Seasonal variabilities of low-latitude mesospheric winds

Observations of mesospheric winds over a period of four years with the partial reflection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70–98 km. The meridional winds exhibit overall transequatorial flow associated with differential heating in the Northern and Southern Hemispheres. At lower altitudes (70–80 km) the mean zonal winds reveal easterly flow during summer and westerly flow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80–98 km) and at all altitudes during equinox periods, the mean zonal flow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at specific harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation.     

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.     

Numerical studies of transient planetary circulations in a wind-driven ocean

Summary The time-dependent primitive equations for a shallow homogeneous ocean with a free surface are solved for a bounded basin on the sphere, driven by a steady zonal wind stress and subject to lateral viscous dissipation. These are the vertically integrated equations for a free-surface model, and are integrated to 60 days from an initial state of rest by an explicit centered-difference method with zero-slip lateral boundary conditions. In a series of comparative numerical solutions it is shown that at least a 2-deg resolution is needed to resolve the western boundary currents adequately and to avoid undue distortion of the transient (Rossby waves. The -plane formulation is shown to be an adequate approximation for the mean circulation in the lower and middle latitudes, but noticeably intensifies the transports poleward of about 50 deg and both slows and distorts the transients in the central basin. The influence of the (southern) zonal boundary on the transport solutions is confined to the southernmost gyre, except in the region of the western boundary currents where its influence spreads to the northern edge of the basin by 30 days. The total boundary current transport is shown to be approximately proportional to the zonal width of the basin and independent of the basin's (uniform) depth, while the elevation of the free water surface is inversely proportional to the basin depth, in accordance with linear theory. The introduction of bottom friction has a marked damping effect on the transient Rossby waves, and also reduces the maximum boundary-current transport. The solutions throughout are approximately geostrophic and are only slightly nonlinear.The root-mean-square (rms) transport variability during the period 30 to 60 days is concentrated in the southwest portion of the basin through the reflection of the transient Rossby waves from the western shore and has a maximum corresponding to an rms current variability of about 3 cm sec^–1. The transport variabilities are about 10 percent of the mean zonal transport and more than 100 percent of the mean meridional transport over a considerable region of the western basin (outside the western boundary current regime). Some 99 percent of the total kinetic energy is associated with the zonal mean and standing zonal waves, which are also responsible for the bulk of the meridional transport of zonal angular momentum. Although the transient Rossby waves systematically produce a momentum flux convergence at the latitude of the maximum eastward current, much in the manner of their atmospheric counterparts, this is only a relatively small contribution to the zonal oceanic momentum balance; the bulk of the mean zonal stress is here balanced by a nearly stationary net pressure torque exerted against the meridional boundaries by the wind-raised water. In an ocean without such boundaries the role of the transient circulations may be somewhat more important.     

Meteor wind radar observations of tidal amplitudes over a low-latitude station Trivandrum (8.5°N, 77°E): Interannual variability and the effect of background wind on diurnal tidal amplitudes

Based on the horizontal winds measured using SKiYMET meteor wind radar during the period of June 2004–May 2007, the seasonal and interannual variability of the diurnal and semidiurnal amplitudes and phases in the mesospheric and lower thermospheric (MLT) region over a low-latitude station Trivandrum (8.5°N) are investigated. The monthly values of amplitudes and phases are calculated using a composite day analysis. The zonal and meridional diurnal tidal amplitudes exhibit both annual and semiannual oscillations. The zonal and meridional components of semidiurnal tide show a significant annual oscillation. The phase values of both diurnal and semidiurnal tides exhibit annual oscillation above 90 km. The effect of background wind in the lower atmosphere on the strength of diurnal tidal amplitudes in the MLT region is studied. The effect of diurnal tides on the background wind in the lower thermosphere is also discussed.     

