Search Results

You are looking at 1 - 10 of 12 items for

  • Author or Editor: Atsuhiko Isobe x
  • All content x
Clear All Modify Search
Atsuhiko Isobe

Abstract

A two-layer numerical model is used to investigate the continental-shelf circulation forced by western boundary currents along shelf edges. Emphasis is placed on an effect by frontal eddies (or frontal waves), which are reproduced in the model with the shelf edge along which a western boundary current is imposed. After long-term integration, the model reaches the equilibrium state in which alternating bands of long-term mean currents are found over the continental shelf. The generation of the band structure is consistent with the observational results obtained by acoustic Doppler current profilers on the shelf of the East China Sea. The western boundary current is accompanied by frontal waves, and so vertical momentum transfer occurs because of the diffusive stretching. Topographic Rossby waves are excited on the shelf slope by the vertical momentum transfer. Thereafter, energy of the waves proceeds to flow toward the lower-frequency limit by resonant interactions. The along-shelf wavenumber also vanishes to satisfy the dispersion relation of the topographic Rossby wave. As a consequence, the alternating bands of mean current are found over the continental shelf.

Full access
Atsuhiko Isobe

Abstract

When freshwater debouches into an adjacent ocean, an anticyclonic eddy (bulge) is formed in front of the river mouth. It is well known that a bulge growing offshore (ballooning) hardly reaches a steady state in the absence of either ambient currents or wind forcing. This study provides a physical interpretation for the ballooning of river-plume bulges by conducting numerical experiments in which a river plume is induced by a coastal freshwater source. Part of the freshwater released to the model ocean undergoes inertial instability. Near-inertial oscillations are predominant when disturbances are not forced in ambient waters of the river plume. These isotropic disturbances are amplified by inertial instability, so that unstabilized freshwater can move in arbitrary directions. Thus, unstabilized freshwater does not need to move toward the coastal boundary current on the right-hand side of the river mouth. Freshwater unstabilized for a long time can stay in the bulge for a long time. Unstabilized freshwater accumulates gradually in the bulge, and so ballooning occurs. When the direction of disturbances is prescribed in ambient waters, unstabilized freshwater is forced to move in the same direction. Thereby, the motion of unstabilized freshwater is restricted in the alongshore direction when background disturbances are induced by alongshore tidal currents. It is therefore concluded that tidal currents play a role in stabilizing the offshore growth of river-plume bulges in coastal and shelf waters.

Full access
Atsuhiko Isobe

Abstract

Observations suggest that a separation branch of the Kuroshio appears southwest of Kyushu, Japan. The observed density and surface current fields clearly show a separation branch in autumn. However, the separation branch disappears in winter and spring when the relatively homogeneous structure develops due to the intense surface cooling. A scenario generating a separation branch is examined using simple two-layer model. The northward Kuroshio along the shelf slope turns to the east abruptly, and leaves the shelf slope around Tokara Strait. A hump (local increase of the upper-layer thickness) is made by the lower-layer flow impinging on the bottom slope. This hump extends to the shallow shelf region due to the combined effect of advection and dissipation of the upper-layer thickness. Then the cross-isobath flow, that is, the separation branch, appears around the hump. When the thin upper layer obstructs the advection process in winter and spring, a separation branch disappears. Moreover, the separation branch disappears when a small damping coefficient is used.

