Variability of the Deep Overflow through the Kerama Gap Revealed by Observational Data and Global Ocean Reanalysis

Herein, the temporal variability of the deep overflow through the Kerama Gap between the East China Sea and the Philippine Sea is investigated based on observational data combined with reanalysis data obtained during 2004–2011. The observations and model results show a strong bottom-intensified flow intruding into the deep Okinawa Trough. The observed deep overflow shows intraseasonal variations that are enhanced from August to November. The variability in the deep overflow via the Kerama Gap is well-correlated with the density changes near its sill depth in the Philippine Sea. Additionally, some portion of the dense water originates from a region east of Miyakojima, which can be related to the northeastward-flowing Ryukyu Current at intermediate depths. In contrast, three extreme deep overflow events indicate that the arriving mesoscale eddies propagated from the east resulted in an increase in the density near the Kerama Gap sill than that on the Okinawa Trough side. The density difference associated with the baroclinic pressure gradient across the Kerama Gap forced the deep overflow into the Okinawa Trough. The volume transport of the deep overflow computed by integrating the cross-sectional velocity and through hydraulic theory are 0.14 and 0.11 Sv (1 Sv = 106 m3/s), respectively.