Estimates of Kuroshio transport using an inverse technique

Abstract

Two CTD/hydrographic sections across the Kuroshio were combined using an inverse technique to estimate the absolute transport. The hydrographic data were obtained as part of a transpacific section across 24°N in 1985. The inverse technique treats the two sections as ends of a channel and conserves mass flowing into and out of the channel as a whole and within certain discrete layers. The strong topographic constraints imposed by the region of the East China Sea resulted in transport estimates independent of the initial reference level for the geostrophic calculation.

The calculated transports were 26.6 Sv northwest of Okinawa and 21.9 Sv across the Tokara Straits. The accuracy of the estimate was approximately 3.3 Sv for the Okinawa section and 5.1 Sv for the Tokara Straits section. The principal errors in the calculation came from lack of knowledge of the flow in the shallow areas of both sections, inadequate sampling of the rapidly varying topography, an estimate of 5 Sv transport in the Tsushima Current and Osumi branch of the Kuroshio and uncertainty over the relative weighting given in the inverse solutions to the different sections.

A set of acoustic Doppler current profiler (ADCP) data taken simultaneously was combined with the inverse model. Because initial mass imbalances were smaller, the combined model gave a better estimate of transport than that of the model using the CTD data alone. Two different methods of using the ADCP data in the inverse model were compared. It was found to be preferable to use the ADCP data as an initial reference for the geostrophic velocities, rather than as a set of separate constraints.