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.     

Latitudinal and longitudinal variability of mesospheric winds and temperatures during stratospheric warming events

Continuous MF and meteor radar observations allow detailed studies of winds in the mesosphere and lower thermosphere (MLT) as well as temperatures around the mesopause. This height region is characterized by a strong variability in winter due to enhanced planetary wave activity and related stratospheric warming events, which are distinct coupling processes between lower, middle and upper atmosphere. Here the variability of mesospheric winds and temperatures is discussed in relation with major and minor stratospheric warmings as observed during winter 2005/06 in comparison with results during winter 1998/99.Our studies are based on MF radar wind measurements at Andenes (69°N, 16°E), Poker Flat (65°N, 147°W) and Juliusruh (55°N, 13°E) as well as on meteor radar observations of winds and temperatures at Resolute Bay (75°N, 95°W), Andenes (69°N, 16°E) and Kühlungsborn (54°N, 12°E). Additionally, energy dissipation rates have been estimated from spectral width measurements using a 3 MHz Doppler radar near Andenes. Particular attention is directed to the changes of winds, turbulence and the gravity wave activity in the mesosphere in relation to the planetary wave activity in the stratosphere.Observations indicate an enhancement of planetary wave 1 activity in the mesosphere at high latitudes during major stratospheric warmings. Daily mean temperatures derived from meteor decay times indicate that strong warming events are connected with a cooling of the 90 km region by about 10–20 K. The onset of these cooling processes and the reversals of the mesospheric circulation to easterly winds occur some days before the changes of the zonal circulation in the stratosphere start indicating a downward propagation of the circulation disturbances from the MLT region to the stratosphere and troposphere during the stratospheric warming events. The short-term reversal of the mesospheric winds is followed by a period of strong westerly winds connected with enhanced turbulence rates and an increase of gravity wave activity in the altitude range 70–85 km.     

Major and minor stratospheric warmings and their interactions on the troposphere

Analyses of evolutions of the kinetic and thermal energy associated with the major and minor stratospheric warmings in the winters of 1976–77 and 1975–76 respectively indicate that the predominant ultra-long waves in the stratosphere oscillated at periods of 10–20 days, whereas in the troposphere the predominant long waves oscillated at periods of 8 to 12 days. These tropospheric long waves are almost out-of-phase with the stratospheric ultra-long waves for the minor warming, but in-phase for the major warming. The kinetic energy of the zonal mean flow in the stratosphere for the minor warming is much greater than that for the major warming, indicating that the occurrence of a major warming depends on the magnitude of the kinetic energy of the zonal mean flow relative to that of the meridional convergence of the poleward flux of sensible heat. In both the major and minor warmings, most of the stratospheric eddy kinetic energy is contained in waves of wavenumbers 1 and 2, whereas the stratospheric available potential energy is primarily contained in waves of wavenumber 1. The kinetic energy associated with waves of wavenumber 1 appeared to be 180° out-of-phase with those of wavenumber 2, indicating that nonlinear transfer of kinetic energy occurred between waves of wavenumbers 1 and 2. The occurrences of wind reversals were accompanied by decouplings of the stratospheric and tropospheric motions, and blockings in the troposphere.     

Variability of the mesospheric wind field at middle and Arctic latitudes in winter and its relation to stratospheric circulation disturbances

Continuous wind observations allow detailed investigations of the upper mesosphere circulation in winter and its coupling with the lower atmosphere. During winter the mesospheric/lower thermospheric wind field is characterized by a strong variability. Causes of this behaviour are planetary wave activity and related stratospheric warming events. Reversals of the dominating eastward directed mean zonal winds in winter to summerly westward directed winds are often observed in connection with stratospheric warmings. In particular, the amplitude and duration of these wind reversals are closely related to disturbances of the dynamical regime of the upper stratosphere.The occurrence of long-period wind oscillations and wind reversals in the mesosphere and lower thermosphere in relation to planetary wave activity and circulation disturbances in the stratosphere has been studied for 12 winters covering the years 1989–2000 on the basis of MF radar wind observations at Juliusruh (55°N, since 1989) and Andenes (69°N, since 1998). Mesospheric wind oscillations with long-periods between 10 and 18 days are observed during the presence of enhanced planetary wave activity in the stratosphere and are combined with a reversal of the meridional temperature gradient of the stratosphere or with upper stratospheric warmings.     

