Numerical study of enhanced energy dissipation near a seamount

Using a two-dimensional primitive equation model, we examine nonlinear responses of a semidiurnal tidal flow impinging on a seamount with a background Garrett-Munk-like (GM-like) internal wavefield. It is found that horizontally elongated pancake-like structures of high vertical wavenumber near-inertial current shear are created both in the near-field (the region over the slope of the seamount) and far-field (the region over the flat bottom of the ocean). An important distinction is that the high vertical wavenumber near-inertial current shear is amplified only at mid-latitudes in the far-field (owing to a parametric subharmonic instability (PSI)), whereas it is amplified both at mid-and high-latitudes (above the latitude where PSI can occur) in the near-field. In order to clarify the generating mechanism for the strong shear in the near-field, additional numerical experiments are carried out with the GM-like background internal waves removed. The experiments show that the strong shear is also created, indicating that it is not caused by the interaction between the background GM-like internal waves and the semidiurnal internal tides. One possible explanation is proposed for the amplification of high vertical wavenumber near-inertial current shear in the near-field where tide residual flow resulting from tide-topography interaction plays an important role in transferring energy from high-mode internal tides to near-inertial internal waves.     

Numerical study of the meridional overturning circulation with “mixing hotspots” in the Pacific Ocean

Using an idealized ocean general circulation model, we examine the effect of “mixing hotspots” (localized regions of intense diapycnal mixing) predicted based on internal wave-wave interaction theory (Hibiya et al., 2006) on the meridional overturning circulation of the Pacific Ocean. Although the assumed diapycnal diffusivity in the mixing hotspots is a little larger than the predicted value, the upwelling in the mixing hotspots is not sufficient to balance the deep-water production; out of 17 Sv of the downwelled water along the southern boundary, only 9.2 Sv is found to upwell in the mixing hotspots. The imbalance as much as 7.8 Sv is compensated by entrainment into the surface mixed layer in the vicinity of the downwelling region. As a result, the northward transport of the deep water crossing the equator is limited to 5.5 Sv, much less than estimated from previous current meter moorings and hydrographic surveys. One plausible explanation for this is that the magnitude of the meridional overturning circulation of the Pacific Ocean has been overestimated by these observations. We raise doubts about the validity of the previous ocean general circulation models where diapycnal diffusivity is assigned ad hoc to attain the current magnitude suggested from current meter moorings and hydrographic surveys.     

Vertical mixing in the bottom mixed layer near the continental shelf break in the East China Sea

The strength of the vertical mixing in the bottom mixed layer near the continental shelf break in the East China Sea was directly measured with the Micro-Scale Profiler (MSP). It has been shown that there is no significant statistical relation between the turbulent energy dissipation and the degree of the stratificationN ². It seems that the vigorous turbulence occurs not constantly but intermittently in the bottom mixed layer so that a large variation of is found depending on the time. In contrast to , the coefficient of the vertical eddy diffusivityK _z is mostly determined byN such thatK _z is large in the bottom mixed layer and small in the thermocline. Large value ofK _z in the bottom mixed layer is also found in the time series ofK _z estimated in terms of Richardson number calculated from the data obtained with electromagnetic current meters. The value ofK _z more than 10 cm²s^–1 frequently occur in the layer of 20–25 m thick just above the bottom.     

Studies on storm surges and tides

Studies on storm surges and tides by the author can be classified into following four categories of works; (1) observational results on storm surges; (2) dynamics of storm surges including their numerical simulations; (3) methods of tidal analysis; and (4) long-period variations of sea level.As for the first subject, distributions of tidal deviations were investigated for some representative typhoons. Peak surges exceeding a definite value were also extracted from tidal records of Kôbe, Ôsaka and other ports. They were used to estimate return periods of storm surges, and to clarify the relations between storm surges and meteorological conditions.Dynamic studies on storm surges were concentrated especially to those produced by progressive meteorological disturbances. Numerical simulations by the primitive method started soon after the damage of Ise Bay Typhoon in 1959. Computations were made also for Ise Bay, Tôkyô Bay, Ôsaka Bay and other basins along the Japanese coast. Numerical simulations were also made for storm surges produced by Hurricane Carla striking the Coast of the Gulf of Mexico in 1961.Tidal analysis were also made by the author, and they included a new method based on the harmonic analysis of consecutive data for 355 days.Seasonal and long-term variations of sea level were also studied.     