Mesospheric mean winds and tides observed by the Imaging Doppler Interferometer (IDI) at Halley,Antarctica

This paper describes the first ever mesospheric wind observations from Halley, Antarctica, over a full year. The recent implementation of an Imaging Doppler Interferometer at Halley is providing a new, high quality and continuous dataset to investigate the dynamics of the Antarctic mesosphere. The mean winds show clear seasonal variations, with reversals in both zonal and meridional components near the equinoxes. The dominant tidal modes have periods of 12 h and 24 h but with significant variations in amplitude during the year. Waves with longer periods are also apparent at certain times of year. The seasonal variations and amplitudes of the winds and tides are compared with other high-latitude sites in the southern and northern hemispheres. It is found that the overall pattern of winds at Halley is broadly similar to that seen at similar geographic latitudes, but with noticeable differences which may be related to it being a southern hemisphere site.     

Dynamical response of low-latitude middle atmosphere to major sudden stratospheric warming events

The zonally averaged UK Meteorological Office (UKMO) zonal mean temperature and zonal winds for the latitudes 8.75°N and 60°N are used to investigate the low-latitude dynamical response to the high latitude sudden stratospheric warming (SSW) events that occurred during winter of the years 1998–1999, 2003–2004 and 2005–2006. The UKMO zonal mean zonal winds at 60°N show a short-term reversal to westward winds in the entire upper stratosphere and lower mesosphere and the low-latitude winds (8.75°N) show enhanced eastward flow in the upper stratosphere and strong westward flow in the lower mesosphere during the major SSW events at high latitudes. The mesosphere and lower thermosphere (MLT) zonal winds acquired by medium frequency (MF) radar at Tirunelveli (8.7°N, 77.8°E) show a change of wind direction from eastward to westward several days before the onset of SSW events and these winds decelerate and weak positive (eastward) winds prevail during the SSW events. The time variation of zonal winds over Tirunelveli is nearly similar to the one reported from high latitudes, except that the latter shows intense eastward winds during the SSW events. Besides, the comparison of daily mean meridional winds over Tirunelveli with those over Collm (52°N, 15°E) show that large equatorial winds are observed over Tirunelveli during the 2005–2006 event and over Collm during the 1998–1999 events. The variable response of MLT dynamics to different SSW events may be explained by the variability of gravity waves.     

On the values of stratospheric circulation index over Natal,Brasil

Summary The SCI values of zonal and meridional winds over Natal, Brasil are calculated. The results are compared with another southern Hemispheric Station Ascension Island and circulation patterns in the Stratosphere are discussed. Also a comparison is made of the zonal winds over Natal with those of an Equatorial Station (Thumba, India) and a station in Subtropics (Sonmiani, Arabian Sea islands).On leave from Dept. of Meteorology and Oceanography, Andhra University, Waltair (India).     

A numerical model of the zonal mean circulation of the middle atmosphere

The annual cycle of the zonally averaged circulation in the middle atmosphere (16–96 km) is simulated using a numerical model based on the primitive equations in log pressure coordinates. The circulation is driven radiatively by heating due to solar ultraviolet absorption by ozone and infrared cooling due to carbon dioxide and ozone (parameterized as a Newtonian cooling). Since eddy fluxes due to planetary waves are neglected in the model, the computed mean meridional circulation must be interpreted as thediabatic circulation, not as the total eulerian mean. Rayleigh friction with a short (2–4 day) time constant above 70 km is included to simulate the strong mechanical dissipation which is hypothesized to exist in the vicinity of the mesopause due to turbulence associated with gravity waves and tides near the mesopause.Computed mean winds and temperatures are in general agreement with observations for both equinox and solstice conditions. In particular, the strong mechanical damping specified near the mesopause makes it possible to simulate the cold summer and warm winter mesopause temperatures without generating excessive mean zonal winds. In addition, the model exhibits a strong semiannual cycle in the mean zonal wind at the equator, with both amplitude and vertical structure in agreement with the easterly phase of the observed equatorial semiannual oscillation.Contribution No. 497, Department of Atmospheric Sciences, University of Washington, Seattle.     

