Three-Dimensional Structure of Frontal Waves and Associated Salinity Minimum Formation along the Kuroshio Extension

Abstract

Abstract High-resolution hydrographic observations were conducted in September 2002 to describe the detailed structure of frontal waves along the Kuroshio Extension. Frontal waves were observed both in the upper and intermediate layers, which corresponded to the depths of North Pacific Subtropical Mode Water and North Pacific Intermediate Water, respectively. The frontal wave in the upper layer preceded the intermediate-depth frontal wave by about 1/4 wavelength. The vertical phase lag created situations in which the low-salinity Oyashio water in the intermediate layer was superimposed below the upper-layer high-salinity Kuroshio water, thus forming a vertical salinity minimum. The frontal waves with a wavelength of about 200 km and a phase speed of 0.2–0.3 m s−1 propagating in the downstream direction were consistent with those from satellite images. Vertical velocity fields were estimated with the quasigeostrophic omega equation. Downwelling around the trough (from the crest to trough) of the intermediate-depth (upper layer) frontal wave and upwelling around the crest (from the trough to the crest) were similar to the features observed in the Gulf Stream meanders. The phase difference between the upper and intermediate layers suggested that the frontal waves were baroclinically unstable waves.