Dichothermal layer deepening in relation with halocline depth change associated with northward shrinkage of North Pacific western subarctic gyre in early 2000s
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In the western subarctic North Pacific, a wind-driven cyclonic circulation, called the western subarctic gyre (WSAG), exists. We examined year-to-year changes of the gyre and hydrographic structures, applying the altimetry-based gravest empirical mode (AGEM) method to hydrographic and altimetric sea surface height (SSH) data, and relation to the in situ variation of the temperature minimum layer, i.e., the dichothermal layer, depth at station K2 (47∘ N, 160∘ E). The AGEM-based geostrophic volume transport and the streamfunction of the WSAG in the top 1000-dbar layer show that the gyre changes substantially. From the late 1990s to the mid-2000s, the gyre shrunk northward. Due to the shrinkage, the halocline bottom, which is equivalent to the top of the main pycnocline, deepens at K2 outside the central part of the gyre. The downward displacement of the dichothermal layer at K2 was found to be significantly related to that of the underlying halocline due to the northward shrinkage of the WSAG.
KeywordsNorth Pacific western subarctic gyre Halocline Dichothermal layer K2 Hydrography Satellite altimetry AGEM
The authors thank the members of the board, the R/V Mirai (cruise number: MR98-K01, MR99-K02, MR00-K01, MR00-K03, MR01-K03, MR01-K04 leg2, MR02-K05 leg2, MR03-K01, MR04-02, MR04-06, MR05-01, MR06-03 leg1, MR07-01, MR08-05, MR10-01, and MR10-06) for collecting the CTD data. The authors also thank the JAMSTEC Argo team for providing Advanced automatic QC Argo Data, and Ssalto/Duacs and AVISO for providing the altimetric SSH data with support from Cnes. The thanks are extended to Prof. Y. Michida (Atmosphere and Ocean Research Institute, The University of Tokyo), Dr. R. J. Greatbatch (Associate Editor of Ocean Dynamics), and anonymous reviewers for their helpful comments. This work was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Grant-in-Aid for Scientific Research on Innovative Areas (22106007, 25106709, 15H02835).
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