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.     

The boreal spring stratospheric final warming and its interannual and interdecadal variability

Based on the daily NCEP/DOE reanalysis II data,dates of the boreal spring Stratospheric Final Warming(SFW) events during 1979–2010 are defined as the time when the zonal-mean zonal wind at the central latitudes(65°–75°N) of the westerly polar jet drops below zero and never recovers until the subsequent autumn.It is found that the SFW events occur successively from the mid to the lower stratosphere and averagely from the mid to late April with a temporal lag of about 13 days from 10 to 50 hPa.Over the past 32 years,the earliest SFW occurs in mid March whereas the latest SFW happens in late May,showing a clear interannual variability of the time of SFW.Accompanying the SFW onset,the stratospheric circulation transits from a winter dynamical regime to a summertime state,and the maximum negative tendency of zonal wind and the strongest convergence of planetary-wave are observed.Composite results show that the early/late SFW events in boreal spring correspond to a quicker/slower transition of the stratospheric circulation,with the zonal-mean zonal wind reducing about 20/5 m s-1 at 30 hPa within 10 days around the onset date.Meanwhile,the planetary wave activities are relatively strong/weak associating with an out-of-/in-phase circumpolar circulation anomaly before and after the SFW events in the stratosphere.All these results indicate that,the earlier breakdown of the stratospheric polar vortex(SPV),as for the winter stratospheric sudden warming(SSW) events is driven mainly by wave forcing;and in contrast,the later breakdown of the SPV exhibits more characteristics of its seasonal evolution.Nevertheless,after the breakdown of SPV,the polar temperature anomalies always exhibit an out-of-phase relationship between the stratosphere and the troposphere for both the early and late SFW events,which implies an intimate stratosphere–troposphere dynamical coupling in spring.In addition,there exists a remarkable interdecadal change of the onset time of SFW in the mid 1990s.On average,the SFW onset time before the mid 1990s is 11 days earlier than that afterwards,corresponding to the increased/decreased planetary wave activities in late winter-early spring before/after the 1990s.     

冬季太阳11年周期活动对大气环流的影响

利用气象场的再分析资料和太阳辐射活动资料,对太阳11年周期活动影响北半球冬季(11月~3月)大气环流的过程进行了统计分析和动力学诊断.根据赤道平流层纬向风准两年振荡(QBO)的东、西风状态对太阳活动效应进行了分类讨论,结果表明：东风态QBO时,太阳活动效应主要集中在赤道平流层中、高层和南半球平流层,强太阳活动时增强的紫外辐射加热了赤道地区的臭氧层,造成平流层低纬明显增温,同时加强了南半球的Brewer-Dobson(B-D)环流,引起南极高纬平流层温度增加;而北半球中高纬的环流主要受行星波的影响,太阳活动影响很小.西风态QBO时,太阳活动效应在北半球更为重要,初冬时强太阳活动除了加热赤道地区臭氧层外,还抑制了北半球的B-D环流,造成赤道平流层温度增加和纬向风梯度在垂直方向的变化,从而改变了对流层两支行星波波导的强度;冬末时在太阳活动调制下,行星波向极波导增强,B-D环流逐渐恢复,造成北半球极地平流层明显增温,同时伴随着赤道区域温度的下降.     

