Norihisa Imasato

A numerical study on the formation of the Kuroshio Counter Current and the Kuroshio Branch Current in the East China Sea

Abstract The Kuroshio Counter Current (KCC) and the Kuroshio Branch Current (KBC) are two unique features of the Kuroshio in the East China Sea. The mechanisms that generate the KCC and the KBC are studied using a barotropic inflow-outflow model with a simplified basin configuration of the East China Sea. The present study shows that the KCC can only exist on a β-plane frame of reference and its flow pattern is independent of the offshore Ryukyu Islands. In the East China Sea, the continental slope plays the role of a western boundary for the Kuroshio. Whether the Kuroshio protrudes onto the slope region, however, is found to have little influence on the KCC. Concerning the formation of the KBC, we found that the planetary β-effect and the existence of Taiwan Island are two indispensible conditions: the planetary β-effect drives part of the Kuroshio inflow to branch southwestward and Taiwan Island blocks this branched current causing it to protrude onto the continental shelf. Based on the numerical calculation, we further found that the branch current is reinforced by topographic Rossby waves induced by the repeated crossing of the Kuroshio over the continental slope.

Tidal Exchange through a Strait: A Numerical Experiment Using a Simple Model Basin

Abstract In order to investigate the mechanism of tidal exchange through a strait, we numerically track the Lagrangian movement of water particles over a full cycle of the M2 tide. As a result, it is found that the spatially rapid changes of the amplitude and the phase lag of the M2 current in the vicinity of the strait cause the exchange of an extremely large amount of water through the strait. The tidally-induced residual circulation in the vicinity of the strait also plays an important role in the water exchange. The calculated exchange ratio over one tidal cycle is ∼87%, i.e., the greater part of the outer water entering into the inner basin through the strait stays in the inner basin while an equal amount of basin water is drawn out after a cycle of the M2 tide. This fact also suggests that the major part of the water exchange through a strait is generated, not by turbulent diffusion, but by the dynamic process of the tidal current.

What is Tide-Induced Residual Current?

Abstract We carried out a numerical experiment to study the velocity field of a two-dimensional tidal current in a simple model basin with a narrow strait. It was found that the tide-induced transient eddy (TITE) originated from the low pressure area that is generated downstream behind a headland by the nonlinearity or the centrifugal force of the tidal current flowing with a large curvature through a narrow channel. The transient eddy is maintained during certain phases of the tide, and therefore the Eulerian tide-induced residual current is the result of the averaging process of transient phenomena; the Eulerian residual current is only a mathematical representation of the tide-induced transient eddy and has no physical reality. We should abandon the concept of residual velocity. The lifetime of TITE depends on the magnitude of vorticity and its dissipation. In an inner basin with large bottom friction, the eddy diminishes within a short time (one or two hours) after the generation, and the pressure gra...

Study of seasonal transport variations in the Indonesian seas

Seasonal transport variations between the Pacific and Indian Oceans via the Indonesian seas were studied by the Euler-Lagrangian method. The velocity field was calculated with a fairly high resolution robust diagnostic model. The model well reproduces the features of seasonal variations in the Indonesian seas. The total volume transport of the Indonesian throughflow is 20±3 Sv (1 Sv = 106 m3 s−1), the maximum being from boreal spring to boreal summer and the minimum in boreal winter. The values are similar to those of previous general circulation models with a wide Indonesian passage despite resolution of the presence of the many small islands in the Indonesian seas. Although a large portion of the net transport is contained in the upper layer, deep transport below 1000-m depth is about 5 Sv. This value corresponds to approximately 25% of the total transport, which means that disregard of the deep transport leads to underestimation of the volume transport of the throughflow. Tracking of numerous labeled particles in the calculated velocity field clarified the sources and pathways of the Indonesian throughflow. The major route is a western one through both the Makassar and Lombok Straits. Most of the North Pacific water supplied from the Mindanao Current passes along this route, entering the Indian Ocean within several months with almost no loss of its properties (intense vertical mixing around the Lombok sill reported by observations could not be reproduced in our model). In contrast, South Pacific water takes the eastern route into the eastern Indonesian seas and subsequently mixes with waters from the North Pacific and Indian Oceans in the Banda Sea, which means that it has a long travel time (at least a few years). Water taking the eastern route therefore loses its original properties before arriving in the Indian Ocean. The transport processes also are significantly affected by seasonal variations in equatorial circulation in the western Pacific. In the surface layer, North Pacific water is vigorously supplied to the western route only from boreal spring to summer in association with the linkage between the current flowing through the Makassar Strait and the Mindanao Current. In other seasons, because the Mindanao Current is strongly linked with the North Equatorial Countercurrent and the New Guinea Coastal Current primarily by northeasterly monsoonal winds, its upper water flows back to the Pacific Ocean. In the subsurface layer, a pronounced inflow of Mindanao Current water into the western route occurs from boreal winter to spring, when the subsurface link between that current and the Equatorial Undercurrent tends to weaken. In the deep, the quasi-steady transport of Pacific water into the Indian Ocean via the eastern route is fed by the westward deep current in the equatorial Pacific.

