Wave Energy Propagation Research Articles

The two leading modes of winter clear-sky days over China and their formation mechanisms

Using gauge data during 1979–2013, the spatial and temporal characteristics of winter clear-sky days (CSDs) in China are investigated. The frequency of winter CSDs in China has two independent and significant modes, which account for 41% and 10% of the total variance, respectively. Based on evidence from observational analysis and numerical experiments, the formation mechanisms of these two leading modes are unraveled. The first mode is a homogenous mode with coherent year-to-year variation of CSDs over most parts of China, and is closely related to a large-scale cyclonic/anticyclonic anomaly over the Asian continent. Anomalous Indian Ocean SST and Arctic sea ice are responsible for the associated large-scale circulation anomalies via inducing a mid-to-high latitude Rossby wave train. The Indian Ocean SST warming can induce wave energy propagation upwards to the stratosphere and then northwards to the polar region, where it moves down to the troposphere and then travels southeastwards from the polar region to East Asia. Meanwhile, the decreased Arctic sea ice can directly induce a Rossby wave train emanating from the polar region that travels southeastwards into East Asia. These two factors jointly lead to a dominant equivalent barotropic cyclonic/anticyclonic anomaly over China, and in turn the homogenous pattern of winter CSDs in the region. The second mode is a seesaw mode with out-of-phase variations of CSDs between northern and southern China. This mode is closely related to a meridional dipole circulation anomaly pattern over the East Asian sector, which is generated by the so-called ‘Mega-ENSO’ SST anomaly over the Pacific Ocean. This Mega-ENSO SSTA pattern can induce convective heating/cooling over the tropical western Pacific, which further leads to the meridional dipole circulation anomaly pattern over the East Asian sector, resulting in the south-to-north seesaw mode of winter CSDs in China.

Instability Processes in Simulated Finite-Width Ocean Fronts

AbstractA simple, isolated front is modeled using a turbulence resolving, large-eddy simulation (LES) to examine the generation of instabilities and inertial oscillations by surface fluxes. Both surface cooling and surface wind stress are considered. Coherent roll instabilities with 200–300-m horizontal scale form rapidly within the front after the onset of surface forcing. With weak surface cooling and no wind, the roll axis aligns with the front, yielding results that are equivalent to previous constant gradient symmetric instability cases. After ~1 day, the symmetric modes transform into baroclinic mixed modes with an off-axis orientation. Traditional baroclinic instability develops by day 2 and thereafter dominates the overall circulation. Addition of destabilizing wind forcing produces a similar behavior, but with off-axis symmetric-Ekman shear modes at the onset of instability. In all cases, imbalance of the geostrophic shear by vertical mixing leads to an inertial oscillation in the frontal currents. Analysis of the energy budget indicates an exchange between kinetic energy linked to the inertial currents and potential energy associated with restratification as the front oscillates in response to the vertically sheared inertial current. Inertial kinetic energy decreases from enhanced mixed layer turbulence dissipation and vertical propagation of inertial wave energy into the pycnocline.

Salient difference of sea surface temperature over the North Atlantic in the spring following three super El Niño events

Despite similar evolution of Niño3.4 index for three strong El Niño events in the tropical Pacific in 1982–1983, 1997–1998 and 2015–2016, divergent sea surface temperature anomalies (SSTAs) were observed in the North Atlantic (NA) in spring following El Niño peak. Strong teleconnection occurred for the first two events in 1982–1983 and 1997–1998, leading to a negative phase of a North Atlantic Oscillation-like circulation over the extratropical NA, and thus a positive tripolar SSTA pattern in the NA. But the teleconnection was weak for the case of 2015–2016 El Niño, the SSTA in spring 2016 in NA being mainly created and maintained by the preconditioning of the NA basin and local atmosphere-ocean interactions. The salient difference among the three events resides in their ways to operate the teleconnection linking the central-eastern equatorial Pacific and the subtropical eastern North Pacific to the extratropical NA, which would be a key to explain the different impacts of the three exceptional El Niño events. It is furthermore shown that the reduced anomalous westerlies over Central America along 30° N around the peak time of 2015–2016 El Niño play a role of inhibition for an efficient Rossby wave energy propagation from the tropics, eastward into the Gulf of Mexico and northward into midlatitudes in the western NA.

