Winter Polar Regions Research Articles

Direct Observational Evidence of Altered Mesosphere Lower Thermosphere Mean Circulation From a Major Sudden Stratospheric Warming

AbstractSudden stratospheric warmings (SSWs) are large‐scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90–125 km altitude. However, the robustness of these effects and the roles of SSW‐induced changes in global‐scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (∼93–106 km altitude) winds near 10°S‐40°N latitude from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ∼3‐day ultra‐fast Kevin wave.

Sub-field of view surface thermal modeling of Cassini CIRS observations of Rhea during south polar winter

Rhea's exosphere is thought to originate from sources of carbon, water ice and other volatiles that arrived at Rhea by bombardment. Its seasonal variability is directly driven by polar surface temperatures allowing surface adsorption, and the persistence of source volatiles require that seasonal temperatures remain sufficiently cold to retain them. Cassini CIRS detected temperatures in Rhea's winter polar region of 23 K, amongst the coldest measured in the Solar System, but with a relatively large footprint we seek to add value to these observations by modeling sub field-of-view (FOV) temperature distribution and examining how the coldest scenes evolve on a seasonal basis.A simple, rough surface 1-dimensional thermal model is developed using a digital elevation map as input to a thermal model. We compared averaged rough and flat modeled FOVs to CIRS temperatures for a set of case study observations in the south polar region in winter darkness and found they both agree within expected CIRS error in all cases. We develop an asymmetric estimate of CIRS FOV temperature uncertainty, which is particularly important for very cold spectra to accurately represent the sensitivity of the instrument. This approach provides a conservative upper temperature limit to spectral fits whilst highlighting there is often little information to constrain the lower bound of temperature uncertainty in these cases.The distribution of modeled sub-FOV facet temperatures is explored for the full range of azimuth angles and slope gradients. More extensive and cooler temperatures were found in the rough model, complemented by fewer but warmer areas than the flat model. We find temperature contrasts of individual model facets of up to 15 K warmer and 11 K colder within some CIRS FOV when the rough model was compared flat, in a case study of scenes in winter darkness. This is due to the persistence of the seasonal thermal wave beneath the surface. We tested model sensitivity to thermal input parameters (thermal inertia and bolometric Bond albedo). These values are challenging to constrain due to limited observations and measurement noise and are expected to vary with subtle changes in surface characteristics. We found that within 20° of the pole, temperatures do not rise significantly above 80 K all year, implying that a variety of simple organic molecules, linear amides and other carbon-containing compounds would remain stable on a quasi-permanent basis, potentially until photolytic or radiolytic mechanisms liberate by-products which provide a source for exospheric constituents. The simple rough model indicates that some facets experience very different summer/winter season lengths than a flat model can represent, which is important in terms of the exospheric sequestration process.We use threshold temperatures of 55 K for CO2 and 30 K for O2 ice as proxies for their relative stability at the surface. The surface coverage of these temperatures with time was compared to modeled exospheric abundance by Teolis and Waite (2016). They are anti-correlated which is expected, but with slight differences in timings of minima and the equinoctial maxima. We find that the rough model suggests larger areas able to adsorb these species than the flat model, and that the cumulative influence of topography would have a direct relationship on the timing and abundance of the exosphere.We place Rhea's polar environment in context with other icy moons in the Saturnian system using the flat model. The satellites Tethys, Dione and Enceladus experience similar polar thermal regimes to Rhea (with exceptionally cold winters) and would potentially benefit from the consideration of topography in relation to exospheric modeling.

