Contrasts of bimodal tropical instability waves(TIWs)-induced wind stress perturbations in the Pacific Ocean among observations,ocean models,and coupled climate models

The coupling between wind stress perturbations and sea surface temperature(SST)perturbations induced by tropical instability waves(TIWs)in the Pacific Ocean has been revealed previously and proven crucial to both the atmosphere and ocean.However,an overlooked fact by previous studies is that the loosely defined“TIWs”actually consist of two modes,including the Yanai wave-based TIW on the equator(hereafter eTIW)and the Rossby wave-based TIW off the equator(hereafter vTIW).Hence,the individual feedbacks of the wind stress to the bimodal TIWs remain unexplored.In this study,individual coupling relationships are established for both eTIW and v TIW,including the relationship between the TIW-induced SST perturbations and two components of wind stress perturbations,and the relationship between the TIW-induced wind stress perturbation divergence(curl)and the downwind(crosswind)TIW-induced SST gradients.Results show that,due to different distributions of eTIW and vTIW,the coupling strength induced by the eTIW is stronger on the equator,and that by the vTIW is stronger off the equator.The results of any of eTIW and vTIW are higher than those of the loosely defined TIWs.We further investigated how well the coupling relationships remained in several widely recognized oceanic general circulation models and fully coupled climate models.However,the coupling relationships cannot be well represented in most numerical models.Finally,we confirmed that higher resolution usually corresponds to more accurate simulation.Therefore,the coupling models established in this study are complementary to previous research and can be used to refine the oceanic and coupled climate models.

Instability waves and low-frequency noise radiation in the subsonic chevron jet

Spatial instability frequency noise radiation at waves associated with low- shallow polar angles in the chevron jet are investigated and are compared to the round counterpart. The Reynolds-averaged Navier-Stokes equations are solved to obtain the mean flow fields, which serve as the baseflow for linear stability analysis. The chevron jet has more complicated instability waves than the round jet, where three types of instability modes are identified in the vicinity of the nozzle, corresponding to radial shear, azimuthal shear, and their integrated effect of the baseflow, respectively. The most unstable frequency of all chevron modes and round modes in both jets decrease as the axial location moves downstream. Besides, the azimuthal shear effect related modes are more unstable than radial shear effect related modes at low frequencies. Compared to a round jet, a chevron jet reduces the growth rate of the most unstable modes at down- stream locations. Moreover, linearized Euler equations are employed to obtain the beam pattern of pressure generated by spatially evolving instability waves at a dominant low frequency St = 0.3, and the acoustic efficiencies of these linear wavepackets are evaluated for both jets. It is found that the acoustic efficiency of linear wavepacket is able to be reduced greatly in the chevron jet, compared to the round jet.

Simulating Tropical Instability Waves in the Equatorial Eastern Pacific with a Coupled General Circulation Model

Satellite observations of SSTs have revealed the existence of unstable waves in the equatorial eastern Pacific and Atlantic oceans. These waves have a 20-40-day periodicity with westward phase speeds of 0.4-0.6 m s^-1 and wavelengths of 1000-2000 km during boreal summer and fall. They are generally called tropical instability waves （TIWs）. This study investigates TIWs simulated by a high-resolution coupled atmosphere-ocean general circulation model （AOGCM）. The horizontal resolution of the model is 120 km in the atmosphere, and 30 km longitude by 20 km latitude in the ocean. Model simulations show good agreement with the observed main features associated with TIWs. The results of energetics analysis reveal that barotropic energy conversion is responsible for providing the main energy source for TIWs by extracting energy from the meridional shear of the climatological-mean equatorial currents in the mixed layer. This deeper and northward-extended wave activity appears to gain its energy through baroclinic conversion via buoyancy work, which further contributes to the asymmetric distribution of TIWs. It is estimated that the strong cooling effect induced by equatorial upwelling is partially （-30%-40%） offset by the equatorward heat flux due to TIWs in the eastern tropical Pacific during the seasons when TIWs are active. The atmospheric mixed layer just above the sea surface responds to the waves with enhanced or reduced vertical mixing. Furthermore, the changes in turbulent mixing feed back to sea surface evaporation, favoring the westward propagation of TIWs. The atmosphere to the south of the Equator also responds to TIWs in a similar way, although TIWs are much weaker south of the Equator.

INSTABILITY WAVES IN THE WALL REGION OF A TURBULENT BOUNDARY LAYER ON A FLAT PLATE

Coherent structures and the bursting phenomena in the wall region of a turbulent boundary layer play a very important role in determining the characteristics of the boundary layer. Yet the nature and the origin of the coherent structures are unclear until now.In this paper, nonlinear stability calculations for the wall region of a turbulent boundary layer have been made.It was found that there do exist instability waves which may be responsible for the coherent structures.