平流层爆发性增温对中国天气气候的影响 及其在ENSO影响中的作用

本文通过分析1957~2002年平流层爆发性增温(SSW)的环流特征,研究平流层爆发性增温可能对我国天气气候的影响.平流层爆发性增温发生后平流层高纬地区有异常的环流变化,但是这种变化并不仅局限于平流层内部,其产生的环流异常能够向下传播,并对对流层的天气和气候产生影响.研究发现,平流层出现强爆发性增温后,平流层异常温度场和位势高度场在中、高纬度形成AO型振荡并向下传播,使得对流层低层西伯利亚高压增强、阿留申低压加深,500 hPa东亚大槽加深且偏西,导致东亚冬季风增强,我国北部大部分地区气温偏低.而在爆发性增温前,强行星波扰动使得东亚大槽加深,西伯利亚高压和阿留申低压同时增强,也可能导致东亚冬季风偏强.El Nio可能激发出强行星波,有利于强SSW事件的发生.通过上述的两个过程可能造成东亚冬季风的加强,这将会对"El Nio事件通过对流层过程而引起东亚冬季风减弱"的结论有一定影响. 因此,ENSO事件影响东亚冬季风及中国的天气气候存在不止一种途径,具体影响情况应该是几种途径的综合结果.     

Influence of short-term changes in solar activity on baric field perturbations in the stratosphere and troposphere

Influence of short-term changes in solar activity on baric (pressure) field perturbations is studied using such characteristics as the Sazonov index (IS), describing the intensity of meridional transfer, the Blinova index (IB), describing the intensity of zonal transfer, and ‘vorticity area index’ (VAI) describing the tropospheric cyclonic perturbations. The epoch superposition method is used to reveal effects of the solar central meridian (CM) passage of active regions, the Forbush decreases (FD) in galactic cosmic rays, and the solar proton (SP) events. The results of the analysis show that influence of short-term changes in the solar activity on baric field perturbations is the most evident in the stratosphere (30 mbar-level). The meridional circulation in case of the FD and SP events begin to increase about 5–7 days before the key date, reaches maximum nearby the key date and decays after the key date. The meridional circulation in case of the solar CM passage of active regions starts to increase after the key date and reaches the maximum by 5–6 days. Fluctuations of baric field within periods of 5–7 days typical of meridional and zonal transfers in troposphere (500 mbar-level) are evidently connected with internal dynamics of the atmosphere, not with the effects of solar activity. VAI characterizing cyclonic activity in the troposphere, shows the striking correspondence to changes of the meridional circulation in the stratosphere. Comparison of changes in the stratospheric perturbations with behavior of the UV irradiance in course of the FD and SP events show their full correspondence at the initial stage of these processes. The conclusion is made that growth of baric perturbations observed in the stratosphere in associations with the FD and SP events before the key date is caused by the solar UV irradiance increase, whereas decay of the baric perturbations after the key date is related to direct influence of the solar energetic corpuscular fluxes on the stratosphere.     

Stratospheric wave activity and the Pacific Decadal Oscillation

Using the monthly mean NCEP/NCAR reanalysis and NOAA Extended Reconstructed sea surface temperature (SST) datasets, strong correlations between the SST anomalies in the North Pacific and calculated three-dimensional Eliassen–Palm vertical fluxes are indicated in December 1958–1976 and 1992–2006. These correlations between the interannual variations of the SST anomalies and the penetration of planetary waves into the stratosphere are much less during the decadal sub-period 1976–1992 in the positive phase of the Pacific Decadal Oscillation (PDO) and the decadal cold SST anomalies in the North Pacific. Interannual variations of the polar jet in the lower stratosphere in January are strongly associated with SST anomalies in the Aleutian Low region in December for the years with positive PDO index. This sub-period corresponds well with that of the violation of the Holton–Tan relationship between the equatorial Quasi-Beinnial Oscillation (QBO) and the stratospheric circulation in the extra-tropics. It is shown that interannual and interdecadal variations of stratospheric dynamics, including stratospheric warming occurrences in January, depend strongly on changes of the upward propagation of planetary waves from the troposphere to the stratosphere over North Eurasia in preceding December. These findings give evidences of a large impact of the decadal SST variations in the North Pacific on wave activity in early winter due to changes of thermal excitation of planetary waves during distinct decadal periods. Possible causes of the decadal violation of the Holton–Tan relationship, its relation to the PDO and an influence of the 11-year solar cycle on the stratosphere are discussed.     

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.     

Resonant interaction between two planetary waves as a precursor for stratospheric warmings?

