Northern Winter Season Research Articles

Vitamin D Status for Chinese Children and Adolescents in CNNHS 2016-2017.

Vitamin D is very important in maintaining children's bone health because of its regulatory role in calcium and phosphate metabolism. To better understand vitamin D status and related risk factors of children and adolescents in China, this study analyzed the 25-hydroxyvitamin-D (25(OH)D) concentration of children and adolescents aged 6-17 years in China and assessed the risk factors of vitamin D deficiency and insufficiency. We analyzed the data of 25(OH)D concentration collected from the China National Nutrition and Health Survey of Children and Lactating Mothers in 2016-2017 (CNNHS 2016-2017). The age, sex, region type, ethnicities, season, weight and height were recorded, measured by unified questionnaire, equipment and standards. The concentration of 25(OH)D was detected by LC-MS/MS. A total of 64,391 participants from the cross-sectional study of CNNHS in 2016-2017 were included in this study. The median serum 25(OH)D concentration was 17.70 (13.20-22.68) ng/mL, 18.70 (14.10-23.80) ng/mL in boys and 16.60 (12.40-21.40) ng/mL in girls. The total prevalence rate of vitamin D deficiency and insufficiency was 65.98% when the cut-off was 20 ng/mL, 60.42% for boys and 71.99% for girls. According to the results of logistic regression analysis, girls aged above 12 y, living in midlands and northern regions, in spring and winter seasons and with abdominal obesity will have a significantly increased risk of vitamin D deficiency and insufficiency among Chinese children and adolescents. The results showed that vitamin D deficiency and insufficiency are very common among children and adolescents aged 6-17 y in China. They should be encouraged to have more effective sunlight exposure, increased intake of vitamin D from food or supplements in their diet, especially for those aged above 12 y, living in the northern or midlands areas, in spring and winter, and abdominally obese.

Characterization of the Thermospheric Mean Winds and Circulation During Solstice Using ICON/MIGHTI Observations

AbstractUsing the horizontal neutral wind observations from the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument onboard NASA's Ionospheric Connection Explorer (ICON) spacecraft with continuous coverage, we determine the climatology of the mean zonal and meridional winds and the associated mean circulation at low‐to middle latitudes (10°S–40°N) for Northern Hemisphere summer solstice conditions between 90 and 200 km altitudes, specifically on 20 June 2020 solstice as well as for a one‐month period from 8 June–7 July 2020 and for Northern winter season from 16 December 2019–31 January 2020, which spans a 47‐day period, providing full local time coverage. The data are averaged within appropriate altitude, longitude, latitude, solar zenith angle, and local time bins to produce mean wind distributions. The geographical distributions and local time variations of the mean horizontal circulation are evaluated. The instantaneous horizontal winds exhibit a significant degree of spatiotemporal variability often exceeding ±150 m s−1. The daily averaged zonal mean winds demonstrate day‐to‐day variability. Eastward zonal winds and northward (winter‐to‐summer) meridional winds are prevalent in the lower thermosphere, which provides indirect observational evidence of the eastward momentum deposition by small‐scale gravity waves. The mean neutral winds and circulation exhibit smaller scale structures in the lower thermosphere (90–120 km), while they are more homogeneous in the upper thermosphere, indicating the increasingly dissipative nature of the thermosphere. The mean wind and circulation patterns inferred from ICON/MIGHTI measurements can be used to constrain and validate general circulation models, as well as input for numerical wave models.

Transverse aeolian ridges in the landing area of the Tianwen-1 Zhurong rover on Utopia Planitia, Mars

