Takahiro Endoh

Numerical simulation of the transient response of the Kuroshio leading to the large meander formation south of Japan

Using a three-dimensional, primitive equation numerical model that takes realistic topography into account, we successfully reproduce the observed transient response of the Kuroshio south of Japan during the transition from the nonlarge meander path to the large meander path. The transient response is triggered by the generation of what is called the “trigger meander” off the southeastern coast of Kyushu resulting from the supply of cyclonic vorticity through vertical stretching caused by the interaction between the Kuroshio and the anticyclonic mesoscale eddy approaching the Tokara Strait. The trigger meander thus generated propagates eastward south of Shikoku while inducing an anticyclone-cyclone pair in the lower ocean. After the trigger meander passes Cape Shiono-misaki it slows down and rapidly amplifies so that the Kuroshio loops back west of the Izu-Ogasawara Ridge. Then the sharpness of the meander trough gradually relaxes, and the large meander path is attained. During the rapid amplification of the trigger meander off Cape Shiono-misaki the abyssal anticyclone develops while being trapped by the local topographic feature, Koshu Seamount, located ∼200 km to the south of Cape Shiono-misaki. This abyssal anticyclone plays a crucial role in intensifying the trigger meander trough in the upper ocean via cross-frontal advection; the intensified trigger meander trough then further amplifies the abyssal anticyclone over Koshu Seamount. This joint evolution of the upper ocean meander trough and the abyssal anticyclone suggests that baroclinic instability enhanced by Koshu Seamount is the dominant mechanism for the rapid amplification of the trigger meander leading to the large meander formation south of Japan.

Thermohaline Structure in the Subarctic North Pacific Simulated in a General Circulation Model

The Miami Isopycnic Coordinate Ocean Model configured with 18 horizontal resolution and 23 layers is used to examine processes that maintain the mesothermal structure, a subsurface temperature inversion, in the subarctic North Pacific. The model successfully reproduces the mesothermal structure consisting of a shallow temperature minimum and an underlying temperature maximum that are called the dichothermal and mesothermal waters, respectively. The mesothermal water is formed through cross-gyre exchange between the subtropical and subarctic gyres, whereas the dichothermal water originates from cold and low-salinity waters formed in the winter mixed layer. The horizontal distribution of the passive tracer injected into the subsurface layers south of Japan shows that warm and saline water of the Kuroshio in the density range of 26.8‐27.0 su is the source of the mesothermal water. There are three pathways through which the Kuroshio waters enter the subarctic region. First, the Kuroshio waters that cross the gyre boundary in the western boundary region are carried to the Alaskan gyre by the northern part of the North Pacific Current. Second, the Kuroshio waters carried by the southern part of the North Pacific Current enter the Alaskan gyre through a cross-gyre window in the eastern basin. Third, the Kuroshio waters that diffuse along the isopycnal in the Kuroshio‐Oyashio Extension enter the western subarctic gyre. The mesothermal water thus formed in the subarctic region is entrained into the winter mixed layer and returns to the subtropics as surface water by the southward Ekman drift, forming the subpolar cell.

The Effect of Koshu Seamount on the Formation of the Kuroshio Large Meander South of Japan

AbstractUsing an inflow–outflow numerical model, the authors demonstrate that the existence of Koshu Seamount, located about 200 km to the south of Cape Shiono-misaki, is essential in creating the large meander (LM) of the Kuroshio. When Koshu Seamount is completely smoothed out, the meander trough propagates away without being amplified to form the LM. In contrast, nearly the same LM as in the case with full topography is formed when the foot of Koshu Seamount remains without being smoothed out and also when the foot of Koshu Seamount is filled in so that the upper part of Koshu Seamount remains. A linear stability analysis applied to the model output shows that the Kuroshio becomes baroclinically most unstable when the water depth decreases offshoreward. The authors therefore conclude that the enhancement of baroclinic instability over the northern slope of Koshu Seamount is a prerequisite to the formation of the LM.

