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Tokai University Graduate School of Marine Science, Japan
Tokai University School of Marine Science and Technology, Japan
Graduate School of Environmental Earth Science, Hokkaido University, Japan
Japan Marine Science and Technology Center, Japan
The origin and the distribution of the North Pacific Intermediate Water (NPIW) has attracted attention of scientists for long years. There are two stories on NPIW. One is that NPIW is not a water mass but merely the interface of sub-polar waters and sub-tropical waters in the western North Pacific. The other is that NPIW is a real water mass which is formed around the western Disturbed Area as a mixture of the sub-polar water and the sub-tropical water. Almost all recent studies about NPIW are based on the latter story. The purpose of this study is the former story to re-examine and to discuss on the relation between the two stories that seem to contradict to each other.
Spatial distribution of characteristics of waters at around depths of the intermediate salinity minimum were described using CTD data sets from WOCE Pre-P2, WHP P-2, and NOPACCS cruises along 32.5°N, 30°N and 175°E, respectively. Salinity, density and dissolved oxygen were examined not only on the intermediate salinity minimum surface but also on the neutral surfaces which originated in western most and eastern most parts of the salinity minimum surface.
On the salinity minimum surface: All of salinity, potential density and dissolved oxygen were much more uniform in the area east of 160°W than in the area west of 160°W. Salinity and potential density showed a general tendency to decrease as it goes east. Dissolved oxygen was much poor in the eastern North Pacific compared to the western North Pacific with an abrupt change its value around 160°W.
On the neutral surfaces: Potential density and salinity became higher as it goes east on the neutral surface originated in the western North Pacific. Nevertheless, potential density became lower and salinity became higher as it goes west on the neutral surface originated in the eastern North Pacific. But in the area east of 160°E, the westward increase in salinity is so small than the neutral surface and the intermediate salinity minimum surface are almost coincide with each other. As a result, characteristics of the intermediate salinity minima were one classified into two regions namely the eastern salinity minimum and the western salinity minimum. The western intermediate salinity minimum may be ascribed to the formation process around the western Disturbed Area, the some other formation process should be proposed for the eastern intermediate salinity minimum.
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