Spectral characteristics of quasi-biennial oscillations of the equatorial stratospheric wind and the problem of synchronization

Spectral characteristics of the quasi-biennial oscillations (QBO) of the zonal velocity in the equatorial stratosphere are investigated in this work on the basis of data from the NCEP/NCAR and ERA40 reanalyses and numerical experiments with the atmospheric general circulation (GCM) model developed at the Institute of Numerical Mathematics, Russian Academy of Sciences (INM RAS). The problem of synchronizing QBO and semiannual oscillations (SAO) of the zonal velocity in the mesosphere is considered. It is shown that the process of synchronization to multiples of SAO periods is identifiable in the transition region between QBO and SAO. For all heights where QBO exist, their synchronization with SAO is expressed in the calculation of the period in terms of differences between the westerly maxima. The INM RAS GCM model is shown to satisfactorily reproduce the main spectral characteristics of QBO and SAO, as well as specific features of the variability of the QBO period obtained from reanalysis data. The possibility of synchronization with SAO or the annual cycle in the upper layers is shown on the basis of an investigation of QBO models with a small number of parameters, both for the absorption mechanism of planetary waves by the mean flow and for the breaking of short gravity waves. The QBO formation from different wave types, together with SAO and the annual cycle, can be considered a unified system of oscillations in the circulation of the equatorial upper atmosphere.     

Simulation of the quasi-biennial oscillations of the zonal wind in the equatorial stratosphere: Part II. Atmospheric general circulation models

The problem of simulating quasi-biennial oscillations (QBOs) of zonal velocity in the equatorial stratosphere in atmospheric general circulation models is considered. In accordance with the results from Part I of this study on the basis of the models developed at the Institute of Numerical Mathematics of the Russian Academy of Sciences (INM RAS), the possibility of implementing (in these models) mechanisms of QBO excitation through both the interaction of planetary waves with the mean flow and breaking of short gravity waves is investigated. A new high-resolution 2° × 2.5° × 80 version of the INM RAS model is designed, a climate simulation with the two 2° × 2.5° × 39 and 2° × 2.5° × 80 versions of the INM RAS model is briefly described, results of spectral analysis of equatorial wave activity are presented, and the QBO formation processes in these models are considered in detail. For the new 2° × 2.5° × 80 model, realistic QBOs of zonal wind are obtained as the result of the action of both mechanisms.     

Quasi-biennial oscillation of wind in the low-latitude stratosphere and the winter wave activity of the atmosphere in the Northern Hemisphere

According to the Holton-Tan hypothesis [1], oscillations of the equatorial stratospheric wind change the conditions of the vertical and meridional propagation of planetary waves in extratropical regions, which can cause quasi-biennial oscillations (QBOs) at middle and polar latitudes. To verify the Holton-Tan hypothesis, the intensity of the winter wave activity of the atmosphere in the Northern Hemisphere was estimated at different phases of the quasi-biennial oscillation of the equatorial stratospheric zonal wind. As it turned out, a higher level of the wave activity expected at the easterly phase of the equatorial QBO is characteristic only of the period when the winter circulation is established. At the end of winter a higher level of the wave activity is observed at the westerly QBO phase, which contradicts the Holton-Tan hypothesis. Small but nevertheless noticeable distinctions in the wave activity at low tropospheric levels suggest that the quasi-biennial periodicity of the wave activity at middle latitudes can be caused by oscillations of synoptic processes between the predominantly zonal and meridional forms of the circulation, as was indicated by Pogosyan and Pavlovskaya [2, 3].     

Zonal distribution features of high frequency planetary waves in the oceans derived from satellite altimeter data

1IntroductionTheavailabilityofnearly9aofhighqual-itysealevelanomaly(SLA)datafromsatellitealtimetersmakesitnowpossibletoestimatethedominantsignalsofvariabilityintheglobalo-ceanmoreaccuratelythanwaspreviouslypos-sible.Moreover,mergingtheT/PandERS-1and2altimeterdatathroughanadvancedglobalobjectiveanalysiscangreatlyimproveoura-bilitytoaccuratelyestimateoceanicvariabili-tycomparedwithusingasinglealtimeter(BoulagerandMenkes,1995).ThealtimeterdatarevealthedominantroleofplanetaryRoss-bywavesinocea…     

Investigation of conditions for the generation and propagation of low-frequency disturbances in the troposphere

Low-frequency disturbances responsible for the excitation of torsional oscillations—variations in the zonal mean flow intensity with a characteristic scale of 15–20 days—propagating along the meridian at mid and low latitudes of both hemispheres are investigated [1]. As data observed over the eastern parts of continents and the western parts of oceans are processed with the lag correlation statistics, traveling waves intersecting the eastern parts of continents from northwest to southeast and then returning to the north along the ocean coasts are identified. In this case, trains of anomalies oriented in the zonal direction periodically appear and are destructed in the western parts of continents. The simulation of the propagation of disturbances in the quasi-geostrophic approximation made it possible to explain the specific features of lag correlation statistics over continents by the dispersion of two-dimensional Rossby waves from traveling sources. The turnover of disturbances over Asia and wave trains to the west from the pole were reproduced. Torsional oscillations caused by the dispersion of two-dimensional Rossby waves have a characteristic form of inclined bands in the latitude-time diagram, whose steepness is controlled by the velocity of displacement of the vorticity source along the meridian.     

