Tomohiro Nakamura

The Generation of Large-Amplitude Unsteady Lee Waves by Subinertial K1 Tidal Flow: A Possible Vertical Mixing Mechanism in the Kuril Straits

Numerical experiments with a two-dimensional nonhydrostatic model are performed to investigate tidally generated internal waves in the Kuril Straits and their effect on vertical mixing. The results show that sill-scale internal waves at the K1 tidal frequency are confined to the sill slopes because the K1 tide is subinertial in the Kuril Straits. In contrast to previous theories, the authors show that intense short internal waves generated at the sill breaks by the subinertial K1 tidal current can propagate upstream as the tidal current slackens. Theoretical considerations identify these short waves as unsteady lee waves, which tend to be trapped at the generation region and grow into large-amplitude waves, eventually inducing vigorous mixing along their ray paths. In particular, superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of ;103 cm2 s21. This quite intense mixing reaches down to the density layer of the North Pacific Intermediate Water (NPIW). In contrast, the M2 tidal current does not cause such strong vertical mixing, because most of generated internal waves propagate away as first-mode internal tides and because the barotropic flow amplitude is small. The authors therefore suggest the possibility that generation of lee waves through interactions between the K1 current and the bottom topography of the Kuril Straits contributes to the observed modification of the Okhotsk Sea water required in the formation of the NPIW.

Tidal Exchange through the Kuril Straits

Abstract The tidal exchange between the Okhotsk Sea and the North Pacific Ocean is studied numerically with particular emphasis on the predominant K1 barotropic component. The calculated harmonic constants of the K1 tide in and around the Okhotsk Sea agree well with those obtained from extensive tide gauge observations. The features of the simulated tidal fields are similar to those reported in the literature. Since the K1 tide is subinertial in the Okhotsk Sea, topographically trapped waves are effectively generated, contributing to strong tidal currents with a maximum amplitude of over 1.5 m s−1 in the Kuril Straits. The structures of tide-induced mean flows in most passages of the straits are characterized by “bidirectional currents” (in which the mean flow exhibits a reversal in direction across the passages). This feature is clearly indicated in NOAA infrared imagery. The mean transport shows significant net exchange of water via several straits in the Kuril Islands. A transport of about 5.0 Sv (1 Sv...

Protein S-Nitrosylation as a Therapeutic Target for Neurodegenerative Diseases

At physiological levels, nitric oxide (NO) contributes to the maintenance of normal neuronal activity and survival, thus serving as an important regulatory mechanism in the central nervous system. By contrast, accumulating evidence suggests that exposure to environmental toxins or the normal aging process can trigger excessive production of reactive oxygen/nitrogen species (such as NO), contributing to the etiology of several neurodegenerative diseases. We highlight here protein S-nitrosylation, resulting from covalent attachment of an NO group to a cysteine thiol of the target protein, as a ubiquitous effector of NO signaling in both health and disease. We review our current understanding of this redox-dependent post-translational modification under neurodegenerative conditions, and evaluate how targeting dysregulated protein S-nitrosylation can lead to novel therapeutics.

Effects of tidal mixing at the Kuril Straits on North Pacific ventilation: Adjustment of the intermediate layer revealed from numerical experiments

[1]xa0The effects of tidal mixing at the Kuril Straits on the North Pacific intermediate layer are investigated using an ocean general circulation model. A comparison of numerical experiments with and without a tidal mixing effect suggests that tidal mixing at the Kuril Straits enhances the ventilation of the North Pacific intermediate layer. The enhanced ventilation results in both freshening and cooling down to ∼27.6 σθ. In particular, the simulated North Pacific Intermediate Water (NPIW) becomes fresher and denser (by 0.3 psu and 0.1 σθ at the maximum) and hence more realistic. The enhanced ventilation is caused both through the supply of the ventilated water from the Kuril Straits, which subsequently spreads along the subarctic and subtropical gyres, and through a modification of the circulation there. The ventilation of the supplied water in turn originates from a combination of tidally enhanced convection in the Okhotsk Sea and downward diffusion at the Kuril Straits as discussed in a previous paper. The former affects the upper part of the NPIW, while the latter is dominant in the denser layers. The circulation is modified through a dynamical adjustment to the mass input into an intermediate layer that is produced by the convergence of diapycnal transport due to the tidally enhanced convection and diffusion. The dynamical adjustment is conducted mainly through Kelvin waves which have the ability to induce intergyre flow along the western boundary and also by eastward moving long Rossby waves. The latter can be present under the influence of the wind-driven gyres for the second and higher baroclinic modes and act to spread information into the interior directly from the western boundary. As a direct consequence of this adjustment, transport of the ventilated water from the Kuril Straits to the subtropical gyre is enhanced by the intergyre flow along the western boundary, which appears as the southward intrusion of the Oyashio Current. The transported water leaves the coast to encircle the interior, and returns to the western boundary, eventually flowing into the equatorial region. Such equatorward transport associated with mass convergence in the intermediate layer is compensated by transport toward the Kuril Straits in the shallower and deeper layers, thereby enhancing both shallow and deep meridional overturning cells by 2–3 Sv. The above dynamical adjustment is thus central to our understanding of the ventilation of the intermediate layer and provides a basis for the analytical model developed in an accompanying paper.

