Solar response in the temperature over the equatorial middle atmosphere

One of the longest temperature records available for the equatorial region is provided by Rocketsonde from Thumba (8°N, 77°E), India during the period 1971–1993. In recent times, these data sets are reanalyzed using the up-to-date regression models, which take care of several corrections and parameters, not accounted for in earlier analyses and hence affecting the conclusions. In this paper, annual mean solar response in this data set along with the seasonal solar coefficient is quantitatively estimated now with improved confidence. A negative solar response in the stratosphere (1–2 K/100 solar flux unit, sfu) and a positive response for the mesosphere (0.5–3 K/100 sfu) are found. The negative stratospheric solar response is in contrast to the solar coefficient reported for low latitudes by earlier workers for other stations.     

The middle atmosphere

The last 50 years have seen enormous advances in our knowledge and understanding of the stratosphere and mesosphere, which together comprise the middle atmosphere. Beginning from a phase of basic discovery, we have now reached the stage where most observed phenomena can be modelled from first principles with a reasonable degree of fidelity, and where there is an overall theoretical framework which can be tested against measurements and models. This review surveys a number of major surprises in middle atmosphere science over the past 50 years. A phenomenological and historical approach is adopted in each case, leading up to the current literature. Along the way, a common thread emerges: the central role of waves, of various types, in redistributing angular momentum within the atmosphere, and the global nature of the atmospheric response to such redistribution.     

Solar cycle changes to planetary wave propagation and their influence on the middle atmosphere circulation

Recent observations suggest that there may be a causal relationship between solar activity and the strength of the winter Northern Hemisphere circulation in the stratosphere. A three-dimensional model of the atmosphere between 10–140 km was developed to assess the influence of solar minimum and solar maximum conditions on the propagation of planetary waves and the subsequent changes to the circulation of the stratosphere. Ultraviolet heating in the middle atmosphere was kept constant in order to emphasise the importance of non-linear dynamical coupling. A realistic thermo-sphere was achieved by relaxing the upper layers to the MSIS-90 empirical temperature model. In the summer hemisphere, strong radiative damping prevents significant dynamical coupling from taking place. Within the dynamically controlled winter hemisphere, small perturbations are reinforced over long periods of time, resulting in systematic changes to the stratospheric circulation. The winter vortex was significantly weakened during solar maximum and western phase of the quasi-biennial oscillation, in accordance with reported 30 mb geopotential height and total ozone measurements.     

Solar activity and dynamic processes in the atmosphere and world ocean heat content

The scenario of climatic changes in the 20th century has been presented in the scope of the developed model concerning the effect of solar activity on the parameters of the climatic system governing the energy flux, outgoing from the Earth into space in the high-latitude regions. The regularities of changes in the circulation in the atmosphere and ocean are discussed. Specific attention is paid to the causes of a “cold snap” in 1940–1976 in the Northern Hemisphere and the nature of an anomalous increase in the heat content in the Earth climatic system (ocean) in 1969–1980. It has been indicated that these phenomena result from changes in the circulation in the atmosphere and ocean (specifically, a change in the thermohaline circulation in the Northern Atlantic), heat exchange between the ocean and the atmosphere and cryosphere.     

Optical measurement in the middle atmosphere

The various measurement techniques and general problems in remote orin situ optical measurement of atmospheric minor gases are described.     

Empirical forcing functions for the large-scale mean disturbances in the atmosphere

Summary Fundamental equations governing the forced response of the atmosphere to the fixed sources and sinks of heat and momentum are reviewed. Empirical forcing functions, computed from data for the winter of 1950, representing the effects only of the horizontal transient eddy transport field, are presented. These appear to be of the same order as is required to account for the observed mean conditions and, hence, should be included in the general theory of the mean state along with other diabatic and mechanical effects which have been considered previously.     