The enigmatic transverse aeolian ridges (TARs), with distinct morphology and albedo, are among the key geological features investigated by China's Tianwen-1 Zhurong rover on southern Utopia Planitia, Mars. Their morphologies and morphometrics are investigated through high-resolution imaging science experiment (HiRISE) orthoimage and Digital Terrain Model (DTM) products. A total of 5089 TARs are identified, with barchan TARs being predominant (97.6%). Morphometric analysis shows these TARs to be small and symmetrical aeolian landforms, with an average crest–ridge lengths of 33.9 ± 20.5 m, profile widths of 9.4 ± 3.8 m, profile heights of 0.4 ± 0.4 m, profile-height-width ratios of 0.04 ± 0.02, and profile symmetry ratios of −0.01 ± 0.13. In-situ observations from the Navigation and Terrain Camera (NaTeCam) show the crests of the TARs to be dark and sharp, while the flanks are interlaced by dark and bright materials. Close-up Multispectral Camera (MSCam) images reveal the TARs to be coated by granules of ∼1.5 mm in diameter. Given the morphometric characteristics and the presence of coating granules, the TARs in the landing area could be categorized as megaripples. Buffered crater counting (BCC) technique-derived absolute model age (AMA) reveals the formation time, or the last active period of the TARs, could be as recent as 1 Ma in the Late Amazonian. The morphometrics and direction of the horns of the barchan TARs suggest the winds for the formation of TARs blew mostly from the north. During the spring-summer transition period (Ls: 50°–93°), the Mars Climate Station (MCS) had recorded local bimodal winds in the landing area, with the speed of the northerly wind in the afternoon being a little stronger than the speed of the southerly wind in the morning. These observations are consistent with the wind fields described in the Mars Climate Database (MCD), which imply the northerly winds during the northern winter season to be responsible for the net sediment transport to the south. Two TARs observed in-situ with secondary NW-SE trending crest-ridges indicate that forked TARs might form given sufficient time (i.e., in the order of millions of years) under modern wind conditions, i.e., the TARs may be currently reworked, if only extremely weakly.

Seasonal Variation Characteristics of Geomagnetic Sq External and Internal Equivalent Current Systems in East‐Asia and Oceania Regions

AbstractGeomagnetic data from 20 observatories in East‐Asia and Oceania regions were used to investigate the seasonal variation characteristics of the external and internal Sq current systems by spherical harmonic analysis (SHA). The seasonal current models of D (northern winter) season, E (equinox) season, and J (northern summer) season have been constructed respectively, and the current parameters of these models were analyzed. The main results are obtained as follows: (1) The local time T of the northern vortex moves from local noon to prenoon from D to E to J season for both external and internal currents; meanwhile, in the southern hemisphere, the T moves from local noon to afternoon. (2) The vortex intensity J increases from local winter to equinox then to local summer. The interseasonal change rate of internal current is larger than that of external current. (3) The latitude Φ of the external current vortex exhibits seasonal variation in the southern hemisphere, while in the northern hemisphere the latitude Φ of the internal current vortex shows obvious seasonal changes. (4) The correlations between the internal and external currents are further analyzed. In the E season the correlation of T is the highest, and in the local summer and winter, the correlation of J is the strongest. The different seasonal behavior of the external and internal current systems is unexpected, and its mechanism remains to be identified in future study.

Disruptive influences of residual noise, network configuration and data gaps on InSAR-derived land motion rates using the SBAS technique

The interferometric synthetic aperture radar (InSAR) small baseline subset (SBAS) technique can be applied to land with varying deformation magnitudes ranging from mm/yr to tens of cm/yr. SBAS defines a network of interferograms that is limited by temporal and spatial baseline thresholds that are often applied arbitrarily, or in apparently subjective ways in the literature. We use simulated SAR data to assess (1) the influence of residual noise and SBAS network configuration on InSAR-derived deformation rates, and (2) how the number of interferograms and data gaps in the time series may further impact the estimated rates. This leads us to an approach for defining a SBAS network based on geodetic reliability theory represented by the redundancy number (r-number). Simulated InSAR datasets are generated with three subsidence signals of linear rates plus sinusoidal annual amplitudes of −2 mm/yr plus 2 mm, −20 mm/yr plus 5 mm and −100 mm/yr plus 10 mm, contaminated by Gaussian residual noise bounded within [−2;+2] mm, [−5;+5] mm and [−10;+10] mm, corresponding to standard deviations of approximately 0.5 mm, 1.5 mm and 3.0 mm, respectively. The influence of data gaps is investigated through simulations with percentages of missing data ranging from 5% to 50% that are selected (1) randomly across the 4-year time series, and (2) for three-month windows to represent the northern winter season where snow cover may cause decorrelation. These simulations show that small deformation rates are most adversely affected by residual noise. In some extreme cases, the recovered trends can be contrary to the signal (i.e., indicating uplift when there is simulated subsidence). We demonstrate through simulations that the r-number can be used to pre-determine the reliability of SBAS network design, indicating the r-values between ~0.8 and ~0.9 are optimal. r-numbers less than ~0.3 can deliver erroneous rates in the presence of noise commensurate with the magnitude of deformation. Finally, the influence of data gaps is not as significant compared to other factors such as a change in the number of interferograms used, although the blocks of “winter” gaps in the SBAS network show a larger effect on the rates than gaps at random intervals across the simulated time series.

