Inter-annual Variability of Nonmigrating Tides Forced by Tropical Convection
Current observations and models demonstrate unequivocally that non-Sun-synchronous (nonmigrating) tides due to deep tropical convection produce large longitudinal and local time variations in bulk ITM properties, i.e., temperature, wind, composition and plasma density, to name a few. We thus stand at an exciting research frontier: understanding how persistent, large-scale tropospheric weather systems affect Earth's upper atmosphere and geospace environment. One science challenge question is the inter-annual variability of the nonmigrating tides. This paper focusses on quasi-biennial and longer term tidal variations presumably induced by the QBO and the solar cycle. Multiple years of satellite-borne tidal temperature and wind observations show considerable amplitude variations on these time-scales, from the MLT region (SABER and TIDI on TIMED) to the upper thermosphere (CHAMP and GRACE). The results are interpreted using an empirical fit model and the WACCM general circulation model. Possible implications for the thermospheric energy budget are discussed.
Longitudinal variability and the geography of wave interference
Longitudinal variability in many observables in the upper atmosphere are now being identified. One candidate for the source of this variability is interference between large scale waves. It is now known that non-migrating tides have amplitudes which are of the same order as the migrating tides and planetary waves in this region of the atmosphere. Interference between these various waves results in geographic variations in the locally determined amplitudes and phases of many waves. Although the exact amplitudes of the various wave components in a GCM do not yet match observations, an examination of the superposition effects in such models does provide an indication of the nature of this interference. In this paper, a run of the extended Canadian Middle Atmosphere model is used to illustrate and quantify the form of the resulting longitudinal variability in wind, temperature, and airglow. Comparison of model geographical variations with the observed variations is a rigorous test for how well a model simulates the large scale wave environment in the atmosphere.
Longitudinal variations in stratospheric/mesospheric temperature at high latitudes
The global structure of the temperature field at high latitudes (60°-80°) in the Northern and Southern hemispheres is examined using temperature data in the altitude range from 10 km to 90 km (from the upper troposphere to the upper mesosphere). The data are obtained from GPS radio occultation measurements by the COSMIC/Formosa-3 satellite constellation and the MLS-Aura satellite. The analysis has revealed strong large-scale planetary wave perturbations both in the stratosphere and mesosphere at high- latitudes. The source of these perturbations and their inter-annual variability and inter-hemispheric coupling is investigated and discussed.
Investigation of Longitudinal Variation by Using Sodium Temperature Lidar Measurements
Recent observation and model studies show not only latitudinal variation but also large longitudinal variations
of density, temperature, and dynamics in the middle atmosphere and ionosphere. Long-term observations by
sodium temperature lidar at the mid-latitude were mainly in the US longitude [e.g., She et al., 2000; Chu et al.,
2005]. In the Western Pacific longitude, occurrence rate of sporadic sodium (Nas) layer were reported by
statistical analysis of sodium density profiles [Nagasawa and Abo, 1995; Gong et al., 2002]. However, vertical
distributions of temperature in the mesosphere and lower-thermosphere (MLT) region were not measured
well. We have observed the temperature and sodium density profiles since August, 2007 at Uji (34.9oN,
135.8oE) located ∼25 km west from Shigaraki Middle and Upper Atmosphere (MU) observatory by a
sodium temperature lidar, which was developed by Shinshu University and National Institute for Polar
Research (NIPR) and was operated at Syowa in Antarctica, in order to reveal these variations in the Western
Pacific longitude. These profiles for 147 nights (more than 1300 hours) have been obtained. Comparisons of
seasonal variations of these profiles between Japan and the US longitudes showed some similarities and
differences. Sodium density in Japan showed similar seasonal variation to that in the US, except for an
enhancement (90-100 km) in June-July caused by Nas in Japan. Temperatures below 98 km showed
semi-annual variation in both longitudes, but months of temperature maximum were slightly different. Because
the long-term observation periods are different between Japan and the US, the differences between two
longitudes include longitudinal variation and also year-to-year variation.
