Spatial variability in long-term temperature trends in the middle atmosphere from SABER/TIMED observations

Temperatures obtained from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite during the period from 2003 to 2019 are analysed to obtain trends in the region from 50°S to 50°N using the method of Empirical Mode Decomposition (EMD). This method is used to remove long-term oscillations from the data to accurately estimate the linear trends. The novelty of the current study is in dividing the entire study region into sub-regions of 5° latitude and 10° longitude to investigate the associated longitudinal variability. The study shows that the global middle atmosphere, including the global mesopause and stratopause, is cooling as a whole. In the stratosphere and lower mesosphere the observed trends are −0.5 K/decade and in the upper mesosphere, the trends increase to −1.0 K/decade. Mesopause temperatures in Southern Hemisphere and stratopause temperatures in Northern Hemisphere are showing stronger cooling trends in comparison to the other hemisphere. Over tropical and subtropical regions, Southern Hemisphere mesopause is showing a stronger cooling trend over Indian Ocean and Asian region while the cooling trends are symmetric around the equator over Pacific and Atlantic Oceans. Two important results obtained in the present study are as follows: (1) A warming trend, reaching 2.5 K/decade, is observed around 77 km over the Equator and adjoining tropical latitudes, which is attributed to processes leading to the mesospheric semi-annual oscillation (MSAO). (2) Anomalous cooling is observed in the stratopause region over the Southern Indian Ocean (SIO) longitudes. Temperature trends are large and negative ranging from −1 to −2 K/decade over SIO while the trends over the Southern Pacific Ocean (SPO) and the Southern Atlantic Ocean (SAO) lying in the same latitudinal band are small and positive, reaching values of 1 K/decade. It is envisaged that the recent accelerated warming of the SIO is probably responsible for the cooling of the stratopause region above.