Origin and formation of the Ryukyu Current revealed by HYCOM reanalysis

Wang, Min; Liu, Zhaojun; Zhu, Xiao‐Hua; Yan, Xiaomei; Zhang, Zhongzhe; Zhao, Ruixiang

doi:10.1007/s13131-018-1329-7

Cited by 11 publications

(15 citation statements)

References 23 publications

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“…Ocean circulation model data can also be used for ocean current verification. The hybrid coordinate ocean model (HYCOM) has been widely used in a variety of ocean current research projects [34][35][36]. Therefore, the HYCOM model data are used to verify the results in this article.…”

Section: Results Validationmentioning

confidence: 99%

Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data

Chong

Sun

et al. 2021

Applied Sciences

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The Kuroshio is the strongest warm current in the western North Pacific, which plays a crucial role in climate and human activities. In terms of this, the accurate acquisition of ocean surface current velocity and direction in the Kuroshio region is of great research value. Gaofen-3 synthetic aperture radar (SAR) provides data support for the study of ocean surface current measurements in the Kuroshio region, but no relevant experimental result has been published yet. In this paper, four available stripmap mode SARs’ data acquired by Gaofen-3 in the Kuroshio region are used for measuring the ocean surface current field. In general, the Doppler centroid anomaly (DCA) estimation is a common method to infer ocean surface currents from single-antenna stripmap data, but only the radial velocity component can be retrieved. In order to measure current vectors, a novel method combining the sub-aperture processing and the least squares (LS) technology is suggested and demonstrated by applying to the Gaofen-3 SAR data processing. The experiment’s results agree well with model-derived ocean current data, indicating that the Gaofen-3 SAR has the capability to accurately retrieve the ocean surface current field in the Kuroshio region and motivate further research by providing more data.

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Section: Results Validationmentioning

confidence: 99%

Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data

Chong

Sun

et al. 2021

Applied Sciences

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“…Because in situ observations are scarce in the RC region, reanalysis data are used as a reference to outline the basic structure and variability of the RC system. The 1/12° global HYCOM reanalysis has been reported to perform well 26,29 in simulating the RC system when compared to previous long-term tall-mooring observations, but its ability to reproduce RC subsurface structure in the upstream region has never before been validated with in situ measurements. Our mooring observations offer a unique opportunity to do this.…”

Section: Discussionmentioning

confidence: 98%

“…Moreover, the mean flow is approximately barotropic removing the eddies' baroclinic velocity structure, suggesting other candidates for the RC's formation mechanism. Based on the analysis of HYCOM 29 , a branch of the Kuroshio east of Taiwan with consistent velocities in the vertical, feeds the RC and may explain the barotropic feature of the RC. And the eddies contribute to its mean VT and variability along the streamline, which is consistent with previous studies upstream 15,16 .…”

Section: Discussionmentioning

confidence: 99%

“…Numerical models such as those of Thoppil et al 26 , You & Yoon 27 , and Nakamura et al 28 are helpful for filling in spatial and temporal gaps in observations and revealing the physical mechanisms of the generation and evolution of the RC. Among these models, the 1/12° HYbrid Coordinate Ocean Model (HYCOM) reanalysis was reported to perform the best, in terms of its consistency with previous observations in the RC region 3,29 . However, the subsurface intensified velocity core in the upstream region of the RC has never before been validated with observations.…”

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confidence: 94%

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

Zhao

Nakamura

Zhu

et al. 2020

Sci Rep

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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.

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“…M. Wang et al. (2019) validated the HYCOM data by comparing the reanalysis data with the mooring observations across the Kuroshio east of Taiwan. They revealed that the Ryukyu Current is a branch of the Kuroshio and intensifies as it flows poleward along the Ryukyu Island chain owing to the added mass influx from the ocean interior.…”

Section: Data Sets and Methodsmentioning

confidence: 99%

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

et al. 2022

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Linear trends in the Ryukyu Current, a part of the western boundary current in the western North Pacific flowing on the seaside of the Ryukyu Island chain, were investigated using reanalysis data during 1993–2018. The subsurface Ryukyu Current has weakened along its path during the recent decades. Two determinant factors for the weakened subsurface Ryukyu Current are as follows. First, the first baroclinic topographic Rossby wave propagated signals emanating from the Tokara Strait (TK) and the Kerama Gap (KG) southward along the eastern slope of the Ryukyu Island chain, which depressed the onshore side of the isopycnal. The weakened Kuroshio in the TK and weakened overflow through the KG during this period led to isopycnal shoaling along the onshore side east of the Ryukyu Island chain, slowing down the Ryukyu Current. Second, anticyclonic eddies emanated from the interior region near 30°N and 170°W increased during this period. These anticyclonic eddies, translating southwestward, reached east of the Ryukyu Island chain, and finally deepened the offshore side of the isopycnal depth of the Ryukyu Current. The isopycnal across the Ryukyu Current velocity core became less steep, thereby weakening the subsurface Ryukyu Current. Moreover, a positive trend of sea surface height anomaly southeast of Miyakojima, driven by wind stress changes in high latitudes (near the Kuroshio Extension band), strengthened the northward current in the upper layer southeast of Miyakojima.

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Origin and formation of the Ryukyu Current revealed by HYCOM reanalysis

Cited by 11 publications

References 23 publications

Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data

Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data

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

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

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