Mixing in the meandering Kuroshio Extension and the formation of North Pacific Intermediate Water

Synoptic hydrographic and velocity data were obtained in the first meander crest and trough of the Kuroshio Extension (KE) from measurements aboard the R/V Soyo-maru in late May and early June 1998. Measurements indicated that the eastward flowing KE had significant (up to 20 cm s−1) flow at 1400 m depth and that a subsurface velocity maximum could be seen on the equatorward side of the current. Large salinity differences were found on potential densities from 26.2 to 27.2 kg m−3 that reflected cold, fresh waters of northern origin and warm, salty subtropical waters. Approaching the first meander crest, northward moving water was saltier than southward moving water at the same density on the downstream side of the crest, approaching the first trough. The velocity-salinity covariance was high and consistent with an eddy diffusivity of order 104 m2 s−2. With an assumption of quasi-steady meanders, supported by altimeter results, density advection can be used to infer vertical velocities of 50–100 m d−1 in the meanders. Ascending water was found to be saltier than the descending water, especially in a potential density range about 26.8 kg m−3 within the core of North Pacific Intermediate Water (NPIW)). However, the two patterns of horizontal velocity-salinity covariance and vertical velocity-salinity covariance were not consistent with one another. The mixing of Oyashio and Kuroshio source waters, thought to be critical to the formation of NPIW, is clearly tied to cross-frontal flows within the meandering KE, with Kuroshio water ejected northward as water parcels approach the first meander crest and Oyashio water injected southward into the flow as parcels approach the first trough.