Analysis Of The Migrating Tides And Zonal Mean Temprature In The Middle Atmosphere

As one of the most important waves in the mesosphere and lower thermosphere(MLT), the migrating tides have been observed in many atmospheric variables like temperature, neutral wind, pressure, and density. A tidal period consists of harmonics of a solar day(i.e. 24 h, 12 h, 8h, 6h…). They are essential to the energy balance and momentum exchange of the atmosphere, as well as the coupling between different atmospheric levels. Thus the researches about the global structure and temporal variations of the migrating tides are of great importance to our understanding of the dynamics in MLT region.In this study, the temperature data observed by SABER/TIMED are used to get the amplitude and phase of the migrating 6-h tide. Then we provide global structure and temporal variations of the migrating 6-h tide, and discuss some possible mechanisms responsible for its generation and the seasonal variations. In addition, we examine the possible modulation of migrating diurnal tide(DW1) in the MLT region by the latitudinal gradient of the zonal mean zonal winds(ζ), which is calculated by the pressure and density data from SABER. At last, we discussed the hemispheric asymmetry of the zonal mean temperature using MERRA data. The main results are as follows:(1). The migrating 6-h tideIn this study, we extract the migrating 6-h tide using the SABER temprature data and provide its global structure and temporal variations for the first time. In addition, we discuss the possible mechanisms that produce the migrating 6-h tide and modulate its seasonal variations. The results can help us to further understand the non-linear wave-wave interaction in the middle atmosphere, as well as to improve atmospheric models. The results are as follows.In general, the amplitude of the migrating 6-h tide increases with altitudes. Below about 70 km, the migrating 6-h tide peaks at 35°N/S. the(4, 6) Hough mode, which has the manifestation with three peaks at the equator and middle latitudes respectively, dominates in the MLT region above 70 km: ①. Between 70 km and 90 km, the migrating 6-h tide peaks at the equator and 35°N/S. The maximum at equator is slightly larger than those at middle latitudes, which indicates the superposition of weak(4, 4) Hough mode. ②. In the lower thermosphere above 90 km, there are three dominate peaks of tidal amplitude. The strongest peak is located at 30°S. A weaker one occurs at the equator. The one near 30°N is the weakest among these three peaks. Besides, there are another two peaks located beyond 40°N/S. Thus there may be more than one higher order Hough modes above 90 km with the(4, 6) mode being the dominant one.Annual(AO), semiannual(SAO), 4-month and 3-month oscillations are the four seasonal oscillations in the amplitude of the migrating 6-h tide. No quasi-biennial oscillation(QBO) is found. The seasonal variations of the migrating 6-h tide are different at different altitudes.By consulting the researches of previous reports, we consider the ozone heating in the stratopause and the background atmosphere as two possible mechanisms that modulate the seasonal variations of the migrating 6-h tide. In addition, the non-linear interaction between the migrating diurnal and terdiurnal tides in the stratosphere and second harmonics of the migrating semidiurnal tide in the lower thermosphere probably also contribute to the generation and the temporal variations of the migrating 6-h tide.(2). Possible influences of the latitudinal gradient of zonal mean zonal wind(ζ)on the seasonal and interannual variations of DW1 in the MLT regionIn this study, we use the observations(temperature, pressure, and density data from SABER) to test the possible modulation of DW1 by the latitudinal gradient of zonal mean zonal wind(ζ), which is one important term in the primitive tidal equation. The results assist us in understanding the dynamics of the middle atmosphere and in the advance of the atmospheric models.There are two possible modulations of DW1 by ζ:①. Local modulation by ζ. In the MLT region, SAO, AO, and QBO are threedominant oscillations in both ζ and the DW1 amplitude at low latitudes between 20°N and 20°S. Both ζ and the DW1 amplitude increase(decrease) near equinoxes(solstices), which are the typical seasonal behavior of DW1 in the tropic MLT region. The maxima of ζ and DW1 in the March-April equinox are stronger. In addition, QBOs of ζ and the DW1 amplitude also resemble to each other, both of which show some reduction in 2006. These closely resembling spatial-temporal features suggest that as a term in the linearized equation governing the tidal winds, the latitudinal gradient of zonal wind at low latitudes in the MLT region may play an important role in modulating the annual, semiannual, and quasi-biennial variations in the DW1 amplitude in the tropic MLT region. The local possible modulation of DW1 by ζ is a new finding here.②. Modulation of DW1 propagation by ζ. In addition, ζ has significant annual variation in the mesosphere. It significantly strengthens in summer hemisphere. Unlike the possible effect in the MLT, ζ in the mesosphere possibly affect the vertical tidal propagation between 30°N and 30°S. Because the 6-month shift in seasons between the two hemispheres, AO of ζ in the mesosphere may produce SAO of DW1 in the lower thermosphere. Thus the SAO of DW1 in the lower thermosphere may be a combined effect of ζ both in the mesosphere and lower thermosphere.(3). The hemispheric asymmetry of the zonal mean temperature in thestratosphere and mesosphereAs one of the most important parameters, the different structure of the zonal mean temperature between the northern(NH) and southern(SH) hemispheres determines the different circulations between the two hemispheres. In this work, we analyze the latitudinal distribution and temporal variations of the asymmetry of the zonal mean temperature between NH and SH in the stratosphere and mesosphere using MERRA reanalysis. The results are as following.First the asymmetric index TNH-SH is define as the difference between the zonal mean temperature in the NH and SH in the same season. The results show that SH summer is warmer than NH summer at latitudes polarward of ~40° throughout the entire stratosphere and lower mesosphere. At low latitudes, NH summer is warmer in the stratosphere but colder in the mesosphere. The asymmetric differences in winter are larger than those in summer. In the stratosphere, SH winter is colder than NH winter with maximum difference about 20 K in the polar region. In the mesosphere, SH winter is warmer at high latitudes with maximum difference-15 K occurring near 60°, 0.3 hpa. In the stratosphere, AO dominates TNH-SH at high(low and middle) latitude, which is positive(negative) in January and negative(positive) in July. In the mesosphere, besides AO, SAO appears at high latitudes. From the stratosphere to the lower mesosphere, QBO of TNH-SH occurs at low and high latitudes, but is much weaker at middle latitudes.