Stratospheric Influences on the MJO-Induced Rossby Wave Train: Effects on Intraseasonal Climate

AbstractThe tropical Madden–Julian oscillation (MJO) excites a northward propagating Rossby wave train that largely determines the extratropical surface weather consequences of the MJO. Previous work has demonstrated a significant influence of the tropospheric El Niño–Southern Oscillation (ENSO) on the characteristics of this wave train. Here, composite analyses of ERA-Interim sea level pressure (SLP) and surface air temperature (SAT) data during the extended northern winter season are performed to investigate the additional role of stratospheric forcings [the quasi-biennial oscillation (QBO) and the 11-yr solar cycle] in modifying the wave train and its consequences. MJO phase composites of 20–100-day filtered data for the two QBO phases show that, similar to the cool phase of ENSO, the easterly phase of the QBO (QBOE) produces a stronger wave train and associated modulation of SLP and SAT anomalies. In particular, during MJO phases 5–7, positive SLP and negative SAT anomalies in the North Atlantic/Eurasian sector are enhanced during QBOE relative to the westerly phase of the QBO (QBOW). The opposite occurs during the earliest MJO phases. SAT anomalies over eastern North America are also more strongly modulated during QBOE. Although less certain because of the short data record, there is some evidence that the minimum phase of the solar cycle (SMIN) produces a similar increased modulation of SLP and SAT anomalies. The strongest modulations of SLP and SAT anomalies are produced when two or more of the forcings are superposed (e.g., QBOE/cool ENSO, SMIN/QBOE, etc.).

A model intercomparison of Titan's climate and low-latitude environment

Cassini-Huygens provided a wealth of data with which to constrain numerical models of Titan. Such models have been employed over the last decade to investigate various aspects of Titan's atmosphere and climate, and several three-dimensional general circulation models (GCMs) now exist that simulate Titan with a high degree of fidelity. However, substantial uncertainties persist, and at the same time no dedicated intercomparisons have assessed the degree to which these models agree with each other or the observations. To address this gap, and motivated by the proposed Dragonfly Titan lander mission, we directly compare three Titan GCMs to each other and to in situ observations, and also provide multi-model expectations for the low-latitude environment during the early northern winter season. Globally, the models qualitatively agree in their representation of the atmospheric structure and circulation, though one model severely underestimates meridional temperature gradients and zonal winds. We find that, at low latitudes, simulated and observed atmospheric temperatures closely agree in all cases, while the measured winds above the boundary layer are only quantitatively matched by one model. Nevertheless, the models simulate similar near-surface winds, and all indicate these are weak. Likewise, temperatures and methane content at low latitudes are similar between models, with some differences that are largely attributable to modeling assumptions. All models predict environments that closely resemble that encountered by the Huygens probe, including little or no precipitation at low latitudes during northern winter. The most significant differences concern the methane cycle, though the models are least comparable in this area and substantial uncertainties remain. We suggest that, while the overall low-latitude environment on Titan at this season is now fairly well constrained, future in situ measurements and monitoring will transform our understanding of regional and temporal variability, atmosphere-surface coupling, Titan's methane cycle, and modeling thereof.

A Comparison of North American Surface Temperature and Temperature Extreme Anomalies in Association with Various Atmospheric Teleconnection Patterns

