Quasi-stationary Planetary Waves Research Articles

Wave-Mean Flow Interaction and Stratospheric Sudden Warming in an Isentropic Model

Abstract A multilayer isentropic model, with constant potential temperature in each model layer, has been developed to investigate the linear and nonlinear characteristics of motions in the stratosphere. The transition from linear to nonlinear behavior in the content of wave-mean flow interaction is studied. The planetary wave in the model is excited by wavelike forcings at the lower boundary. It propagates upward into the middle atmosphere. Ale planetary wave breaking and the mean zonal flow modification in the model are closely associated with critical layer of the quasi-stationary planetary wave. However, the region with strong mean flow deceleration and severe potential vorticity (PV) contour deformation is broad. As the forcing amplitude increases, this region shifts poleward and the maximum center of mean flow deceleration extends upward. In cases with large forcing amplitudes, the polar vortex is pushed away from the pole and easterly winds are found in the polar region. The responses of the model ...

The structure and propagation of stationary planetary wave packet in the barotropic atmosphere

Monthly or seasonally mean anomalies of large-scale atmospheric circulation are better represented by wave packets or their combination. Both qualitative and quantitative analyses of equations of wave packet dynamics, which are obtained by the use of WKB approximation, are very helpful for the understanding of structure, formation and propagation of stationary and quasi-stationary planetary wave packet patterns in the atmosphere. Indeed, these equations of wave packet dynamics can be directly solved by the method of characteristic lines, and the results can be simply and clearly interpreted by physical laws. In this paper, a quasi-geostrophic barotropic model is taken for simplicity, and the wave packets superimposed on several ideal profiles of the basic current and excited by some ideal forcings are investigated in order to make comparison of the accuracy of calculation with the analytical solution. It is revealed that (a) the rays of stationary planetary wave packet do not coincide with but go away from the great circle with significant difference if the shear of the basic zonal flow is not too small; (b) being superimposed on a westerly jet flow with positive shear (∂U λ/∂y>0), the stationary wave packets excited by low-latitudinal forcing are first intensified during their northeastward propagation in the Northern Hemisphere, then reach their maximum of amplitude at some critical latitude, and after that weaken again; (c) the connected line of extremes (the positive and negative centres) of wave packet does not coincide with but crosses the ray by an angle, the larger the scale of external forcing, the larger the angle; and (d) the whole pattern of a trapped stationary wave packet is complicated by the interference between the incident and reflected waves.

The effect of spatial structure character of heat source on the ray path and the evolution of wave energy of meridional wave train

This paper studies correlations between the spatial structure character of thermal forcing and deformation and the amplitude of rays of meridional wave train. It is shown that if thermal forcing appears a meridional linear variation the rays of quasi-stationary planetary wave may propagate along oblique lines and if the meridional variability of heat source has second order term the rays show distinct deformation as a great circular route. Additionally, the inhomogeneous distribution may cause lower frequency oscillations in mid- and low-latitudes. The combination of zonal and meridional wave numbers and distributive character of heat source may form an inverse mechanism of variational trend of generized wave energy, reflecting in some degree the physical process of transition between meridional and zonal flow patterns.

Recent stratospheric warmings at mid-latitudes and planetary-scale wave activity

Recent stratospheric warmings are herewith examined in association with the influence of long-period planetary waves. Investigations on height and time variations of geopotential and wind data obtained in the winters of 1980–87 at different tropo-stratospheric levels from messages of the radiosondes situated in the Central Europe take necessarily into account the propagation of quasi-stationary and free planetary waves which in proper conditions mutually interfere with each other. The definition and evolution of the different types of warming are connected with the structure of these waves and enable one to interpret more completely coupling phenomena among different regions of the neutal atmosphere during the most disturbed winters. Distinct spectral analysis methods permit to extract from geopotential data series prominent wave periods within 1÷3 weeks. Westerly winds in the stratosphere reach unusual amplitudes before a sudden stratospheric warming, and this amplification is seen to be in connection with transient normal modes which are able to interact even with the mean circulation of the mesosphere and lower thermosphere.

Interhemispheric Asymmetry and Annual Synchronization of the Ozone Quasi-biennial Oscillation

The quasi-biennial oscillation (QBO) in total column ozone has been examined at several tropical stations. The ozone QBO at Mauna Loa (19.5°N) was found to have a remarkable annual synchronization. Both positive and negative extremes in the deseasonalized ozone time series almost always occur between December and March. The annual cycle-QBO phase locking is much more pronounced in this ozone record than it is for the familiar QBO in the prevailing tropical stratospheric winds. This result is taken as evidence that the dynamical QBO acts to modulate a strong seasonal ozone transport from midlatitudes to the tropics. If this transport is connected with quasi-stationary planetary waves, then this interpretation offers an obvious explanation for the interhemispheric asymmetry in the ozone QBO that has been noted in many earlier studies.

