Decadal Variability of North Pacific Subtropical Mode Water using a Long-term High-resolution Ocean Reanalysis Product

Shiro Nishikawa1, Yoichi Ishikawa1, Tsuyoshi Wakamatsu2, Yusuke Tanaka1, Hiromichi Igarashi1, Takahiro Toyoda3 and Norihisa Usui4, (1)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, (2)Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan, (3)Meteorological Research Institute, Japan Meteorological Agency, Oceanography and Geochemistry Research Department, Tsukuba, Japan, (4)Meteorological Research Institute, Ibaraki, Japan
Abstract:
We studied the decadal variability of North Pacific subtropical mode water (STMW) using a newly created ocean reanalysis product (named FORA-WNP30). This is a high-resolution (about 0.1 degrees) reanalysis of the western North Pacific ocean from 1982 to 2012, which assimilated various up-to-date observational data including Argo and satellite altimetry using a sophisticated (4D-Var) assimilation scheme. We confirmed that this dataset has a good reproducibility of Kuroshio, its extension, ocean surface properties, and major water mass structures through the comparison with several observational data. The analysis showed that the time series of volume and mean density of STMW from 1982 to 2012 contain decadal variability. This decadal variability was closely correlated with that of the Kuroshio Extension (KE) state, as indicated by past studies. It was indicated that the stable (unstable) KE state leads to deeper (shallower) mixed layer depth in the STMW formation region and thicker (thinner) STMW with the time lag of 1-2 years. Such relation was recently pointed out by an observational study based on the Argo data (Oka et al. 2015, JO) and the present result is consistent with this. We will also examine a variability structure of the STMW subduction.
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