An estimate for the vertical motion associated with long-period waves such as the tides can be determined from the temperature perturbations assuming long-period wave motions are adiabatic. A remarkable increase of sodium column density of 70-120 km and OI (557.7 nm) emission were observed on December 9, 2007, simultaneously. The sodium lidar temperature profiles showed long-period wave with a downward phase propagation, and vertical displacement of atomic oxygen, oxygen, and nitrogen estimated from the temperature profiles could explain the OI enhancement. The long-period wave was probably a part of tides, which had unusual large amplitude shown by TIMED/SABER temperature measurements. Further studies of detailed comparison with the Na lidar results in the US, as well as investigation of atmospheric stability characteristics and its longitudinal dependencies are being carried out under collaboration between Japan, the US and China.
Planetary Wave Influence on Wintertime OH Meinel Longitudinal Variation?
We report on very unusual conditions in the upper mesosphere during the boreal winters of 2004 and 2006. Unusually bright OH volume emissions, as measured by TIMED/SABER, occurred in the region north of 60N. These emissions also occurred at unusually low altitudes, while at the same time very high temperatures characterized the upper mesosphere. These large perturbations allowed us to see more clearly longitudinal spatial and temporal variations that were present in the emissions. The affected areas varied in size and location on time scales of a few days and had a distinct planetary-wave wave-1 structure. We present data demonstrating the variability in the emissions and temperatures throughout the polar region and the correlations among them, and we contrast their behavior with that in normal years. The underlying cause of the correlations and longitudinal structure appears to be greatly enhanced downwelling in the upper mesosphere, which in turn was produced by unusual dynamical conditions in the lower atmosphere, consisting of stratospheric warmings and perturbations of wave structures within the polar vortex.
Gravity waves in polar mesospheric clouds measured by Odin/OSIRIS since 2002
The Optical Spectrograph and InfraRed Imager System instrument (OSIRIS) on the limb-viewing Odin satellite observes Polar Mesospheric Clouds (PMCs) in both hemispheres since November, 2001. The orbit period of Odin is 96 minutes and the maximum latitudinal coverage in the orbit plane is between 82.2 N and 82.2 S. In this work, the longitudinal distribution of Odin/OSIRIS PMC brightness in each hemisphere during a 4-week period around the summer solstice from 2002 until 2009 is analyzed. It is found that in the southern hemisphere, the cloud brightness was consistently up to 30% lower around 250-300ºE (70-120ºW) - above the Antarctic peninsular. In the northern hemisphere, the PMC brightness was systematically 20-30% lower around 50-130ºE - above Ural Mountains. Similar results have been obtained for the PMC 2007-08 season in Antarctica and PMC-2007 Arctic season by one of the instruments on a recently launched Aeronomy of Ice in the Mesosphere (AIM) satellite. We attribute this effect to the influence of gravity waves generated by the Earth's terrain.
Seasonal Anomaly Observed by FORMOSAT-3/COSMIC Ionospheric Radio Occultation Measurements
Seasonal anomaly (winter anomaly) that the daytime electron density of F2-layer in winter is higher than in summer is observed by FORMOSAT-3/COSMIC ionospheric radio occultation (COSMIC RO) measurements. Ionospheric electron density profiles retrieved from RO measurements provide the global feature of ionospheric variations. COSMIC RO measurements from November 2006 to February 2009 in geomagnetic quiet conditions are used to study the global aspect of seasonal anomaly. Electron density variations with the height in America, African/European and Asian sector are analyzed for the longitudinal variation of electron density in the winter and the summer season of the northern hemisphere. The result from COSMIC RO in northern hemisphere at mid-latitudes is compared and validated with electron density measurements of Incoherent Scatter Radar (ISR) in Millstone Hill, typically known as a winter anomaly station.