Water temperature and salinity range in the transition region between the Oyashio and Kuroshio currents (mixed waters)
The following chart shows the approximate water mass classification in the surface to middle layers of the Oyashio-Kuroshio transition region (off-Sanriku) near the Japanese Islands.
(Hanawa and Mitsudera, Journal of the Oceanographical Society of Japan, 42, 435-446, 1987)
The lower the temperature and the higher the salinity, the higher the density, so the deeper the water as it moves toward the lower right. In this figure, water with a density lower than 26.7σ (plotted in the upper left side) represents surface water※.
KW：High-salinity subtropical Kuroshio water (>34.2 salinity)
TW：Tsugaru Warm Current water (33.7-34.2 salinity), slightly low-salinity due to the influence of river water after the Kuroshio diverges into the Sea of Japan
OW：Low-salinity Oyashio-water (<33.7 salinity)
s-OW：OW warmed summer Oyashio surface water (< 33.7 salinity, > 7°C temperature)
(Even though the low-temperature Oyashio Water (OW) originates in the subarctic if warmed by solar radiation in summer, it will warm up and become s-OW while maintaining low salinity)
CL：Water with a density of 26.7σ or greater is water in the core layer (CL), which is deeper than the surface layer
Deep water in the Pacific Ocean is subducted in the North Atlantic high latitudes, so at depth it converges to the same CL T-S range regardless of location.
Note that this classification is only for looking at water mass changes off-Sanriku. TW is defined because water can flow through the Tsugaru Straits into the off-Sanriku. Even in the northwestern Pacific, TW cannot be defined in places where Tsugaru Warm Current water does not flow in. In such places, salinity above 34 is defined as Kuroshio water and below 34 as Oyashio water.
※The density of 26.7σ is approximately the boundary between the surface and core layer. In the subarctic, it corresponds to a depth of about 200 m. In the subtropical, it corresponds to a depth of about 700 m. The subtropical has a thicker surface layer. Please learn about it in the chapters "Surface Layer" and "Subduction," which will be explained later. Note that surface mixed layer and surface layer have different meanings.
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Distribution of salinity and temperature in the Northwest Pacific Ocean (summary)
Surface salinity
Subtropical surface layer (Kuroshio water)：Salinity 34‰ or more
Surface seawater density is approximately less than 26.7σ
Subtropical surface less than 26.7σ：0～700 m