Phosgene distribution derived from MIPAS ESA v8 data: intercomparisons and trends

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measured the middle-infrared limb emission spectrum of the atmosphere from 2002 to 2012 on board ENVISAT, a polar-orbiting satellite. Recently, the European Space Agency (ESA) completed the final reprocessing of MIPAS measurements, using version 8 of the level 1 and level 2 processors, which include more accurate models, processing strategies, and auxiliary data. The list of retrieved gases has been extended, and it now includes a number of new species with weak emission features in the MIPAS spectral range. The new retrieved trace species include carbonyl chloride (COCl2), also called phosgene. Due to its toxicity, its use has been reduced over the years; however, it is still used by chemical industries for several applications. Besides its direct injection in the troposphere, stratospheric phosgene is mainly produced from the photolysis of CCl4, a molecule present in the atmosphere because of human activity. Since phosgene has a long stratospheric lifetime, it must be carefully monitored as it is involved in the ozone destruction cycles, especially over the winter polar regions. In this paper we exploit the ESA MIPAS version 8 data in order to discuss the phosgene distribution, variability, and trends in the middle and lower stratosphere and in the upper troposphere. The zonal averages show that phosgene volume mixing ratio is larger in the stratosphere, with a peak of 40 pptv (parts per trillion by volume) between 50 and 30 hPa at equatorial latitudes, while at middle and polar latitudes it varies from 10 to 25 pptv. A moderate seasonal variability is observed in polar regions, mostly between 80 and 50 hPa. The comparison of MIPAS–ENVISAT COCl2 v8 profiles with the ones retrieved from MIPAS balloon and ACE-FTS (Atmospheric Chemistry Experiment – Fourier Transform Spectrometer) measurements highlights a negative bias of about 2 pptv, mainly in polar and mid-latitude regions. Part of this bias is attributed to the fact that the ESA level 2 v8 processor uses an updated spectroscopic database. For the trend computation, a fixed pressure grid is used to interpolate the phosgene profiles, and, for each pressure level, VMR (volume mixing ratio) monthly averages are computed in pre-defined 10∘ wide latitude bins. Then, for each latitudinal bin and pressure level, a regression model has been fitted to the resulting time series in order to derive the atmospheric trends. We find that the phosgene trends are different in the two hemispheres. The analysis shows that the stratosphere of the Northern Hemisphere is characterized by a negative trend of about −7 pptv per decade, while in the Southern Hemisphere phosgene mixing ratios increase with a rate of the order of +4 pptv per decade. This behavior resembles the stratospheric trend of CCl4, which is the main stratospheric source of COCl2. In the upper troposphere a positive trend is found in both hemispheres.

Latitudinal and Seasonal Asymmetries of the Helium Bulge in the Martian Upper Atmosphere

AbstractIn the present study, we investigate the characteristics of helium (He) bulges in the Martian upper atmosphere using He densities and winds measured by the Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. The observations are compared with those predicted by the Mars Global Ionosphere Thermosphere Model (M‐GITM). The results of the present study show that the nightside He bulge is a persistent feature of the Martian upper atmosphere in all seasons. The He densities inside the bulges are 1–2 orders of magnitude greater than those on the dayside. In solstices, the bulges are observed in the winter polar region which is in accordance with the model predictions. In equinoxes, however, the bulges are observed to extend from mid‐latitudes into the southern polar regions (>60°S), which is contrary to the model predictions at mid‐latitudes. These anomalous bulges are predominantly observed in the northern spring equinox and are 10–30 × greater than the modeled ones. During the autumnal equinox, the observed winds depart from the modeled winds. Furthermore, the observed winds point to the southern polar regions where the bulges are observed. Thus, the results of the present study indicate that in equinoxes the regions of local vertical advection, that are responsible for the formation of the bulges, are displaced toward the southern polar regions. The results of the present study point to the need of a larger wind database from NGIMS in southern polar region, particularly during equinoxes.