Counteracting effects on ENSO induced by ocean chlorophyll interannual variability and tropical instability wave-scale perturbations in the tropical Pacific

Large perturbations in chlorophyll(Chl)are observed to coexist at interannual and tropical instability wave(TIW)scales in the tropical Pacific;at present,their combined effects on El Ni?o-Southern Oscillation(ENSO)through ocean biologyinduced heating(OBH)feedbacks are not understood well.Here,a hybrid coupled model(HCM)for the atmosphere and ocean physics-biogeochemistry(AOPB)in the tropical Pacific is adopted to quantify how ENSO can be modulated by Chl perturbations at interannual and TIW scales,individually or collectively,respectively.The HCM-based sensitivity experiments demonstrate a counteracting effect on ENSO:the bio-climate feedback due to large-scale Chl interannual variability acts to damp ENSO through its impact on upper-ocean stratification and vertical mixing,whereas that due to TIW-scale Chl perturbations tends to amplify ENSO.Because ENSO simulations are sensitively dependent on the ways Chl effects are represented at these different scales,it is necessary to adequately take into account these related differential Chl effects in climate modeling.A bias source for ENSO simulations is illustrated that is related with the Chl effects in the tropical Pacific,adding in a new insight into interactions between the climate system and ocean ecosystem on different scales in the region.These results reveal a level of complexity of ENSO modulations resulting from Chl effects at interannual and TIW scales,which are associated with ocean biogeochemical processes and their interactions with physical processes in the tropical Pacific.

Effect of spin on the instability of THz plasma waves in field-effect transistors under non-ideal boundary conditions

Terahertz(THz) radiation can be generated due to the instability of THz plasma waves in field-effect transistors(FETs). In this work, we discuss the instability of THz plasma waves in the channel of FETs with spin and quantum effects under non-ideal boundary conditions. We obtain a linear dispersion relation by using the hydrodynamic equation, Maxwell equation and spin equation. The influence of source capacitance, drain capacitance, spin effects, quantum effects and channel width on the instability of THz plasma waves under the non-ideal boundary conditions is investigated in great detail. The results of numerical simulation show that the THz plasma wave is unstable when the drain capacitance is smaller than the source capacitance;the oscillation frequency with asymmetric boundary conditions is smaller than that under non-ideal boundary conditions;the instability gain of THz plasma waves becomes lower under non-ideal boundary conditions. This finding provides a new idea for finding efficient THz radiation sources and opens up a new mechanism for the development of THz technology.

Nonlinear Planetary Wave Instability and Blocking

The instability of geostrophic wave circulations related to the nonlinear processes involved in the zonal mean heat balance equations is studied. It is found that the planetary waves may be destabilized by thermal forcing in specific baroclinic layers, called the breaking layers. The critical conditions of the instability will be given. In the troposphere, these conditions may be provided in blocking regions and the development of planetary perturbations is characterized distinctly by the unset, maintenance and decay of observed blocks. The whole blocking episode cannot be described as either the barotropic or baroclinic process only. The limitations on the study of wave-wave interaction using spectral models or spectrum analyses will be discussed also.

Magnetohydrodynamics instability of interfacial waves between two immiscible incompressible cylindrical fluids

The problem of nonlinear instability of interfacial waves between two immiscible conducting cylindrical fluids of a weak Oldroyd 3-constant kind is studied. The system is assumed to be influenced by an axial magnetic field, where the effect of surface tension is taken into account. The analysis, based on the method of multiple scale in both space and time, includes the linear as well as the nonlinear effects. This scheme leads to imposing of two levels of the solvability conditions, which are used to construct like-nonlinear Schr6dinger equations （1-NLS） with complex coefficients. These equations generally describe the competition between nonlinearity and dispersion. The stability criteria are theoret- ically discussed and thereby stability diagrams are obtained for different sets of physical parameters. Proceeding to the nonlinear step of the problem, the results show the appearance of dual role of some physical parameters. Moreover, these effects depend on the wave kind, short or long, except for the ordinary viscosity parameter. The effect of the field on the system stability depends on the existence of viscosity and differs in the linear case of the problem from the nonlinear one. There is an obvious difference between the effect of the three Oldroyd constants on the system stability. New instability regions in the parameter space, which appear due to nonlinear effects, are shown.

The instability of dust acoustic waves in inhomogeneous dusty plasmas with non-adiabatic dust charge fluctuation

This paper investigates the propagation of linear dust acoustic waves in inhomogeneous dusty plasmas due to spatial gradients of dust charge, plasma densities. A linear dispersion relation is obtained with the non-adiabatic dust charge iguctuation and the non-thermally distributed ions. The numerical results show that the inhomogeneity, nonthermal ions and non-adlabatic dust charge iguctuatlon have strong iniguence on the frequency and the damping rate of waves.