Temperature variability over the tropical middle atmosphere

A study on the variability of temperature in the tropical middle atmosphere over Thumba (8 32’ N, 76 52’ E), located at the southern part of India, has been carried out based on rocket observations for a period of 20 years, extending from 1970 to 1990. The rocketsonde-derived mean temperatures over Thumba are corrected prior to 1978 and then compared with the middle atmospheric reference model developed from satellite observations and Solar Mesosphere Explorer (SME) satellite data. Temperature variability at every 1 km interval in the 25–75 km region was analysed. The tropical stratosphere is found to be highly stable, whereas considerable variability is noted in the middle mesosphere. The effect of seasonal cycle is least in the lower stratosphere. Annual and semi-annual oscillations in temperature are the primary oscillations in the tropical middle atmosphere. Annual temperature oscillations are dominant in the mesosphere and semi-annual oscillations are strong in the stratosphere. The stratopause region is noted to be the part of the middle atmosphere least sensitive to the changes in solar activity and long-term variability.     

Truncated model of a barotropic atmosphere with a periodic forcing

The effect of variations in time of the zonal flow is investigated by the study of a simplified truncated model of a barotropic atmosphere in the presence of an oscillating zonal forcing. Long-time numerical simulations of a triadic model in spherical geometry are carried out for various values of both the frequency and the amplitude of the oscillating part of the zonal forcing. It is found that the reaction of the system to simple sinusoidal forcing is characterized, as happens for strongly nonlinear systems, by complicated trajectories in the phase-space and that the spectrum of the zonal component is much more complicated than that of the forcing function, with interesting relative maxima in the range of very low climatological frequencies. Moreover it is shown that, for proper values of both the frequency and the amplitude of the sinusoidally oscillating part of the forcing function, our simplified model of the large-scale planetary circulation oscillates between an essentially zonal regime (a flow pattern dominated by the zonal flow component) and a wave regime (a flow pattern characterized by significant values of the meridional component of the velocity field associated with the wave components). The transitions between the two regimes are strongly asymmetric: in fact, the time needed for a wave-like flow to evolve into an essentially zonal one is, in the limit of our model, typically 4 to 5 times greater than that needed for the inverse transition. The results are intuitively interpreted in the limit of very long periods of the oscillations of the forcing function. Other interesting features of the results are considered.     

Modeling of nonmigrating tides in the middle atmosphere

On the basis of calculations using the general circulation model of the middle and upper atmosphere, the relative role of sources of nonmigrating tides distributed in atmosphere has been investigated. It is shown that in winter, when planetary waves in stratosphere are well developed, the main contribution to the generation of nonmigrating tides is caused by nonlinear interaction between migrating tides and a quasi-stationary planetary wave with zonal wave number 1 (SPW1). Taking into account the longitudinal ozone inhomogeneities in the model leads to the occurrence of additional sources of nonmigrating tides caused by longitudinally inhomogeneous heating of the atmosphere, the contribution of which can be comparable to that from nonlinear interaction under an attenuating amplitude of SPW1 in the stratosphere.     

Satellite solar occultation sounding of the middle atmosphere

This paper discusses the principles, achievements, and prospects for satellite solar occultation sounding of the middle atmosphere. Advantages, disadvantages, and spatial and temporal coverage capabilities are described. Progress over the past 15 years is reviewed, and results from a recent satellite aerosol experiment are presented. Questions with regard to Doppler shift, atmosphric refraction, instrument pointing, pressure sensing, and measurement of diurnally active species are addressed. Two experiments now orbiting on the Nimbus-7 and AEM-B satellites, and approved experiments under development for future flights on Spacelab and the Earth Radiation Budget Satellite, are also described. In some cases more than one experiment is scheduled to be flown on the same spacecraft, and the advantages and synergistic effects of these applications are discussed.     