Atmospheric transport into polar regions on Mars in different orbital epochs

The variation of the transport of air into Martian polar regions with obliquity is examined using a series of MarsWRF general circulation model simulations which include idealized age of air and exponential decay tracers. These show substantial changes in the circulation, polar vortex structure, and tracer transport with changes in obliquity. These changes are non-monotonic with respect to obliquity, as well as differing between the summer and winter polar regions. In particular, there is an increase in the rate of vertically-integrated transport of air to the winter pole as obliquity increases from 5° to 25° followed by a decrease from 25° to 55°, with generally the reverse trends for transport to the summer pole. This behavior suggests that the amount of dust transport into polar regions will vary with obliquity, and could contribute to the apparent cyclical deposition of dust in polar regions as expressed in the polar layered deposits. Furthermore, the non-monotonic variation with obliquity suggests that visible changes in the appearance of polar layering may vary on a cycle with half the period of the cycle over the full obliquity variation range.

Two-dimensional radiative transfer for the retrieval of limb emission measurements in the martian atmosphere

The remote sounding of infrared emission from planetary atmospheres using limb-viewing geometry is a powerful technique for deriving vertical profiles of structure and composition on a global scale. Compared with nadir viewing, limb geometry provides enhanced vertical resolution and greater sensitivity to atmospheric constituents. However, standard limb profile retrieval techniques assume spherical symmetry and are vulnerable to biases produced by horizontal gradients in atmospheric parameters. We present a scheme for the correction of horizontal gradients in profile retrievals from limb observations of the martian atmosphere. It characterizes horizontal gradients in temperature, pressure, and aerosol extinction along the line-of-sight of a limb view through neighboring measurements, and represents these gradients by means of two-dimensional radiative transfer in the forward model of the retrieval. The scheme is applied to limb emission measurements from the Mars Climate Sounder instrument on Mars Reconnaissance Orbiter. Retrieval simulations using data from numerical models indicate that biases of up to 10K in the winter polar region, obtained with standard retrievals using spherical symmetry, are reduced to about 2K in most locations by the retrieval with two-dimensional radiative transfer. Retrievals from Mars atmospheric measurements suggest that the two-dimensional radiative transfer greatly reduces biases in temperature and aerosol opacity caused by observational geometry, predominantly in the polar winter regions.

Validation of the global distribution of CO2 volume mixing ratio in the mesosphere and lower thermosphere from SABER

AbstractThe Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite has been measuring the limb radiance in 10 broadband infrared channels over the altitude range from ~ 400 km to the Earth's surface since 2002. The kinetic temperatures and CO2 volume mixing ratios (VMRs) in the mesosphere and lower thermosphere have been simultaneously retrieved using SABER limb radiances at 15 and 4.3 µm under nonlocal thermodynamic equilibrium (non‐LTE) conditions. This paper presents results of a validation study of the SABER CO2 VMRs obtained with a two‐channel, self‐consistent temperature/CO2 retrieval algorithm. Results are based on comparisons with coincident CO2 measurements made by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE‐FTS) and simulations using the Specified Dynamics version of the Whole Atmosphere Community Climate Model (SD‐WACCM). The SABER CO2 VMRs are in agreement with ACE‐FTS observations within reported systematic uncertainties from 65 to 110 km. The annual average SABER CO2 VMR falls off from a well‐mixed value above ~80 km. Latitudinal and seasonal variations of CO2 VMRs are substantial. SABER observations and the SD‐WACCM simulations are in overall agreement for CO2 seasonal variations, as well as global distributions in the mesosphere and lower thermosphere. Not surprisingly, the CO2 seasonal variation is shown to be driven by the general circulation, converging in the summer polar mesopause region and diverging in the winter polar mesopause region.