Diagnostic calculation for circulation and water mass movement in the deep Pacific

The steady circulation of the deep Pacific is estimated with a robust diagnostic model, which is internally constrained by hydrographic data. It is shown that the input data should be modified to fit the model in inverse proportion to the Coriolis parameter because a density field inconsistent with the model generates unrealistic geostrophic flows. The model reproduces most of the deep currents previously reported, such as the deep western boundary current east of New Zealand. In addition, as a new feature, the present model diagnoses an anticyclonic circulation around the East Pacific Rise. This circulation is discovered to be associated with a rise of isopycnals at middepth. Tracking of many particles in the diagnosed velocity field reveals that two water masses enter the Southwest Pacific Basin. One is the deep water of the South Indian Basin which enters through a gap to the south of New Zealand. The other is the upper water of the Antarctic Circumpolar Current; this water becomes dense near the Ross Sea and sinks into the deep Southeastern Pacific Basin. The anticyclonic circulation around the East Pacific Rise transports it to the Southwest Pacific Basin. These waters supply comparable volumes to the Southwest Pacific Basin; the residence time is estimated to be 86 years. The deep water in the Southwest Pacific Basin is brought northward rapidly by the deep western boundary current east of New Zealand; it takes only a few decades to move from the east of New Zealand to the North Pacific.

Kuroshio path variation south of Japan: 1. Barotropic inflow‐outflow model

We examined the dynamics of the variation in the path of the Kuroshio current along the south coast of Japan by use of a barotropic inflow-outflow ocean model. The model basin has an inclined coastline and includes a part of the East China Sea so that we could investigate the effect of Kyushu on the Kuroshio. We obtained three regimes of the Kuroshios path variation according to the magnitude of its inlet velocity Vmax, similar to the previous study. The Kuroshio takes a straight path along the south coast of Japan in regime I with a low Vmax, a meandering path in regime II with a high Vmax, and both of these paths (multiple-equilibrium state) in regime III with an intermediate Vmax. The transition from the straight state to the meandering state occurs with an increase in Vmax from regime I (III) to II, preceded by an eastward progression of a small meander which is produced as a separated vortex by the nonlinear effect of the flow and the effect of Kyushu. It takes 120 days, consistent with observations. The nonlinearity of the flow has two effects in the transition. One causes a downstream shift of the small meander, and the other enlarges it by pulling out positive vorticity from the no-slip coast. With an increase in the meander scale due to the latter effect, the former effect becomes small to be balanced with the beta effect, causing an upstream shift of the meander. Thus a large-meander path is formed. A small meander does not progress eastward to develop into a large meander until Vmax exceeds a critical value. This behavior causes a multiple-equilibrium state in Regime III and only the meandering path in Regime II.