Coda-wave decorrelation sensitivity kernels in 2-D elastic media: a numerical approach

SUMMARYProbabilistic sensitivity kernels based on the analytical solution of the diffusion and radiative transfer equations have been used to locate tiny changes detected in late arriving coda waves. These analytical kernels accurately describe the sensitivity of coda waves towards velocity changes located at a large distance from the sensors in the acoustic diffusive regime. They are also valid to describe the acoustic waveform distortions (decorrelations) induced by isotropically scattering perturbations. However, in elastic media, there is no analytical solution that describes the complex propagation of wave energy, including mode conversions, polarizations, etc. Here, we derive sensitivity kernels using numerical simulations of wave propagation in heterogeneous media in the acoustic and elastic regimes. We decompose the wavefield into P- and S-wave components at the perturbation location in order to construct separate P to P, S to S, P to S and S to P scattering sensitivity kernels. This allows us to describe the influence of P- and S-wave scattering perturbations separately. We test our approach using acoustic and elastic numerical simulations where localized scattering perturbations are introduced. We validate the numerical sensitivity kernels by comparing them with analytical kernel predictions and with measurements of coda decorrelations on the synthetic data.

Modelling and simulation of a heaving wave energy converter based PEM hydrogen generation and storage system

The research on wave energy systems has been ongoing for decades. However, there are not many operational wave energy converters in use. The hydrogen energy systems also have a great potential. The proposed solution is to combine wave energy system with hydrogen energy system. The study provides details of simulation models and related simulation results. It is environmentally friendly, safe, feasible and effective. The results indicate that the proposed system model has a very high potential. With the use of low to medium energy density sea states, it is appears to be possible to generate (for DS1, DS2 and DS3, mH2 = 350.8 kg, 623.9 kg and 2124 kg, respectively) a considerable amount of hydrogen in 20-min. The presented results include WEC motion properties, instantaneous and moving average value of other system parameters. The future promising simulations results indicate that next generation wave energy converter systems could be accompanied by hydrogen generation and storage systems.

Design and fabrication of wave generator using an oscillating wedge

This paper describes the design and fabrication of a wave flume and associated equipment. The water wave generation by a triangle wedge is examined in this paper. Wave flume is fabricated by a concrete flume, which is 0.75-meter-wide, 1.3-meter-tall, and 11 meters long. One side of the flume is made of concrete, and the other side is made of clear mica sheets for observe the water wave profile. Wave flume is equipped with a triangle wedge located at one end of the wave flume, and this triangle wedge can be move along the rail of the wave flume. The passive wave absorber, which is made of honeycomb, which is located at the other end of the wave flume for absorbing energy waves generated from the wavemaker. The wedge is controlled by a desktop computer through Matlab software. The wedge can be controlled move up and down at a prescribed speed and oscillation amplitude corresponding to the desired wave height and wave frequency. At the middle flume is equipped microlaser distance sensor, which provides the data-logging capability. The microlaser distance sensor can collect wave height and wave period through a small ball, which is motioned in the tube, which is assembled on the water surface, and the small ball is very slight to avoid kinetic inertia energy. At the wavemaker is equipped with an Eccentricity Sensor, which is used to measure the wedge position to feedback for a desktop computer. Wave flume can generate the largest waves are about 0.2 meter high, a period about 1 second, and a wavelength of about 1.5 meters. The waves generated by an oscillating wedge have been measured, analyzed to consider the generated wave energy.