Numerical forecasting of the time interval between successive M8 earthquakes along the Nankai Trough,southwest Japan,using ocean bottom cable network data

One possible approach to estimating the time interval between large-scale Tōnankai (Tōkai) and Nankai earthquakes on the Japan arc is sequential assimilation of crustal deformation data. We conducted numerical modeling of sequential assimilation using surface deformation calculated from earthquake generation cycle simulations along the Nankai Trough. To account for observation noise, we used measured ocean bottom pressure gauge data, excluding tidal modulation, from a station on the ocean bottom cable network Dense Oceanfloor Network System for Earthquakes and Tsunamis in the Kumano basin. We used sequential importance sampling as our data assimilation method. We found that as the amount of data increased, the estimated time interval between the Tōnankai and Nankai earthquakes approached the “true” observed interval. In addition, the noise in the pressure gauge data was sufficiently small that simulated crustal deformation patterns could be distinguished for different time intervals.     

Effects of mass source/sink at the western boundary on the wind-driven gyres: Implications for the ventilation of the north pacific intermediate layer through convection in the Okhotsk Sea and tidal mixing at the Kuril Straits

A steady quasi-geostrophic 2.5-layer model, forced by both Ekman pumping and a mass source/sink situated at the western boundary has been constructed to investigate the effect of diapycnal transport due to convection in the Okhotsk Sea and tidal mixing at the Kuril Straits on the intermediate layer in the North Pacific. The model illustrates a combined effect of the wind-driven and mass-driven circulations. First, net mass input induces a “barotropic” mode inter-gyre flow along the western boundary through the dynamical influence of Kelvin waves. This flow creates characteristic curves (geostrophic contours) that facilitate inter-gyre communication through the western boundary layer from the location of the mass source to the subtropical gyre. Due to the effect of wind-driven circulation, the offshore part turns eastward into the interior, encircles the outer rim of the region (which would otherwise be the pool region in the absence of mass input), and then encounters the western boundary. Eventually, the water fed into the lower layer flows mostly along this path and later flows away to the equatorial region. Conversely, in the upper layer, water is fed from the equator to the subtropics, and to the subpolar interior region through the western boundary current. The water then circulates along the outer rim and is absorbed into the mass sink. The model is controlled mainly by three nondimensional parameters: (1) the ratio of net mass input rate to the maximum Sverdrup transport (Q/T _Sv ^max ), which affects the inter-gyre communication by altering the paths of geostrophic contours, (2) the ratio of a mass input rate into the lower layer to that in total (Q ₂/Q), which controls the vertical structure of the inter-gyre flow, and (3) the measure of the wind forcing effect relative to the β effect, which determines the horizontal extent of the area influenced by the mass input. The other parameter regimes with respect to Q/T _Sv ^max and Q ₂/Q are also presented.     

Study of internal wave generation by tide-topography interaction

The generation mechanism of internal waves by a relatively strong tidal flow over a sill is clarified analytically. Special attention is directed to the role of the tidal advection effect, which is examined by use of characteristics. An internal wave which propagates upstream is gradually formed through interference of infinitesimal amplitude internal waves (elementary waves) emanating from the sill at each instant of time. In the accelerating (or decelerating) stage of the tidal flow, the effective amplification of the internal wave takes place as the internal Froude number exceeds (or falls below) unity because during this period the internal wave slowly travels downstream (or upstream) while crossing over the sill where elementary waves are efficiently superimposed. In fact, the variability in the internal wave field actually observed in a realistic situation (Stellwagen Bank in Massachusetts Bay) is shown to be satisfactorily interpreted in terms of this mechanism. Furthermore, by using this analytical model, the relation between the strength of the tidal advection effect and the resulting internal waveform is clarified. This theory is easily extended to include a vertically sheared steady flow. In this case, although the fundamental generation mechanism is the same as above, the amplitude of the elementary wave varies with time depending on the relative direction of the tidal flow and steady shear flow, so that the internal wave field over the sill differs markedly between the ebb and flood tidal phases. As an example, the internal wave generation process over the sill in the Strait of Gibraltar is qualitatively discussed on the basis of this analytical model. The effect of vertical mixing caused by breaking of these large-amplitude internal waves on the coastal environment is also pointed out. In particular, a brief discussion is made on the control of water exchange by the fortnightly modulation of tidal mixing processes at the sills and constrictions in channels connecting freshwater sources with the ocean.