Full access
Atsuhiko Isobe and Shiro Imawaki

Abstract

A two-layer numerical model driven by the wind stress is used to explain the observed annual variation of the Kuroshio transport south of Japan. Special attention is given to the effect of a ridge, representing the Izu–Ogasawara Ridge, on the generation of the baroclinic activity through the coupling of the barotropic and baroclinic modes of motion. For annual variation, lower-layer motion is found in areas surrounding the ridge because isostasy (a state of no motion in the lower layer) is not achieved within such a short timescale. Thus, the lower-layer flow impinges on the bottom slope. This impinging process generates anomalies of the upper-layer thickness especially on the eastern side of the ridge. Thereafter, anomalies move westward with characteristic velocities composed of the vertically averaged flow and westward propagation of the long baroclinic Rossby wave forced above the ridge. As anomalies of the upper-layer thickness move westward above the ridge, isostasy is accomplished with respect to these anomalies. As a result, the positive (negative) anomaly of the upper-layer thickness carries the information about the positive (negative) anomaly of the volume transport as it reaches the western edge of the ridge. Thereafter, anomalies of the volume transport are released to the west of the ridge. This experiment shows that the annual range of the volume transport east of the ridge is around 40 Sv, which is nearly equal to the zonal integration of the Sverdrup transport there. The annual range west of the ridge, however, reduces to around 10 Sv, which is mostly caused by the baroclinic activity generated above the ridge. Results are compared with the observed Kuroshio transport across the ASUKA line south of Japan. The annual range west of the ridge is consistent with that estimated from the observation.

Full access
Yasuyuki Miyao and Atsuhiko Isobe

Abstract

A novel observational technique to map surface ocean currents at high spatial resolution in narrow regions is developed. Low-altitude remote sensing using a digital camera suspended from a vessel-towed balloon is used to track trajectories of floating buoys deployed on the ocean. Surface-current velocities are thereafter computed by sequentially moving buoy locations on photo images converted into ground (Cartesian) coordinates. Field experiments were conducted in July and August 2013 using a balloon towed by a research vessel on the Seto Inland Sea. The image-derived currents were compared with those derived from buoy locations recorded by GPS receivers attached to each floating buoy. It was found that surface currents computed using GPS data contain unrealistic values arising from stochastic fluctuations in those data. However, the image-derived currents reproduced well convergent flows and a cyclonic eddy that accumulated foam and marine debris, as actually observed during the surveys. This performance is attributed to the fact that the image processing acts as a filter to remove erroneous buoy locations in computing surface currents. The estimated error was 4.1 cm s−1, sufficiently small to measure snapshots of surface coastal currents with magnitudes greater than several tens of centimeters per second.

Full access
Youn-Jong Sun and Atsuhiko Isobe

Abstract

Front locations are identified as the local maximum of the sea surface temperature gradient revealed in the continuously ferryboat-monitored data in the Seto Inland Sea, Japan. In addition, Simpson and Hunter’s H/U 3 values, where H is the water depth and U is the tidal-current amplitude, are computed at each cell using a fine-resolution, finite-volume ocean model to find the values at the fronts (i.e., Eulerian critical H/U 3 values). Moreover, Lagrangian critical H/U 3 values are computed using the same model in conjunction with particle-tracking experiments in which the particle’s H/U 3 values saved along their trajectories are all averaged at each cell. In comparison with the Eulerian H/U 3 values, it is revealed that the standard deviation of the Lagrangian critical values considerably reduces, especially for the fronts near straits. This accurate estimate of the critical H/U 3 values shows that critical values depend on the tidal-current amplitude at the front locations. The dependency of critical H/U 3 values on the tidal-current amplitude is likely due to dependency of the efficiency factor for vertical mixing in the original Simpson and Hunter formula on the amplitude. It is suggested that the efficiency factor for vertical mixing must be small (large) at locations with the large (small) tidal-current amplitude.

Full access
Atsuhiko Isobe and Shin’ichiro Kako

Abstract

To investigate whether the relatively warm Yellow and East China Seas play an active role in the deepening of extratropical cyclones over East Asia during winter, surface wind vectors downloaded from the Quick Scatterometer (QuikSCAT) website are used to compute the standard deviation of surface vorticity at ¼° resolution. In addition, a regional numerical atmospheric model is adopted to find atmospheric and/or oceanic conditions favorable for development of extratropical cyclones over the study area. These satellite-derived and modeled vorticity fields demonstrate that, on average, extratropical cyclone activity is moderate over the warm Yellow and East China Seas. This is because enhanced lower-level baroclinicity over these ocean areas is transferred as far as the shelf break of the East China Sea by strong northwesterly surface winds. Based on the numerical model results, the weak northwesterly surface wind condition is required for enhancing lower-level baroclinicity over the Yellow and East China Seas. This baroclinicity may contribute to enhancing cyclone development near Japan, with a simultaneous increase of lower-level baroclinicity over the Sea of Japan.

