Genta Mizuta

Structure and Seasonal Variability of the East Sakhalin Current

Abstract In order to clarify the structure and seasonal variability of the flow field near the western boundary of the Sea of Okhotsk, long-term mooring measurements were carried out from 1998 to 2000 in this region. In most of the mooring period a persistent southward flow (the East Sakhalin Current) was observed, which extends from the surface to a depth around 1000 m. The speed of this southward flow clearly changed seasonally. The peak monthly mean speed along 53°N at a depth of 200 m attained a maximum of 37 ± 9 cm s−1 in January and a minimum of 10 ± 8 cm s−1 in July. Three different cores of intense flow were identified in the southward flow. The first core was centered over the continental slope and had rather large vertical extent, reaching the bottom on the slope. The second core was trapped over the shelf near the surface and was observed from October to November. This core was associated with less saline surface water affected by the Amur River discharge. The third core was intensified toward ...

Sverdrup Balance and the Cyclonic Gyre in the Sea of Okhotsk

Abstract It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean–Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s−1 near the western boundary and slow northward flow with an average speed of approximately 2 cm s−1 in the east. Climatological data show that along zonal sections at 50°–53°N isopycnal surfaces g...

Winter oceanographic conditions in the southwestern part of the Okhotsk Sea and their relation to sea ice

In the southwestern part of the Okhotsk Sea, oceanographic and sea-ice observations on board the icebreaker Soya were carried out in February 1997. A mixed layer of uniform temperature nearly at the freezing point extending down to a depth of about 300 m was observed. This is much deeper than has previously been reported. It is suggested that this deep mixed layer originated from the north (off East Sakhalin), being advected along the shelf slope via the East Sakhalin Current, accompanied with the thick first-year ice (average thickness 0.6 m). This vertically uniform winter water, through mixing with the surrounding water, makes the surface water more saline (losing a characteristic of East Sakhalin Current Water) and the water in the 100–300 m depth zone less saline, colder, and richer in oxygen (a characteristic of the intermediate Okhotsk Sea water). The oceanographic structure and a heat budget analysis suggest that new ice zone, which often appears at ice edges, can be formed through preconditioning of thick ice advection and subsequent cooling by the latent heat release due to its melting.

Structure of the Circulation Induced by a Shoaling Topographic Wave

Abstract The structure of the potential vorticity flux and a mean flow induced by a topographic wave incident over a bottom slope are investigated analytically and numerically, with focus on the case that bottom friction is the dominant dissipation process. In this case it is shown that the topographic wave cannot be a steady source of the potential vorticity outside the bottom Ekman layer. Instead, the distribution of potential vorticity is determined from the initial transient of the topographic wave. This potential vorticity and the heat flux by the topographic wave at the bottom determine the mean flow and give a relation between the horizontal and vertical scales of the mean flow. When the horizontal scale of the mean flow is larger than the internal deformation radius and the potential vorticity is not so large, the mean flow is almost constant with depth independent of whether the topographic wave is bottom intensified. Then the mean flow is proportional to the divergence of the vertically integrat...

Sea-ice drift characteristics revealed by measurement of acoustic Doppler current profiler and ice-profiling sonar off Hokkaido in the Sea of Okhotsk

Abstract In the southwestern part of the Sea of Okhotsk off Hokkaido, sea-ice drift characteristics are investigated using the ice and water velocities obtained from a moored upward-looking acoustic Doppler current profiler (ADCP) during the winters of 1999–2001. Using hourly-mean values of these data along with the wind data measured at a nearby coastal station, the wind factor and turning angle of the relative velocity between the ice and water velocities with respect to the wind are calculated assuming free drift under various conditions. Since the simultaneous sea-ice draft data are also available from a moored ice-profiling sonar (IPS), we examine the dependence of drift characteristics on ice thickness for the first time. As ice thickness increases and wind decreases, the wind factor decreases and the turning angle increases, as predicted by the theory of free drift. This study clearly shows the utility of the moored ADCP measurement for studying sea-ice drift, especially with the simultaneous IPS measurement for ice thickness, which cannot be obtained by other methods.

Three-Dimensional Structure of Thermohaline Circulation Steered by Bottom Topography

An investigation based on the expansion in terms of diffusive vertical modes is made on the effect of an isolated seamount on the three-dimensional structure of thermohaline circulation driven by an inflow of deep water. While the circulation has a relatively simple structure dominated by the first baroclinic mode in a flat ocean, it becomes rather complicated in an ocean with a seamount. Nevertheless its three-dimensional structure is reconstructed and understood well by analyzing the dynamics of the vertical modes. It is shown that the vertical modes respond to the topography-induced stretching in a similar way irrespective of the mode number. When synthesized in the deep layer, therefore, the vertical modes interfere constructively to yield the same pattern of circulation as a single vertical mode or the diffusive reduced-gravity model. It is thus confirmed that the abyssal circulation certainly has the features predicted by the diffusive reduced-gravity model such as cyclonic circulation over the mountain or downwelling in the geostrophic island, in which geostrophic contours close on themselves. Close to the surface, however, different modes of different vertical structures interfere destructively so that the horizontal distribution of topography-induced circulation in the deep layer decays, upward shifting its center westward.

