Global variability of mesospheric temperature: Planetary‐scale perturbations at equatorial and tropical latitudes

Abstract

The study examines the source of a cold temperature perturbation observed in the Wind Imaging Interferometer (WINDII) daytime daily zonal mean temperatures at 87 km height during March/April 1993, also seen to some extent in 1995 and 1997 over the latitude range 25°S to 25°N. The perturbation appears centered at 10°S as a departure of 10K to 35K below the semiannual climatological mean, which peaks at equinox. After accounting for tidal contributions to the observations, by adopting tidal parameters determined from the Microwave Limb Sounder (MLS) temperature observations the cold temperature signature still remained in the residual data. The perturbation is a global phenomenon immediately following March equinox and lasts over a period of 2 weeks. The period is marked by the presence of mesospheric temperature inversions over the altitude range of 70–90 km. Harmonic analysis reveals the presence of planetary waves of wave number 1 with periods of 3 days, 4–5 days and 7 days. A cross section of the temperature monthly zonal mean data and month of the year over the period from January 1992 to December 1995 revealed a distinct MSAO, modulated by a QBO with strong cold anomalies in March/April 1993, 1995 and 1997 in excellent agreement with the MSAO easterly phase of correlative MF radar wind observations at Tirunelveli (8.7°N, 77.8°E). It is shown that the observed temperature anomalies can be produced by the residual circulation associated with the wave driving of the MSAO itself.