High‐resolution measurements of radiocarbon (14C) in corals can be used to reconstruct past variability in ocean conditions. Here we report seasonal Δ14C changes in coral from Ishigaki Island, Japan, and compare with previously reported data from Palau and Guam. Our data clearly indicate a significant increase in Δ14C from 1947 to 1998 related to atmospheric nuclear bomb testing. The three early Δ14C spikes related to the atmospheric nuclear bomb tests in the US Proving Grounds at Bikini and Enewerak atoll conducted in 1954, 1956, and 1958 were detected from the Ishigaki coral. After 1976, variability in the Mindanao Dome region related to North Equatorial Current (NEC) bifurcation latitude migration affected the Δ14C difference between Palau and Guam, whereas the difference between Ishigaki and Guam was not correlated with the bifurcation latitude. The Δ14C difference between Ishigaki and Guam may be due to mesoscale eddies in the Kuroshio area. On the decadal scale, the northward shift of NEC bifurcation latitude after 1976, the year as known as Pacific Decadal Oscillation regime shift from negative to positive, was concurrent with the abundant westward‐propagating mesoscale eddies in the Subtropical Countercurrent region and stronger Kuroshio transport off the east Taiwan, which may be represented by a smaller Δ14C difference between Ishigaki and Guam after 1976.

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The North Equatorial Current (NEC) bifurcates at the eastern coast of the Philippines and moves northward as the Kuroshio, a North Pacific western boundary current. The NEC bifurcation point and Kuroshio variability are known to be affected by changes in climate such as the El Niño-Southern Oscillation and the pacific decadal oscillation. However, observational data are not sufficient to examine the mechanisms of decadal fluctuation. Here, we report seasonal radiocarbon data recorded from 1968 to 1995 in coral skeletons northwest of Luzon Island. The data suggest that the East Asian winter monsoon is a dominant factor in the seasonal fluctuations in water mass northwest of Luzon Island. Compared with other coral records reported for Guam, Ishigaki, Con Dao, and Hon Tre Island, the data suggest that the area of the Kuroshio loop current through the Luzon Strait decreased from the 1970s to 1980s as a result of the change in Kuroshio transport and the migration of the NEC bifurcation latitude after a regime shift in 1976.

Radiocarbon (14C) dating is a widely used and powerful tool for determining the ages of samples in studies of palaeoclimatology, palaeoseismology and archaeology. 14C ages of marine samples often require correction for local reservoir age (ΔR). Although ΔR in the mid‐ to low latitudes has varied through time, few spatiotemporal reconstructions of ΔR have been published for those regions. Here we report on new and previously published ΔR data for the early 1900s to 1950 obtained from coral skeletons from the Kuroshio region off southern Japan. The data presented here show a marked positive‐to‐negative shift of ΔR from the early 1900s to the 1940s, which has important implications for calibration of radiocarbon ages in the region and for understanding the relationship of ΔR in the western Pacific with both the El Niño–Southern Oscillation and the Pacific Decadal Oscillation.