The current system east of the Ryukyu Islands as revealed by a global ocean reanalysis

The structure and variability of the Ryukyu Current System (RCS), which forms the western boundary current along the eastern slope of the Ryukyu Islands, are studied using results from a 32-layer, 1/12.5° global HYbrid Coordinate Ocean Model (HYCOM) reanalysis for the period 1993-2012. The reanalysis realistically reproduces salient features of the velocity observations at three locations southeast of Miyakojima, Okinawa and Amami-Oshima. The mean velocity across the three sections shows a well-developed subsurface RCS core between 700 and 900 m. The current core southeast of Amami-Oshima shows year-to-year variations with cyclonic (anticyclonic) circulation east of Amami-Oshima generating weak (strong) velocity cores. RCS interactions with anticyclonic eddies often produce a two-core velocity structure, with a surface core in the upper 300 m and a deeper core near 700-900 m. The horizontal structure of the RCS at 15 m depth shows a well-developed northeastward current northeast of Okinawa, which is partly fed by the southwestward extension of the anticyclonic recirculation gyre. The RCS forms a continuous northeastward current from Miyakojima to Amami-Oshima below 500 m with shoreward intensification. A quasi-steady southwestward current is present below ~2000 m with its origin at 30°N, triggered by a cyclonic eddy. The volume transports across these three sections have respectively the mean values of 0.6, 6.2 and 12.4 Sv (Sv = 10⁶ m³ s^-1) and standard deviations of 10.2, 7.1 and 11.3 Sv. They have dominant seasonal variations with the maximum in winter and spring and minimum in summer. On interannual time scales the transport anomaly, which co-varies with the RCS core, results from the westward propagating mesoscale eddies from the east.