A technique to identify vortex air using carbon monoxide observations

This study uses observations of carbon monoxide (CO) from the Microwave Limb Sounder instrument to identify vortex air in the winter polar regions. In particular, we use the probability distribution function (PDF) of the CO data to delineate CO concentrations characteristic of the interior of the vortex core as a function of space and time. This is achieved by fitting two Gaussian distributions to the PDF for a specific period and vertical level. These Gaussian fits are then examined to determine whether two chemically distinct regions exist by inspecting the intersection area between the Gaussians relative to their individual areas. When chemically distinct regions exist, the values of the fitted mean CO concentrations are representative of the interior and exterior of the polar vortex. To prove this point, a domain‐filling analysis is performed to produce high‐resolution maps. Comparison of these maps with the vortex edge derived using the equivalent latitude technique, and a statistical analysis over a range of years and isentropic levels, shows that the fitting scheme can be used to characterize measurements made inside the vortex during periods when chemically distinct populations exist. The statistical analysis also suggests that a threshold CO concentration can be derived based on the Gaussian fits which defines a good mixture between reducing the quantity of CO observations incorrectly identified as vortex interior air, whilst also minimizing the number of observations misclassified as vortex exterior air. This technique therefore provides an alternative to the dynamically derived calculation of values characteristic of the vortex interior.

Extensive MRO CRISM observations of 1.27 μm O2 airglow in Mars polar night and their comparison to MRO MCS temperature profiles and LMD GCM simulations

The Martian polar night distribution of 1.27 μm (0–0) band emission from O2 singlet delta [O2(1Δg)] is determined from an extensive set of Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectral Mapping (CRISM) limb scans observed over a wide range of Mars seasons, high latitudes, local times, and longitudes between 2009 and 2011. This polar nightglow reflects meridional transport and winter polar descent of atomic oxygen produced from CO2 photodissociation. A distinct peak in 1.27 μm nightglow appears prominently over 70–90NS latitudes at 40–60 km altitudes, as retrieved for over 100 vertical profiles of O2(1Δg) 1.27 μm volume emission rates (VER). We also present the first detection of much (×80 ± 20) weaker 1.58 μm (0–1) band emission from Mars O2(1Δg). Co‐located polar night CRISM O2(1Δg) and Mars Climate Sounder (MCS) (McCleese et al., 2008) temperature profiles are compared to the same profiles as simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation/photochemical model (e.g., Lefèvre et al., 2004). Both standard and interactive aerosol LMD simulations (Madeleine et al., 2011a) underproduce CRISM O2(1Δg) total emission rates by 40%, due to inadequate transport of atomic oxygen to the winter polar emission regions. Incorporation of interactive cloud radiative forcing on the global circulation leads to distinct but insufficient improvements in modeled polar O2(1Δg) and temperatures. The observed and modeled anti‐correlations between temperatures and 1.27μm band VER reflect the temperature dependence of the rate coefficient for O2(1Δg) formation, as provided in Roble (1995).

SEASONAL DISAPPEARANCE OF FAR-INFRARED HAZE IN TITAN'S STRATOSPHERE

A far-infrared emission band attributed to volatile or refractory haze in Titan's stratosphere has been decreasing in intensity since Cassini's arrival in 2004. The 220 cm(sup -1) feature, first seen by the Voyager Infrared Interferometer Spectrometer, has only been found in Titan's winter polar region. The emission peaks at about 140 km altitude near the winter stratospheric temperature minimum. Observations recorded over the period 2004-2012 by the Composite Infrared Spectrometer on Cassini show a decrease in the intensity of this feature by about a factor of four. Possible seasonal causes of this decline are an increase in photolytic destruction of source chemicals at high altitude, a lessening of condensation as solar heating increased, or a weakening of downwelling of vapors. As of early 2012, the 220 cm(sup -1) haze has not yet been detected in the south. The haze composition is unknown, but its decrease is similar to that of HC3N gas in Titan's polar stratosphere, pointing to a nitrile origin.