A new method to determine near‐sea surface air temperature by using satellite data

We present a new algorithm with which to determine near-sea surface air temperature from satellite observations. A relationship between air temperature, sea surface temperature (SST), wind speed, and humidity is obtained from the aerodynamical equation and the bulk formula. We solve air temperature from this relationship by giving other variables with the observations and without explicit assumption of boundary layer parameters. Our new method is validated using observed monthly mean data at the Japan Meteorological Agency (JMA) and Tropical Ocean-Global Atmosphere (TOGA)-Tropical Atmosphere Ocean (TAO) buoys. Air temperature and sensible heat flux can be determined with accuracies of 0.0° ± 1.2°C and 0.1 ± 8.7 W/m2, respectively, when SST, wind speed, and humidity are given by in situ observations. In order to retrieve near-sea surface air temperature and sensible heat flux from satellite data, SST is obtained from multi channel sea surface temperature (MCSST) of advanced very high resolution radiometer (AVHRR), wind speed is obtained from special sensor microwave/imager (SSM/I), and humidity is obtained from SSM/I together with the empirical equation proposed by Liu [1986]. An error of monthly mean satellite-derived air temperature is −0.3° ± 3.1°C, and that of sensible heat flux is 10.0 ± 37.6 W/m2. Errors of both satellite-derived values are enlarged, possibly because the satellite observations have seasonably systematic error. Time evolution of the interannual variation of air temperature estimated by satellite agrees well with that of in situ measurements. The estimation error of the interannual variation of air temperature is 0.0° ± 1.5°C, and that of sensible heat flux is 0.3 ± 19.0 W/m2. At an interannual timescale the satellite-derived sensible heat flux catches the time evolution of the in situ observation. Both the air temperature and the sensible heat flux obtained by using our method have fewer errors than estimations made by assuming relative humidity as the climatological mean value.

Numerical study of shelf water motion driven by the Kuroshio: barotropic model

Abstract The barotropic response of the shelf and coastal regions south of Japan to short-term variations in the Kuroshio was studied numerically with an inflow–outflow model. The onshore–offshore movements of the stream axis of the Kuroshio due to changes in the upstream volume transport have an important effect on shelf and coastal circulations off the coast of Japan. When the Kuroshio comes near the shelf south of Japan, topographic eddies of about 1.0 × 105 m diameter are produced on the shelf behind the capes by the separation of the Kuroshio from the subsurface tip of spurs at a depth of about 200 m projecting from the capes into the sea. On the other hand, when the Kuroshio moves away from the shelf, the eddies disappear. This implies that the periodic formation and disappearance of the eddies takes place on the shelf due to the combined effect of the short-term onshore–offshore movements of the Kuroshio axis and the irregular topography of the continental margin. Phenomena similar to the present e...

Tidal exchange through Naruto, Akashi and Kitan Straits

In order to re-evaluate the water volume exchange through Naruto Strait, we have performed a numerical experiment (nonlinear barotropic model including the actual depth of water and the details of shoreline) where trajectories of a number of labeled particles are calculated during a full cycle of the M2 tidal current. The ratio of water volume exchanged through Naruto Strait to that through Akashi Strait is found to be twice as large as the previously estimated value. The calculated water exchange rate is 104 % for Naruto Strait, 52 % for Akashi Strait and 28 % for Kitan Strait. In the case where the tide-induced residual current is excluded from the calculated velocity field (i.e. considering only the M2 current), the calculated exchange rate maintains the level of 68 % for Naruto Strait, 18 % for Akashi Strait and 11 % for Kitan Strait, respectively. The mechanism of tidal exchange through these three straits is discussed, and it is shown that a suitable exchange rate is obtained by starting the calculation of trajectories of labeled particles at the time of either a maximum ebb- or flood-current.

Measurement of the sea surface emissivity

The sea surface emissivity in the infrared region is determined on the basis of data analyses. Net radiation, surface irradiance and other oceanographical and meteorological variables are measured throughout most of the year at the oceanographical observatory tower in Tanabe Bay, Japan. We have found that 0.984±0.004 is a reliable emissivity value from the night time data. Surface emission radiates not from the subsurface water but from the sea surface. The thermal skin layer on the sea surface, however, is disturbed and disappears under high wind speed over 5 m/s through the analyses of the radiation observation using the emissivity value of 0.984. Under low wind speed, the sea surface can be cooler or warmer than the subsurface due to overlying thermal conditions and the skin layer can be neutral as the transient process between them. By using an emissivity value of 0.984, the temperature difference between the sea surface temperature and the temperature determined from surface irradiance that has been reported in the satellite data analyses is found to be reduced by half.