Instability analysis of three-dimensional ocean shear waves

Based on the continuously stratified quasi-geostrophic vorticity equation. the present paper analyses the instabilityof three-dimensional shear waves.The cause that most shear waves occur on the shelfside of strong current near the west boundaries of the oceans is presented. The growth rate of small perturbations relies on the stratification charctters, and a maximum value of growth rate exists for certain stratification.

Topographically Forced Rossby Wave Instability and the Development of Blocking in the Atmosphere

In this paper, the linear stability of disturbance superimposed on basic state Rossby wave forced by topography is investigated, and pointed out that when a certain criterion is satisfied by the basic flow and the height of topography for the subresonance, the small disturbance may be unstable. Furthermore, we also compare the evolution of the instability disturbance with the development of blocking in the Pacific, and we suggested that the topographically forced Rossby wave instability may provide a possible mechanism for the development of blocking in the Pacific.

An evaluation of input/dissipation terms in WAVEWATCH Ⅲ using in situ and satellite significant wave height data in the South China Sea

A WAVEWATCH III version 3.14（WW3） wave model is used to evaluate input/dissipation source term packages WAM3, WAM4 and TC96 considering the effect of atmospheric instability. The comparisons of a significant wave height acquired from the model with different packages have been performed based on wave observation radar and HY-2 altimetry significant wave height data through five experiments in the South China Sea domain spanning latitudes of 0°–35°N and longitudes of 100°–135°E. The sensitivity of the wind speed correction parameter in the TC96 package also has been analyzed. From the results, the model is unable to dissipate the wave energy efficiently during a swell propagation with either source packages. It is found that TC96 formulation with the ＂effective wind speed＂ strategy performs better than WAM3 and WAM4 formulations. The wind speed correction parameter in the TC96 source package is very sensitive and needs to be calibrated and selected before the WW3 model can be applied to a specific region.

ON THE INSTABILITY IN GAS ATOMIZATION

The instability theory of fluid flow is applied in gas atomization and the results show that the instability of interfacial wave is the main cause of gas atomization. The size of the droplets and its change with parameters are also studied, the results are compatible with the experiments.

The initialization of nonlinear water waves in a numerical wave flume

This study investigates the initialization of nonlinear free-surface simulations in a numerical wave flume. Due to the mismatch between the linear input wavemaker motion and the kinematics of fully nonlinear waves, direct numerical simulations of progressive waves, generated by a sinusoidally moving wavemaker, are prone to suffering from high-frequency wave instability unless the flow is given sufficient time to adjust. A time ramp is superimposed on the wavemaker motion at the start that allows nonlinear free-surface simulations to be initialized with linear input. The duration of the ramp is adjusted to test its efficiency for short waves and long waves. Numerical results show that the time ramp scheme is effective to stabilize the wave instability at the start of the simulation in a wave flume.

Spectral Broadening of Ion Bernstein Wave Due to Parametric Decay Instabilities

The parametric decay instabilities （PDIs） of ion Bernstein wave with different input power levels are investigated via particle-in-cell simulation. It is found that the number of decay channels increases with the input power. Resonant mode-mode couplings dominate for a low input power. With increasing the input power, the nonresonant PDIs appear to dissipate the energy of the injected wave and give rise to edge ion heating. The generated child waves couple with each other as well as the injected wave and /or act as a pump wave to excite new decay channels. As a result, the frequency spectrum is broadened with the increase of the input power.

Parametric Instabilities of Parallel Propagating Circularly Polarized Alfven Waves:One-Dimensional Hybrid Simulations

By performing one-dimensional （l-D） hybrid simulations, we analyze in detail the parametric instabilities of the Alfv^n waves with a spectrum in a low beta plasma. The parametric instabilities experience two stages. In the first stage, the density modes are excited and immediately couple with the pump Alfv6n waves. In the second stage, each pump Alfv^n wave decays into a density mode and a daughter Alfv6n mode similar to the monochromatic cases. Ftlrthermore, the proton velocity beam will also be formed after the saturation of the parametric instabilities. When the plasma beta is high, the parametric decay in the second stage will be strongly suppressed.