Stratospheric warmings are attributed to an enhanced planetary wave activity, occurring nearly each winter – at least in the northern hemisphere – with different strengths. The generation of stratospheric warmings is not totally understood. One of the most promising explanations is the interaction of planetary waves: in many cases, the amplitude of the quasi-stationary planetary wave 1 builds up, until it transmits its momentum and energy to the background wind field. The role of wave 2 is usually considered to be less important.Based on ERA-40 and DYANA temperature data (January–February 1990), we found evidence that a resonant wave–wave interaction between a travelling and a stationary wave 2 was responsible for a minor stratospheric warming in February 1990. The interaction being observed during four weeks can eventually be used as an indication for an upcoming stratospheric warming.     

Latitude and Altitude Dependence of the Interannual Variability and Trends of Atmospheric Temperatures

—The 4-season (12-month) running means of temperatures at five atmospheric levels (surface, 850–300 mb, 300–100 mb, 100–50 mb, 100–30 mb) and seven climatic zones (60°N–90°N, 30°N–60°N, 10°N–30°N, 10°N–10°S, 10°S–30°S, 30°S–60°S, 60°S–90°S) showed QBO (Quasi-biennial Oscillation), QTO (Quasi-triennial Oscillation) and larger periodicities. For stratosphere and tropopause, the temperature variations near the equator and North Pole somewhat resembled the 50mb low latitude zonal winds, mainly due to prominent QBO. For troposphere and surface, the temperature variations, especially those near the equator, resemble those of eastern equatorial Pacific sea-surface temperatures, mainly due to prominent QTO. In general, the temperature trends in the last 35 years show stratospheric cooling and tropospheric warming. But the trends are not monotonic. For example, the surface trends were downward during 1960–70, upward during 1970–82, downward during 1982–85 and upward thereafter. Models of green-house warming should take these non-uniformities into account.     

Solar wind influence on atmospheric processes in winter Antarctica

Galactic cosmic rays (GCRs) altered by solar wind are traditionally regarded as the most plausible agent of solar activity influence on the Earth's atmosphere. However, it is well known that severe reductions in the GCRs flux, known as Forbush decreases (FDs), are caused by solar wind of high speed and density, which sweeps away the GCRs on its way. Since the FD beginnings are registered at the Earth's orbit simultaneously with dramatic disturbances in the solar wind, the atmospheric effects, assigned to FDs, can be, in reality, the results of the solar wind influence on the atmospheric processes. This paper presents a summary of the experimental results demonstrating the strong influence of the interplanetary electric field on atmospheric processes in central Antarctica, where the large-scale system of vertical circulation is formed during winter seasons. The influence is realized through acceleration of the air masses, descending into the lower atmosphere from the troposphere, and the formation of cloudiness above the Antarctic Ridge, where the descending air masses enter the surface layer. The acceleration is followed by a sharp increase of the atmospheric pressure near-pole region, which gives rise to the katabatic wind strengthening above the entire Antarctica. The cloudiness formation results in the sudden warmings in the surface atmosphere, since the cloud layer efficiently backscatters the long wavelength radiation from the ice sheet, but does not affect the adiabatic warming process of the descending tropospheric air masses. When the drainage flow strengthening the circumpolar vortex around the periphery of the Antarctic continent decays, the surface easterlies typical of the coast stations during the winter season are replaced by southerlies and the cold Antarctic air masses flow out to the Southern ocean.     

Sudden stratospheric warmings: the role of normal atmospheric modes

The role of normal atmospheric modes in the beginning and development of sudden stratospheric warming (SSW) events is studied on the basis of calculations with the use of the general circulation model of the middle and upper atmosphere. The analysis of the effect of a phase of quasi-biennial oscillations on the dynamics of the extratropical stratosphere has shown that the conditions for SSW commencement are more favorable and the SSW events are more intense during the easterly phase of these oscillations as compared to the westerly phase. The conclusion has been drawn that fundamental normal atmospheric modes can be recorded in the temperature field at mesopause altitudes during ground-based optical measurements.     

