Mooring Observations of the current system southeast of Miyakojima Island

The origin, structure, and variability of the Ryukyu Current (RC) has long been debated, mostly due to limited observations. Using 2-year records from a mooring array southeast of Miyakojima Island, we illustrate the velocity structure and variability of the RC. Observations showed that the shoreward intensified currents flowed northeastward. The subsurface core of the RC was located near the 1000 m isobath, with a maximum of 19.4 cm s^-1 at 500 m. The observed velocity structure was reproduced well by the HYCOM reanalysis, except that the observed current core was stronger and shallower. The mean estimated volume transport across the observation section was 9.0 Sv (1 Sv = 10⁶ m³ s^-1) and a standard deviation of 8.7 Sv, with near 100-day variability dominant in the upper layer but absent in the deep layer. This study presents the existence of Ryukyu Current in its upstream region for the first time.