Study on circulation and water mass formation in the intermediate and deep layers of the Okinawa Trough

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Circulation and water mass formation in the intermediate and deep layers of the Okinawa Trough were investigated using comprehensive datasets for 1) water properties on the minimum salinity surface mainly derived from Argo float profiles, 2) velocity data in the Kerama Gap measured by moored current-meters from 2009 to 2011, and 3) CTD/LADCP data over the southern Okinawa Trough measured in Junes 2011 and 2013.Intermediate-water properties are largely modified in the Okinawa Trough due to strong diapycnal diffusion (6.8-21.5 cm2/s based on an advection-diffusion equation). Deep water below the sill depth of the Kerama Gap (~1100 m), is ventilated by the bottom-overflow in the Kerama Gap down to the deepest layer (~2000 m) in the southern Okinawa Trough. A box model predicts that this bottom-overflow (0.18-0.35 Sv) causes strong upwelling in the southern Okinawa Trough, which must be maintained by buoyancy gain of the deep water due to strong diapycnal diffusion (4.8-9.5 cm2/s).