Winter Pole Research Articles

The ENSO Signal in the Stratosphere

Although the El Niño-Southern Oscillation (ENSO) is a tropospheric phenomenon, its effects are also observed in the stratosphere. Traditionally, the study of ENSO above the troposphere has been difficult because of the lack of global observations at high altitudes and also because of the presence of other sources of variability whose signals are difficult to disentangle from ENSO effects. Recent work with general circulation models that isolate the ENSO signal have demonstrated its upward propagation into the stratosphere. Here we review the literature in this field and show results from the most recent version of the Whole Atmosphere Community Climate Model to illustrate the propagation and the mechanisms whereby the signal manifests itself in the stratosphere. The ENSO signal propagates upward to about 40 km by means of large-scale Rossby waves. The propagation is strongly influenced by the zonal mean zonal winds. Most of the strong ENSO events tend to peak in the boreal winter and so the ENSO signal is observed mainly at high latitudes during the Northern Hemisphere winter where the winds are westerly and allow Rossby wave propagation. The ENSO signal is also identified at polar latitudes in the Northern Hemisphere winter in the form of warmer temperatures and weaker winds during a strong El Niño event. This signal shows a zonally homogeneous behavior from the intensification of the stratospheric meridional circulation (in which air rises in the tropics and moves toward the winter pole where it descends) forced by anomalous propagation and dissipation of Rossby waves at middle latitudes during strong ENSO events.

Digging into the surface of the icy dwarf planet Eris

We describe optical spectroscopic observations of the icy dwarf planet Eris with the 6.5-m MMT telescope and the Red Channel Spectrograph. We report a correlation, that is at the edge of statistical significance, between blue shift and albedo at maximum absorption for five methane ice bands. We interpret the correlation as an increasing dilution of methane ice with another ice component, probably nitrogen, with increasing depth into the surface. We suggest a mechanism to explain the apparent increase in nitrogen with depth. Specifically, if we are seeing Eris 50 degrees from pole-on [Brown, M.E., Schaller, L., 2008. Science 316, 1585], the pole we are seeing now at aphelion was in winter darkness at perihelion. Near perihelion, sublimation could have built up atmospheric pressure on the sunlit (summer) hemisphere sufficient to drive winds toward the dark (winter) hemisphere, where the winds would condense. Because nitrogen is more volatile and scarcer than methane, it sublimated from the sunlit hemisphere relatively early in the season, so the early summer atmosphere was nitrogen rich, and so was the ice deposited on the winter pole. Later in the season, much of the nitrogen was exhausted from the summer pole, but there was plenty of methane, which continued to sublimate. At this point, the atmosphere was more depleted in nitrogen, as was the ice freezing out on top of the earlier deposited nitrogen rich ice. Our increasing nitrogen abundance with depth apparently contradicts the Licandro et al. [Licandro, J., Grundy, W.M., Pinilla-Alonso, N., Leisy, P., 2006. Astron. Astrophys. 458, L5–L8] result of a decreasing nitrogen abundance with depth. A comparison of observational, data reduction, and analysis techniques between the two works, suggests the difference between the two works is real. If so, we may be witnessing the signature of weather on Eris. The work reported here is intended to trigger further observational effort by the community.

Numerical simulations of the three‐dimensional distribution of meteoric dust in the mesosphere and upper stratosphere

Micrometeorites that ablate in the lower thermosphere and upper mesosphere are thought to recondense into nanometer‐sized smoke particles and then coagulate into larger dust particles. Previous studies with one‐dimensional models have determined that the meteoric dust size distribution is sensitive to the background vertical velocity and have speculated on the importance of the mesospheric meridional circulation to the dust spatial distribution. We conduct the first three‐dimensional simulations of meteoric dust using a general circulation model with sectional microphysics to study the distribution and characteristics of meteoric dust in the mesosphere and upper stratosphere. We find that the mesospheric meridional circulation causes a strong seasonal pattern in meteoric dust concentration in which the summer pole is depleted and the winter pole is enhanced. This summer pole depletion of dust particles results in fewer dust condensation nuclei (CN) than has traditionally been assumed in numerical simulations of polar mesospheric clouds (PMCs). However, the total number of dust particles present is still sufficient to account for PMCs if smaller particles can nucleate to form ice than is conventionally assumed. During winter, dust is quickly transported down to the stratosphere in the polar vortex where it may participate in the nucleation of sulfate aerosols, the formation of the polar CN layer, and the formation of polar stratospheric clouds (PSCs). These predictions of the seasonal variation and resulting large gradients in dust concentration should assist the planning of future campaigns to measure meteoric dust.