Efficient Nonlinear Hydrodynamic Models for Wave Energy Converter Design—A Scoping Study

This review focuses on the most suitable form of hydrodynamic modeling for the next generation wave energy converter (WEC) design tools. To design and optimize a WEC, it is estimated that several million hours of operation must be simulated, perhaps one million hours of WEC simulation per year of the R&D program. This level of coverage is possible with linear potential flow (LPF) models, but the fidelity of the physics included is not adequate. Conversely, while Reynolds averaged Navier–Stokes (RANS) type computational fluid dynamics (CFD) solvers provide a high fidelity representation of the physics, the increased computational burden of these models renders the required amount of simulations infeasible. To scope the fast, high fidelity options, the present literature review aims to focus on what CFD theories exist intermediate to LPF and RANS as well as other modeling options that are computationally fast while retaining higher fidelity than LPF.

The Relationship Between Abnormal Meiyu and Medium-Term Scale Wave Perturbation Energy Propagation Along the East Asian Subtropical Westerly Jet

The East Asian subtropical westerly jet (EASWJ) is one of the most important factors modulating the Meiyu rainfall in the Yangtze-Huaihe River Basin, China. This article analyzed periods of the medium-term EASWJ variation, wave packet distribution and energy propagation of Rossby waves along the EASWJ during Meiyu season, and investigated their possible influence on abnormal Meiyu rain. The results showed that during the medium-term scale atmospheric dynamic process, the evolution of the EASWJ in Meiyu season was mainly characterized by the changes of 3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves. The strong perturbation wave packet and energy propagation of the 3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves are mostly concentrated in the East Asian region of 90°-150°E, where the two wave trains of perturbation wave packets and wave-activity flux divergence coexist in zonal and meridional directions, and converge on the EASWJ. Besides, the wave trains of perturbation wave packet and wave-activity flux divergence in wet Meiyu years are more systematically westward than those in dry Meiyu years, and they are shown in the inverse phases between each other. In wet (dry) Meiyu year, the perturbation wave packet high-value area of the 10-15 d low-frequency variability is located between the Aral Sea and the Lake Balkhash (in the northeastern part of China), while over eastern China the wave-activity flux is convergent and strong (divergent and weak), and the high-level jets are strong and southward (weak and northward). Because of the coupling of high and low level atmosphere and high-level strong (weak) divergence on the south side of the jet over the Yangtze-Huaihe River Basin, the low-level southwest wind and vertically ascending motion are strengthened (weakened), which is (is not) conducive to precipitation increase in the Yangtze-Huaihe River Basin. These findings would help to better understand the impact mechanisms of the EASWJ activities on abnormal Meiyu from the perspective of medium-term scale Rossby wave energy propagation.

Investigation of the energy of wave motions in a three-layer hydrodynamic system

In this paper, the propagation of the energy of internal waves in a three-layer hydrodynamic liquid system ‘a layer with a solid bottom–a layer–a layer with a lid’ is investigated. The problem statement is executed in a dimensionless form. Using the method of multidimensional development, the first three linear problems were obtained. For the first approximation problem, a dispersion equation of the first order is derived, and two pairs of independent solutions are obtained. The dependence of total energy on the wave number and thickness at different values of physical parameters were graphically illustrated and analyzed. The limiting case in which the obtained results are compared with the calculation of the energy that transferring the wave in the hydrodynamic system ‘liquid layer–a layer with a lid’, which was investigated earlier is considered. Graphically illustrates the transition of this system to a two-layer one. Data obtained as a result of the study of the problem of propagation of internal wave energy in a liquid three-layer hydrodynamic system can be used in the study of similar areas in the ocean and in the development of appropriate technologies and devices using the energy of water waves or converted into electricity.