The atmospheric teleconnection pattern reflects large-scale variations in the atmospheric wave and jet stream, and has pronounced impacts on climate mean and extremes over various regions. This study compares those patterns that have significant circulation anomalies over the North Pacific–North American–North Atlantic sector, which directly influence surface temperature and temperature extremes over North America. We analyze the pattern associated anomalies of surface temperature and warm and cold extremes over North America, during the northern winter and summer seasons. In particular, we assess the robustness of the regional temperature and temperature extreme anomaly patterns by evaluating the field significance of these anomalies over North America, and quantify the percentages of North American temperature and temperature extreme variances explained by these patterns. The surface temperature anomalies in association with the Pacific–North American pattern (PNA), Tropical–Northern Hemisphere pattern (TNH), North Pacific pattern (NP), North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Western Pacific pattern (WP), circumglobal teleconnection (CGT), and Asian–Bering–North American (ABNA) patterns are similar to those reported in previous studies based on various datasets, indicating the robustness of the results. During winter, the temperature anomaly patterns considered are field significant at the 5% level over North America, except the WP-related one. These pattern associated anomalies explained about 5–15% of the total interannual temperature variance over North America, with relatively high percentages for the ABNA and PNA patterns, and low for the WP pattern. The pattern associated warm and cold extreme anomalies resemble the corresponding surface mean temperature anomaly patterns, with differences mainly in magnitude of the anomalies. Most of the anomalous extreme patterns are field significant at the 5% level, except the WP-related patterns. These extreme anomalies explain about 5–20% of the total interannual variance over North America. During summer, the pattern-related circulation and surface temperature anomalies are weaker than those in winter. Nevertheless, all of the pattern associated temperature anomalies are of field significance at the 5% level over North America, except the PNA-related one, and explain about 5–10% of the interannual variance. In addition, the temperature extreme anomalies, in association with the circulation patterns, are comparable in summer and winter. Over North America, the NP-, WP-, ABNA-, and CGT-associated anomalies of warm extremes are field significant at the 5% level and explain about 5–15% of the interannual variance. Most of the pattern associated cold extreme anomalies are field significant at the 5% level, except the PNA and NAO related anomalies, and also explain about 5–15% of the interannual variance over North America.

Numerical simulation of the mean meridional circulation in the middle atmosphere at different phases of stratospheric warmings and mountain wave scenarios

In this study, numerical simulations have been performed to estimate the transformation of the mean meridional circulation in altitude range 0–100 km at different phases of simulated stratospheric warming (SW) events in January–February including and excluding impact of mesoscale orographic gravity waves (OGWs). To obtain an ensemble of 12 pairs of model runs with and without a parameterization of OGW effects, the numerical middle and upper atmosphere model (MUAM) has been used. Obtained results demonstrate weakening of the zonal mean meridional circulation at altitudes up to 100 km during and after simulated SWs compared to the time interval before SWs. At altitudes below 50 km, southward mean meridional winds decrease (up to 15%) before and after simulated SWs. OGW effects may increase the mean northward wind at altitudes above 60 km up to 10–15%. The most significant changes of the meridional circulation in the middle atmosphere are detected at the middle and high latitudes of the Northern Hemisphere: the southward meridional circulation increases at altitudes above 40 km and decreases below 40 km. Thus, the global-scale mean meridional circulation in the middle atmosphere may significantly depend on different phases of SW events during the northern winter season. It is also quite sensitive to the dynamical and thermal OGW impacts.

(Online First)A study of temperature profiles and trends as revealed by COSMIC RO technique and balloon –borne radiosonde instrument

This research presents atmospheric temperature profiles and trends retrieved using COSMIC RO techniqueand balloon-borne radiosonde instrument in 2007 and a few cases during 2017. By effectively using ‘wet’ temperatureproduct available at COSMIC Data Analysis and Archive Center (CDAAC) website, an analysis has been made topresent temperature profiles and trends at various regions including, Indian, Taiwan and Japan. A one-to-onecorrespondence is, clearly, seen between temperature profiles retrieved with COSMIC RO and radiosonde instrument. But, few and dominant differences in temperature profiles are found below at an altitude of ~5 km and above aroundtropopause (~16-17 km). The dominant differences found at below ~5km could be due to the inhomogeneousdistribution of humidity present, generally, at the tropical regions, whereas above the tropopause altitudes, differencesmight be due to the ionospheric residual correction as reported by other researchers. Further, temperature monthlytrends at various regions show distinct characteristics including, a sharp temperature inversion up to tropopause altitude. In addition, it is also observed maximum temperatures (peaks) during the northern summer seasons (May, June, July, and August) and minimum temperatures (troughs) during the northern winter seasons (November, December, January, and February) near to the surface of the Earth. Interestingly, although it is generally observed that the tropopausealtitude is located at ~ 16-17 km at various regions, a keen observation reveals that distinct seasonal and latitudinalvariations can be witnessed. With this case study, it may be concluded that the COSMIC RO technique is able toprovide very accurate measurement, which reiterates its importance as a powerful tool to explore the Earth’s atmosphereon the local and global scale.