Impact of improved representation of horizontal and vertical cloud structure in a climate model

Many studies have investigated the effects that misrepresentation of sub-grid cloud structure can have on the radiation budget. In this study, we perform 20-year simulations of the current climate using an atmosphere-only version of the Met Office Unified Model to investigate the effects of cloud approximation on model climate. We apply the “Tripleclouds” scheme for representing horizontal cloud inhomogeneity and “exponential-random” overlap, both separately and in combination, in place of a traditional plane-parallel representation with maximum-random overlap, to the clouds within the radiation scheme. The resulting changes to both the radiation budget and other meteorological variables, averaged over the 20 years, are compared. The combined global effect of the parameterizations on top-of-atmosphere short-wave and long-wave radiation budget is less than 1 W m−2, but changes of up to 10 W m−2 are identified in marine stratocumulus regions. A cooling near the surface over the winter polar regions of up to 3°C is also identified when horizontal cloud inhomogeneity is represented, and a warming of similar magnitude is found when exponential-random overlap is implemented. Corresponding changes of the same sign are also found in zonally averaged temperature, with maximum changes in the upper tropical troposphere of up to 0.5°C. Changes in zonally averaged cloud fraction in this location were of opposite sign and up to 0.02. The individual effects on tropospheric temperature of improving the two components of cloud structure are of similar magnitudes to about 2% of the warming created by a quadrupling of carbon dioxide.

Evaluation of a Statistical Model of Cloud Vertical Structure Using Combined CloudSat and CALIPSO Cloud Layer Profiles

Abstract A model of the three-dimensional distribution of clouds was developed from the statistics of cloud layer occurrence from the International Satellite Cloud Climatology Project (ISCCP) and the statistics of cloud vertical structure (CVS) from an analysis of radiosonde humidity profiles. The CVS model associates each cloud type, defined by cloud-top pressure of the topmost cloud layer and total column optical thickness, with a particular CVS. The advent of satellite cloud radar (CloudSat) and lidar [Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)] measurements (together C&C) of CVS allows for a quantitative evaluation of this statistical model. The zonal monthly-mean cloud layer distribution from the ISCCP CVS agrees with that from C&C to within 10% (when normalized to the same total cloud amount). The largest differences are an overestimate of middle-level cloudiness in winter polar regions, an overestimate of cloud-top pressures of the highest-level clouds, especially in the tropics, and an underestimate of low-level cloud amounts over southern midlatitude oceans. A more severe test of the hypothesized relationship is made by comparing CVS for individual satellite pixels. The agreement of CVS is good for isolated low-level clouds and reasonably good when the uppermost cloud layer is a high-level cloud; however, the agreement is not good when the uppermost cloud layer is a middle-level cloud, even when ISCCP correctly locates cloud top. An improved CVS model combining C&C and ISCCP may require classification at spatial scales larger than individual satellite pixels.

On Forcing the Winter Polar Warmings in the Martian Middle Atmosphere during Dust Storms

Using a Martian general circulation model, we investigated the changes in the meridional circulation during planet-encircling dust storms on Mars that produce strong temperature vertical inversions in the middle atmosphere over winter polar regions. It is shown that vigorous poleward and downward transport, and, consequently, the adiabatic heating are caused by dissipating thermal tides, planetary and resolved small-scale gravity waves and eddies in almost equal degree. The increase of tidal forcing is mainly due to a stronger excitation in the summer hemisphere. Contribution of the stationary planetary wave (SPW) with the zonal wavenumber s ¼ 1i ncreases during dust storms due to intensified generation in the lower atmosphere as well as due to more favorable vertical propagation. SPW ðs ¼ 2Þ varies less with the dust load, dissipates lower, and contributes to the warming only below @ 0.1 mb. Transient planetary wave (s ¼ 1, period @ 5 sols) with a barotropic/baroclinic vertical structure provides up to 1/3 of the forcing by SPW ðs ¼ 1Þ. For the first time, we demonstrated a significance of small-scale gravity waves and eddies in maintaining the meridional circulation in Martian middle atmosphere, at least in high winter latitudes during dust storms.