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.

Global and temporal distribution of meteoric smoke: A two‐dimensional simulation study

Meteoric material entering Earth's atmosphere ablates in the mesosphere and is then expected to recondense into tiny so‐called “smoke particles.” These particles are thought to be of great importance for middle atmosphere phenomena like noctilucent clouds, polar mesospheric summer echoes, metal layers, and heterogeneous chemistry. Commonly used one‐dimensional (1‐D) meteoric smoke profiles refer to average global conditions and yield of the order of a thousand nanometer sized particles per cubic centimeter at the mesopause, independent of latitude and time of year. Using the first two‐dimensional model of both coagulation and transport of meteoric material we here show that such profiles are too simplistic, and that the distribution of smoke particles indeed is dependent on both latitude and season. The reason is that the atmospheric circulation, which cannot be properly handled by 1‐D models, efficiently transports the particles to the winter hemisphere and down into the polar vortex. Using the assumptions commonly used in 1‐D studies results in number densities of nanometer sized particles of around 4000 cm−3 at the winter pole, while very few particles remain at the Arctic summer mesopause. If smoke particles are the only nucleation kernel for ice in the mesosphere this would imply that there could only be of the order of 100 or less ice particles cm−3 at the Arctic summer mesopause. This is much less than the ice number densities expected for the formation of ice phenomena (noctilucent clouds and polar mesospheric summer echoes) that commonly occur in this region. However, we find that especially the uncertainty of the amount of material that is deposited in Earth's atmosphere imposes a large error bar on this number, which may allow for number densities up to 1000 cm−3 near the polar summer mesopause. This efficient transport of meteoric material to the winter hemisphere and down into the polar vortex results in higher concentrations of meteoric material in the Arctic winter stratosphere than previously thought. This is of potential importance for the formation of the so‐called stratospheric condensation nuclei layer and for stratospheric nucleation processes.

Sensitivity of the boreal winter circulation in the middle atmosphere to the quasi‐biennial oscillation in MAECHAM5 simulations

The polar vortex in the Northern Hemisphere exhibits high intraseasonal and interannual variability which, to some degree, may be controlled by the quasi‐biennial oscillation (QBO) in the tropical stratosphere. Here we analyze the QBO signal in the Northern Hemisphere polar vortex in a model simulation using the general circulation model MAECHAM5 (Middle Atmosphere European Center Hamburg Model), which simulates the QBO as an internal mode of variability. Composites have been computed for the westerly and easterly QBO phases from early winter to late winter (November–December, December–January and January–February) of a 50‐year experiment containing 20 complete QBO cycles. This method identifies tropical and midlatitude patterns in wind and temperature that are related to the secondary meridional circulation of the QBO extending towards the winter pole. In the tropics, significant QBO signature is observed in the mesosphere up to 0.05 hPa only in early winter and mainly in the easterly QBO phase forced by the parameterized gravity waves‐mean flow interaction. At high latitudes, MAECHAM5 shows a significantly warmer (colder) polar stratosphere in the easterly (westerly) QBO phase at 30 hPa accompanied by a weaker (stronger) polar vortex in the late winter months, from December to February (December to January) in the easterly (westerly) phase of the QBO. In early winter there is no significant change in temperature and zonal mean zonal wind in the polar stratosphere. The analysis of EP fluxes shows a different behavior between QBO phases, with changes in the waveguide. Wave propagation occurs upwards and polewards in the easterly QBO phase at 30 hPa, while waves are refracted equatorward in the westerly phase. No relationship has been found between the tropical QBO and the final warming at the end of the boreal winter.

