Long‐Term Trends in the Low‐Latitude Middle Atmosphere Temperature and Winds: Observations and WACCM‐X Model Simulations

Abstract

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 ~−1.7 ± 0.5 K/decade between 30 and 80 km is noticed. Large decreasing trend (~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 (O3). 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.

Long‐Term Variability and Tendencies in Middle Atmosphere Temperature and Zonal Wind From WACCM6 Simulations During 1850–2014

“…In addition, observational studies have assessed the variability with the measurements available (e.g., Lee et al, 2010; Sridharan et al, 2009, 2010; Venkat Ratnam et al, 2013, 2019); see the reviews by Beig et al (2003) and Laštovička (2017). Both simulations and observational analyses are important to improve our understanding of the atmospheric response to the various external and anthropogenic changes that have occurred and are expected to continue.…”

Introduction to Special Issue on “Long‐Term Changes and Trends in the Middle and Upper Atmosphere”

“…Han et al () suggest that the residual circulation is changing driven by climate change, resulting in changing transport of trace gases in the stratosphere. Venkat Ratnam et al () found a large decreasing trend (~5 m/s per decade) in the eastward winds between 70 and 80 km in Indian region, using multi‐instrumental data. So far, the reports on local wind or wave trends are often contradictory (Jacobi et al, ; Liu et al, ).…”

Progress in investigating long-term trends in the mesosphere, thermosphere and ionosphere

“…Venkat Ratnam et al (2019) carefully merged data on the middle atmosphere over India obtained by various measuring techniques (rockets, High-Resolution Doppler Imager (HRDI)/ Upper Atmosphere Research Satellite (UARS), (HALOE)/UARS, SABER/TIMED and Mesosphere-Stratosphere-Troposphere (MST) radars) over more than 25 years. The observational analysis was accompanied by WACCM-X model simulations.…”