Microwave measurements of the ozone content in winter arctic stratosphere

As a result of the long-term observations lead in region of Kola Peninsula, connection between character of variations of the ozone content in a stratosphere of Arctic regions and behavior and structure of a winter polar vortex is established. During winter seasons with well developed cyclone and duration of stable existence not less than 1.5–2 months were observed extremely low ozone number density at heights 20–25 km connected, apparently, with its chemical destruction. On the other hand, during disturbances of the vortex, accompanied strong stratospheric warming, was registered almost double increase of ozone amount in a high-altitude interval from 20 up to 40 km. Comparison of results of ground-based microwave monitoring of an ozone layer to data of the satellite instrument EOS MLS installed on satellite AURA is lead. In most cases comparison has shown satisfactory within the limits of an error of measurements coincidence of results. However in conditions of atmospheric disturbances when arose significant spatial heterogeneity, the discrepancy of results of comparison was marked. The possible reasons which cause the detected disagreement in results are discussed.     

The influence of tropical Indian Ocean warming on the Southern Hemispheric stratospheric polar vortex

During the past decades, concurrent with global warming, most of global oceans, particularly the tropical Indian Ocean, have become warmer. Meanwhile, the Southern Hemispheric stratospheric polar vortex (SPV) exhibits a deepening trend. Although previous modeling studies reveal that radiative cooling effect of ozone depletion plays a dominant role in causing the deepening of SPV, the simulated ozone-depletion-induced SPV deepening is stronger than the observed. This suggests that there must be other factors canceling a fraction of the influence of the ozone depletion. Whether the tropical Indian Ocean warming (IOW) is such a factor is unclear. This issue is addressed by conducting ensemble atmospheric general circulation model (AGCM) experiments. And one idealized IOW with the amplitude as the observed is prescribed to force four AGCMs. The results show that the IOW tends to warm the southern polar stratosphere, and thus weakens SPV in austral spring to summer. Hence, it offsets a fraction of the effect of the ozone depletion. This implies that global warming will favor ozone recovery, since a warmer southern polar stratosphere is un-beneficial for the formation of polar stratospheric clouds (PSCs), which is a key factor to ozone depletion chemical reactions. Supported by National Natural Science Foundation of China (Grant Nos. 40775053 and 90711004), National Basic Research Program of China (Grant No. 2009CB421401), and Innovation Key Program of Chinese Academy of Sciences (Grant Nos. KZCXZ-YW-Q11-03, KZCZ2-YW-Q03-08)     

Transport and the seasonal variation of ozone

The transport mechanisms responsible for the seasonal behavior of total ozone are deduced from the comparison of model results to stratospheric data. The seasonal transport is dominated by a combination of the diabatic circulation and transient planetary wave activity acting on a diffusively and photochemically determined background state. The seasonal variation is not correctly modeled as a diffusive process. The buildup of total ozone at high latitudes during winter is dependent upon transient planetary wave activity of sufficient strength to cause the breakdown of the polar vortex. While midwinter warmings are responsible for enhanced ozone transport to high latitudes, the final warming marking the transition from zonal mean westerlies to zonal mean easterlies is the most important event leading to the spring maximum. The final warming is not followed by reacceleration of the mean flow; so that the ozone transport associated with this event is more pronounced than that associated with midwinter warmings.     

Relation between the intensity of the stratospheric circumpolar vortex and the accumulation of ozone in the winter hemisphere

Summary The intensity of the polar vortex at 10 mb is used to calculate theoretical values of mean total ozone north of 40° latitude. A satisfactory fit is attained between the development in time of the theoretical ozone and that of the mean of the observed total ozone. The results lead to the conclusion, that a one-cell mean meridional motion relative to the polar night vortex is important for the transport of heat and ozone.     