Cassini CIRS update on stratospheric ices at Titan's winter pole

A strong, broad spectral emission feature at 85° N latitude centered at 221 cm −1 remains unidentified after candidate ices of H 2O and pure crystalline CH 3CH 2CN are unambiguously ruled out. A much shallower weak emission feature starts at 160 cm −1 and blends into the strong feature at ∼190 cm −1. This feature is consistent with one formed by an HCN ice cloud composed of ⩽5 μm radius particles that resides in the lower stratosphere somewhere below an altitude of 160 km. Titan's stratospheric aerosol appears to have a spectral emission feature at about 148 cm −1. The aerosol abundance at 85° N is about a factor 2.2 greater than at 55° S.

Influence of the Solar Cycle on the General Circulation of the Stratosphere and Upper Troposphere

The record of dynamical structure reveals a systematic variation that operates coherently with the 11-yr variation of UV irradiance. Involving periods shorter than 5 years, the systematic variation reflects the influence of the QBO on the polar-night vortex. It has the same basic structure as interannual changes associated with the residual mean circulation of the stratosphere. A signature of the solar cycle also appears in the direct correlation to solar flux, as recovered through regression of the entire monthly record. That signature, however, is sharply enhanced around solstice, when the residual circulation is active, and during extremal phases of the QBO. In the tropics, the solar signature follows, throughout the year, from a decadal modulation in the frequency of the QBO. The modulation is manifested to either side of the QBO’s mean frequency, in two spectral peaks where the QBO dwells: one at (24 months)−1, reflecting a Biennial Oscillation (BO), and another at (36 months)−1. Intrinsic to the QBO, those peaks are separated from its mean frequency by ∼11 years−1. Through the QBO’s residual circulation, the decadal modulation introduces anomalies in the subtropics, with symmetry about the equator. Accompanying anomalous temperature in the subtropics is a stronger signature over the winter pole. Discovered by Labitzke and van Loon (1988), that solar signature reflects anomalous downwelling of the Brewer-Dobson circulation. It is shown to follow through the BO, which is intrinsic to the QBO and its modulation of the polar-night vortex.

Middle atmosphere polar warmings on Mars: Simulations and study on the validation with sub-millimeter observations

Temperature inversions and the warmings over the winter poles in the Martian atmosphere occur due to the adiabatic heating associated with the downward branch of the Hadley circulation. We present results of simulations with a recently developed GCM which suggest that the warmings are the manifestations of the global meridional transport, and are strongly related to atmospheric eddies (planetary waves and tides). To date, sets of data required for the validation of the predicted warmings and the corresponding circulation patterns do not extend far enough into the middle atmosphere of Mars. This motivated our radiative and retrieval simulations to demonstrate that sub-millimeter observations can provide the required fields to validate and constrain the GCM results.

Three-dimensional on-line chemical modeling in a Mars general circulation model

In this paper we present the design and some results of an on-line chemical model of the atmosphere of Mars using a three-dimensional dynamical–chemical model. A chemistry module has been added to the 3D Mars Global Multiscale Model (GM3), a general circulation model based on the Canadian weather forecast model. As water is a major fuel for chemistry on Mars, we have taken care to simulate the atmospheric water vapor content, taking into account evaporation from and condensation to the polar caps and in the atmosphere we allow for freezing and formation of a bulk cloud and sedimentation. The chemical module includes the main neutral carbon–oxygen–hydrogen–nitrogen species and associated relevant reactions. The outputs of the model are analyzed in the light of our current understanding of the chemistry on Mars. The results are in general accordance with 1D models and with two other 3D models with more limited chemistry. The model does not require elevated diffusion coefficients to fit observational data in the thermosphere as was suggested in some 1D studies. The large scale transport and the downward thermospheric transport in the winter pole appear to have a significant role in the concentrations of some trace species.

