Tempo-spatial variations of the Ryukyu Current southeast of Miyakojima Island determined from mooring observations

Abstract

The origin, structure, and variability of the Ryukyu Current (RC) have long been debated, mostly due to limited observations. A mooring array, deployed for two years southeast of Miyakojima in the southern portion of the Ryukyu Island chain, has provided, for the first time, data confirming the existence and revealing the characteristics of the RC in that upstream region, including its velocity structure and variability. The observations show a shoreward-intensified current flowing northeastward, with a subsurface core located near the 1,000 m isobath and having a record-long mean speed of up to 19.4 cm s −1 at 500 m depth. Estimated volume transport across the observation section had mean 9.0 Sv (1 Sv = 10 6 m 3 s −1) and standard deviation 8.7 Sv. The RC shows significant barotropic character compared with other similar mid-latitude currents. The Kuroshio is the strongest western boundary current in the Pacific Ocean. Consequently it has long been the focus of many scientific studies 1,2. After it enters the East China Sea (ECS), the Kuroshio flows northeastward, west of the Ryukyu Island chain, until it reaches the region south of Kyushu, Japan and exits through the Tokara Strait (Fig. 1a). However, studies 3,4 have also indicated the existence of another strong portion of the western boundary current, namely the Ryukyu Current (RC) flowing northeastward just east of the Ryukyu Island chain, accounting for the large difference in volume transport of the Kuroshio between the ECS (19 − 28 Sv; 1 Sv = 10 6 m 3 s −1) 5-7 and the sea off southern Japan (42~65 Sv) 8,9. Compared to the Kuroshio west of the Ryukyu Island chain, which is surface intensified and relatively stable, the RC features a strong subsurface velocity core 10,11 and large spatio-temporal variability caused by mesoscale eddies propagating from the Pacific 12,13. Previous studies have also suggested the RC's variability may be related to that of the Kuroshio East of Taiwan 14-16. Interactions between the RC and the Kuroshio via the Kerama Gap (Fig. 1b) contribute to the variability of the Kuroshio in the ECS and play a key role in water mass exchange between the ECS and the North Pacific 17-20. The RC carries a large mass of water, heat, and nutrients poleward 21,22 , greatly enhancing the Kuroshio when the two currents merge east of the Tokara Strait. Although the RC is important to the circulation of the North Pacific, its mean structure and spatio-temporal variability remain unclear, due to limited observations. Snapshots of hydrographic casts 23,24 can be greatly affected by mesoscale eddies; therefore, long-term mooring deployments are needed to remove aliases. Previous mooring observations have revealed the velocity structure and variability of the RC southeast of Amami-Oshima 3 and Okinawa 21,25 , which are in the downstream region of the RC. In the upstream region, two moored current meters showed a persistent northeastward current southeast of Miyakojima 26 but were not able to determine the structure of the RC there.

Response of the Ryukyu Current to Climate Change During 1993–2018: Is There a Robust Trend?

“…To further evaluate the capability of the HYCOM reanalysis in the upstream Ryukyu Current, we used two sets of current‐measuring mooring systems belonging to an international cooperative research project “Joint Kuroshio and Ryukyu Current System Study” (JKRYCSS, Zhao et al., 2020). These two stations, RCM2 and RCM3, were positioned on the Ryukyu Current path southeast of Miyakojima from June 2015 to June 2017 (Figure 1b), with RCM2 near the Ryukyu Current core and RCM3 near the edge (Figure 2b).…”

Anthropocene North Western Pacific Oceanography Recorded as Seasonal‐Resolution Radiocarbon in Coral From Kikai Island, Japan

“…Kikai Island is located along the path of the Kuroshio and Ryukyu currents, where mesoscale eddies propagating from the Pacific cause complex current variability (Nagano et al, 2007;Zhao et al, 2020). In addition, due to the existence of seamounts and small islands in the Tokara Strait near Kikai Island, turbulent mixing induced by these abrupt topographical features may influence Kikai ocean currents (Chang et al, 2016;Hasegawa et al, 2021;Ichikawa, 2001;Kobari et al, 2020;Nagai et al, 2019Nagai et al, , 2021Tanaka et al, 2021;Tsutsumi et al, 2017).…”

“…Recently, the high‐resolution systematic oceanic investigation for the KC by Argo profiling floats, covering the last 20 years. The observation and research of the RC started even later, and observational data were obtained only after the 1990s (Andres et al., 2008; Ichikawa et al., 2004; Jin et al., 2010; Liu et al., 2022; Nagano et al., 2007; Thoppil et al., 2016; Zhao et al., 2020; Zhu et al., 2006). These data cannot provide oceanic information regarding climate changes such as El Niño‐Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) and hence other proxies are necessary to reconstruct long‐term variation of the Kuroshio and Ryukyu currents.…”

The Intensifying East China Sea Kuroshio and Disappearing Ryukyu Current in a Warming Climate

“…Further investigation shows that the HR models largely reproduces the observed KRS characteristics, including the ECS‐Kuroshio with transport of 25 Sv across PN section and the 12 Sv Ryukyu Current with core locating at 500 m across RN section (Figures 2a and 2c), consisting well with observations and reanalysis data (Table S2 in Supporting Information ). In addition to PN and RN, two additional sections, PCM1 and RCM1, are also involved to describe the upstream KRS (Johns et al., 2001; Zhao et al., 2020; location of the sections are shown in Figure 1a). It is found that the ECS‐Kuroshio exhibits a 21 Sv transport across PCM1 section and the Ryukyu Current displays a 6 Sv transport across RCM1 section, both are consistent with observation/reanalysis values.…”