Anomalous wind circulation over Taipei, Taiwan during the northern winter seasons of 2004 and 2005- A case study

This research reports, for the first time, an anomalous wind circulation over Taipei (Latitude 25.030N, Longitude 121.510E), Taiwan during the northern hemisphere winter season (December, January, and February) of years 2004 and 2005. The anomalous wind circulation of meridional winds, which showed southward directions during the winter seasons of 2004 and 2005 instead of northward winds, is noticed from one kilometer altitude range (lower troposphere) and that trend continued till around 20 km altitude range (lower stratosphere). To ascertain whether such a disturbed nature of wind pattern existed over nearby locations to Taipei, we have analyzed radiosonde-measured meridional and zonal winds over four nearby stations station to Taipei including, Roig, Xiamen, Minami and Fuzhou. Surprisingly, no anomalous wind behavior is seen except over Taipei during the northern winter seasons of 2004 and 2005. On the other hand, the European Centre for Medium-Range Weather Forecasts (ECMWF) model-predicted winds do not show any anomalous wind patterns over Taipei and other nearby stations, possibly due to the large averaging of internal variabilities of reanalysis databases. The plausible physical mechanisms of these disturbed meridional wind patterns are not understood at this juncture, but it is believed that local winds and atmospheric pollutants might have created an amicable environment as to provide such a disturbed meridional wind pattern over Taipei, Taiwan in the winter season of 2004 and 2005.

Limb clouds and dust on Mars from images obtained by the Visual Monitoring Camera (VMC) onboard Mars Express

The Visual Monitoring Camera (VMC) onboard the Mars Express (MEx) spacecraft is a simple camera aimed to monitor the release of the Beagle-2 lander on Mars Express and later used for public outreach. Here, we employ VMC as a scientific instrument to study and characterize high altitude aerosols events (dust and condensates) observed at the Martian limb. More than 21,000 images taken between 2007 and 2016 have been examined to detect and characterize elevated layers of dust in the limb, dust storms and clouds. We report a total of 18 events for which we give their main properties (areographic location, maximum altitude, limb projected size, Martian solar longitude and local time of occurrence). The top altitudes of these phenomena ranged from 40 to 85 km and their horizontal extent at the limb ranged from 120 to 2000 km. They mostly occurred at Equatorial and Tropical latitudes (between ∼30°N and 30°S) at morning and afternoon local times in the southern fall and northern winter seasons. None of them are related to the orographic clouds that typically form around volcanoes. Three of these events have been studied in detail using simultaneous images taken by the MARCI instrument onboard Mars Reconnaissance Orbiter (MRO) and studying the properties of the atmosphere using the predictions from the Mars Climate Database (MCD) General Circulation Model. This has allowed us to determine the three-dimensional structure and nature of these events, with one of them being a regional dust storm and the two others water ice clouds. Analyses based on MCD and/or MARCI images for the other cases studied indicate that the rest of the events correspond most probably to water ice clouds.

Changes in the normal mode energetics of the general atmospheric circulation in a warmer climate