Vertical profiles of HCN, HC3N, and C2H2 in Titan's atmosphere derived from Cassini/CIRS data

Mid-infrared limb spectra in the range 600–1400 cm−1 taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, HC3N, C2H2, and temperature in Titan's atmosphere. Both high (0.5 cm−1) and low (13.5 cm−1) spectral resolution data were used. The 0.5 cm−1 data gave profiles at four latitudes and the 13.5 cm−1 data gave almost complete latitudinal coverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesosphere were observed at Titan's northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell. On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiation and increased IR emission from volatile enriched air. HC3N had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole. A boundary observed at 60° N between enriched and un-enriched air is consistent with a confining polar vortex at 60° N and HC3N's short lifetime. In the far north, layers were observed in the HC3N profile that were reminiscent of haze layers observed by Cassini's imaging cameras. HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from 13.5 cm−1 data indicated a HCN enriched layer at 200–250 km, extending into the southern hemisphere. This could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for HC3N due to HCN's longer photochemical lifetime. C2H2 appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistent with observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not C2H2 towards the north pole.

GCM simulation of balloon trajectories on Titan

Possible trajectories of passive balloons in Titan's troposphere are simulated with the instantaneous wind field predicted by a GCM (general circulation model). In most areas the basic motion of a balloon is a predominantly eastward or westward drift, depending on altitude, latitude and season of the balloon release point. Some meridional oscillation is always superposed on this basic motion, resulting in a wavy trajectory, with a maximum extent (of 40 ∘ ) at high latitudes of the winter hemisphere. As a general rule, the meridional oscillation can be maximised if a balloon is deployed in altitudes and latitudes where the mean zonal wind is eastward and smaller than the phase speed of Saturn's gravitational tide on Titan. A balloon's groundtrack does not repeat as it makes successive circuits around Titan, but rather makes a spiral or braided pattern. The summer pole is rather difficult to access for a balloon not directly introduced there because of small meridional oscillation, while the winter pole can be readily accessed and left several times. A preferred zonal drift direction can be achieved by choosing a proper altitude and hemisphere, but choosing a preferred meridional direction is not possible.

Relationship of the quasi‐biennial oscillation to the stratospheric signature of the solar cycle

Records of analyzed dynamical structure indicate a correlation with the 11‐year variation of solar flux Fs. In the monthly record, anomalous temperature maximizes in the stratosphere, with symmetry about the equator. However, even there, the signature is weak and scarcely significant. Sampling the same record around solstice and during extremal phases of the QBO sharply enhances the strength, significance, and symmetry of the solar signature. In the tropics the solar signature follows from a decadal modulation in the frequency of the quasi‐biennial oscillation (QBO). In instantaneous power spectra the modulation is manifested to either side of the QBO's mean frequency, in two spectral peaks where the QBO dwells: one at (24 months)−1, reflecting a biennial oscillation (BO), and another at (36 months)−1. Intrinsic to the QBO, those peaks are separated from its mean frequency by ∼11 yr−1. The decadal modulation produces a strong correlation to Fs and, through the residual circulation of the QBO, symmetry about the equator. Augmenting this high‐frequency contribution is a low‐frequency contribution to the solar signature, one that simply tracks the 11‐year variation of Fs. Characterized by broad symmetry about the equator, it reflects the involvement of the Brewer‐Dobson circulation. Accompanying anomalous temperature in the subtropics is a stronger signature over the winter pole, where interannual variance is large. Discovered by Labitzke and van Loon (1988), that solar signature reflects anomalous downwelling of the Brewer‐Dobson circulation. As strong as the RMS interannual variation, it is accounted for entirely by the high‐frequency contribution associated with the QBO. The extratropical signature is shown to follow through the BO, which is transmitted poleward by planetary waves. Salient features of the observed behavior are reproduced by a simple model of the QBO's modulation and its interaction with the extratropical circulation.

Prediction of the structure of the martian upper atmosphere for the Mars Reconnaissance Orbiter (MRO) mission