The equatorial undercurrent, meridional overturning circulation, and their roles in mass and heat exchanges during El Niño events in the tropical Pacific ocean

The equatorial undercurrent (EUC), the shallow meridional overturning cells feeding it, and their role in El Niño and decadal variability in the equatorial Pacific are studied using both in situ data and an ocean general circulation model. Using temperature and current data from the TAO/TRITON moorings at the equator, their data gaps are filled and it was shown that continuous time series of mass transport, temperature, depth, and kinetic energy of the EUC could be constructed for the period 1980–2002 with an excellent accuracy. This dataset was analysed and used to validate the output from an oceanic general circulation model (OGCM). The OGCM was then used to find that variations in the strength of the EUC, shallow meridional overturning (pycnocline convergence and surface divergence), and equatorial upwelling had the same variations in mass transport on interannual and longer time scales within the period 1951–1999. These variations are all caused by variations of the zonal wind stress zonally integrated, in agreement with simple linear and steady dynamics theories. Impact of these mass transport variations and of temperature variations on heat budgets in the entire equatorial band of the Pacific and in its eastern part are quantified.     

On the meridional circulation and balance of momentum in the Southern Ocean of POP

 The circulation of the Southern Ocean is studied in the eddy-resolving model POP (Parallel Ocean Program) by an analysis of zonally integrated balances. The TEM formalism (Transformed Eulerian Mean) is extended to include topography and continental boundaries, thus deviations from a zonally integrated state involve transient and standing eddies. The meridional circulation is presented in terms of the Eulerian, eddy-induced, and residual streamfunctions. It is shown that the splitting of the meridional circulation into Ekman and geostrophic transports and the component induced by subgrid and Reynolds stresses is identical to a particular form of the zonally integrated balance of zonal momentum. In this balance, the eddy-induced streamfunctions represent the interfacial form stresses by transient and standing eddies and the residual streamfunction represents the acceleration of the zonal current by density fluxes in a zonally integrated frame. The latter acceleration term is directly related to the surface flux of density and interior fluxes due to the resolved and unresolved eddies. The eddy-induced circulation is extremely vigorous in POP. In the upper ocean a shallow circulation, reversed in comparison to the Deacon cell and mainly due to standing eddies, appears to the north of Drake Passage latitudes, and in the Drake Passage belt of latitudes a deep-reaching cell is induced by transient eddies. In the resulting residual circulation the Deacon cell is largely cancelled and the residual advection of the zonal mean potential density is balanced by diapycnal eddy and subgrid fluxes which are strong in the upper few hundred meters but small in the ocean interior. The balance of zonal momentum is consistent with other eddy-resolving models; a new aspect is the clear identification of density effects in the zonally integrated balance. We show that the wind stress and the stress induced by the residual circulation drive the eastward current, whereas both eddy species result in a braking. Finally, we extend the Johnson–Bryden model of zonal transport to incorporate all relevant terms from the zonal momentum balance. It is shown that wind stress and induction by the residual circulation carry an eastward transport while bottom form stress and the stress induced by standing eddies yield westward components of transport. Received: 26 June 2001 / Accepted: 2 November 2001     

A modelling study of the latitudinal variations in the nighttime plasma temperatures of the equatorial topside ionosphere during northern winter at solar maximum