Long-period fluctuations in ionospheric absorption and their relation with planetary activity in the stratosphere

By investigating the quasi-regular variations in ionospheric absorption along three distant radio paths with approximately one and the same equivalent frequency during the winters 1983/84 and 1984/85, an attempt is made to study the stratospheric-mesospheric relations at typical geographic mid latitudes. It is shown that the quasi-regular oscillations of ionospheric absorption, investigated in this paper, reach maximum development in each time series at one and the same time. This means, that if they are provoked by an external source when it starts acting, a spectrum of frequencies is generated rather than a definite fixed frequency. By studying the variation of the instantaneous amplitudes of the quasi-regular oscillations in ionospheric absorption with time, their connection with the amplification of the quasi-stationary planetary waves in the stratosphere with a zonal wave number 2 during the winter periods under investigation is shown.

A DIAGNOSTIC STUDY OF FORMATION AND MAINTENANCE OF BLOCKING SITUATION ON THE NORTHERN HEMISPHERE USING E-P FLUX

In this paper, the variations of the mean flow, the E-P flux and its divergence of planetary waves in the process of the formation, maintenance and collapse of the blocking situation in the second half of February, 1979 are analysed with the transformed Eulerian mean-motion equations.Analysed results show that because the basic flow changes from the easterly into the westerly in the lower troposphere at high latitudes, the planetary wave for wavenumber 2 strongly propagates upwards, and because of the interaction between the upward propagating planetary wave and the basic flow, the westerty is weakened and approaches to the resonant flow of wavenumber 2 in the middle and upper troposphere (then, in the lower and middle Stratosphere). This may cause the anomalous amplification of planetary wave for wavenumber 2, and moreover make the mean flow change from the westerly into the easterly in the lower and middle stratosphere, following the upper troposphere. Therefore, the blocking situation can be formed and maintained on the Northern Hemisphere. Thus, the upward propagations of quasi-stationary planetary waves and the mutual interaction with the basic flow are also one of the reasons of the formation and maintenance of the blocking situation.

Mean winds in the winter middle atmosphere above northern Scandinavia

Wind measurements which were carried out during the MAP/WINE Campaign in northern Scandinavia between 2 December 1983 and 24 February 1984 are used to derive background winds and monthly as well as winter mean values from the ground up to 90 km altitude. These mean winds compare favourably to the wind field proposed for the revised CIRA 86, which is deduced from satellite measurements. The vertical structure of the zonal monthly means is similar in both data sets during January and February. The winter mean zonal winds are observed to be slightly stronger in the stratosphere and lower mesosphere during the MAP/WINE winter than the satellite winds proposed for CIRA 86. The long term mean meridional winds are in good agreement up to 60 km. They indicate a dominant influence of quasistationary planetary waves up to 90 km and an ageostrophic poleward flow between 60 km and 85 km over northern Scandinavia, which maximizes at 76 km at about 8 m s −1. The observed short term variability of the wind is discussed with respect to a possible impact of saturating gravity waves on the momentum budget of the middle atmosphere.

Quasi-stationary planetary waves and temperature reference atmosphere

A middle atmosphere temperature reference model from satellite measurements has recently been proposed. Comparisons are made with the Air Force Reference Atmosphere and the Reference model from satellite data. Large temperature deviations between two models at 60 N, 90 W in winter are found and the role of quasi-stationary planetary waves for these deviations is dicussed.

PHYSICAL MECHANISM OF INFLUENCE OF HEAT SOURCE ANOMALY OVER LOW LATITUDES ON GENERAL CIRCULATION OVER NORTHERN HEMISPHERE IN WINTER

In this paper, the propagating laws of quasi-stationary planetary waves forced in low latitudes in a realistic current are discussed by using the refractive index squared of waves in a spherical coordinate system during the Nothern Hemisphere winter. It is pointed out that anomaly of heat source over low latitudes can cause the general circulation anomalies over high and middle latitudes during the Northern Hemisphere winter. In this paper, a quasi-geostrophic, 34-level spherical coordinto model with Rayleigh fraction, Newtouian cooling and horizontal eddy thermal diffusivity is used to investigato the propagations of the-anomaly of disturbance pattern due to an idealized heat source anomaly during the Northern Hemisphere winter. The computed results show that because of the heat source anomaly over the tropical Pacific, the PNA (the Pacific/North America) pattern anomalies in the winter circulation over the Northern Hemisphere will be caused.

A Linear Stability Analysis of the Stratospheric and Mesospheric Zonal Mean State in Winter and Summer

Abstract The linear normal modes of large-scale atmospheric motions are computed in a spherical quasi-geostrophic model extending from the surface up to 80 km to investigate the stability of zonal motions in the stratosphere and mesosphere. The basic states are taken from the Northern Hemisphere winter and summer solstitial conditions. The winter basic state is able to generate growing planetary waves 1 and 2, which extend into the stratosphere and the mesosphere at high latitudes. They are identified as Green modes which propagate very slowly. The structure of the Green mode compares favorably with the observed quasi-stationary planetary waves. Waves 4–6, generated in the troposphere, can also penetrate into the lower stratosphere but an trapped them and resemble Charney modes. Energetics analysis indicates that the ratio of the local energy conversion versus energy flux from below is about 1:2 for the Omen modes waves 1 and 2 in the stratosphere while the mean flow and the planetary waves in the mesosph...