全球海洋高频波动主振荡周期的纬向带状分布特征

基于1992年10月至2000年12月的TOPEX/Poseidon和ERS-1/2卫星高度计资料,分析了周期短于150d的高频波动及其能量的空间分布特征.在功率谱密度计算基础上,分析特定频段所含能量占周期短于150d的高濒波动总能量的百分比,发现该频段最强振荡(下文称主振荡)所对应的周期从近赤道的1个月逐步增加至南北纬30°附近的4个月,进一步分析表明主振荡周期的这种变化是连续的,与西向行星Rossby波的相速随纬度增加而变慢相对应.主振荡所对应的周期在全球海洋中呈显著的带状分布.进而表明在14°N(S)附近60d周期的振荡除通常所认为的由于潮汐混淆所造成的虚假信号之外,主要是行星Rossby波形态的真实信号.     

Preliminary study on the dynamic mechanism of the deep equatorial jets

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Alternating zonal flows in a two-layer wind-driven ocean

Alternating zonal flows in an idealized wind-driven double-gyre ocean circulation have been investigated using a two-layer shallow-water eddy-permitting numerical model. While the alternating zonal flows are found almost everywhere in the time-mean zonal velocity field, their meridional scales differ from region to region. In the subpolar western boundary region, where the energetic eddy activity induces quasi two-dimensional turbulence, the alternating zonal flows are generated by the inverse energy cascade and its arrest by Rossby waves, and the meridional scale of the flows corresponds well to the Rhines scale. In the eastern part of the basin, where barotropic basin modes are dominant, the zonal structure is formed through the nonlinear effect of the basin modes and is wider than the Rhines scale. Both effects are likely to form zonal structure between the two regions. These results show that Rossby basin modes become an important factor in the formation of alternating zonal flows in a closed basin in addition to the arrest of the inverse energy cascade by Rossby waves. The wind-driven general circulation associated with eddy activities plays an essential role in determining which mechanism of the alternating zonal flows is possible in each region.     

Variability of the Baltic Sea level and floods in the Gulf of Finland

The statistical analysis of the long-term data on the variability of the Baltic Sea level has revealed the complicated character of the wave field structure. The wave field formed by the variable winds and the disturbances of the atmospheric pressure in the Baltic Sea is a superposition of standing oscillations with random phases. The cross spectral analysis of the synchronous observation series of the level in the Gulf of Finland has shown that the nodal lines of the standing dilatational waves are clearly traced with frequencies corresponding to the distance from the nodal line to the top of the gulf (a quarter of the wave length). Several areas of the water basin with clearly expressed resonant properties may be distinguished: the Gulfs of Finland, Riga, and Bothnia, Neva Bay, etc. The estimations of the statistical correlation of the sea level oscillations with the variation of the wind and atmospheric pressure indicate the dominant role of the zonal wind component during the formation of the floods in the Gulf of Finland. The probable reason for the extreme floods in St. Petersburg may be the resonance rocking of the eigenmode oscillations corresponding to the basic fundamental seiche mode of the Gulf of Finland with a period of 27 h when the repeated atmospheric disturbances in the Baltic Sea occur with a period of 1–2 days.     

Height-season structure of the available potential energy and kinetic energy in the lower and middle atmosphere

The distributions of kinetic energy (KE) and available potential energy (APE) in the lower and middle atmosphere of the Northern and Southern hemispheres over the period 1992–2003 are investigated. Annual mean values of the amplitude and phase of annual and semiannual oscillations in the zonal and eddy forms of KE and APE are calculated in the height range 0–55 km (1000–0.316 hPa) for the 21st layer. A clearly pronounced annual cycle of the zonal and eddy components of KE and APE with maxima in the winter season are observed in the troposphere of both hemispheres. In the lower stratosphere, the annual-cycle maximum is shifted toward the summer season because of the meridional gradient of the zonal mean temperature. In the stratosphere of both hemispheres, along with annual oscillations, semiannual oscillations are present in all forms of energy. The intensity of these oscillations for the zonal KE and APE at the upper-stratosphere heights is comparable to the intensity of annual oscillations. A local structure of the energy regime of the upper mesosphere-lower thermosphere is investigated against the background of the global energy regime from the data of meteor sounding in Kazan. It is shown that, for both the global and regional regimes, specific features of the phase profiles of energy characteristics can be explained by the presence of barriers during the propagation of wave disturbances along the vertical.     