Mesospheric bore events in the equatorial middle atmosphere

An all-sky CCD imager designed to measure wave structure of the OH, O₂b(0,1) and OI557.7 nm airglow emission layers in the mesosphere and lower thermosphere (MLT) region has been operated near the equatorial region at São João do Cariri (Cariri), Brazil, (7.5°S, 36.5°W). A large number of gravity wave was observed from September 2000 to September 2002 and among them 64 wave events were identified as mesospheric bores. The bore front shows a horizontal extension greater than 1000 km, and observed in the airglow layers as a complementary brilliance between the three emissions. At the first time mesospheric bore events were observed and analyzed in the equatorial region. Their predominant characteristics, occurrence, local time dependency, morphology and propagation direction will be presented and discussed.     

Short-period planetary waves in the Antarctic middle atmosphere

Planetary waves with periods between two and four days in the middle atmosphere over Antarctica are characterized using one year of data from the medium-frequency spaced antenna (MFSA) radars at Scott Base, Rothera, and Davis. In order to investigate the origin of the observed waves, the ground-based data are complemented by temperature measurements from the Earth Observing System Microwave Limb Sounder (EOS MLS) instrument on the Aura satellite as well as wind velocity data from the United Kingdom Met. Office (UKMO) stratospheric assimilation. Observed characteristics of waves with a period of approximately two days in summer are consistent with the quasi-two-day wave (QTDW) generally found after the summer solstice at low- and mid-latitudes. The Scott Base observations of the QTDW presented here are the highest-latitude ground-based observations of this wave to date. Waves with preferred periods of two and four days occur in bursts throughout the winter with maximum activity in June, July, and August. The mean of the two- and four-day wave amplitudes is relatively constant, suggesting constant wave forcing. When several waves with different periods occur at the same time, they often have similar phase velocities, supporting suggestions that they are quasi-non-dispersive. In 2005, a “warmpool” lasts from late July to late August. An alternative interpretation of this phenomenon is the presence of a structure propagating with the background wind. Consideration of the role of vertical shear (baroclinic instabilities) and horizontal shear (barotropic instabilities) of the zonal wind suggests that instabilities are likely to play a role in the forcing of the two- and four-day waves, which are near-resonant modes and thus supported by the atmosphere.     

Solar forcing on the 7Be-air concentration variability at ground level

This paper analyzes the correlation between the temporal and spatial variability of ⁷Be-air concentration at ground level to precipitation. Data, obtained from 26 stations distributed throughout North and South America, Australia and Antarctica, were analyzed. Variations in the data were extracted by the empirical orthogonal function (EOF) and principal component (PC) analysis. The results presented here show that the variability of ⁷Be-air concentration at ground level is influenced simultaneously both by solar cycle and atmospheric processes, such as precipitation, turbulent transport and advection. Solar forcing dominates ⁷Be annual variability worldwide. On the other hand, atmospheric processes influence ⁷Be air–concentration at ground level regionally and seasonally.     

Solar forcing of the Northern hemisphere land air temperature: New data

It has previously been demonstrated that the mean land air temperature of the Northern hemisphere could adequately be associated with a long-term variation of solar activity as given by the length of the approximately 11-year solar cycle. Adding new temperature data for the 1990s and expected values for the next sunspot extrema we test whether the solar cycle length model is still adequate. We find that the residuals are now inconsistent with the pure solar model. We conclude that since around 1990 the type of Solar forcing that is described by the solar cycle length model no longer dominates the long-term variation of the Northern hemisphere land air temperature.     

Quasi-stationary planetary waves in the middle atmosphere of Mars

Using the temperature profiles retrieved from the Mars Climate Sounder(MCS) instrument onboard Mars Reconnaissance Orbiter(MRO) satellite between November 2006 and April 2013, we studied the seasonal and interannual variability of QuasiStationary Planetary Waves(QSPWs) in the Martian middle atmosphere. The QSPW amplitudes in the Southern Hemisphere(SH) high latitudes are significantly stronger than those in the Northern Hemisphere(NH). Seasonal variation with maximum amplitude near winter solstice of each hemisphere is clearly seen. The vertical structure of the QSPW in temperature shows double-layer feature with one peak near 50 Pa and the other peak near 1 Pa. The QSPW in geopotential height is clearly maximized in the region between two temperature peaks. The maximum amplitude of QSPW for s=1 is ~8–10 K in temperature and ~1 km in geopotential height in the SH high latitudes. The maximum amplitude at the SH high latitudes in Mars Year(MY) 31 is ~2 K stronger than those in other MYs, suggesting the clear interannual variability. We compared the satellite results with those obtained from the Mars Climate Database(MCD) simulation version 5.0; a reasonable agreement was found. The MCD simulation further suggested that the variability of dust might partially contribute to the interannual variability of QSPW amplitude.     

