Temperature and salinity data from Argo profiling floats in the North Pacific during 2003-08 have been analyzed to study the structure of winter mixed layer north of the Kuroshio Extension and the subsurface potential vorticity distribution in the subtropical gyre in relation to the formation and subduction of the central mode water(CMW). In late winter, two zonally elongated bands of deep mixed layer extend at 33°-39° and 39°-43°N, from the east coast of Japan to 160°W. These correspond to the formation region of the lighter variety of CMW (L-CMW) and that of the denser variety of CMW (D-CMW) and the recently identified transition region mode water (TRMW), respectively. In the western part of the L-CMW and D-CMW-TRMW formation regions west of 170°E, the winter mixed layer becomes deeper and lighter to the east (i.e., to the downstream).As a result, the formed mode water is reentrained into the mixed layer in the farther east in the following winter and modified to the lighter water and is thus unable to be subducted to the permanent pycnocline. In the eastern part of the formation regions between 170°E and 160°W, on the other hand, the winter mixed layer becomes shallower and lighter to the east. From these areas, the L-CMW with potential density of 25.7-26.2 kg m-3 and the D-CMW-TRMW (mostly the former) of 26.1-26.4 kg m-3 are subducted to the permanent pycnocline, and they are then advected anticyclonically in the subtropical gyre. These results imply that during the analysis period large-scale subduction to the permanent pycnocline occurs in the density range up to 26.4 kg m-3 in the open North Pacific, whereas the winter mixed layer density reaches the maximum of 26.6 kg m-3. This is supported by the vertical distribution of apparent oxygen utilization in a hydrographic section in the subtropical gyre.
- Mixed layer
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