Climate change over the Mediterranean and current destruction of marine ecosystem

The Mediterranean is one of the most vulnerable regions to climate change and its summer climate is known to be affected by the South Asian summer monsoon (SASM) through the monsoon–desert teleconnection. In future, rainfall is expected to increase not only over the SASM area but also over the East Asian summer monsoon (EASM) and equatorial Atlantic regions. Here we show that the remote forcing regions affect the Mediterranean climate in the future. A subset of CMIP5 climate simulations exhibits an increase in the descending motion over the Western Mediterranean in the future. This strengthened subsidence comes from the SASM, EASM, and Atlantic forcings: the SASM and EASM heating induces the Gill-type Rossby wave response, and the Atlantic forcing causes the northeastward wave energy propagation. The sea surface temperature change over the Western Mediterranean is consistent with the subsidence change both in the future and in the recent decades. The chlorophyll-a concentration and fisheries landings have decreased in the recent period along with sea surface temperature warming. Our results suggest that special attention is required to conserve the marine ecosystem in the Mediterranean as climate warms.

Relationships between convective activity in the Maritime Continent and precipitation anomalies in Southwest China during boreal summer

Using the outgoing long-wave radiation (OLR) data from NOAA, the ERA-Interim reanalysis products from ECMWF, and daily observations at 756 stations provided by National Meteorological Information Center of China Meteorology Administration, we have examined the relationships of interannual variations of summer precipitation in Southwest China with the convective activity in the Maritime Continent (MC) region by employing the singular value decomposition (SVD) method and the regional climate model RegCM4.4. The first SVD mode (here after SVD1) indicates that high precipitation anomalies in Southwest China correspond to abnormally weak convective activity in northeastern MC and strong convective activity in Southwestern MC if the time-series of coefficients of SVD1 is in positive phase. When convective activity are anomalously strong in Southern MC, the anomalous divergence at 700 hPa and convergence at 200 hPa are observed over the tropical western Pacific and South China Sea. The propagation of wave energy at 700 hPa from Western Europe and the tropics provide a favorable condition for inducing more precipitation in most of Southwest China. The atmospheric water vapor transport from northern Indochina and the South China Sea to Southwest China intensifies while the western Pacific subtropical high is stronger than normal and extends more westward. All these results along with the simulations demonstrate that the summer precipitation in Southwest China is significantly affected by the convective activity over the MC region. These results above are helpful for our better understanding the role of the MC in regulating the summer climate in Southwest China.

Wave energy attenuation in fields of colliding ice floes – Part 1: Discrete-element modelling of dissipation due to ice–water drag

Abstract. The energy of water waves propagating through sea ice is attenuated due to non-dissipative (scattering) and dissipative processes. The nature of those processes and their contribution to attenuation depends on wave characteristics and ice properties and is usually difficult (or impossible) to determine from limited observations available. Therefore, many aspects of relevant dissipation mechanisms remain poorly understood. In this work, a discrete-element model (DEM) is used to study one of those mechanisms: dissipation due to ice–water drag. The model consists of two coupled parts, a DEM simulating the surge motion and collisions of ice floes driven by waves and a wave module solving the wave energy transport equation with source terms computed based on phase-averaged DEM results. The wave energy attenuation is analysed analytically for a limiting case of a compact, horizontally confined ice cover. It is shown that the usage of a quadratic drag law leads to non-exponential attenuation of wave amplitude a with distance x, of the form a(x)=1/(αx+1/a0), with the attenuation rate α linearly proportional to the drag coefficient. The dependence of α on wave frequency ω varies with the dispersion relation used. For the open-water (OW) dispersion relation, α∼ω4. For the mass loading dispersion relation, suitable for ice covers composed of small floes, the increase in α with ω is much faster than in the OW case, leading to very fast elimination of high-frequency components from the wave energy spectrum. For elastic-plate dispersion relation, suitable for large floes or continuous ice, α∼ωm within the high-frequency tail, with m close to 2.0–2.5; i.e. dissipation is much slower than in the OW case. The coupled DEM–wave model predicts the existence of two zones: a relatively narrow area of very strong attenuation close to the ice edge, with energetic floe collisions and therefore high instantaneous ice–water velocities, and an inner zone where ice floes are in permanent or semi-permanent contact with each other, with attenuation rates close to those analysed theoretically. Dissipation in the collisional zone increases with an increasing restitution coefficient of the ice and with decreasing floe size. In effect, two factors contribute to strong attenuation in fields of small ice floes: lower wave energy propagation speeds and higher relative ice–water velocities due to larger accelerations of floes with smaller mass and more collisions per unit surface area.

