Kosei Komatsu

Hydrographic structure and transport of the Oyashio south of Hokkaido and the formation of North Pacific Intermediate Water

Hydrographic structure and transport of the Oyashio south of Hokkaido were described with conductivity-temperature-depth and lowered acoustic Doppler current profiler (LADCP) survey performed in June 1998. The southwestward Oyashio transport just off the Hokkaido coast was 10.1 Sv in the density of 26.6–27.5σθ, in which 2.5 Sv was from the Okhotsk Sea and 7.6 Sv was from the Western Subarctic Gyre (WSAG). The Oyashio northeastward countercurrent was 4.9 Sv. The cross-gyre Oyashio transport in the area from the east coast of Hokkaido to the Subarctic Front was estimated to be 5.2 Sv; 2.4 Sv in 26.6–27.0σθ was mainly composed of low potential vorticity Okhotsk Sea water and 2.8 Sv in 27.0–27.5σθ mostly from WSAG, suggesting that the Okhotsk Sea water (WSAG water) would contribute to the formation of the upper (lower) part of the North Pacific Intermediate Water (NPIW). The Oyashio water was lower in oxygen than in the subtropical areas south of the Subarctic Front in the density of 26.9–27.6σθ, possibly because of the absence of the Antarctic Intermediate Water (AAIW) influence with the relatively high oxygen water that might be transported along the western boundary of the North Pacific Ocean and along the Kuroshio Extension, increasing the oxygen in the areas south of the Subarctic Front. Just south of the Subarctic Front, a cold- and fresh-core anticyclonic eddy was observed with a salinity minimum in the core, suggesting one possible formation process of NPIW. The density of the potential vorticity minimum in the southwestward Oyashio near Hokkaido was at around 26.65σθ, which was lower than 26.8–26.9σθ observed in the early 1990s. This is possibly because the potential vorticity vertical profiles in the Okhotsk Sea and WSAG significantly changed corresponding to the water mass regime shift occurred in the Subarctic Pacific in the mid-1990s [Kawasaki, 1999].

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.

Current-Induced Modulation of the Ocean Wave Spectrum and the Role of Nonlinear Energy Transfer

Abstract Numerical simulations were performed to investigate current-induced modulation of the spectral and statistical properties of ocean waves advected by idealized and realistic current fields. In particular, the role of nonlinear energy transfer among waves in wave–current interactions is examined. In this type of numerical simulation, it is critical to treat the nonlinear transfer function (Snl) properly, because a rigorous Snl algorithm incurs a huge computational cost. However, the applicability of the widely used discrete interaction approximation (DIA) method is strictly limited for complex wave fields. Therefore, the simplified RIAM (SRIAM) method is implemented in an operational third-generation wave model. The method approximates an infinite resonant quadruplet with 20 optimized resonance configurations. The performance of the model is assessed by applying it to fetch-limited wave growth and wave propagation against a shear current. Numerical simulations using the idealized current field reve...