A study of stratospheric chlorine partitioning based on new satellite measurements and modeling

Two recent satellite instruments, the Microwave Limb Sounder (MLS) on Aura and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) on SCISAT‐1, provide an unparalleled opportunity to investigate stratospheric chlorine partitioning. We use measurements of ClO, HCl, ClONO2, and other species from MLS and ACE‐FTS to study the evolution of reactive and reservoir chlorine throughout the lower stratosphere during two Arctic and two Antarctic winters characterizing both relatively cold and relatively warm and disturbed conditions in each hemisphere. At middle latitudes, and at high latitudes at the beginning of winter, HCl greatly exceeds ClONO2, representing ∼0.7–0.8 of estimated total inorganic chlorine. Nearly complete chlorine activation is seen inside the winter polar vortices. In the Arctic, chlorine recovery follows different paths in the two winters: In 2004/2005, deactivation initially takes place through reformation of ClONO2, then both reservoirs are produced concurrently but ClONO2 continues to significantly exceed HCl, and finally slow repartitioning between ClONO2 and HCl occurs; in 2005/2006, HCl and ClONO2 rise at comparable rates in some regions. In the Antarctic, chlorine deactivation proceeds in a similar manner in both winters, with a rapid rise in HCl accompanying the decrease in ClO. The measurements are compared to customized runs of the SLIMCAT three‐dimensional chemical transport model. Measured and modeled values typically agree well outside the winter polar regions. In contrast, partly because of the equilibrium scheme used to parameterize polar stratospheric clouds, the model overestimates the magnitude, spatial extent, and duration of chlorine activation inside the polar vortices.

Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model

The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194–2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s −1 at high winter latitudes between 200 and 300 km altitude (0.1–1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C 2H 2 and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15° S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80° N), some compounds (C 4H 2, C 3H 4) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40–50° S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.

Spacecraft Observations of the Martian Atmosphere

Recent spacecraft observations of the Martian atmosphere show that the current climate has a distinct and generally repeatable pattern of seasonal and spatial variations. The half of the year that Mars is closest to the Sun is characterized by warm, dusty conditions, whereas the opposite season is relatively cool, free of dust, and dominated by a low-latitude band of water ice clouds. Water vapor is most abundant at high latitudes in the summertime of both hemispheres, whereas a polar hood of clouds forms over the winter polar region. The greatest year-to-year variations are caused by intermittent large dust storms, which also affect atmospheric temperature and circulation patterns on a global scale.

Global distribution ofCT2at altitudes 30–50 km from space-borne observations of stellar scintillation

[1] Locally isotropic turbulence in the stratosphere consists of isolated sporadic patches with random values of temperature structure characteristic CT2. Stellar scintillations measured aboard GOMOS/ENVISAT through the Earth atmosphere provided the first global distribution of the effective characteristic CT,eff2 averaged along sounding ray. For zonal mean, the largest values are achieved in winter polar regions. This observed intense turbulence is probably related to the polar night jet. Relatively weak turbulence is observed at low latitudes. Turbulence intensity map is shown for altitude 42 km in the latitude band ±35°, where the maxima follow the sub-solar latitude, with enhancements located mainly over continents. Turbulence enhancements are not related with orography. Despite a noticeable correlation with typical regions of deep convection, the overall distribution of CT,eff2 displays a more complicated structure. Analyzed data suggest that the main turbulence sources, at altitudes ∼40 km, are instabilities of stratospheric circulation and gravity wave breaking.