A Study on the Effects of Bottom Topography on Deep Circulation with a Diffusive Reduced-Gravity Model

Abstract Abyssal circulation around a sea mountain and a depression in a midlatitude basin is investigated with a reduced-gravity model characterized by vertical and horizontal diffusion of both momentum and density. Above either topography a low with the associated cyclonic circulation is induced. The low, however, extends to the west at poleward latitudes for the sea mountain and equatorward at latitudes for the depression. These features are explained well by a perturbation theory to the second order with respect to the height of a sea mountain (depression). Even when topography is steep enough to generate a region called a geostrophic island or a planetary island, the resulting circulation remains qualitatively the same. One difference is that the current is concentrated to make a jet along the border and the tail of the geostrophic island, whore the tail is defined by the branch of the critical geostrophic contour extending westward. Inside the geostrophic island, upwelling becomes quite feeble or ev...

Atlantic–Pacific asymmetry of subsurface temperature change and frontal response of the Antarctic Circumpolar Current for the recent three decades

For the 32-year period from 1979 to 2010, trends of surface and subsurface temperature and meridional motion of the current system in the Antarctic Circumpolar Current (ACC) region are studied with in situ observations and an eddy-resolving general circulation model. The observed and simulated surface temperature shows a similar pattern between the Atlantic and Pacific: warming to the north of the Subantarctic/Subtropical Fronts in the Atlantic and of the Subtropical Front in the Pacific and cooling to the south of those fronts. The subsurface temperature trend, again from both observation and model, reveals an asymmetric pattern between the Atlantic and Pacific: subsurface warming is dominant over the whole ACC region in the Atlantic, while both warming and cooling are significant in the Pacific, the former located to the north of the Subantarctic Front and the latter to the south. The model reveals that the ACC has generally shifted poleward in the Atlantic, while it has shifted equatorward around Subantarctic Front and Polar Front in the Pacific. The ACC shift is consistent with the overall subsurface temperature trend. The basin-scale difference of the ACC response can be related to the different regime of the trend in meridional gradient of the zonal wind stress to the north and south of 50–55°S and suggests a coupling of the ACC and overlying westerly on the multi-decadal time scale.

Role of the Rossby Waves in the Broadening of an Eastward Jet

To investigate the effect of the Rossby waves on an eastward jet such as the Kuroshio or Gulf Stream Extensions, a series of numerical experiments is conducted using a primitive equation model. In these experiments, an inflow and an outflow imposed on the western and eastern boundaries drive an unstable narrow jet and a broad interior flow in the western and eastern regions of the model domain, respectively. The barotropic Rossby waves are radiated from the transient region between the two regions. The eddy potential vorticity flux by the waves tends to compensate for the difference in the mean potential vorticity along mean streamlines between both sides of the transient region. Instability of the jet is insufficient for this compensation andweakens themeanpotential vorticitygradienttoo much.Moreover,as thepotential vorticityof the outflowisincreased,theRossbywavesareintensifiedinordertocompensatefortheincreasein thedifference in the mean potential vorticity. These features strongly suggest that the Rossby waves are substantial in matching ajetwith aninteriorflow.Thespeedof thewavesandpropertiesofeddiesin recirculations of thejet areconsistentwithatwo-layeranalyticmodel,whichindicatesthattheRossbywavesareradiatedfromeddies in recirculations. These eddies as well as the Rossby waves increase in amplitude with the transport of the recirculation nearthe surface presumably because of mean advection. Therefore, the mean potentialvorticity of the interior flow, the intensity of the Rossby waves, and the transport of the recirculation change consistently with one another.

Observations of frazil ice formation and upward sediment transport in the Sea of Okhotsk: A possible mechanism of iron supply to sea ice

In the Sea of Okhotsk, sediment incorporation, transport and release by sea ice potentially plays important roles in the bio-related material (such as iron) cycle and ecosystem. The backscatter strength data of bottom-mounted Acoustic Doppler Current Profilers have suggested signals of frazil ice down to 30 m depth, and signals of upward sediment transport throughout the water column simultaneously in the region northeast of Sakhalin, with a water depth of ∼100 m. Such events occurred under turbulent conditions with strong winds of 10–20 m s−1. During such events, newly formed ice was present near the observational sites, shown by satellite microwave imagery. Sediment dispersion from the bottom occurred in association with strong currents of 1.0–1.5 m s−1. During these events, the mixed layer reaches near the bottom due to wind-induced stirring, inferred from the high frequency component of vertical velocity. Thus the winter time turbulent mixing brings re-suspended sediment up to near the ocean surface. This study provides the first observational evidence of a series of processes on the incorporation of sedimentary materials into sea ice: sedimentary particles are dispersed by the strong bottom current, subsequently brought up to near the surface by winter time mixing, and finally incorporated into sea ice through underwater interaction with frazil ice and/or flooding of sea ice floes. This wintertime incorporation of bottom sediment into sea ice is a possible mechanism of iron supply to sea ice which melts in spring, and releases bio-reactive iron into the ocean.