Latitudinal variations in the nighttime plasma temperatures of the equatorial topside ionosphere during northern winter at solar maximum have been examined by using values modelled by SUPIM (Sheffield University Plasmasphere Ionosphere Model) and observations made by the DMSP F10 satellite at 21.00 LT near 800 km altitude. The modelled values confirm that the crests observed near 15° latitude in the winter hemisphere are due to adiabatic heating and the troughs observed near the magnetic equator are due to adiabatic cooling as plasma is transported along the magnetic field lines from the summer hemisphere to the winter hemisphere. The modelled values also confirm that the interhemispheric plasma transport needed to produce the required adiabatic heating/cooling can be induced by F-region neutral winds. It is shown that the longitudinal variations in the observed troughs and crests arise mainly from the longitudinal variations in the magnetic meridional wind. At longitudes where the magnetic declination angle is positive the eastward geographic zonal wind combines with the northward (summer hemisphere to winter hemisphere) geographic meridional wind to enhance the northward magnetic meridional wind. This leads to deeper troughs and enhanced crests. At longitudes where the magnetic declination angle is negative the eastward geographic zonal wind opposes the northward geographic meridional wind and the trough depth and crest values are reduced. The characteristic features of the troughs and crests depend, in a complicated manner, on the field-aligned flow of plasma, thermal conduction, and inter-gas heat transfer. At the latitudes of the troughs/crests, the low/high plasma temperatures lead to increased/decreased plasma concentrations.     

Seasonal variation of the auroral E-region neutral wind for different solar activities

Seasonal variations in the auroral E-region neutral wind for different solar activity periods are studied. This work is based on neutral wind data obtained over 56 days between 95–119 km altitude under geomagnetic quiet conditions (A_p<16) during one solar cycle by the European Incoherent Scatter radar located in northern Scandinavia. In general, the meridional mean wind shifts northward, and the zonal mean wind increases in eastward amplitude from winter to summer. The zonal mean wind blows eastward in the middle and lower E-region for each season and for each solar condition except for the equinox, where the zonal mean wind blows westward at and below 104 km. Solar activity dependence of the mean wind exists during the winter and equinox seasons, while in summer it is less prominent. Under high solar activity conditions, the altitude profiles of the horizontal mean winds in winter and the equinoxes tend to resemble those in summer. The horizontal diurnal tide is less sensitive to solar activity except during summer when the meridional amplitude increases by ∼10 m s⁻¹ and the corresponding phase shifts to a later time period (1–2 h) during high solar activity. Seasonal dependence of the semidiurnal tide is complex, but is found to vary with solar activity. Under low solar activity conditions the horizontal semidiurnal amplitude shows seasonal dependence except at upper E-region heights, while under high solar activity conditions it becomes less sensitive to seasonal effects (except for the meridional component above 107 km). Comparisons of mean winds with LF and UARS observations are made, and the driving forces for the horizontal mean winds are discussed for various conditions.     

Combined solar and QBO effects on the modes of low-frequency atmospheric variability in the Northern Hemisphere

We examine joint effects of the solar activity and phase of the quasi-biennial oscillation (QBO) on modes of low-frequency variability of tropospheric circulation in the Northern Hemisphere in winter. The winter months (December–March) are stratified by the solar activity into two (below/above median) classes, and each of these classes is subdivided by the QBO phase (west or east). The variability modes are determined by rotated principal component analysis of 500 hPa heights separately in each class of solar activity and QBO phase. Detected are all the modes known to exist in the Northern Hemisphere. The solar activity and QBO jointly affect the shapes, spatial extent, and intensity of the modes; the QBO effects are, however, generally weaker than those of solar activity. For both solar maxima and minima, there is a tendency to the east/west phase of QBO to be accompanied by a lower/higher activity of zonally oriented modes and increased meridionality/zonality of circulation. This means that typical characteristics of circulation under solar minima, including a more meridional appearance of the modes and less activity of zonal modes, are strengthened during QBO-E; on the other hand, circulation characteristics typical of solar maxima, such as enhanced zonality of the modes and more active zonal modes, are more pronounced during QBO-W. Furthermore, the zonal modes in the Euro-Atlantic and Asian sectors (North Atlantic Oscillation, East Atlantic pattern, and North Asian pattern) shift southwards in QBO-E, the shift being stronger in solar maxima.     