Long-period waves in the vicinity of Onagawa Bay (I)

To investigate the response characteristics of a bay to tsunamis, field measurements of long-period waves have been carried out at Onagawa and Okachi Bays, both of which face the Pacific Ocean in northern Japan. In Onagawa Bay, the observed transfer function is in good agreement with the prediction based on the one-dimensional numerical model, in the period range larger than about 15 minutes. The response of shorter periods seems to be influenced by the two-dimensionality of the bay. The oscillations within Onagawa Harbor are also discussed with respect to the relative amplitude and phase at two stations inside the harbor and it is estimated that the reflection coefficient at the waterfront is about 0.7. In Okachi Bay, the oscillations in the period range larger than about 10 minutes could be explained by a Y-shaped model of the bay. The dominant oscillations offshore of these two-bays are found to be the mode with the motion predominantly in the direction normal to the shelf orientation, and the estimated power spectral density of incoming waves in deep water varies asf ^–2,f being the frequency. The waves of lateral modes, such as edge waves on the shelf, are small and of minor importance to generate bay oscillations of longer periods.     

Quasi-biennial variations in ozone and meteorological parameters over western Europe from ozonesonde data

Quasi-biennial variations in vertical profiles of ozone, temperature, air pressure, and zonal and meridional wind velocities are analyzed from ozonesonde data obtained at the western European stations of Lindenberg, Hohenpeissenberg, and Payerne. The effect of quasi-biennial variations manifests itself variously in different variables and is nonuniform in altitude. The period of quasi-biennial variations is not constant, and the values of the mean period group mainly around 2 and 2.5 years. As in the North American region, the effects of quasi-biennial variations in different parameters of the stratosphere and troposphere over western Europe are due to a combination of the effects of the quasi-biennial oscillation (QBO) in the equatorial stratosphere, the El Niño-Southern Oscillation (ENSO), and the North Atlantic Oscillation (NAO). The observed 2.5-year variations in stratospheric ozone are related to the equatorial QBO to a larger extent in comparison with variations in other variables. It seems likely that a determining influence on variations in stratospheric wind and temperature is exerted by the ENSO. Variations in tropospheric and stratospheric parameters with a mean period of about 2 years are due to the ENSO and NAO effects.     

Influence of offshore topography on the amplification of infragravity oscillations within a harbor

The main purpose of this article is to systematically investigate the influence of offshore fringing reef topography on the infragravity-period harbor oscillations. The infragravity (IG) period oscillations inside an elongated harbor induced by normally-incident bichromatic wave groups are simulated using a fully nonlinear Boussinesq model, FUNWAVE 2.0. Based on an IG wave decomposition method, effects of plane reef-face slopes, reef-face profile shapes and the existence of reef ridge on bound and free IG waves and their relative components inside the harbor are comprehensively studied. For the given harbor and reef ridge, the range of the reef-face slopes and the various profile shapes studied in this paper, results show that the amplitude of the free IG waves inside the harbor increases with the increasing of the reef-face slope; while the bound IG waves inside the harbor seem insensitive to it. The effects of the profile shapes on the IG period waves inside the harbor are closely related to the width of the reef face. The existence of the reef ridge can relieve the bound IG waves to some extent when the incident short wave amplitudes are relatively large, while its effects on the free IG waves are negligible.     

Seasonal features of quasi-biennial variations of NO<Subscript>2</Subscript> stratospheric content derived from ground-based measurements

Seasonal and latitudinal distributions of amplitudes of quasi-biennial variations in total NO₂ content (NO₂ TC), total ozone content (TOC), and stratospheric temperature are obtained. NO₂ TC data from ground-based spectrometric measurements within the Network for the Detection of Atmospheric Composition Change (NDACC), TOC data from satellite measurements, and stratospheric temperature data from ERA-Interim reanalysis are used for the analysis. The differences in the NO₂ TC diurnal cycles are identified between the westerly and easterly phases of the quasi-biennial oscillations (QBO) of equatorial stratospheric wind. The QBO effects in the NO₂ TC, TOC, and stratospheric temperature in the Northern (NH) and Southern (SH) hemispheres are most significant in the winter–spring periods, with essential differences between the NH and SH. The NO₂ TC in the Antarctic is less for the westerly phase of the QBO than that for the easterly phase, and the NO₂ TC quasi-biennial variations in the SH mid-latitudes are opposite of the variations in the Antarctic. In the NH, the winter values of the NO₂ TC are generally less during the westerly QBO phase than during the easterly phase, whereas in spring, on the contrary, the values for the westerly QBO phase exceed those for the easterly phase. Along with NO₂, the features of the quasi-biennial variations of TOC and stratospheric temperature are discussed. Possible mechanisms of the quasi-biennial variations of the analyzed parameters are considered for the different latitudinal zones.     

