Freshwater spreading far offshore the Japanese coast

River discharge to the ocean influences the transport of salts and nutrients and is a source of variability in water mass distribution and the elemental cycle. Recently, using an underwater glider, we detected thick, low-salinity water offshore for the first time, probably derived from coastal waters, in the central-eastern Sea of Japan, whose primary productivity is comparable to that of the western North Pacific. Thereafter, we aimed to investigate the offshore advection and diffusion of coastal water and its variability and assess their impact. We examined the effects of river water discharge on the flow field and biological production. Numerical experiments demonstrated that low-salinity water observed by the glider in spring was discharged from the Japanese coast to offshore regions. The water is discharged offshore because of its interaction with mesoscale eddies. A relationship between the modeled low-salinity water transport to the offshore region and the observed chlorophyll-a in the offshore region was also observed, indicating the influence of river water on offshore biological production. This study contributes to understanding coastal-offshore water exchange, ocean circulation, elemental cycles, and biological production, which are frontiers in the Sea of Japan and throughout the world

Turbulent bottom Ekman boundary layer measured over a continental shelf

An acoustic Doppler current profiler was deployed in summer and autumn in 2007-2009 at two stations on the East China Sea shelf. Clear velocity spirals that basically correspond to theoretical Ekman spirals were identified for both mean and tidal currents. From these spirals and the corresponding Ekman equation, we estimated the time-averaged eddy viscosity () profiles. The estimate dμ was largest (2-3 × 10-3 m2 s -1) around 5 m from the bottom and decreased almost exponentially with height. A qualitatively similar profile of the eddy diffusivity was also inferred from the acoustic Doppler current profiler data and microstructure profiler data. The flux Richardson number was estimated as 0.11±0.10 ∼ 0.46 0.17, indicating relatively large buoyancy contribution to the turbulent kinetic energy budget.Yoshikawa, Y., Endoh, T., Matsuno, T., Wagawa, T., Tsutsumi, E., Yoshimura, H., Morii, Y., (2010), Turbulent bottom Ekman boundary layer measured over a continental shelf, Geophysical Research Letters, 37(15), L15605, DOI:10.1029/2010GL044156. To view the published open abstract, go to http://dx.doi.org and enter the DOI

Low ocean-floor rises regulate subpolar sea surface temperature by forming baroclinic jets

Sea surface temperature (SST) fronts in mid-to high-latitude oceans have significant impacts on extratropical atmospheric circulations and climate. In the western subarctic Pacific, sharp SST fronts form between the cold subarctic water and the recently found quasi-stationary jets that advect warm waters originating in the Kuroshio northeastward. Here we present a new mechanism of the jet formation paying attention to the propagation of baroclinic Rossby waves that is deflected by eddy-driven barotropic flows over bottom rises, although their height is low (similar to 500 m) compared with the depth of the North Pacific Ocean (similar to 6000 m). Steered by the barotropic flows, Rossby waves bring a thicker upper layer from the subtropical gyre and a thinner upper layer from the subarctic gyre, thereby creating a thickness jump, hence a surface jet, where they converge. This study reveals an overlooked role of low-rise bottom topography in regulating SST anomalies in subpolar oceans

Bathymetric Influences of the Emperor Seamounts upon the North Pacific Subarctic Gyre : Boundary Current along the Eastern Side of the Emperor Seamounts

To make clear bathymetric influences of the Emperor Seamounts upon the North Pacific subarctic gyre, numerical experiments were performed using two-layer model with an idealized wind and ridge. Seasonally varying interior Sverdrup transport to the east of the ridge partly returns along the eastern side of the ridge as a western boundary current. A ratio of the boundary transport to the interior transport depends on a ridge height (H_R) and stratification (N). For smaller H_R (H_R/H_0 < 0.3 where H_0 is background water depth), the ratio increases with H_R, but it changes little with N. For larger H_R (HR_/H_0 > 0.3), the ratio decreases with N but is independent of H_R, Spatial distribution of geostrophic contours is a key to understand the dependence of the ratio on H_R and N. For smaller H_R, the ratio almost coinsides with a ratio of the number of closing geostrophic contours to one of all geostrophic contours. For larger H_R at which all geotstrophic contours in the eastern region cannot cross over the ridge, bottom pressure torque decreases with N to allow more transport across the ridge

Spatial variations in zooplankton community structure along the Japanese coastline in the Japan Sea: influence of the coastal current

This study evaluates spatial variations in zooplankton community structure and potential controlling factors along the Japanese coast under the influence of the coastal branch of the Tsushima Warm Current (CBTWC). Variations in the density of morphologically identified zooplankton in the surface layer in May were investigated for a 15-year period. The density of zooplankton (individuals per cubic meter) varied between sampling stations, but there was no consistent west–east trend. Instead, there were different zooplankton community structures in the west and east, with that in Toyama Bay particularly distinct: Corycaeus affinis and Calanus sinicus were dominant in the west and Oithona atlantica was dominant in Toyama Bay. Distance-based redundancy analysis (db-RDA) was used to characterize the variation in zooplankton community structure, and four axes (RD1–4) provided significant explanation. RD2–4 only explained  &lt;  4.8 % of variation in the zooplankton community and did not show significant spatial difference; however, RD1, which explained 89.9 % of variation, did vary spatially. Positive and negative species scores on RD1 represent warm- and cold-water species, respectively, and their variation was mainly explained by water column mean temperature, and it is considered to vary spatially with the CBTWC. The CBTWC intrusion to the cold Toyama Bay is weak and occasional due to the submarine canyon structure of the bay. Therefore, the varying bathymetric characteristics along the Japanese coast of the Japan Sea generate the spatial variation in zooplankton community structure, and dominance of warm-water species can be considered an indicator of the CBTWC