Simulations of stratospheric sudden warmings in the Berlin troposphere-stratosphere-mesosphere GCM

Stratospheric sudden warming events in the Northern Hemisphere of the Berlin TSM GCM are investigated. In about 50% of the simulated years (13 out of 28), major midwinter warmings occur. This agrees well with observations but, whereas real events tend to occur approximately every second season, those in the model are clustered, most of them occur in the period between years 15/16 and years 24/25. In most other years, minor warming events take place. The warming events are found earlier in the winter than in reality. Many of the observed characteristics of warming events are well captured by the model: pulses of wave activity propagate out of the troposphere; these transient events force the zonal-mean zonal wind in the stratosphere and coincide with increases of the temperature at the North Pole and cooling at low levels in the tropics; temperature changes of opposite sign are modelled at higher levels. Synoptically, the modelled stratosphere evolves quite realistically before the warmings: the cyclonic vortex is displaced from the Pole by an amplifying anticyclone. After minor warmings, the stratosphere remains too disturbed as the cyclonic centre does not return to the North Pole as quickly as in reality. In the aftermath of major warmings the cyclonic vortex is not fully eroded and the anticyclonic circulation does not develop properly over the Pole; furthermore, the wintertime circulation is not properly restored after the event.     

Influence of wave activity on the composition of the polar stratosphere

The planetary wave impact on the polar vortex stability, polar stratosphere temperature, and content of ozone and other gases was simulated with the global chemical–climatic model of the lower and middle atmosphere. It was found that the planetary waves propagating from the troposphere into the stratosphere differently affect the gas content of the Arctic and Antarctic stratosphere. In the Arctic region, the degree of wave activity critically affects the polar vortex formation, the appearance of polar stratospheric clouds, the halogen activation on their surface, and ozone anomaly formation. Ozone anomalies in the Arctic region as a rule are not formed at high wave activity and can be registered at low activity. In the Antarctic Regions, wave activity affects the stability of polar vortex and the depth of ozone holes, which are formed at almost any wave activity, and the minimal ozone values depend on the strong or weak wave activity that is registered in specific years.     

Experiments on instability of columnar vortex pairs in rotating fluid

We present results from a new series of experiments on the geophysically important issue of the instability of anticyclonic columnar vortices in a rotating fluid in circumstances such that the Rossby number exceeds unity. The vortex pair consisting of a cyclonic and an anticyclonic vortex is induced by a rotating flap in a fluid which is itself initially in a state of solid-body rotation. The anticyclonic vortex is then subject to either centrifugal or elliptical instability, depending on whether its initial ellipticity is small or large, while the cyclone always remains stable. The experimental results demonstrate that the perturbations due to centrifugal instability have a typical form of toroidal vortices of alternating sign (rib vortices). The perturbations due to elliptical instability are of the form of sinuous deformation of the vortex filament in the plane of maximal stretching which corresponds to the plane of symmetry for the vortex pair. The initial perturbations in both cases are characterized by a definite wave number in the vertical direction. The characteristics of the unstable anticyclone are determined by the main nondimensional parameter of the flow - the Rossby number. The appearance of both centrifugal and elliptical instabilities are in accord with the predictions of theoretical criteria for these cases.     

Pattern of vertical velocity in the Lofoten vortex (the Norwegian Sea)

Mean radial distributions of various dynamic characteristics of the permanently existing anticyclonic Lofoten vortex (LV) in the Norwegian Sea are obtained from an eddy-permitting regional hydrodynamic MIT general circulation model. It is shown that the model adequately reproduces the observed 3D thermohaline and dynamic structure of the vortex. The obtained radial distribution of the mean vertical velocity is found to form a complex structure: with the upward fluxes along the axis in and above the anticyclonically rotating LV core, compensated by the downward fluxes in the vortex skirt. These vertical motions maintain the vortex potential energy anomaly against dissipation. This secondary circulation is generated by the centrifugal force and, to a lesser extent, by the horizontal dispersion of the vortex energy, both intensified towards the sea surface. Below the vortex core, the maximum downward vertical velocity converges towards the vortex axis with depth. At these depth levels, the secondary circulation is forced by Ekman divergence in the bottom mixed layer. The theory of columnar vortices with helical structure, applied to the LV, relate the radial profiles of the vertical velocity with those of the horizontal circulation. The theoretically predicted the radial patterns of the mean vertical velocity in the LV were close to those, obtained from the primitive equation ocean model, when approximating the radial patterns of the azimuthal velocity with the Rayleigh profile.     