Ozone distribution in winter at high latitudes in the northern hemisphere

The ozone winter maximum at high latitudes in the northern hemisphere is not evenly distributed along the longitudes. This is mainly due to the upper air circulation, both horizontally and vertically. In addition it is also strongly influenced by the largest mountain ranges. During the last two decades the air circulation in the North Atlantic has intensified. This has led to ascending motion in the upper troposphere and the lower stratosphere, which in turn has resulted in a reduced total ozone column in Northwest Europe.The large mounter ranges in Asia are initiating standing waves, with descending motions in the atmosphere behind the mountains. The descending motion leads to adiabatic warming of the lower stratosphere and the upper troposphere. Ozone-rich air is transported downwards to lower levels and stored there, where the ozone is less affected by heterogeneous chemical destruction.     

The effect of the 11-year solar cycle on the temperature in the lower stratosphere

Two temperature datasets are analyzed for quantifying the 11-year solar cycle effect in the lower stratosphere. The analysis is based on a regression linear model that takes into account volcanic, Arctic Oscillation (AO), Quasi-Biennial Oscillation (QBO) and El Nino Southern Oscillation (ENSO) effects. Under solar maximum conditions, temperatures are generally warmer for low- and mid-latitudes than under solar minimum, with the effect being the strongest in northern summer. At high latitudes, the vortex is generally stronger under solar maximum conditions, with the exception of February and to a lesser extent March in the Northern Hemisphere; associated with this positive signal at high latitudes, there is a significant negative signal at the equator. Observations also suggest that contrary to the beginning of the winter, in February–March, the residual circulation in the Northern Hemisphere is enhanced. A better understanding of the mechanisms at work comes from further investigations using the ERA-40 reanalysis dataset. First, a consistent response in terms of temperature and wind is obtained. Moreover, considering Eliassen-Palm (EP) flux divergence and residual circulation stream functions, we found an increased circulation in the Northern Hemisphere in February during solar maxima, which results in more adiabatic warming at high latitudes and more adiabatic cooling at low latitudes, thus demonstrating the dynamical origin of the response of the low stratosphere to the solar cycle.     

External impact on wind in the mesosphere/lower thermosphere region

Regular measurements of the velocity and direction of the horizontal wind in the mesosphere/lower thermosphere (MLT) region at a height of ∼95 km have been conducted since 1975 over Eastern Siberia (Badary observatory near Irkutsk), using the spaced-diversity reception method in the LF range. The accumulated database of measurement results (for more than 20 years, from 1974 to 1996) makes it possible to get information on the impact on wind in the MLT region from both below (stratospheric warmings) and above (geomagnetic storms as a consequence of magnetospheric disturbances) with sufficient statistical reliability. Effects of stratospheric warmings and strong geomagnetic storms in the prevailing wind and amplitude of the semidiurnal tide are evaluated by the superposed epoch method. It is shown that the effects of stratospheric warmings depend on the type (intensity) of stratospheric warming and on the phase of quasi-biennial oscillations of the wind in the equatorial stratosphere at the 30 hPa level. The response of MLT winds to external impacts is different for the 21st and 22nd cycles of solar activity. Effects of geomagnetic storms (A _p > 100) are manifested in the decrease in the eastward prevailing wind and increase in the semidiurnal tide amplitude.     

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.     

大气季节内振荡的活动与江淮流域夏季旱涝

观测资料的分析极为清楚地表明,江淮流域的夏季降水有着极为明显的低频变化,周期为30-60d和近20d的振荡是其最基本的特征,尤其是在多雨的年份.对应江淮夏季多雨(涝)年和少雨(旱)年,大气环流的分析表明其大气季节内振荡(IS0)的形势有着显著的差异.例如在多雨(少雨)年,在长江以南的850hPa上为一个低频(IS0)反气旋(气旋)性环流控制,而中国北部和日本一带为气旋(反气旋)性环流,从而在江淮流域形成较强的低频辐合(辐散)气流;在200hPa的青藏高原上却为一个低频气旋(反气旋)性环流所控制.分析还表明,对应多雨年,在江淮流域有明显的由中高讳度向南传播和由低玮度向北传播的大气低频振荡的汇合情况;而对应于少雨年,由中高纬度向南传播的低频系统较不明显,在江淮流域低频系统的汇合也较为不清楚.