Tracer transport in cold-core rings pinched off from the Kuroshio Extension in an eddy-resolving ocean general circulation model

Abstract

[1] Using an eddy-resolving ocean general circulation model, we examine the cold-core rings pinched off from the Kuroshio Extension (KE), focusing on their tracer transport. The origin of the tracers in the rings and how they are trapped are directly investigated by releasing two passive tracers, corresponding to the KE water and the water in the Mixed-Water Region (MWR) to the north of the KE. The cold-core rings carry the tracer in the MWR wrapped by the KE water from the surface to the bottom at the pinch-off. In and above the upper thermocline ( σθ < 26.4), these tracers are trapped in the ring and move along the ring until the rings rejoin the Kuroshio. Even in the lower thermocline ( σθ ∼ 26.4–27.2), a substantial part of the tracers moves along the ring, while gradual leakage occurs. Vorticity balance in the rings is consistent with the passive tracer experiment, indicating that water in the upper thermocline moves along the rings, while the synchronous movement in and below the lower thermocline is severely contaminated by baroclinic Rossby waves. These direct estimations are compared with the indirect diagnoses using PV, the ratio of drift velocity of ring to its tangential velocity (nonlinear parameter), and the largest closed contour of stream function in the comoving frame of the ring. These indirect diagnoses are consistent with the direct estimation in the upper thermocline, but are not good indicators below the lower thermocline. The cause of their insufficient performance is discussed.