A three-dimensional model comparison of PSC processing during the Arctic winters of 1991/1992 and 1992/1993

A three-dimensional transport model has been used to compare and contrast the extent of processing by polar stratospheric clouds during the northern hemisphere winters of 1991/1992 and 1992/1993. The model has also been used to compare the potential for ozone loss between these two winters. The TOMCAT off-line model is forced using meteorological analyses from the ECMWF. During winter 1992/1993 polar stratospheric clouds (PSCs) in the model persisted into late February/early March, which is much later than in 1991/1992. This persistence of PSCs should have resulted in much more ozone loss in the later winter. Interestingly, however, the extent of PSC processing and ozone loss was greater in January 1992 than January 1993. In January 1992 PSCs occurred at the edge of a distorted polar vortex whilst in January 1993 the PSCs were located at the centre of a much more zonally symmetrical vortex. In March 1993, distortions of the vortex led to the tearing off of vortex air and its mixing into midlatitudes.     

Numerical Study on Vortices in the Middle Layer of Flow around a Large Mountain under Rotating Stratified Conditions

Generation of cyclonic vortices in the middle layer of flow around a large mountain like Tibet and Rocky was investigated by means of a 3-D nonhydrostatic meteorological prognostic model. Special attention was paid to the effects of the earth’s rotation and stratification on the vortices detached successively from the slope of a high and large horizontal scale mountain. It was found the successive formation and detachment of such ‘von Karman-like vortices’ occurred in the flow regime at high Rossby numbers Ro and low Froude numbers Fr. It was successfully divided by the criterion of baroclinic instability. This means that if the condition is unstable baroclinically, a lee vortex is destabilized into a three-dimensional one, while under baroclinically stable conditions the lee vortex with vertical axis retains its standing structure and remains long lasting in the middle layer.     

Influence of strong sudden stratospheric warmings on ozone in the middle stratosphere according to millimeter wave observations

This paper reports the study data on variations in the ozone content in the middle stratosphere over Moscow based on millimeter wavelength observations during a range of midwinter sudden stratospheric warmings that occurred in the past two decades. The relation of ozone with planetary waves and the intensity of the polar stratospheric vortex has been established. The ozone vertical distribution has been monitored with a highly sensitive spectrometer with a two-millimeter wave band. The discovered phenomena of a relatively long-term lower ozone content in December in the considered cold half-year periods are related to the higher amplitude of the planetary wave with n = 1. Such phenomena preceded the development of strong midwinter stratospheric warmings, which, in turn, were accompanied by a significant increase in the ozone content in January. This ozone enrichment was related to the lower amplitude of the wave with n = 1 and higher amplitude of the wave with n = 2 and was accompanied by geopotential H _c.v. growth in the polar vortex center. Specific features of variations in the ozone content under the influence of the major atmospheric processes are observed not only in certain cold half-year periods but are also well seen in the general averaged pattern for winters with strong stratospheric warmings.     

Development of a chemistry module for GCMs: first results of a multiannual integration

The comprehensive chemistry module CHEM has been developed for application in general circulation models (GCMs) describing tropospheric and stratospheric chemistry, including photochemical reactions and heterogeneous reactions on sulphate aerosols and polar stratospheric clouds. It has been coupled to the spectral atmospheric GCM ECHAM3. The model configuration used in the current study has been run in an –off-line mode, i.e. the calculated chemical species do not affect the radiative forcing of the dynamic fields. First results of a 15-year model integration indicate that the model ECHAM3/CHEM runs are numerically efficient and stable, i.e. that no model drift can be detected in dynamic and chemical parameters. The model reproduces the main features regarding ozone, in particular intra- and interannual variability. The ozone columns are somewhat higher than observed (approximately 10%), while the amplitude of the annual cycle is in agreement with observations. A comparison with HALOE data reveals, however, a serious model deficiency regarding lower-stratosphere dynamics at high latitudes. Contrary to what is concluded by observations, the lower stratosphere is characterized by slight upward motions in the polar regions, so that some of the mentioned good agreements must be considered as fortuitous. Nevertheless, ECHAM3/CHEM well describes the chemical processes leading to ozone reduction. It has been shown that the mean fraction of the northern hemisphere, which is covered by polar stratospheric clouds (PSCs) as well as the temporal appearance of PSCs in the model, is in fair agreement with observations. The model results show an activation of chlorine inside the polar vortex which is stronger in the southern than in the northern winter hemisphere, yielding an ozone hole over the Antarctic; this hole, however, is also caused to a substantial degree by the dynamics. Interhemispheric differences concerning reformation of chlorine reservoir species HCl and ClONO₂ in spring have also been well reproduced by the model.     