Intra-seasonal variability of the abyssal currents in COMRA's contract area in the Clarion-Clipperton Zone

In this paper,the intra-seasonal variability of the abyssal currents in the China Ocean Mineral Resources Association(COMRA)polymetallic nodule contact area,located in the western part of the Clarion and Clipperton Fraction Zone in the tropical East Pacific,is investigated using direct observations from subsurface mooring instruments as well as sea-surface height data and reanalysis products.Mooring observations were conducted from September 13,2017 to August 15,2018 in the COMRA contact area(10°N,154°W).The results were as follows:(1)At depths below 200 m,the kinetic energy of intra-seasonal variability(20-100 d)accounts for more than 40%of the overall low-frequency variability,while the ratio reaches more than 50%below 2000 m.(2)At depths below 200 m,currents show a synchronous oscillation with a characteristic time scale of 30 d,lasting from October to the following January;the energy of the 30-d oscillation increases with depth until the layer of approximately 4616 m,and the maximum velocity is approximately 10 cm/s.(3)The 30-d oscillation of deep currents is correlated with the tropical instability waves in the upper ocean.

Dependence of anomalous resistivity on bulk drift velocity of electrons in the reconnecting current sheets in solar flares

Anomalous resistivity is critical for triggering fast magnetic reconnection in the nearly collisionless coronal plasma. Its nonlinear dependence on bulk drift velocity is usually assumed in MHD simulations. However, the mechanism for the production of anomalous resistivity and its evolution is still an open question. We numerically solved the one dimension Vlasov equation with the typical solar coronal parameters and realistic mass ratios to infer the relationship between anomalous resistivity and bulk drift velocity of electrons in the reconnecting current sheets as well as its non- linear characteristics. Our principal findings are summarized as follows： 1） the relationship between the anomalous resistivity and bulk drift velocity of electrons relative to ions may be described as ηmax=0.03724（vd/ve）^5.702Ωm for vd/ve in the range of 1.4-2.0 and ηmax=0.8746（vd/ve）^1.284Ωm for vd/ve in the range of 2.5-4.5;2）if drift velocity is just slightly larger than the threshold of ion-acoustic instability, the anomalous resistivity due to the wave-particle interactions is enhanced by about five orders as compared with classic resistivity due to Coulomb collisions, With the increase of drift velocity from 1.4ve to 4.5Ve, the anomalous resistivity continues to increase 100 times; 3） in the rise phase of unstable waves, the anomalous resistivity has the same order as the one estimated from quasi-linear theory; after saturation of unstable waves, the anomalous resistivity decreases at least about one order as com- pared with its peak value; 4） considering that the final velocity of electrons ejected out of the reconnecting current sheet （RCS） decreases with the distance from the neutral point in the neutral plane, the anomalous resistivity decreases with the distance from the neutral point, which is favorable for the Petschek-like reconnection to take place.

INSTABILITY OF THE OCEANIC WAVES IN THE TROPICAL REGION

In this paper, the effects of the large-scale mean sea temperature fields of the tropical ocean and the zonal current field (southern equatorial current) have been comprehensively entered in consideration on the basis of Chao and Ji (1985), and Ji and Chao (1986), the equatorial oceanic waves of the tropical ocean have been discussed by use of linearized primitive equations, then, the significant influence of the climatic back- ground fields of the tropical ocean upon the oceanic waves of this region has been further testified. When very cold water appears in the tropical region, and the southern equatorial current is also relatively strong, the effect of the Rossby wave weakens, as a consequence, there are substitutive slow waves (i.e. thermal waves) which travel in opposite direction (eastward) to the Rossby wave. The characteristics of the slow wave are similar to those of Rossby waves, only the travelling direction is opposite. Under a certain environ- mental background field, the slow wave and the modified Rossby wave may be instable. With this conclu- sion, the mechanism of the occurrence, development and propagation of El Nino events has been studied. It is pointed out that the opposite travelling direction of the thermal wave and Rossby wave will bring re- pectively into action under different marine environmental background fields. The physical causes for that the abnormal warm water inclines to occur along the South American coast have also been explored in this paper.

PDRK:A General Kinetic Dispersion Relation Solver for Magnetized Plasma

A general,fast,and effective approach is developed for numerical calculation of kinetic plasma linear dispersion relations.The plasma dispersion function is approximated by J-pole expansion.Subsequently,the dispersion relation is transformed to a standard matrix eigenvalue problem of an equivalent linear system.Numerical solutions for the least damped or fastest growing modes using an 8-pole expansion are generally accurate;more strongly damped modes are less accurate,but are less likely to be of physical interest.In contrast to conventional approaches,such as Newton＇s iterative method,this approach can give either all the solutions in the system or a few solutions around the initial guess.It is also free from convergence problems.The approach is demonstrated for electrostatic dispersion equations with one-dimensional and twodimensional wavevectors,and for electromagnetic kinetic magnetized plasma dispersion relation for bi-Maxwellian distribution with relative parallel velocity flows between species.