Changes in the normal mode energetics of the general atmospheric circulation are assessed for the northern winter season (DJF) in a warmer climate, using the outputs of four climate models from the Coupled Model Intercomparison Project, Phase 3. The energetics changes are characterized by significant increases in both the zonal mean and eddy components for the barotropic and the deeper baroclinic modes, whereas for the shallower baroclinic modes both the zonal mean and eddy components decrease. Significant increases are predominant in the large-scale eddies, both barotropic and baroclinic, while the opposite is found in eddies of smaller scales. While the generation rate of zonal mean available potential energy has globally increased in the barotropic component, leading to an overall strengthening in the barotropic energetics terms, it has decreased in the baroclinic component, leading to a general weakening in the baroclinic energetics counterpart. These global changes, which indicate a strengthening of the energetics in the upper troposphere and lower stratosphere (UTLS), sustained by enhanced baroclinic eddies of large horizontal scales, and a weakening below, mostly driven by weaker baroclinic eddies of intermediate to small scales, appear together with an increased transfer rate of kinetic energy from the eddies to the zonal mean flow and a significant increase in the barotropic zonal mean kinetic energy. The conversion rates between available potential energy and kinetic energy, C, were further decomposed into the contributions by the rotational (Rossby) and divergent (gravity) components of the circulation field. The eddy component of C is due to the conversion of potential energy of the rotational adjusted mass field into kinetic energy by the work realized in the eddy divergent motion. The zonal mean component of C is accomplished by two terms which nearly cancel each other out. One is related to the Hadley cell and involves the divergent component of both wind and geopotential, while the other is associated to the Ferrel cell and incorporates the divergent wind with the rotationally adjusted mass field. Global magnitude increases were found in the zonal mean components of these two terms for the warmer climate, which could be the result of a strengthening and/or widening of both meridional cells. On the other hand, the results suggest a strengthening of these conversion rates in the UTLS and a weakening below, that is consistent with the rising of the tropopause in response to global warming.

Interannual variability in Saharan dust over the North Atlantic Ocean and its relation to meteorological fields during northern winter

In this study, we focus on interannual variability in Saharan dust transport over the North Atlantic Ocean during northern winter using the Aerosol Index data from the Total Ozone Mapping Spectrometer (TOMS). Since the TOMS observations provide a long-term global record, they are useful for considering interannual variations of the Saharan dust in relation to the North Atlantic Oscillation (NAO). We calculated correlation coefficients between the NAO Index and the TOMS AI averaged over the northern winter season. Using the correlation map, we defined three regions: Region 1 is in positive correlation over the North Atlantic Ocean, Region 2 is in insignificant correlation over the west Sahara, and Region 3 is in negative correlation over the east Sahara. Focusing on Region 1, we considered the relationship between the regional mean TOMS AI time series and certain meteorological parameters. We used sea level pressure and wind speed fields to represent the synoptic circulation, and virtual temperature and lifting condensation level (LCL) to represent the mixed layer condition. When the TOMS AI in Region 1 is large, the easterly winds over the west Sahara are stronger than usual compared to the NAO, and the virtual temperature is higher than average over the ocean. As a result, the LCL is higher near 20°W–0° and is lower than average over the desert. When dust outflow events are observed over the North Atlantic Ocean, strong winds with a high pressure blow in the west Sahara, and dust containing warm air parcels is carried by these strong winds to ocean through areas near the coast (20°N–30°N, 15°W–10°W) from the west Sahara. We summarized the features of meteorological fields during dust outflow events.

Response of migrating tides to the stratospheric sudden warming in 2009 and their effects on the ionosphere studied by a whole atmosphere‐ionosphere model GAIA with COSMIC and TIMED/SABER observations

This paper compares results from a whole atmosphere‐ionosphere coupled model, GAIA, with the COSMIC and TIMED/SABER observations during the 2008/2009 northern winter season. The GAIA model has assimilated meteorological reanalysis data by a nudging method. The comparison shows general agreement in the major features from the stratosphere to the ionosphere including the growth and decay of the major stratospheric sudden warming (SSW) event in 2009. During this period, a pronounced semidiurnal variation in the F region electron density and its local‐time phase shift similar to the previous observations are reproduced by the model and COSMIC observation. The model suggests that the electron density variation is caused by an enhanced semidiurnal variation in the E × B drift, which is probably related to an amplified semidiurnal migrating tide (SW2) in the lower thermosphere. The model and TIMED/SABER observation show that the SW2 tide amplifies at low latitudes from the stratosphere to the thermosphere as well as the phase variation. Possible mechanisms for the SW2 variability in the low latitude stratosphere could be the change of its propagation condition, especially the (2, 2) mode, due to changing zonal background wind and meridional temperature gradient, and/or an enhancement of its source due to redistribution of stratospheric ozone. Present results also show a prominent long‐term variation of the terdiurnal migrating component (TW3) in the ionosphere and atmosphere.