基态位涡径向分布对浅水涡旋系统演变的影响分析Part Ⅱ:扰动发展和结构变化

本文在基态位涡（Potential Vorticity,PV）径向分布和基态涡旋强度对热带气旋（Tropical Cyclone,TC）类涡旋系统稳定性特征影响的研究基础上,结合理想试验和数值模拟诊断分析基态PV径向分布对扰动增长和系统结构变化的影响.基于线性正压浅水模型,设计三种典型基态PV中空结构下基态涡旋强度对系统稳定性影响的敏感性试验.结果表明：基态涡旋的强度主要影响稳定性的强弱,强度越强,不稳定增长率越大,而基态PV径向分布对系统最不稳定波动性质起着决定作用.分析不同波数下扰动的发展及不同波数间扰动的相互作用可知,对于宽且实的PV环,系统稳定性主要取决于低波数不稳定,且最不稳定波数扰动的发展具有明显的优势地位;对于窄且空的PV环,系统稳定性主要取决于高波数不稳定,且多个高波数下增长最快模态的不稳定增长率值十分接近.利用模态线性叠加法讨论扰动增长对系统结构变化的影响表明：最不稳定波数的扰动发展对系统结构变化有关键影响,而多个波数的扰动不稳定增长相当时,不同波数的扰动发生相互作用从而影响系统结构变化.最后,利用实际个例模拟资料分析基态PV径向分布及其变化对TC结构和强度的影响表明：TC内核区出现的多边形眼墙结构与当前时刻基态PV径向分布所决定的最不稳定波数有很好的对应关系,同时基态PV径向分布变化所反映出的系统动力稳定性强弱与TC强度发展阶段具有很好的相关性.     

利用GS流场重构方法研究磁尾等离子体片涡流

2000年9月30日Geotail卫星分别于17∶54∶36~18∶09∶00UT和18∶59∶00~19∶30∶00UT在磁尾晨侧等离子体片内(n≈0.4 cm^-3,T≈6 keV)观测到等离子体涡流事件.本文采用Grad-Shafranov (GS)流场重构技术再现了这些涡流的二维速度场、离子数密度和离子温度的分布图像.结果显示:从地心太阳磁层坐标系(GSM)赤道面上面看, 涡流的尺度约为5000 km×1400 km , 朝地球的运动速度约为15~25 km/s.所有5个涡流的旋转方向都为顺时针方向,旋转周期约为6~11 min.相邻涡流的相互作用导致它们之间的磁场强度增强.考察观测数据发现,涡流内不仅包含等离子体片热等离子体成分,也包含较大通量的类似源自磁鞘的冷等离子体成分(T<1 keV).这与观测到涡流等离子体的平均温度(T≈4 keV)较磁尾等离子体片等离子体的典型温度(T≈6 keV)明显偏低的事实是一致的.不仅如此,离子数密度和温度在结构内的分布也不均匀,数密度在涡流内部偏离中心的位置比较低而在每个涡流的边缘位置比较高,温度的分布大体上与密度相反.分析认为观测到的磁尾等离子体涡流事件可能由发生在低纬边界层的Kelvin-Helmholtz不稳定性引起,涡流结构内的冷等离子体可能来自磁层顶外部的磁鞘.     