Background: The Mars Reconnaissance Orbiter (MRO) spacecraft was launched toward Mars from the Kennedy Space Center in Florida on August 12, 2005, and arrived at Mars on March 10, 2006. Aerobraking in the Martian lower thermosphere (~100-200 km) is planned for 6-months after arrival, enabling the desired mapping orbit to be achieved. Knowledge of the Mars upper atmosphere structure is important for ensuring a safe and successful aerobraking process. Method: Previous Mars Global Surveyor (MGS) density data, acquired during Phase 2 (MGS2) aerobraking operations, is used to validate new coupled MGCM-MTGCM (General Circulation Model) simulations. Confidence gained from this analysis is used to make new predictions for anticipated MRO densities during its aerobraking phase. Specific Mars conditions approaching the aphelion season are prescribed in the MGCM-MTGCM inputs. 110) 30 ~ ( − S L Conclusion: Predicted densities for MRO are simulated to decrease toward the southern winter pole, in accord with the general trend observed during MGS2. However, it is clear that MRO predicted densities at 120 km are smaller than observed MGS2 densities (especially at similar latitudes near ). The distinct solar local time sampling planned for MRO, during a different interval of the solar cycle, may contribute to these differences. The new MRO MGCM-MTGCM predictions described in this paper are being used to support MRO aerobraking activities. 85 ~ S L

Interaction between the Brewer–Dobson Circulation and the Hadley Circulation

Abstract Interannual changes of the stratospheric circulation are studied in relation to coherent changes of the tropospheric circulation. Emerging over the winter pole is a clear signature of adiabatic warming and anomalous downwelling. Reflecting an intensification of the Brewer–Dobson circulation, the signature of anomalous downwelling extends from stratospheric levels into the troposphere. Compensating for it at subpolar latitudes is a signature of adiabatic cooling and anomalous upwelling. Equally coherent, the signature of anomalous upwelling occupies the same levels as the signature of anomalous downwelling. Inside the tropical troposphere, anomalous cooling is replaced by anomalous warming. It reflects an intensification of organized convection and the Hadley circulation, one that accompanies the intensification of the Brewer–Dobson circulation. These signatures of anomalous vertical motion represent changes that operate coherently in the stratosphere and troposphere. They share major features with the Arctic Oscillation. Extending across the tropopause, they couple the stratosphere and troposphere through a transfer of mass. By modifying vertical motion inside the Tropics, anomalous upwelling influences organized convection. Support for this interpretation comes from anomalous divergence in the tropical upper troposphere; it is shown to vary coherently with anomalous downwelling in the Arctic stratosphere. Exhibiting analogous behavior are changes of the tropical tropopause. Coupled to stratospheric changes, these variations of the tropical circulation act to organize convection about the equator, favoring a split ITCZ. They reflect as much as 40% of the interannual variance of tropical divergence, representing an important complement to ENSO. Much of the covariance between the polar stratosphere and the tropical troposphere is concentrated at periods shorter than 5 yr. Included is variability that is associated with the quasi-biennial oscillation (QBO) in the tropical stratosphere. Also included is biennial variability, which accompanies the QBO in the polar stratosphere. These stratospheric variations involve the same time scales as biennial variability in the tropical troposphere, which likewise influences convection.

CO as an important high‐altitude tracer of dynamics in the polar stratosphere and mesosphere

We present new ground‐based measurements of polar stratospheric and mesospheric CO, made with a millimeter‐wave spectrometer at Thule, Greenland (76.5°N, 68.7°W). Almost daily measurements were made between 17 January and 4 March 2002 and again between 5 January and 22 February 2003. We stress here the retrieval and analysis of CO mixing ratios in the 50–80 km altitude range, though it can be monitored at lower altitudes as well. Since CO exhibits a strong positive latitude gradient from the summer to the winter pole, it is an excellent tracer for poleward transport from lower latitudes. Moreover, the mixing ratio of CO increases rapidly from ∼40 km to at least 100 km at midlatitudes, providing a good tracer for high‐altitude vertical transport as well. Our profiles indicate that in winter near the poles the CO mixing ratio decreases above ∼70 km because of subsidence of air and minimal high‐altitude photoproduction at high latitudes. Our data also show large variations in mixing ratio and vertical distribution, yielding a good picture of stratospheric and mesospheric dynamics‐induced changes on a scale of hours to days. These observations verify that CO serves as an excellent tracer of vortex‐related dynamics in the 30–80 km altitude range, where other information, particularly above ∼40 km, may be sparse, unreliable, or nonexistent. Our results are in general agreement with analyses of 1991–1992 Improved Stratospheric and Mesospheric Sounder (ISAMS) satellite data by Lopez‐Valverde et al. [1993, 1996] and by Allen et al. [1999, 2000]. We show the contrast between CO over the summer pole and CO over the winter pole with the aid of trial observations made at the South Pole during the austral summer of 1999–2000. Our Thule data indicate that large concentrations of CO generally exist in winter just outside the vortex boundary. The large rapid variations in vertical profile that are found in our data in 2002 appear to be well correlated with vortex position in the lower stratosphere. In 2003 this correlation appeared to be much weaker, but early 2003 was also a period of vortex splitting in the Arctic on three occasions during our observation period, accompanied by generally more complex vortex dynamics.

