Atsuhiko Isobe

On the origin of the Tsushima Warm Current and its seasonality

Abstract There are two different schools of thought with regard to the origin of the Tsushima Warm Current. One school of thought believes that it comes from the Taiwan Strait, while the other believes that it enters the East China Sea from the Kuroshio region southwest of Kyushu, Japan, crossing the steep shelf slope. Using the seasonally averaged distributions of the wind (30-year mean of Comprehensive Ocean Atmosphere Data Set) and density (data from 1961 to 1990 supplied by Japan Oceanographic Data Center) in the East China and Yellow Seas, we determine the volume transport distribution diagnostically through a numerical model in which we solve the vorticity equation of the vertically averaged flow. Then, we try to identify the origin of the volume transport of the Tsushima Warm Current and its seasonality. The results except for the autumn indicate the existence of the so-called Taiwan–Tsushima Warm Current System (Fang et al., 1991), i.e. a continuity of the volume transport between the Taiwan and Tsushima/Korea Straits. However, this current system breaks down in the autumn. According to this study, about 66% of the volume transport of the Tsushima Warm Current comes directly from the Kuroshio region in autumn, crossing the shelf edge of the East China Sea.

Seasonal variability in the Tsushima Warm Current, Tsushima-Korea Strait

To study seasonal variability in the Tsushima Warm Current, flowing to the northeast in the Tsushima-Korea Strait, ADCP surveys were undertaken about every 2 months from 1990 to 1991. The survey line extended from Kawajiri-misaki, Japan, to some 30 km off Ulsan, Korea. The tidal current component was removed from raw ADCP data to understand the regional residual current. The total volume transport towards the northeast, across the Strait, was at a maximum of 5.6 Sv (1 Sv = 1 × 106 m3 s−1), in September. Total volume transports were around 1.0 Sv during the other months. Seasonal variability in the current structure across the Strait has also been investigated. The intensified core of the northeasterly current formed near the Korean coast in September. This resulted in the maximum total volume transport in the same month.

Selective transport of microplastics and mesoplastics by drifting in coastal waters

The quantity and size distributions of small plastic fragments in the Seto Inland Sea, Japan were investigated using field surveys and a numerical particle-tracking model. The model was used to interpret the distributions of small plastic fragments and the possible transport processes in coastal waters. Of note, the size and quantity of mesoplastics (approximately >5mm) gradually increased close to the coast irrespective of the existence of river mouths, which probably act as a major source of anthropogenic marine debris. Additionally, microplastics were more dominant as we moved further offshore. The numerical model reproduced the near-shore trapping of mesoplastics, suggesting that mesoplastics are selectively conveyed onshore by a combination of Stokes drift and terminal velocity, dependent on fragment sizes. It is suggested that mesoplastics washed ashore on beaches degrade into microplastics, and that the microplastics, which are free from near-shore trapping, are thereafter spread offshore in coastal waters.

Seasonal variability of the barotropic and baroclinic motion in the Tsushima-Korea Strait

The aim of this study is to elucidate the seasonal variation in the volume transport through the Tsushima-Korea Strait using the sea level difference across the Strait. The sea level difference associated with the baroclinic motion is estimated from the geostrophic current profile, which is calculated as its vertical integrated transport is zero, using the CTD data from 1988 to 1990. The sea level difference associated with the barotropic motion is estimated by subtracting the sea level difference associated with the baroclinic motion from the observed one. The range (maximum-minimum) of the seasonal variation in the volume transport is evaluated about 0.7 Sv on the average, using the sea level difference associated with the barotropic motion. It is one third of the seasonal variation in the volume transport which is estimated from observed sea level difference on the assumption that no baroclinic component exists. Such analyses also indicate that the volume transport was at a maximum in early winter and at a minimum in early spring from 1988 to 1990. The negative correlation is also found between the volume transport through the eastern channel and that through the western channel. Moreover, it is noticed that the seasonal variation in the surface current velocity in the Strait largely contains baroclinic motions which are locally caused in the Tsushima-Korea Strait.

Ballooning of River-Plume Bulge and Its Stabilization by Tidal Currents

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.

Sequential monitoring of beach litter using webcams

This study attempts to establish a system for the sequential monitoring of beach litter using webcams placed at the Ookushi beach, Goto Islands, Japan, to establish the temporal variability in the quantities of beach litter every 90 min over a one and a half year period. The time series of the quantities of beach litter, computed by counting pixels with a greater lightness than a threshold value in photographs, shows that litter does not increase monotonically on the beach, but fluctuates mainly on a monthly time scale or less. To investigate what factors influence this variability, the time derivative of the quantity of beach litter is compared with satellite-derived wind speeds. It is found that the beach litter quantities vary largely with winds, but there may be other influencing factors.

Two-Layer Model on the Branching of the Kuroshio Southwest of Kyushu, Japan

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.

Using aerial photography and in situ measurements to estimate the quantity of macro-litter on beaches.

This study has demonstrated a reliable method of quantifying the total mass of litter on a beach. It was conducted on Ookushi beach, Goto-Islands, Japan, and uses a combination of balloon-assisted aerial photography and in situ mass measurements. The total mass of litter over the beach was calculated to be 716±259kg. This figure was derived by multiplying the litter-covered area (calculated using balloon-assisted aerial photography) by the mass of litter per unit area. Light plastics such as polyethylene made up 55% of all plastic litter on the beach, although more work is needed to determine whether lighter plastics are transported to beaches more readily by winds and ocean currents compared with heavier plastics, or whether lighter plastics comprise a greater percentage of marine litter. Finally, the above estimates were used to calculate the total mass of metals released into coastal ecosystems via plastic litter on beaches.

East Asian seas: A hot spot of pelagic microplastics.

To investigate concentrations of pelagic micro- (<5mm in size) and mesoplastics (>5mm) in the East Asian seas around Japan, field surveys using two vessels were conducted concurrently in summer 2014. The total particle count (pieces km(-2)) was computed based on observed concentrations (pieces m(-3)) of small plastic fragments (both micro- and mesoplastics) collected using neuston nets. The total particle count of microplastics within the study area was 1,720,000 pieces km(-2), 16 times greater than in the North Pacific and 27 times greater than in the world oceans. The proportion of mesoplastics increased upstream of the northeastward ocean currents, such that the small plastic fragments collected in the present surveys were considered to have originated in the Yellow Sea and East China Sea southwest of the study area.

Microplastics in the Southern Ocean

A field survey to collect microplastics with sizes <5mm was conducted in the Southern Ocean in 2016. We performed five net-tows and collected 44 pieces of plastic. Total particle counts of the entire water column, which is free of vertical mixing, were computed using the surface concentration (particle count per unit seawater volume) of microplastics, wind speed, and significant wave height during the observation period. Total particle counts at two stations near Antarctica were estimated to be in the order of 100,000pieceskm-2.