An analytic investigation of oscillations within a harbor of constant slope

Longitudinal and transverse oscillations within a harbor of constant slope are analyzed. Based on the linear shallow water approximation, longitudinal oscillations are described with Bessel equations. Ignoring friction, oscillations are forced using the period of the incident perpendicular wave field by the method of matched asymptotics. The analytic results show that the varying depth shifts the resonant wave numbers to lower values than those for the same geometric harbor with constant depth. Furthermore, we extend the shallow water equations to a linear, weakly dispersive, Boussinesq-type equation by modifying the offshore velocity component, and then use it to investigate possible existing transverse oscillations in the harbor of constant slope. These oscillations are types of standing edge waves. Their character is quite sensitive to the boundary condition at the backwall of the harbor.     

Intraseasonal variability of the surface zonal current in the equatorial Indian Ocean: Seasonal differences and causes

Using observations and numerical simulations, this study examines the intraseasonal variability of the surface zonal current(u ISV) over the equatorial Indian Ocean, highlighting the seasonal and spatial differences, and the causes of the differences. Large-amplitude u ISV occurs in the eastern basin at around 80°–90°E and near the western boundary at 45°–55°E. In the eastern basin, the u ISV is mainly caused by the atmospheric intraseasonal oscillations(ISOs), which explains 91% of the standard...     

Effects of the equatorial quasi-biennial oscillation in the total ozone content over Russia

On the basis of ground-based measurements of total ozone content (TOC) over Russia and a number of neighboring states during 1973–2002, the amplitudes and phases of TOC variations caused by the quasi-biennial oscillation (QBO) of wind in the equatorial stratosphere are estimated for different regions and for the whole area. The seasonal dependence of the QBO effect in the TOC is analyzed. It is shown that the magnitude and even the sign of the effect depend on the relation between the equatorial QBO phase and the season. The regional empirical models of seasonally dependent QBO effects are constructed. It is found that the seasonal dependence of regional effects accounts for 4% (in the north of the area) to 20% (in the south) of the interannual variability of the TOC. The relation between the QBO effect and the 11-year cycle of solar activity is analyzed. Significant differences are revealed in the effects under the conditions of maximum and minimum solar activity. The QBO effects obtained from observations at Russian stations, satellite measurements with a TOMS instrument, and spectrometric observations of the TOC at western European stations are compared, and their satisfactory agreement is shown. An analysis of the results suggests that the QBO effects in the TOC over Russia are caused by several interacting factors and apparently reflect their regional properties.     

Generation of surface and internal waves in a bounded basin by a moving baric front

We consider a plane problem of generation of surface and internal waves in a bounded rotating basin of variable depth by a front of atmospheric pressure moving over the basin. The fluid is assumed to be two-layer. The system of nonlinear long-wave equations is solved numerically by the method of finite differences for the distribution of depths corresponding to a zonal section of the Black-Sea basin. It is shown that the baric front moving over the basin generates barotropic and baroclinic oscillations of the fluid. The intensity of disturbances depends on the velocity of motion and the width of the front. There exists a velocity of motion of the front for which internal waves are generated especially efficiently. When the front leaves the basin, we observe the formation of a packet of internal waves propagating from one lateral boundary of the basin to the other boundary with reflections from the boundaries.     

Harbor resonance induced by subaerial landslide-generated impact waves

Past studies of harbor resonance have mainly been restricted to the quasi-steady oscillations induced by steady wave conditions. This paper investigates the response of a rectangular harbor to subaerial landslide-generated impact waves based on physical models, in order to compare the oscillations induced by steady and transient waves. In response to steady incident waves, oscillations within the harbor need to experience a long process to obtain their maximum value before the input energy and the losses are balanced. Landslide-generated impact waves usually include components with solitary wave characteristics and also components with dispersive wave characteristics. Each component travels with a different celerity. Usually, solitary wave components propagate faster, and arrive in the harbor first. Oscillations attain their maximum status as soon as these components arrive. The subsequently arriving components with dispersive characteristics do not enhance the resonance oscillations. So the waves with solitary characteristics are considered to play an important role in harbor resonance. Numerical experiments, using the FUNWAVE model, were conducted in order to further verify these conclusions.