Full access
Atsuhiko Isobe and Robert C. Beardsley

Abstract

The interannual variation in cold-air outbreak activity over the Japan Sea is investigated using Japan Meteorological Agency buoy 21002 and Quick Scatterometer (QuikSCAT) wind data, Japan Oceanographic Data Center sea surface temperature (SST) data, NCEP–NCAR reanalysis surface wind and sea level pressure (SLP) data, and the winter Arctic Oscillation (AO) index of Thompson and Wallace. Cold-air outbreaks occur during the “winter” November–March period, and wind data for this season for the 19-winter period 1981–2000 were analyzed. Wavelet spectra averaged between 5- and 15-day periods were used to evaluate the intensity of cold-air outbreaks quantitatively. The winter mean wavelet spectra exhibited a clear interannual variation and a significant positive correlation with the AO index, indicating that intensive cold-air outbreaks frequently occur during relatively warm winters caused by a quasi-decadal AO. Based on the SST and SLP data, the low atmospheric surface pressure disturbances tend to develop over the warm East China Sea in warm winters in the positive AO phase. As these low SLP disturbances advance toward the northern Japan islands during the positive AO phase, they intensify more, leading to stronger cold-air outbreaks over the Japan Sea and increased sea surface cooling over the northern Japan Sea.

Full access
Atsuhiko Isobe, Shin’ichiro Kako, and Shinsuke Iwasaki

Abstract

Atmospheric responses to biological heating caused by the spring phytoplankton bloom in the Sea of Japan are investigated. Sea surface temperature (SST) is first computed using a mixed-layer model with an ocean reanalysis product. Satellite-derived surface chlorophyll concentrations representing phytoplankton population are input to an equation for attenuation coefficients of solar radiation penetrating the mixed layer. Two sets of SST are obtained by this model, using the attenuation coefficients with and without phytoplankton. It is found that the phytoplankton bloom increases SST by up to 0.8°C by mid-May, especially in the northern Sea of Japan. Thereafter, two experiments using a regional atmospheric numerical model are conducted for April and May. One imposes SST synthesized by multiple satellite observations on the lower boundary of the model (the green case). The satellite-derived SST includes influences of biological heating by phytoplankton in the actual ocean. The other uses SST reduced by differences between SSTs computed by the mixed-layer model with and without phytoplankton (the blue case). Under modest wind conditions, extratropical cyclones east and south of the Japan Islands in the blue case develop more rapidly than in the green case. Cyclones are likely initiated by the cool and dry air mass that enhances lower-level baroclinicity above oceanic fronts. This cool and dry air mass is transported from the Sea of Japan, where SST decreases in the absence of phytoplankton. Therefore, incorporating ocean biology is potentially capable of improving regional atmospheric and ocean general circulation models.

Full access
Shinsuke Iwasaki, Atsuhiko Isobe, and Shin’ichiro Kako

Abstract

A regional atmosphere–ocean coupled model is developed, based on the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model in conjunction with the Princeton Ocean Model, to investigate atmosphere–ocean coupled processes that might occur over the Yellow and East China Sea shelves in winter. To examine how the coupled processes actually work in the ocean, sea surface temperatures (SSTs) computed in both coupled and uncoupled models are compared with SSTs synthesized from multiple satellite observations. The results indicate that the coupled model significantly improves the negative SST bias in shallow waters around the Chinese coast produced by the uncoupled model. Cool and dry northerly winds from the Asian landmass reduce SST in these shallow waters through intensive upward heat loss. Thereafter, the horizontal gradient of sea level pressure (SLP) around the Chinese coast moderates because the land–ocean heat contrast weakens owing to the reduced SST in the coastal waters. As a result, the wind speed weakens, in line with the moderated horizontal SLP gradient. Moreover, northerly winds can reduce the transport of cool and dry air from the Asian landmass. Hence, upward heat flux around the coastal waters is reduced because of the weakening of the northerly winds and the decreased cool and dry air. This negative feedback thereby suppresses excessive SST cooling along the Chinese coast during winter.

Full access