Global S4 index variations observed using FORMOSAT‐3/COSMIC GPS RO technique during a solar minimum year

In this paper, presented for the first time the three‐dimensional global morphology and seasonal variations of scintillation index (S4 index) measured from the signal‐to‐noise ratio (SNR) intensity fluctuations of L1 channel of GPS radio occultation (RO) signals using FORMOSAT‐3/COSMIC (in short, F3/C) satellites for a low solar activity year 2008. The S4 index, which confined around ±30° magnetic latitudes, is found to start around post‐sunset hours (1900 MLT, magnetic local time) and often persists till post‐midnight hours (0300 MLT) between 150 and 350 km altitudes during equinox and northern winter seasons while no activity is observed during southern winter season. However, high latitudes are characterized with no scintillation activity beyond 150 km during any season, which implying that in the solar minimum period the drives of instabilities in the auroral, cusp and polar cap regions, namely the gradient drift and velocity shear, are absent. The S4 index at F region altitudes during magnetically quiet times is more intense and extends to higher latitudes than that observed during disturbed time consistent with earlier studies. The equatorial S4 index appears below the peak of F2 layer (hmF2) during most of the seasons although the associated intensities and the time of maximum occurrences are relatively higher and earlier during vernal equinox followed by autumn equinox. This equinoctial asymmetry could be primarily attributed to the asymmetries in eastward drift velocities, thermospheric meridional winds and plasma densities. Further, the global maps of S4 index at E region altitudes (between 75 and 125 km) show strong seasonal variations with highest activity during northern and southern summer solstice in the middle latitudes while it appears on both sides of magnetic equator with less or no activity at and around the equator during equinox seasons. The absence of S4 index along the equator can be understood in terms of the vanishing vertical component of the magnetic field lines that can inhibit the vertical movement and layered deposition of ionized particles of thin irregular electron density layers such as Es‐layers. Keeping in view the importance of these valuable database, we would like to emphasize that the F3/C GPS RO technique can be used to study the ionospheric irregularities at GHz frequency globally directly from the high‐rate L1 data, which reiterating its importance as a powerful tool to explore the terrestrial ionosphere on a global scale.

The Lower-Stratospheric Response to 11-Yr Solar Forcing: Coupling to the Troposphere–Ocean Response

Abstract The origin of the tropical lower-stratospheric response to 11-yr solar forcing and its possible coupling to a troposphere–ocean response is investigated using multiple linear regression (MLR) analyses of stratospheric ozone and temperature data over the 1979–2009 period and tropospheric sea level pressure (SLP) data over the 1880–2009 period. Stratospheric MLR results, comparisons with simulations from a chemistry–climate model, and analyses of decadal variations of meridional eddy heat flux indicate that the tropical lower-stratospheric response is produced mainly by a solar-induced modulation of the Brewer–Dobson circulation (BDC), with a secondary contribution from the Hadley circulation in the lowermost stratosphere. MLR analyses of long-term SLP data confirm previous results indicating a distinct positive response, on average, during the northern winter season in the North Pacific. The mean response in the Northern Hemisphere resembles a positive Arctic Oscillation mode and can also be characterized as “La Niña–like,” implying a reduction of Rossby wave forcing, a weakening of the BDC, and an increase in tropical lower-stratospheric ozone and temperature near solar maxima. However, MLR analyses of different time periods show that the Pacific SLP response is not always present during every cycle; it was most clearly detected mainly during the ~1938–93 period when 11-yr solar variability was especially strong. During the 1979–93 period, the SLP response was strongly present when the lower-stratospheric responses were large. But during the 1994–2009 period, the SLP response was much less significant and the lower-stratospheric responses were weak, supporting the hypothesis that the lower-stratospheric and surface climate responses are dynamically coupled.

Lunar tide in the equatorial electrojet in relation to stratospheric warmings

[1] The relationship between sudden stratospheric warmings (SSWs) and large-amplitude lunar tides in the equatorial electrojet (EEJ) is studied. Analysis of ground magnetometer data shows that the lunar tide in the EEJ is maximum during the northern winter season except in the Pacific Ocean region. Since SSWs are also a northern winter phenomenon, it is suggested that the relation between the large lunar tide in the EEJ and the SSW may possibly be coincidental. The lunar tide in the geomagnetic variations at Huancayo is anomalously large compared with other EEJ stations. An examination of geomagnetic variations at EEJ stations during SSW events shows that afternoon counter-electrojets are frequently present at new moon and full moon, though the relationship is sometimes broken. The observation of large lunar EEJs when no SSW is present and of various different delay times suggests that other atmospheric processes are likely to be in play.