On the nonlinear evolution of columnar vortices in a rotating environment

We examine the three-dimensional, nonlinear evolution of columnar vortices in a rotating environment. As the initial vorticity distribution, a wavetrain of finite amplitude Kelvin-Helmholtz vortices in shear is employed. Through direct numerical simulation of the Navier-Stokes equations we seek to better understand the process of maturation of the various three-dimensional modes of instability to which such vortical flows are subject, especially those which exist as a consequence of the action of the Coriolis force. In the absence of rotational influence, we thereby demonstrate that the nonlinear evolution of columnar vortices is most strongly controlled by one or the other of two mechanisms. One mechanism of instability is identifiable as a so-called elliptical instability, which promotes the initial bending of vortex tubes in a sinusoidal fashion, while the other is a hyperbolic mode, which is responsible for the development of streamwise vortex streaks in the "braids" between adjacent vortex cores. In the rotating case, anticyclonic vortices are strongly destabilized by weak background rotation, while rapid rotation stabilizes both the cyclones and anticyclones. The strong anticyclones are subject to two distinct forms of instability, namely a Coriolis force modified elliptical instability and an inertial (centrifugal) instability. The former instability is very similar to the nonrotating form of the elliptical instability as it promotes bending of vortex tubes, while the latter instability grows on the edge of the vortex core and generates streaks of vorticity, which surround the vortex core itself. These results of direct numerical simulation fully verify the results of previous linear stability analyses. Taken together, they provide a simple explanation for the broken symmetry that is often observed to be characteristic of the von Karman vortex streets that develop in the atmospheric lee of oceanic islands.     

Role of wave–mean flow interaction in sun–climate connections: Historical overview and some new interpretations and results

Quasi-decadal variations in solar irradiance – termed the 11-year solar cycle (SC) – have been linked to variations in a variety of atmospheric circulation features, including the polar vortex, the Brewer–Dobson circulation, and the quasi-biennial oscillation. These features share an underlying commonality: they are all rooted in wave–mean flow interaction. The purpose of this paper is to provide a historical overview of the connection between the SC and wave–mean flow interaction and to propose a more complete theoretical framework for solar modulated wave–mean flow interaction that includes both zonal-mean and zonally asymmetric ozone as intermediaries for communicating variations in solar spectral irradiance to the climate system. We solve a quasi-geostrophic model using the WKB formalism to highlight the physics connecting the SC to planetary wave-drag. Numerical results show the importance of the zonally asymmetric ozone field in mediating the effects of solar variability to the wave-driven circulation in the middle atmosphere.     

Radiating Vortices in Geophysical Fluid Dynamics

The dynamics of a single vortex on a beta-plane is discussed in this paper. A barotropic, an equivalent barotropic, one-and-a half and two-layer models are considered. The momentum and energy balances are used to describe the evolution of a vortex. A quasi-stationary balance of the Rossby, Zhukovsky-Kutta forces and the force induced by Rossby-wave radiation, describes the dynamics of the barotropic vortex. A net Coriolis force occurs if the fluid is stratified. The difference between the dynamics of cyclones and anticyclones results directly from the Coriolis force acting on a single vortex in a stratified fluid.All vortices radiate Rossby waves in the quasigeostrophic approximation but intense anticyclones propagate steadily in a one-and-a half layer model. A critical amplitude that bounds radiating and steadily propagating anticyclones is found. Steady propagation of anticyclones in general is impossible in a two-layer fluid due to the radiation of a barotropic Rossby-wave. Some solutions of solitary wave type which are known for a two-layer model, survive owing to wave interference.A single vortex can extract energy from a Rossby wave if synchronism conditions are satisfied. The wave interference again plays a crucial role in this case. The wave interference also determines the energy exchange of vortices located at larger distances. If the distance between the vortices is shorter than the length of the radiated waves, modon may be formed due to a small energy loss.The unbounded monotonic variation of the planetary vorticity is a characteristic feature of a beta-plane approximation. As a result, a single vortex propagates up to a 'rest latitude' where it disappears. The evolution of a single barotropic vortex over bottom topography provides another example of a background vorticity distribution with a local extremum above hills (valleys) or ridges (troughs). Physics of its movement differs from a beta-plane case, but if a vortex lies over broad topography, equations are similar and the evolution of a vortex manifests the same typical features. Particularly, a cyclonic vortex tends to drift to the top of a hill or a ridge. An anticyclonic vortex, on the contrary, slides to the bottom of a valley or a trough.An interaction of a barotropic vortex with a broad mean flow is tractable qualitatively on the basis of previous results. Numerical examples illustrating absorption of a small vortex by a larger one and a vortex movement across the flow, are direct analogies of the vortex evolution over a hill and a ridge, respectively. At the same time, strong influence of strain drastically changes the vortex structure.     