Global empirical wind model for the upper mesosphere/lower thermosphere. I. Prevailing wind

An updated empirical climatic zonally averaged prevailing wind model for the upper mesosphere/lower thermosphere (70/110 km), extending from 80°N to 80°S is presented. The model is constructed from the fitting of monthly mean winds from meteor radar and MF radar measurements at more than 40 stations, well distributed over the globe. The height-latitude contour plots of monthly mean zonal and meridional winds for all months of the year, and of annual mean wind, amplitudes and phases of annual and semiannual harmonics of wind variations are analyzed to reveal the main features of the seasonal variation of the global wind structures in the Northern and Southern Hemispheres. Some results of comparison between the ground-based wind models and the space-based models are presented. It is shown that, with the exception of annual mean systematic bias between the zonal winds provided by the ground-based and space-based models, a good agreement between the models is observed. The possible origin of this bias is discussed.     

Long term variations in the mesospheric mean flow observed at Grahamstown (South Africa) and Adelaide (Australia)

The seasonal and interannual behaviour of monthly mean winds at a height of 90 km recorded at Grahamstown (33.3°S, 26.5°E) and Adelaide (34.5°S, 138.5°E) between 1987 and 1994 are compared. The zonal wind is found to be consistently stronger at Grahamstown and is always eastward, whereas at Adelaide it sometimes reverses. Maxima tend to occur near the solstices, the primary maximum during summer at Grahamstown, in agreement with satellite results, and during winter at Adelaide. The meridional wind also tends to be stronger at Grahamstown, but at both stations is predominantly northward with a maximum in summer and generally not as strong as the zonal component. This seasonal behaviour is reasonably well understood in terms of the interaction of the mean flow with gravity waves propagating up from below, with coriolis forces also playing an important role in the case of the meridional wind. Satellite observations do not generally support the idea that longitudinal differences between the stations could be attributed to the presence of a tropospheric/stratospheric stationary wave. It is suggested that these differences are more probably associated with local effects. Interannual zonal wind patterns at the two sites are similar over the summer months but are less well correlated during the rest of the year. The underlying causes of this variability are not well understood but are most probably global in nature, at least during the summer.     

Mixed-layer water oscillations in tropical Pacific for ENSO cycle

The main modes of interannal variabilities of thermocline and sea surface wind stress in the tropical Pacific and their interactions are investigated,which show the following results.(1) The thermocline anomalies in the tropical Pacific have a zonal dipole pattern with 160°W as its axis and a meridional seesaw pattern with 6-8°N as its transverse axis.The meridional oscillation has a phase lag of about 90° to the zonal oscillation,both oscillations get together to form the El Ni?o/La Ni?a cycle,which be-haves as a mixed layer water oscillates anticlockwise within the tropical Pacific basin between equator and 12°N.(2) There are two main patterns of wind stress anomalies in the tropical Pacific,of which the first component caused by trade wind anomaly is characterized by the zonal wind stress anomalies and its corresponding divergences field in the equatorial Pacific,and the abnormal cross-equatorial flow wind stress and its corresponding divergence field,which has a sign opposite to that of the equatorial region,in the off-equator of the tropical North Pacific,and the second component represents the wind stress anomalies and corresponding divergences caused by the ITCZ anomaly.(3) The trade winds anomaly plays a decisive role in the strength and phase transition of the ENSO cycle,which results in the sea level tilting,provides an initial potential energy to the mixed layer water oscillation,and causes the opposite thermocline displacement between the west side and east side of the equator and also between the equator and 12°N of the North Pacific basin,therefore determines the amplitude and route for ENSO cycle.The ITCZ anomaly has some effects on the phase transition.(4) The thermal anomaly of the tropical western Pacific causes the wind stress anomaly and extends eastward along the equator accompanied with the mixed layer water oscillation in the equatorial Pacific,which causes the trade winds anomaly and produces the anomalous wind stress and the corresponding divergence in favor to conduce the oscillation,which in turn intensifies the oscillation.The coupled system of ocean-atmo-sphere interactions and the inertia gravity of the mixed layer water oscillation provide together a phase-switching mechanism and interannual memory for the ENSO cycle.In conclusion,the ENSO cycle essentially is an inertial oscillation of the mixed layer water induced by both the trade winds anomaly and the coupled ocean-atmosphere interaction in the tropical Pacific basin between the equator and 12°N.When the force produced by the coupled ocean-atmosphere interaction is larger than or equal to the resistance caused by the mixed layer water oscillation,the oscillation will be stronger or maintain as it is,while when the force is less than the resistance,the oscillation will be weaker,even break.     