A note on the semidiurnal non-migrating tide at polar latitudes

Abstract In the Antarctic upper mesosphere and lower thermosphere around 90 km, meteor radar observations at the South Pole have detected a significant semidiurnal wind component in summer which is found to be non-migrating with zonal wavenumber s = 1. It has been surmised that this component might possibly be excited through the non-linear interaction of the migrating semidiurnal tide with stationary planetary waves with zonal wavenumber s = 1 prevailing at stratospheric heights. The Kyushu University GCM has been successful in elucidating very unambiguously this conjecture. In the present paper, linearized steady tidal modeling is carried out in this connection to reproduce, by a fairly simplified but explicit model, trans-equatorial propagation of non-migrating semidiurnal tide forced in the opposite winter hemisphere and to compare latitudinal structures of migrating and non-linearly excited intermittent tides in view of polar latitudes where non-migrating tide tends to dominate over migrating tide. It is also shown that the re-analysis meteorological data for almost 10 years clearly supports the well-known N-S asymmetry in stationary planetary wave activity in winter polar stratospheric regions, possibly due to the difference in surface topography between two hemispheres. We suggest that there might be significant asymmetry in the summertime enhancement of semidiurnal non-migrating tide between both polar regions. This phenomenon may be in a context similar with the N-S asymmetry of gravity wave activity in the polar regions, possibly giving rise to N-S disparity of cold summertime mesopause temperature. Clarification awaits intensive bi-polar studies by coordinated radar and optical observations which are running both in the Arctic and Antarctic regions.

Satellite Measurements of Middle Atmospheric Impacts by Solar Proton Events in Solar Cycle 23

Solar cycle 23 was extremely active with seven of the largest twelve solar proton events (SPEs) in the past forty years recorded. These events caused significant polar middle atmospheric changes that were observed by a number of satellites. The highly energetic protons produced ionizations, excitations, dissociations, and dissociative ionizations of the background constituents in the polar cap regions (>60 degrees geomagnetic latitude), which led to the production of HOx (H, OH, HO2) and NOy (N, NO, NO2, NO3, N2O5, HNO3, HO2NO2, BrONO2, ClONO2). The HOx increases led to short-lived ozone decreases in the polar mesosphere and upper stratosphere due to the short lifetimes of the HOx constituents. Polar middle mesospheric ozone decreases greater than 50% were observed and computed to last for hours to days due to the enhanced HOx. The NOy increases led to long-lived polar stratospheric ozone changes because of the long lifetime of the NOy family in this region. Upper stratospheric ozone decreases of >10% were computed to last for several months past the solar events in the winter polar regions because of the enhanced NOy.

Observation of NOx enhancement and ozone depletion in the Northern and Southern Hemispheres after the October–November 2003 solar proton events

The large solar storms in October–November 2003 produced enormous solar proton events (SPEs) where high energetic particles reached the Earth and penetrated into the middle atmosphere in the polar regions. At this time, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) was observing the atmosphere in the 6–68 km altitude range. MIPAS observations of NOx (NO + NO2) and O3 of the period from 25 October to 14 November 2003 are the first global measurements of NOx species, covering both the summer (daylight) and winter (dark) polar regions during an SPE. Very large values of NOx in the upper stratosphere of 180 ppbv (parts per billion by volume) have been measured, and a large asymmetry in northern and southern polar cap NOx enhancements was found. Arctic mean polar cap (>60°) NOx enhancements of 20 to 70 ppbv between 40 to 60 km lasted for at least 2 weeks, while the Antarctic mean NOx enhancement was between 10 and 35 ppbv and was halved after 2 weeks. Ozone shows depletion signatures associated with both HOx (H + OH + HO2) and NOx enhancements but at different timescales. Arctic lower mesospheric (upper stratospheric) ozone is reduced by 50–70% (30–40%) for about 2 weeks after the SPEs. A smaller ozone depletion signal was observed in the Antarctic atmosphere. After the locally produced Arctic middle and upper stratospheric as well as mesospheric NOx enhancement, large amounts of NOx were observed until the end of December. These are explained by downward transport processes. These enhancements drastically declined with the mid‐December stratospheric warming. Significant O3 depletion was observed inside the polar vortex in a wide altitude range during this period. From mid‐January until the end of March 2004, MIPAS observed extraordinary high values of NO2 in the upper stratosphere of the northern polar region (mean in‐vortex values up to 350 ppbv at ∼54 km), which seem to be caused by the unusually strong vortex and downward transport at that time together with an uncommonly large auroral activity starting with the solar storms in October–November and continuing over the winter. In‐vortex ozone was observed to significantly decline in the mid‐February to late March period above the 1750 K potential temperature level.