Vortex pairs and power station cooling water

Cooling water discharged from power stations in the U.K. is frequently released from an outlet in an estuary or the sea. The warm water forms a thermal plume which is slightly buoyant and which spreads horizontally over the water surface while mixing vertically downwards with the cooler ambient water. In this paper, the possibility of vortex pair production at the cooling water outlet is considered as a mechanism contributing to this spreading of the warm water.The motion of a vortex pair contained between two rigid plane boundaries is an idealization of the flow between the water surface and the sea bed. The resulting motion is calculated from potential theory and viscous effects are neglected. The problem of deciding what strength to assign the vortices is discussed and specific consideration of shear and buoyancy at the outlet is detailed. It is observed that bifurcation of the vortex pair is determined by the initial position of the vortices and is unlikely to occur in conditions relevant to U.K. power station discharges.It is calculated that, in the absence of turbulence, the motion of such vortex pairs would result in horizontal spreading of the warm water which is greater than that observed at site surveys. It is concluded that turbulence in the ambient receiving water is sufficient to destroy vortices produced by the discharge during the early stages of the plume development.     

The role of the stratosphere in Iberian Peninsula rainfall: A preliminary approach in February

This paper attempts to establish a connection between stratospheric anomalies in the North Pole and rainfall on the Iberian Peninsula through the occurrence of major midwinter warmings (MMWs) and cold events (CEs), taking February as a preliminary approach. We define the MMWs as the warmings which break down the polar vortex, whereas the CEs are the episodes in which the polar vortex remains cold and undisturbed. Both anomalies lead to a wind anomaly around the north polar stratosphere, which is connected with a shortly lagged tropospheric anomaly through a stratosphere–troposphere coupling in winter. A T-mode principal component analysis (PCA) was used as an objective pattern classification method for identifying the main daily surface-level pressure (SLP) patterns for February for the 1961–1990 reference period. Subsequently, those February months with an MMW or a CE influence in the troposphere are identified in the whole study period (1958–2000) by means of the Arctic Oscillation Index (AOI). Thus, performing the same analysis for the selected February months, new principal patterns for detecting changes in surface circulation structure and morphology are obtained. The results show a significant decrease in the westerlies and a southward shift of the storm tracks in Western Europe some weeks after an MMW occurrence, leading to an increase in precipitation in western Iberia and a slight decrease on the eastern Mediterranean fringe. The results are quite the opposite under a CE influence: the westerlies are strengthened and shifted northwards due to the displacement of the Atlantic anticyclone towards Central Europe; dry conditions are established throughout Iberia, except for the Mediterranean fringe, where precipitation shows a considerable increase due to the greater frequency of the northeasterly winds. Finally, an 11-year sunspot cycle–quasi-biennial oscillation (QBO) modulation might be demonstrated in Iberian rainfall in February through the occurrence of these stratospheric anomalies.     

Sensitivity experiments with a quasi-geostrophic model of stratospheric sudden warming

We describe a series of sensitivity experiments with a quasi-geostrophic model of the interaction of stationary planetary waves with the mean zonal flow in the stratosphere and mesosphere. The model is of the Matsuno type, which neglects wave-wave interaction and includes only a single zonal harmonic of the planetary wave spectrum in each simulation. We employed the model to investigate the source of the double-layer structure previously obtained by several authors for the stratospheric sudden warming with wavenumber one. Our results suggest that this characteristic of the model-produced warming is a property only of models without damping. When reasonable dissipation is included in the model, the double-layer structure disappears. This implies the importance of the drag parameterization in properly simulating warming events and, since the actual drag very probably is effected by breaking internal waves, it suggests that future analysis should include a specific representation of this effect. We also investigated the dependence of stratospheric warming on the structure of the zonal wind field. Our analyses show in particular that substantial reduction of the height of the polar night jet mitigates strongly against the occurrence of a sudden warming event.