CMIP5模式对东亚冬季风指数变化及其与冬季大气环流和气温关系的模拟评估

本文评估了44个CMIP5模式对东亚冬季风环流系统,特别是东亚冬季风指数及其对应的环流和气温特征的模拟能力.结果表明:CMIP5模式对地表气温和500 hPa位势高度场模拟效果最好,对200 hPa纬向风的模拟次之,而对海平面气压和850 hPa经向风的模拟相对较差.与单个模式相比,多模式集合(MME)的模拟能力要更优,其能够很好地再现西伯利亚高压、阿留申低压、东亚低层偏北风、中层东亚大槽、高层东亚西风急流以及地表气温的空间分布.不过,模拟的环流系统偏强,造成东亚地表气温总体偏低.对于东亚冬季风指数,分别选取基于300 hPa纬向风(I_(Jhun))、850 hPa风场(I_(Wang))、500 hPa位势高度(I_(Cui))、以及海平面气压(I_(Guo))定义的四个指数表征东亚冬季风强度.MME能很好地模拟I_(Cui)和I_(Wang)指数的长期变化,还能合理再现四个指数所指示的东亚冬季风环流和气温的变化特征:对应冬季风偏强年份,西伯利亚高压、阿留申低压、东亚沿岸低层北风、东亚大槽和高空西风急流加强,东亚大陆地表气温和极端低温降低,但变化的幅度比观测结果偏弱.     

On the maintenance of the axisymmetric part of the flow in the atmosphere

Summary The maintenance of the axisymmetric component of the flow in the atmosphere is investigated by means of a steady-state, quasi-geostrophic formulation of the meteorological equations. It is shown that the meridional variations in the time-averaged axisymmetric variables can be expressed as the sum of three contributions, one being due to the eddy heat transport, another to the eddy momentum transport, and a third to the convective-radiative equilibrium temperature which enters the problem through the specification of a Newtonian form of diabatic heating. The contributions by the large scale eddies are evaluated through the use of observed values for the eddy heat and momentum transports.The contributions from each of the three forcing mechanisms to the temperature and zonal wind fields are invstigated individually and found to be of about equal importance. The sum of the three contributions are also presented for the temperature, the zonal wind, the stream function associated with the mean meridional circulation and the corresponding vertical motion. Although the results fail to reproduce the main observed features of the lower stratosphere, they are found to be in good agreement with observations in the middle latitude troposphere. At any pressure level, for example, the computed mean zonal wind has a jet-like profile and the axis of the jet is found to slope to the south with height, as observed in the atmosphere.Based in part on a thesis submitted by the first author as partial fulfillment of the requirements for the Ph.D. degree at the University of Michigan. — Publication No. 194 from the Department of Meteorology and Oceanography, The University of Michigan.     

On the mean zonal and meridional circulations and the flux of moisture in the northern hemisphere during the summer season

Summary The mean zonal and meridional wind components of the northern hemisphere at different pressure levels for the summer season June–August have been determined and the mean meridional mass circulation has been computed as a function of latitude. From the mass circulation the meridional flux of moisture is computed for the latitudinal belt 0°–45° N. Using the horizontal divergence of this flux the average difference between precipitation and evapotranspiration from the earth's surface is evaluated.