito Toshio Suga Kimio Hanawa Tomowo Watanabe |
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| Institution: | (1) Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan;(2) Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan;(3) National Research Institute of Far Seas Fisheries, Fishery Research Agency, Shizuoka 236-8648, Japan |
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| 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 26.5–26.7 σ
θ
, 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. |
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| Keywords: | Pycnostad transition region mode water mixed layer Kuroshio Bifurcation front subarctic front isopycnal intrusion |
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