Titan’s Atmosphere at High Resolution

Fig. 1.—Example images of some of the phenomena discovered and observed during this work. (a) Accumulation of ethylene in Titan’s south polar stratosphere is apparent in this 10.7 mm image, which is sensitive to thermal emission from haze and ethylene (C2H4) in Titan’s stratosphere. (b) Titan’s troposphere and surface are obscured from view because of strong methane absorption in this mm filter image. The global stratospheric haze and the bright south polar 1.702 0.008 collar of haze are seen in reflected sunlight. (c) and (e) In sections of the K filter (1.95–2.30 mm), Titan’s atmosphere is relatively transparent and numerous surface features are seen in reflected sunlight. (d) and ( f ) In a 2.108–2.140 mm filter, the surface is blocked from view because of moderate methane absorption. This filter probes down into the troposphere, and cloud activity near the south pole is apparent. The resolution in (b)–( f ) is ∼300 km. Titan’s apparent diameter is 0 .83 in (a) and is 0 .87 in (b)–( f ). Titan stands out in our solar system as the sole moon with a substantial atmosphere. Until recently, the only spatially resolved observations of Titan were from the Voyager spacecraft flybys in 1980 and 1981, when the season was early northern spring on Titan. The advent of large ground-based telescopes and adaptive optics allows us to study Titan and its atmosphere with spatial resolutions of a few hundred kilometers. We report nearand mid-infrared observations of Titan’s atmosphere with the Keck and Gemini telescopes in late southern spring on Titan. These observations represent the start of a long-term ground-based monitoring campaign that will extend over a significant fraction of Titan’s 30 Earth-year seasonal cycle, and in doing so will lead us to a better understanding of the seasonal variations. Observing at mid-infrared wavelengths (8–13 mm) with the Long-Wavelength Spectrometer (LWS) on the Keck I Telescope, we found a large buildup of ethylene (C2H4) in the south polar stratosphere (see Fig. 1a, which is from H. G. Roe, I. de Pater, & C. P. McKay 2003, Icarus, submitted). We link this buildup to chemistry that occurs in the shadow of the winter pole. We also developed a new line-by-line radiative-transfer model for Titan at 8–13 mm. This model can easily be extended to longer wavelengths and will in the future be useful for several Titan-related projects, including long-term monitoring of south polar ethylene. At near-infrared wavelengths (1–2.5 mm) using adaptive optics on the Keck II Telescope, we found a collar of haze near the tropopause at 70 –75 south latitude (see Fig. 1b, which is from H. G. Roe et al. 2002, Icarus, 157, 254). This collar is most likely the remnant of stratospheric nitrile chemistry during the long polar winter night. We report a spatially resolved detection of clouds on Titan (see Figs. 1c–1f, which are from H. G. Roe et al. 2002, ApJ, 581, 1399). These observations were made using adaptive optics on the Gemini North and Keck II telescopes. We found the discrete cloud features to vary on timescales of hours to days. All clouds were near the south pole, with the most northern cloud appearing at 61 south. We find each cloud covers an area of Titan’s surface of to km. From a limited 4 5 2 # 10 3 # 10 temporal sample, these clouds appear to be common in the south polar region in late southern spring. We discuss several possible seasonal triggering mechanisms for these clouds. Finally, in appendices we present a new telluric transmission model and discuss the implications of differential atmospheric refraction for adaptive-optics observations (H. G. Roe 2002, PASP, 114, 450).

