Mechanism of interannual to decadal sea level variability along the Japanese coast

Sea level variability along the coast of Japan on interannual to decadal timescales was investigated using a long-term ocean reanalysis dataset, named Four-dimensional variational (4DVR) Ocean ReAnalysis for the western North Pacific over 30-year (FORA-WNP30). The reanalysis data was produced by using an eddy-resolving 4DVAR assimilation system, which enables us to analyze realistic mesoscale variability as well as large-scale low-frequency variability. An Empirical Orthogonal Function (EOF) analysis was performed to extract principal modes for sea-level changes along the Japanese coast. The first mode that explains about 65% of the total sea-level variance exhibits a simultaneous sea-level change along the whole Japanese coast. In contrast, the second mode explaining about 19% of the total variance has a prominent dipole pattern along the south coast of Japan, whose bifurcation is located at the southern tip of the Kii Peninsula, central Japan. These features seen in FORA-WNP30 are in good agreement with tide-gauge observations. It is also shown that the leading two EOF modes have a close relationship with changes in the Kuroshio Current system. To be specific, the EOF1 variability is well correlated with the Kuroshio Extension (KE) variability on interannual to decadal timescales. A case study suggests that coastal trapped waves tend to be induced and cause sea-level rise along the Japanese coast when the latitudinal position of the Kuroshio path at the eastern flank of the Izu-Ogasawara Ridge, controlled by the KE variability, shifts northward to the Japanese coast. In contrast, the second mode is closely related to the bimodal state of the Kuroshio path south of Japan, that is, the large meander or not.