Acoustic source localization in a highly anisotropic plate with unknown orientation of its axes of symmetry and material properties with numerical verification

Development of acoustic source localization techniques in anisotropic plates has gained attention in the recent past and still has scope of improvement. Most of such techniques existing in the literature either require known material properties or assume a straight line propagation of wave energy from the acoustic source to a sensor. These limitations have been overcome in recent years by employing wave front shape-based techniques. However, the existing wave front shape-based techniques are applicable in situations where the orientation of the axes of symmetry of the anisotropic plate is known beforehand. In the present study, a modified version of these techniques, namely, elliptical and parametric curve-based techniques, is proposed. This new version is useful when the angle between the axes of symmetry and the reference Cartesian coordinate system is unknown. In the new definition of the objective function, the orientation of the axes of symmetry of the anisotropic plate is treated as an input in the objective function in addition to the other unknowns like the source coordinates and the curve parameters. A numerical study illustrates how the modified new techniques can localize the acoustic source with sufficient accuracy in an anisotropic plate with unknown orientation of the axes of symmetry and its material properties.

Tropical Synoptic‐Scale Waves Propagating Across the Maritime Continent and Northern Australia

AbstractThe structure and characteristics of tropical synoptic‐scale wave disturbances over the Maritime Continent (MC) and northern Australia during the Southern Hemisphere summer are analyzed. The tropical synoptic‐scale waves are identified by an extended empirical orthogonal function analysis on 2‐ to 8‐day‐filtered daily 850‐hPa meridional wind anomalies during December–February for the period 1979–2016. Extended empirical orthogonal function‐based composite analyses of various fields reveal a synoptic‐scale wave train, with a well‐organized structure, which is coupled with deep convection anomalies. The composite waves are considered to be tropical depression‐type waves that develop along a strong meridional gradient of the mean monsoon westerly flow. These waves propagate westward at approximately 8 m/s, with zonal wavelengths of about 3,000–4,000 km. Eastward amplification of wave troughs and ridges due to group propagation occurs within the synoptic‐scale wave train. Wave activity diagnostics confirm that downstream wave energy propagation facilitates the development of the synoptic‐scale wave train along the monsoon westerly flow. Rainfall is enhanced over the MC and the ocean in the vicinity of the north coast of Australia, which is associated with the passage of the wave trough. The development of this synoptic‐scale wave is found to play an important role in day‐to‐day weather variation over the MC. Extratropical forcing of the tropical synoptic‐scale waves is also explored. Southerly surges that originated in the midlatitude Indian Ocean could intensify the tropical synoptic‐scale wave trough. The southerly surges are induced by the development of midlatitude synoptic‐scale baroclinic disturbances over the eastern Indian Ocean.

The Interdecadal Change of Summer Water Vapor over the Tibetan Plateau and Associated Mechanisms

Abstract In recent decades, long-term changes of the Tibetan Plateau (TP) water vapor and the associated mechanisms have not been investigated fully. This study aims to examine the interdecadal change of summer TP water vapor using the monthly mean European Centre for Medium-Range Weather Forecasts interim reanalysis during 1979–2014. The results show a drier phase in the TP during 1979–94, with a subsequent wetter phase, which suggests an interdecadal variation of summer TP water vapor around the middle of the 1990s. This interdecadal variation is mainly due to a significant change of the water vapor export on the eastern boundary of the TP, which is closely associated with a summer atmospheric circulation anomaly near Lake Baikal. When a cyclonic (an anticyclonic) anomaly occurs near Lake Baikal, there is less (more) water vapor over the TP. On the interdecadal scale, the atmospheric circulation anomaly near Lake Baikal is related to an extratropical large-scale anomalous wave train over the northwestern Atlantic–East Asian region, with an eastward propagation of the anomalous wave energy from the Atlantic to East Asia. Climate model simulations further demonstrate an impact of sea surface temperature (SST) anomalies in the northwestern Atlantic on the anomalous wave train. Both the extratropical tropospheric anomalous wave train and the anomalous atmospheric circulation near Lake Baikal are successfully simulated by changing the summer northwestern Atlantic SST. Therefore the warming northwestern Atlantic is an important factor contributing to the wetting TP in recent decades.