Response of the mesosphere to human‐induced perturbations and solar variability calculated by a 2‐D model

We have used the improved NCAR interactive 2‐D model (SOCRATES) to investigate the chemical and thermal response of the mesosphere to composition changes from the preindustrial era (∼1850) to the present, to doubling the CO2 concentration, and to the 11‐year solar flux variability. The calculations show that all regions in the model mesosphere have cooled relative to the preindustrial times. The mesopause region has cooled by ∼5 K and the winter pole by up to 9 K near 60 km. Ozone mixing ratio has decreased by about 5% in the lower mesosphere and by about 30% near the summer mesopause region (caused by a dramatic increase in [OH]). Doubling the CO2 abundance cools the whole mesosphere by about 4–16 K and has a complicated effect on O3, which exhibits an alternating increase/decrease behavior from the lower mesosphere up to the mesopause region. Similar results are obtained, in both magnitude and structure, for the O3 response to a decrease in solar UV flux. Similarities are also found in the response of T, OH, and H to these two perturbations. These results lead to the conclusion that the long‐term increase in the well‐mixed greenhouse gases, in particular CO2, alters the thermal structure and chemical composition of the mesosphere significantly and that these anthropogenic effects are of the same magnitude as the effects associated with the 11‐year solar cycle. Thus, the difference in the timescales involved suggests that the anthropogenic signal over periods of typically 10 years is smaller than the signal generated by the 11‐year solar variability. Finally, analysis of the results from a simulation of the combined perturbations (2 × CO2 + 11‐year solar variability) shows that, for the most part, the solar variability does not interact with increasing CO2 and vice versa; that is, the two effects are additive.

A model study of the connection between polar and midlatitude ozone loss in the Northern Hemisphere lower stratosphere

A numerical model has been used to quantify halogen‐induced ozone loss at the winter pole and at middle latitudes. This loss is compared with that due to nonhalogen ozone‐destroying cycles. The three‐dimensional off‐line chemical transport model, SLIMCAT, was run at 3.75° latitude by 3.75° longitude, with United Kingdom Meteorological Office analyzed winds and temperatures. The contribution of polar processes to ozone loss at middle latitudes was investigated with novel tracers mapped to equivalent latitudes. Novel tracers were also included in SLIMCAT to follow ozone loss by reactions with ClOx, BrOx, NOx, and HOx, and to follow ozone production by oxygen photolysis. The interannual variability of the different processes was studied for five winters in the 1990s, which covered a variety of meteorological conditions in and around the polar vortex. Analysis of the ozone loss tracers shows large interannual variability in the relative strengths of particular chemical ozone loss mechanisms depending on the meteorology of the given year. In all cases, the ClO‐BrO cycle dominates at polar latitudes. The role of mixing between the pole and middle latitudes also varies with the meteorological conditions. In winters with a cold, stable vortex, like 1996/1997, there is little impact of polar processes on midlatitude loss. In contrast, the cold, disturbed vortex of 1999/2000 contributed significantly to the ozone loss in middle latitudes. In all winters, ozone loss from cycles involving halogens was an important contributor (40–70%) to the modeled midlatitude ozone loss.

Assimilation of Mars Global Surveyor atmospheric temperature data into a general circulation model

We examined the observed temperature data from Thermal Emission Spectrometer (TES) between heliocentric longitude Ls = 141° and 146° (∼10 Martian days in northern summer) during the mapping phase, then compared them with the simulated results using the NASA/Ames Mars general circulation model. Both show a strong polar vortex at the winter pole, higher equatorial temperatures near the ground and larger tropospheric lapse rates during daytime than at night. However, the simulation is colder than the observation at the bottom and top of the atmosphere and warmer in the middle. The highest wave activities are found in the polar front in both the simulations and the observations, but it is at a much higher altitude in the former. Experiments show that larger dust opacity improves the temperature field in the lower atmospheric levels. Using a steady state Kalman filter, we attempted to obtain a model state that is consistent with the observations. The assimilation did achieve better agreement with the observations overall, especially over the north pole. However, it is hard to make any further improvement. Dust opacity is the key factor in determining the temperature field; correcting temperature alone improves the spatial and temporal variations, it degrades the mean state in the south pole. Assimilation cannot improve the simulation further, unless more realistic dust opacity and its vertical profile are considered.