Intermediate and deep water formation in the Okinawa Trough

Abstract

Water mass formation in the intermediate and deep layers of the Okinawa Trough is investigated using two distinct data sets: a quasi‐climatological data set of the water properties of the minimum salinity surface produced from Argo float profiles and historical CTD data, and a velocity data set in the Kerama Gap measured by moored current meters during June 2009 to June 2011. The formation process of Okinawa Trough Intermediate Water is explained on the basis of horizontal advection and mixing of North Pacific Intermediate Water (NPIW) and South China Sea Intermediate Water (SCSIW). The salinity‐minimum water intruding into the Okinawa Trough through the channel east of Taiwan is approximately composed of 45% NPIW and 55% SCSIW, while that through the Kerama Gap is 75% NPIW and 25% SCSIW. Salinities of these water masses increase in the Okinawa Trough due to strong diapycnal diffusion; its coefficient is estimated as 6.8–21.5 × 10 −4 m 2 s −1 based on a simple advection‐diffusion equation. On the other hand, deep water in the Okinawa Trough, below the sill depth of the Kerama Gap (∼1100 m), is ventilated by overflow in the bottom layer of the Kerama Gap down to the deepest layer (∼2000 m) in the southern Okinawa Trough. A simple box model predicts that this bottom overflow (0.18–0.35 Sv) causes strong upwelling (3.8–7.6 × 10 −6 m s −1 ) 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 × 10 −4 m 2 s −1 ).