Oceanic fronts and jets around Japan: a review

This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

A Growth Mechanism for Topographic Internal Waves Generated by an Oscillatory Flow

On the basis of ray tracing of individual waves generated at various phases of the tidal flow, an amplification mechanism is presented for a new class of topographically generated internal waves identified by Nakamura et al., which develop across a broad latitude range and can exist even above the critical latitude where the tidal frequency equals the inertial frequency. The results show that unsteady lee waves are always amplified when the maximum frequency is much smaller than the buoyancy frequency because their phase speeds (amplitudes) are equal (proportional) to the tidal flow speed at their time of generation. Fast mixed tidal‐lee waves are also effectively amplified, when the rotation effect is significant. Accordingly, amplification of unsteady lee and fast mixed tidal‐lee waves can occur even if the requirements of previous theories (e.g., the critical slope and critical Froude number conditions) are not satisfied. Since the result here covers the generation and amplification processes of topographic internal waves across a broader parameter range than earlier theories, it should contribute to a better understanding of boundary mixing processes, especially in high-latitude regions.

Elevated glucose and oligomeric β-amyloid disrupt synapses via a common pathway of aberrant protein S-nitrosylation.

Metabolic syndrome (MetS) and Type 2 diabetes mellitus (T2DM) increase risk for Alzheimers disease (AD). The molecular mechanism for this association remains poorly defined. Here we report in human and rodent tissues that elevated glucose, as found in MetS/T2DM, and oligomeric β-amyloid (Aβ) peptide, thought to be a key mediator of AD, coordinately increase neuronal Ca2+ and nitric oxide (NO) in an NMDA receptor-dependent manner. The increase in NO results in S-nitrosylation of insulin-degrading enzyme (IDE) and dynamin-related protein 1 (Drp1), thus inhibiting insulin and Aβ catabolism as well as hyperactivating mitochondrial fission machinery. Consequent elevation in Aβ levels and compromise in mitochondrial bioenergetics result in dysfunctional synaptic plasticity and synapse loss in cortical and hippocampal neurons. The NMDA receptor antagonist memantine attenuates these effects. Our studies show that redox-mediated posttranslational modification of brain proteins link Aβ and hyperglycaemia to cognitive dysfunction in MetS/T2DM and AD.

S-nitrosylation of Cdk5: potential implications in amyloid-β-related neurotoxicity in Alzheimer disease.

Aberrant activation of Cdk5 has been implicated in the process of neurodegenerative diseases such as Alzheimers disease (AD). We recently reported that S-nitrosylation of Cdk5 (forming SNO-Cdk5) at specific cysteine residues results in excessive activation of Cdk5, contributing to mitochondrial dysfunction, synaptic damage, and neuronal cell death in models of AD. Furthermore, SNO-Cdk5 acts as a nascent S-nitrosylase, transnitrosylating the mitochondrial fission protein Drp1 and enhancing excessive mitochondrial fission in dendritic spines. However, a molecular mechanism that leads to the formation of SNO-Cdk5 in neuronal cells remained obscure. Here, we demonstrate that neuronal nitric oxide synthase (NOS1) interacts with Cdk5 and that the close proximity of the two proteins facilitates the formation of SNO-Cdk5. Interestingly, as a negative feedback mechanism, Cdk5 phosphorylates and suppresses NOS1 activity. Thus, together with our previous report, these findings delineate an S-nitrosylation pathway wherein Cdk5/NOS1 interaction enhances SNO-Cdk5 formation, mediating mitochondrial dysfunction and synaptic loss during the etiology of AD.

Dysfunctional Mitochondrial Dynamics in the Pathophysiology of Neurodegenerative Diseases

Mitochondrial dysfunction occurs in neurodegenerative diseases, however molecular mechanisms underlying this process remain elusive. Emerging evidence suggests that nitrosative stress, mediated by reactive nitrogen species (RNS), may play a role in mitochondrial pathology. Here, we review findings that highlight the abnormal mitochondrial morphology observed in many neurodegenerative disorders including Alzheimer’s, Parkinson’s, and Huntington’s diseases. One mechanism whereby RNS can affect mitochondrial function and thus neuronal survival occurs via protein S-nitrosylation, representing chemical reaction of a nitric oxide (NO) group with a critical cysteine thiol. In this review, we focus on the signaling pathway whereby S-nitrosylation of the mitochondrial fission protein Drp1 (dynamin-related protein 1; forming S-nitrosothiol (SNO)-Drp1) precipitates excessive mitochondrial fission or fragmentation and consequent bioenergetic compromise. Subsequently, the formation of SNO-Drp1 leads to synaptic damage and neuronal death. Thus, intervention in the SNO-Drp1 pathway may provide therapeutic benefit in neurodegenerative diseases.

Dependence of surface oxidation on hydrogen absorption and desorption behaviors of Ti-6Al-4V alloy

Abstract The hydrogen absorption and desorption behaviors of surface oxidized and degassed Ti–6Al–4V alloys under a condition of hydrogen pressure near to the pressure during glow discharge conditioning in reactor were evaluated by thermal desorption spectroscopy (TDS). The equilibrium hydrogen concentration, C eq , of degassed Ti–6Al–4V alloy decreased with increase of temperature. The C eq was about half of that of pure Ti in the temperature region of 673–873 K. The desorption peak of degassed Ti–6Al–4V alloy appeared at the temperature of 790 K. The peak temperature was almost the same even when the absorption amount changed. The absorption amount, C ab , of surface oxidized alloy largely decreased with the increase of atomic ratio (O/Ti) on the top surface and the thickness of the oxide layer when adsorption time was 1 h. In addition, the desorption peak shifted to a higher temperature region with the increase of O/Ti ratio. In the surface oxidized Ti–6Al–4V alloy with an oxide layer of about 10 nm, the C ab was two orders of magnitude smaller than that of the degassed sample when the absorption temperature and time were kept at lower than 650 K and 1 h, respectively. However, the C ab of surface oxidized alloy became comparable with that of degassed sample when the absorption temperature was higher than 800 K because the oxide layer disappeared in the higher temperature region.