[M-IS14] Paleoclimatology and paleoceanography

[MIS14-P27] Relationship between Kuroshio Current Characteristics and Decadal-Scale Variability Based on Skeletal Records from a Porites Colony in the Ryukyu Islands

*Yuta Oue¹, Ryuji Asami¹, Hideko Takayanagi^1,4, Shusaku Sugimoto¹, Tohru Naruse², Osamu Abe³, Yasufumi Iryu⁴ (1.Graduate School of Science, Tohoku University, 2.Tropical Biosphere Research Center, University of the Ryukyus, 3.Graduate School of Environmental Studies, Nagoya University, 4.Advanced Institute for Marine Ecosystem Change(WPI-AIMEC))

Keywords:Porites colony, oxygen isotope, seawater temperature, salinity, Kuroshio Current, Pacific Decadal Oscillation

Understanding past and present climate and oceanic variations is crucial for accurately predicting long-term climate change. However, high-quality, continuous oceanic data from satellite observations and instrumental measurements have been available only since 1950, highlighting the need for long-term records derived from paleoenvironmental analyses. One of the useful proxies for reconstructing oceanic records is hermatypic corals. The coral skeleton is composed of aragonite, which records environmental information during their formation. Due to its rapid skeletal growth and longevity, it enables the reconstruction of long-term, high-resolution records, and its skeletal annual bands allow age determination. The oxygen isotope ratio in coral skeletons strongly reflects seawater temperature and seawater oxygen isotope ratio, which can be a useful proxy for reconstructing sea surface temperature (SST) and sea surface salinity (SSS) variations in the coral growth site. Long-term coral records have been reported primarily from the South Pacific, Atlantic, and Indian Oceans. However, there are relatively few datasets from the Kuroshio Current region, which is the northern limit of coral reef distribution.

The Kuroshio Current is a western boundary current of the North Pacific subtropical gyre, characterized by relatively warm and high salinity waters. It plays a crucial role in driving the climate of the northwestern Pacific by transporting significant heat and water vapor from the Indo-Pacific warm pool. The Kuroshio path and volume exhibit seasonal and interdecadal variability, and its relationships with the Pacific Decadal Oscillation (PDO), El Niño-Southern Oscillation (ENSO), and monsoons have been suggested. While studies on the characteristics of the Kuroshio Current and its link to long-term climate variability have been currently reported, the underlying mechanisms remain understood. One of the reasons is the limited availability of long-term time series Kurosshio Current and climate data.

In this study, we analyzed the monthly resolved time-series oxygen isotope ratio in a massive Porites sp. coral from Kume Island to reconstruct variations in the marine environment of the Kuroshio region over the past 100 years. Additionally, we examined the relationships among SST and SSS in Kume Island, Kuroshio Current variability, and long-term climate oscillations in the Pacific Ocean. Coral records obtained from this study showed a long-term decreasing trend in the oxygen isotope ratio, indicating a SST increase and a SSS decrease over the past 100 years. Furthermore, we observed changes in the relationship between SST/SSS in Kume Island and Kuroshio Current transport in response to PDO phase shifts. This suggests that variations in Kuroshio Current volume and transport associated with PDO variability can influence the Kuroshio path. During the negative PDO phase from 1947 to 1976, Kuroshio Current transport appeared to affect variations in summer SSS and winter SST at Kume Island.