TIMED/SABER observations of lower mesospheric inversion layers at low and middle latitudes

Abstract

We present the global distribution, seasonal, and interannual variations of the lower mesospheric inversion layers (MILs) using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) temperature data. We show that both the characteristics and the formation mechanisms of large spatiotemporal‐scale lower MILs are latitude dependent. At low latitudes, the monthly zonal mean amplitude of the lower MILs exhibits a semi‐annual cycle and reaches a maximum of ∼40 K in spring and a secondary maximum of ∼30 K in autumn. On the equator, the semi‐annual oscillations in the background and diurnal‐migrating‐tide temperatures could contribute more than 12 and 25 K, respectively, suggesting they are the key causes of large spatiotemporal‐scale lower MILs at low latitudes. At middle latitudes, the monthly zonal mean amplitude of the lower MILs exhibits an annual cycle with its maximum in the range 24–33 K in winter. In addition, their longitudinal distribution and daily variation in winter are closely correlated with the transient structure of a composite wave composed of stationary and westward‐propagating quasi‐16‐day planetary waves with zonal wave number 1. The correlation coefficient between the lower MILs and the composite wave can sometimes reach unity. The composite planetary wave could contribute temperature enhancements of at least 15–20 K to the lower MILs. Thus, we believe that the transient structure of planetary waves is also an important cause of the large spatiotemporal‐scale lower MILs in winter at middle latitudes, in addition to previously proposed mechanisms.