Long-term trends in the low-latitude middle atmosphere temperature and winds: Observations and WACCM-X model simulations

In recent years, the middle atmosphere has evoked great scientific interest as long-term changes can be clearly captured owing to the large perturbation amplitudes at these altitudes. In the present study, more than 25 years of the data are used to investigate the long-term trends in the middle atmosphere by suitably combining the observations from different techniques (Rocketsonde, High-Resolution Doppler Imager (HRDI)/Upper Atmosphere Research Satellite (UARS), Halogen Occultation Experiment (HALOE)/UARS, Sounding of the Atmosphere using Broadband Emission Radiometry (SABER)/Thermosphere Ionosphere Mesosphere Energetic Dynamics (TIMED), and Mesosphere-stratosphere-troposphere (MST) radar) from Indian region. As different instruments/techniques are used and the time periods are not the same, extreme care has been taken while merging various data sets to obtain meaningful long-term trends. To understand the observed long-term trends, Whole Atmosphere Community Climate Model-eXtended model simulations for the Indian low-latitude regions are also used. A significant cooling trend of similar to-1.7 +/- 0.5 K/decade between 30 and 80 km is noticed. Large decreasing trend (similar to 5 m/s/decade) in the eastward winds is noticed which is significant between 70 and 80 km only changing from strong eastward wind in 1970s to weak westward wind in recent decade. No significant trends are observed in the meridional wind. These observations are well captured by the Whole Atmosphere Community Climate Model-eXtended model simulations while considering changes in concentrations of greenhouse gases including carbon dioxide (CO2), methane (CH4), water vapor (H2O), and chlorofluorocarbon species that cause depletion of stratospheric ozone (O-3). Thus, it is prudent to conclude that long-term decreasing trends in the zonal winds and cooling trends in the temperature in the middle atmosphere are caused to a large extent by greenhouse gases suggesting the role of anthropogenic changes in the dynamics of the middle atmosphere.