Vertical structure of Convectively Coupled Equatorial Waves (CCEWs) during Boreal Summer and Winter

Atmospheric variability in the tropics on the daily to intraseasonal timescale is often related to convectively coupled equatorial waves (CCEWs), including Kelvin, MRG, ER, and TD-type waves. This study aims toanalyze the observed vertical structures of the CCEWsduring boreal summer and winter fromthe NCEP-DOE reanalysis dataset. Consistent with a linear wave theory, the vertical structure of CCEWs is characterized by a distinctive phase tilt, indicatinga distinct direction of wave energy propagation. The vertical structure of Kelvin, MRG and ER waves is signified by westward phase tilts with height, especially in the upper troposphere when the waves become dry (i.e. there is noeffect of moisture), while in the lower troposphere, the phase tiltsdeviatesignificantly from the linear wave theory, whenthe waves become moist. On the other hand, the TD-type waves exhibit a perpendicular, baroclinic structure with height. The vertical structures of the Kelvin, MRG and TD-Type waves are more clearly observed during boreal summer, while ER waves are more observed during boreal winter. This is consistent with the increasing wave amplitudesdue to a stronger wave sourceduring those periods. Future studies are still required in order to understand how the vertical modulation of CCEWs on RH and wind fields can be used to better improve weather predictions in the tropics.

Experimental investigation of the dissipation characteristic of sandwich structures with periodically perforated viscoelastic damping material core

Wave energy can be dissipated gradually when it is propagated in viscoelastic damping material (VDM) composite structures. In this paper, after the specimens with different opening ratios (ORs) of VDM layer are prepared, the elastic wave energy propagation and dissipation characteristic of periodically perforated VDM cored sandwich structures are investigated by an experimental method. The sandwich structures are discretized into several testing points in our experiment. When the complex velocity, equivalent effective mass, and external excitation forces have been obtained at each testing point by sensors, the energy dissipation in the sandwich structure is determined based on the energy dissipation mechanism of wave transmission in solid. The experimental results are then compared with theoretical and numerical simulation results. By analyzing the computational accuracy of theoretical and numerical results using experimental data, it is shown that high consistency between theoretical, numerical, and experimental results can be achieved, especially in the medium-frequency and high-frequency ranges. Thus, our experimental results demonstrate that periodically perforated VDM sandwich structures can be applied to engineering practice for their good performance of dissipation characteristics in the middle-frequency and high-frequency ranges.

Regional Swell Transformation by Backward Ray Tracing and SWAN

Beach erosion and wave-induced flooding models are often initialized in O(10)-m depth, seaward of the surfzone, with wave conditions estimated from regional nonlinear spectral wave models [e.g., Simulating Waves Nearshore (SWAN)]. These models are computationally expensive for high-resolution, long-term regional O(100)-km hindcasts, and they limit examination of the effect of different climate scenarios on nearshore processes. Alternatively, computationally fast models with reduced linear wave physics enable long-term hindcasts at high spatial (<100 m) resolution. Linear models, that efficiently transform complete spectral details from deep water through complex offshore bathymetry, are appropriate for low-frequency swell wave energy propagation. Here, two numerically different linear methods are compared: backward ray-tracing and stationary linear SWAN simulations. The methods yield similar transformations from deep water (seaward of offshore islands in Southern California) to the nearshore, O(10)-m depth. However, SWAN is sensitive to model spatial resolution, especially in highly sheltered regions, where with typical (1–2 km) resolution SWAN estimates of nearshore energy vary by over a factor of 2 relative to ray tracing. Alongshore radiation stress estimates from SWAN and ray tracing also differ, and in some cases the climatological means have opposite signs. Increasing the SWAN resolution to 90 m, higher than usually applied to regional models, yields the nearshore transforms most similar to ray tracing. Both accurate rays and high-resolution SWAN require significant computation time; however, ray tracing is more efficient if transforms are needed at relatively few locations (compared with every grid point), or if computer memory is limited. Though presently less user friendly than SWAN, ray tracing is not affected by numerical diffusion or limited by model domain size or spatial resolution.