Estimates of the turbulent kinetic energy budget in the oceanic convective boundary layer

The terms of the steady-state turbulent kinetic energy (TKE) budget in the oceanic convective boundary layer (CBL) are estimated by use of microstructure data obtained over the continental shelf of the East China Sea. The dissipation term is calculated from the micro-scale vertical shear of horizontal velocity measured directly using a freely-falling microstructure profiler, whereas the buoyancy flux and shear production terms are estimated indirectly by integrating vertically the one-dimensional conservation equation of density and by applying similarity theory, respectively. The transport term, calculated as the residual of the other three terms, vertically redistributes the TKE from the upper half of the CBL to the lower half, consistent with the TKE budgets in the atmospheric CBL and in shear-free and slightly-sheared CBLs simulated by large eddy-simulation models. The relatively large contribution of the transport term to the TKE budget shows that a local equilibrium form of the TKE equation is not appropriate for the TKE budget in the oceanic CBL.

Sensitivity of the Ventilation Process in the North Pacific to Eddy-Induced Tracer Transport*

Abstract A coarse-resolution isopycnal model coupled with a bulk mixed layer model is used to examine the effect of isopycnal thickness diffusion, which parameterizes the subgrid-scale eddy-induced tracer transport, on ventilation of the North Pacific Ocean. Three numerical experiments with thickness diffusivities of 0 m2 s−1 and around 500 and 2000 m2 s−1 are carried out. The model successfully reproduces a deep winter mixed layer in the subarctic North Pacific, leading to well-formed mode waters and the subtropical countercurrent in the experiment with thickness diffusivity around 500 m2 s−1. The annual-mean subduction rate has peaks at densities of 25.0–25.4 and 26.4 σθ. The former peak spans the densities of North Pacific Subtropical Mode Water and North Pacific Eastern Subtropical Mode Water, whereas the latter peak is centered near the density of North Pacific Central Mode Water. The annual mean obduction rate also has the former peak and a slight enhancement corresponding to the latter peak. The Ku...

Slope-induced tidal straining: Analysis of rotational effects

Tidal straining is known to be an important factor for the generation of residual currents and transports of suspended matter in the coastal ocean. Recent modeling studies and field experiments have revealed a new type of “slope-induced” tidal straining, in which the horizontal density gradient required for this process is induced by the presence of a slope rather than by river runoff (as in classical tidal straining). Slope-induced tidal straining is investigated here with the help of an idealized numerical model, and results are compared to a recent data set from the East China Sea providing first direct observational evidence. The focus of this study is on the effect of rotation that was ignored in previous investigations. The model is shown to reproduce the key features of the observations, in particular the strain-induced generation of unstable stratification in the bottom boundary layer during periods of upslope flow. Rotation effects are found to significantly reduce the upslope tidal pumping of suspended material and also give rise to a newly identified pumping mechanism that results in a vigorous transport of suspended material along the slope. It is shown that slope-induced tidal straining is likely to be relevant for a wide range of oceanic slopes exposed to tidal motions.

Estimating the Eddy Viscosity Profile from Velocity Spirals in the Ekman Boundary Layer

AbstractTurbulent mixing induced by tidal currents near the sea bottom plays a key role in coastal and shallow sea environments. Many attempts have been made to quantify turbulent mixing near the seabed, such as velocity microstructure measurements with microstructure profilers and turbulent Reynolds stress measurements using acoustic Doppler current profilers (ADCPs). This study proposes an alternative method in which the Ekman balance equations are solved with measured velocity spirals to estimate the eddy viscosity profile. Three schemes (schemes 1, 2, and 3) are described in this paper; schemes 1 and 2 were used in previous studies, while scheme 3 is newly proposed in the present study. The performance of the three schemes was tested using velocity spirals simulated with an idealized eddy viscosity profile, showing that scheme 2 is useful if the random measurement errors are small, while scheme 3 is useful when the errors in the Ekman balance are small. The performance was also evaluated using measure...