The Role of Transient Motions in the Formation of Quasi-Stationary Planetary Waves

Abstract We investigate numerically, using bifurcation theory, the formation of quasi-stationary planetary waves in the winter troposphere, i.e., of time-mean waves with large energy conversion from eddy available potential to eddy kinetic energy. The models used are quasi-geostrophic, two-level, highly truncated spectral ones, with topography of gradually increasing complexity. An external heating of newtonian type is given. The meridional difference of the radiative equilibrium temperatures (δT) is assumed to be the only variable parameter, all other external parameters taking constant, realistic values. How the nature of solutions changes with ΔT is examined. The conclusions are as follows: The originally stationary solutions pass through two Hopf bifurcations when ΔT increases. Periodic solutions appear at the first Hopf bifurcation and, at the next bifurcation, chaotic solutions are generated. The quasi-stationary waves (the time-mean waves) in the chaotic state show properties distinct from truly st...

The sun's rotation and perturbations of geopotential height and temperature fields in the stratosphere

The morphology of a solar activity effect apparently connected with the Sun's rotation and showing up in 25-day and 13.6-day oscillations of stratospheric geopotential and temperature fields is analysed in this study. The used data cover the height range between roughly 20 and 30 km and a timespan from July 1965 to October 1971. Most prominent responses are found for zonal harmonic wave number 1 at the oscillation period of 25 days (solar rotation period modulated by seasonal changes) and for the zonally averaged meteorological quantities at the oscillation period of 13.6 days. Additional statistically significant effects show up in the zonal harmonics with wave number 1 and 3 at half the solar rotation period and in the zonal means with periodicities near 25–27 days. The results point towards a modulation of the quasistationary stratospheric planetary wave with a positive geopotential anomaly around roughly 180° longitude by solar activity changes. The direct physical mechanisms of this Sun-climate relationship are not yet clear, but it can be concluded that atmospheric dynamics is an important factor for its morphology and that downward propagation of such effects seems possible and should be investigated in future studies.

準定常プラネタリー波によるEliassen-Palmフラックスの観測データによる評価

準定常プラネタリー波によるEliassen-Palmフラックスの観測データによる評価

The Response of the Zonally Averaged Circulation to Stratospheric Ozone Reductions

The effects of various ozone density reductions of the zonally averaged circulation are evaluated with a numerical quasi-geostrophic model. If the ozone perturbations are confined to the polar regions and are minuscule on a global basis as was characteristic of the August 1972 solar proton event, then the calculations indicate a negligible effect on the mean circulation. For global ozone perturbations by predicted halocarbon pollution, about 10% reduction in the zonal jet strength and less than a 5% change in global mean stratospheric temperature are calculated. Large, uniform ozone reductions (above 50%) produce significant effects on the mean circulation: a substantial collapse of the stratosphere due to cooler temperatures, and a weak polar night jet. The reflection and transmission of quasi-stationary planetary waves in the middle atmosphere are computed to be insensitive to solar activity as extreme as the August 1972 solar proton event. It thus seems improbable that planetary waves are a viable mechanism for solar-weather interactions that involve perturbations of the zonally averaged circulation by ozone density reductions.

冬の準定常プラネタリー波の鉛直構造

Zonal harmonic analysis of the structure of quasi-stationary planetary waves in the winter stratosphere of the Northern Hemisphere made by Hirota and Sato (1969) is extended to the cases of eight winters (1963-1970) inclusive of 500mb data. Each winter is formed of three months: January, February and March.From the analysis of monthly mean states of the amplitude, the position of trough and the wave energy, it is found that the ″node-like structure″ of planetary waves of zonal wavenumber 1 appears at about 100mb level at 60°N in many months among 24 months. The presence of nodes and the intensity of the zonal westerlies at upper levels are highly correlated; the node appears when the intensity of the mean zonal wind at 10mb level exceeds about 40meter/sec. This suggests that the node appears as the result of the interferences of waves propagating upwards and downwards. In such situation, reflective indices of amplitudes are simply estimated to be between 35 and 60%. The presence of the node-like structure is not evident in the wave 1 at 40°N. It is not found in waves 2 and 3.The vertical distribution of the wave energy of waves 2 and 3 shows the maximum at about 300-500mb level and decreases with increasing height above the level.The year to year variation of the vertical structure of planetary waves is discussed, and is also compared with the result of van Loon et al. (1973).

SOME METEOROLOGICAL ASPECTS OF DROUGHT

The problem of drought is examined as a manifestation of anomalous patterns of the atmosphere's general circulation. Special consideration is given to the quasi-stationary planetary wave ensembles which are responsible for extensive summertime drought over the United States. The critical importance of the great Atlantic and Pacific anticyclones for encouraging or discouraging the development of the great North American upper level anticyclone, the immediate drought producing cell, is emphasized.