Titan’s cloud seasonal activity from winter to spring with Cassini/VIMS

Since Saturn orbital insertion in July 2004, the Cassini orbiter has been observing Titan throughout most of the northern winter season (October 2002–August 2009) and the beginning of spring, allowing a detailed monitoring of Titan’s cloud coverage at high spatial resolution with close flybys on a monthly basis. This study reports on the analysis of all the near-infrared images of Titan’s clouds acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during 67 targeted flybys of Titan between July 2004 and April 2010. The VIMS observations show numerous sporadic clouds at southern high and mid-latitudes, rare clouds in the equatorial region, and reveal a long-lived cloud cap above the north pole, ubiquitous poleward of 60°N. These observations allow us to follow the evolution of the cloud coverage during almost a 6-year period including the equinox, and greatly help to further constrain global circulation models (GCMs). After 4 years of regular outbursts observed by Cassini between 2004 and 2008, southern polar cloud activity started declining, and completely ceased 1 year before spring equinox. The extensive cloud system over the north pole, stable between 2004 and 2008, progressively fractionated and vanished as Titan entered into northern spring. At southern mid-latitudes, clouds were continuously observed throughout the VIMS observing period, even after equinox, in a latitude band between 30°S and 60°S. During the whole period of observation, only a dozen clouds were observed closer to the equator, though they were slightly more frequent as equinox approached. We also investigated the distribution of clouds with longitude. We found that southern polar clouds, before disappearing in mid-2008, were systematically concentrated in the leading hemisphere of Titan, in particular above and to the east of Ontario Lacus, the largest reservoir of hydrocarbons in the area. Clouds are also non-homogeneously distributed with longitude at southern mid-latitudes. The n = 2-mode wave pattern of the distribution, observed since 2003 by Earth-based telescopes and confirmed by our Cassini observations, may be attributed to Saturn’s tides. Although the latitudinal distribution of clouds is now relatively well reproduced and understood by the GCMs, the non-homogeneous longitudinal distributions and the evolution of the cloud coverage with seasons still need investigation. If the observation of a few single clouds at the tropics and at northern mid-latitudes late in winter and at the start of spring cannot be further interpreted for the moment, the obvious shutdown of the cloud activity at Titan’s poles provides clear signs of the onset of the general circulation turnover that is expected to accompany the beginning of Titan’s northern spring. According to our GCM, the persistence of clouds at certain latitudes rather suggests a ‘sudden’ shift in near future of the meteorology into the more illuminated hemisphere. Finally, the observed seasonal change in cloud activity occurred with a significant time lag that is not predicted by our model. This may be due to an overall methane humidity at Titan’s surface higher than previously expected.

A composite study of the MJO influence on the surface air temperature and precipitation over the Continental United States

The influence of the MJO on the continental United States (CONUS) surface air temperature (SAT) and precipitation is examined based on 30 years of daily data from 1979–2008. Composites are constructed for each of the eight phases of the Wheeler-Hendon MJO index over 12 overlapping three-month seasons. To ensure that the MJO signal is distinguished from other patterns of climate variability, several steps are taken: (a) only days classified as “MJO events” are used in the composites, (b) statistical significance of associated composites is assessed using a Monte Carlo procedure, and (c) intraseasonal frequencies are matched to the unfiltered data. Composites of other fields are also shown in order to examine how the SAT and precipitation anomalies are associated with large-scale circulations providing a link between the tropics and extratropics. The strongest and most significant MJO effects on SAT are found during the northern winter seasons. When enhanced convection is located over the equatorial Indian Ocean, below-average SAT tends to occur in New England and the Great Lakes region. As enhanced tropical convection shifts over the Maritime continent, above-average SAT appears in the eastern states of the US from Maine to Florida. The MJO influence on precipitation is also significant during northern winter seasons. When enhanced convection is located over the Maritime continent, more precipitation is observed in the central plains of the US. Enhanced precipitation also occurs over the west coast of the US when convective activity is stronger over the Indian Ocean. During the northern summer and fall, the MJO impact on precipitation is mainly significant at lower latitudes, over Mexico and southeastern US.