Ionic to covalent glass network transition: Effects on elastic and vibrational properties according to ultrasonic echography and Raman spectroscopy

In this study, ultrasonic echography and Raman spectroscopy were used to investigate the dependence of the elastic and vibrational properties on the composition of glass-forming pseudo-binary xSr(PO3)2-(1-x)(0.62MgF2-0.38AlF3) fluorophosphate glasses. The variation in Tg as the phosphate content increased indicated that the network in the glass structure was more interconnected. The decrease in the density with higher PO3−/F− ratios can be understood in terms of the less dense packing of the phosphate chains relative to the fluoride network and changes in the anionic volume in the glass network. The Raman spectra obtained for the fluorophosphate glasses confirmed the presence of PO4 tetrahedral species configurations with 2, 3, and 4 non-bridging oxygen atoms analogous to multicomponent mixed fluoride-phosphate glasses. A well-resolved Boson peak dominated the low-frequency Raman spectra. The transversal and longitudinal sound velocities exhibited the same composition-induced changes in the glass network as the density measurements. The variation in the bulk modulus as the PO3−/F− ratio increased indicated the transition from a strong (ionic-fluoride limit) to a less ionic (covalent-phosphate limit) structure, which was related to changes in the atomic volume. The glass network was characterized by a low cross-linking density according to the variations in Poisson's ratio. The incorporation of phosphate groups in the fluoride structure decreased the strength of the average chemical bond, as well as leading to a less interconnected network in the glass structure and reducing the strong ionic character of these fluorophosphate glasses, which was confirmed by the trend in the electronegativity difference and the attenuated propagation of the ultrasonic wave energy.

Influence of the Boreal Summer Intraseasonal Oscillation on Extreme Temperature Events in the Northern Hemisphere

The impact of the boreal summer intraseasonal oscillation (BSISO) on extreme hot and cool events was investigated, by analyzing the observed and reanalysis data for the period from 1983 to 2012. It is found that the frequency of the extreme events in middle and high latitudes is significantly modulated by the BSISO convection in the tropics, with a 3–9-day lag. During phases 1 and 2 when the BSISO positive rainfall anomaly is primarily located over a northwest–southeast oriented belt extending from India to Maritime Continent and a negative rainfall anomaly appears in western North Pacific, the frequency of extreme hot events is 40% more than the frequency of non-extreme hot events. Most noticeable increase appears in midlatitude North Pacific (north of 40°N) and higher-latitude polar region. Two physical mechanisms are primarily responsible for the change of the extreme frequency. First, an upper-tropospheric Rossby wave train (due to the wave energy propagation) is generated in response to a negative heating anomaly over tropical western North Pacific in phases 1 and 2. This wave train consists of a strong high pressure anomaly center northeast of Japan, a weak low pressure anomaly center over Alaska, and a strong high pressure anomaly center over the western coast of United States. Easterly anomalies to the south of the two strong midlatitude high pressure centers weaken the climatological subtropical jet along 40°N, which is accompanied by anomalous subsidence and warming in North Pacific north of 40°N. Second, an enhanced monsoonal heating over South Asia and East Asia sets up a transverse monsoonal overturning circulation, with large-scale ascending (descending) anomalies over tropical Indian (Pacific) Ocean. Both the processes favor more frequent extreme hot events in higher-latitude Northern Hemisphere. An anomalous atmospheric general circulation model is used to confirm the tropical heating effect.