Spatial Distribution of Diffusivity Coefficients and the Effects on Water Mass Modification in the North Pacific

Abstract Three‐dimensional velocity, diapycnal mixing, and isopycnal mixing fields were estimated based on long‐term mean (2001–2015) salinity distributions obtained with Argo float arrays on neutral‐density (γ) surfaces. Strong estimated isopycnal mixings were associated with the Kuroshio Extension, subtropical boundary, and the west coast of North America, and strong estimated diapycnal mixing was associated with the Izu‐Ogasawara and Hawaiian ridges, which are internal wave‐generation spots. Patterns of the mixing coefficient distributions were consistent with previous estimates by other methods. We used the mixing coefficient fields to reveal the spatial patterns of volume transports and salinity budgets on isopycnal layers. Whereas relatively large eddy transports associated with thickness diffusivity contributed to the spatial patterns of volume transports, the contributions of spatial changes in diapycnal mixing were small. Spatial changes of diapycnal mixing strongly affected those of salinity, especially in the layers around North Pacific Intermediate Water (26.5–27.0γ and 27.0–27.5γ), whereas spatial changes of isopycnal mixing were important for water mass modification in all layers. This analysis suggests that the clear salinity minimum formed regionally along the Kuroshio Extension decays because of locally strong diapycnal mixing as well as isopycnal mixing. Although the results support the utility of using the spatial distributions of water properties to infer water mass modifications, assessments based on direct measurements of mixing and fine‐scale parameterization are needed to clarify the localized effects of mixing on water mass modifications because the inversion errors in this study were not small enough to enable detection of high‐resolution spatial changes.