Lidar observations of middle atmosphere temperature variability

We discuss 223 middle atmosphere lidar temperature observations. The record was collected at Frascati (42°N-13°E), during the 41-month period January 1989-May 1992, corresponding to the maximum of solar cycle 22. The choice of this interval was aimed at minimizing the temperature variability induced by the 11-year solar cycle. The average climatology over the 41-month period and comparison with a reference atmosphere (CIRA86) are presented. Monthly temperature variability over the full period, during opposite quasi-biennial oscillation phases and on a short-term scale (0.5-4 h), is analyzed. Results indicate the 50-55-km region as less affected by variability caused by the natural phenomena considered in the analysis. Due to this minimum in natural noise characterizing the atmospheric temperature just above the stratopause, observations of that region are well suited to the detection of possible temperature trends induced by industrial activities.     

Solar faculae and waves propagating in the solar atmosphere

Solar faculae are among the most common manifestations of solar activity and can play an important role in the energy transfer from the lower solar atmosphere into the corona. However, the mechanisms by which energy is transferred remain insufficiently studied. Our work is based on observational data obtained with the AST telescope of the Sayan solar observatory. Simultaneous observations were performed in the Hα 6563 Å and FeI 6569 Å, BaII 4554 Å and FeI 4551.6 Å, and CaII 8542 Å and FeI 8538 Å pairs of spectral lines. The studies indicated that the spectral composition of the line-of-sight (LOS) velocity oscillations in faculae is inhomogeneous and is affected by the chromospheric network’s structural elements. The possible presence of short and low magnetic loops in the facula region makes it difficult to determine the unambiguous phase relations between chromospheric and photospheric oscillations. The LOS velocity oscillation signals registered at the chromospheric level both lead and lag behind the signals registered at the photospheric level. At the same time, signs of propagating waves are evidently registered in the chromosphere of individual faculae.     

Trends in planetary wave activity in the upper middle atmosphere inferred from the nighttime LF radio wave absorption measurements

Summary The nighttime LF radio wave absorption in the lower ionosphere measured at two frequencies in central Europe over 1963–1985 is used to infer planetary wave activity and its long-term trend in the upper middle atmosphere (∼90–100 km). The observed positive trend is roughly consistent with results based on daytime absorption. Nighttime results are less pronounced and less statistically significant probably due to perturbing effects of geomagnetic activity. The observed trends, which are probably of anthropogenic origin, are together with the daytime results [3,4] the first evidence of long-term trends in planetary wave activity in the upper middle atmosphere.     

Constrains on gravity wave induced diffusion in the middle atmosphere

A review of the important constraints on gravity wave induced diffusion of chemical tracers, heat, and momentum is given. Ground-based microwave spectroscopy measurements of H₂O and CO and rocket-based mass spectrometer measurements of Ar constrain the eddy diffusion coefficient for constituent transport (K _zz ) to be (1–3)×10⁵ cm²s^–1 in the upper mesosphere. Atomic oxygen data also limitsK _zz to a comparable value at the mesopause. From the energy balance of the upper mesosphere the eddy diffusion coefficient for heat transport (D _H ) is, at most 6×10⁵ cm²s^–1 at the mesopause and decreasing substantially with decreasing altitude. The available evidence for mean wind deceleration and the corresponding eddy diffusion coefficient for momentum stresses (D _M ) suggests that it is at least 1×10⁶ cm²s^–1, in the upper mesosphere. Consequently the eddy Prandtl number for macroscopic scale lengths is >3.     

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.