Mesospheric semiannual oscillation in temperature and nightglow emission

Abstract

The study examines the seasonal variability of mesospheric temperature along with atomic oxygen and hydroxyl nightglow observed by the Wind Imaging Interferometer (WINDII) at equatorial and tropical latitudes over the period from January 1992 until December 1995. Cross sections of the zonal monthly mean temperature and airglow profiles in the latitude range ± 15 ∘ show a distinct mesospheric semiannual oscillation (MSAO), its relationship with the wind MSAO and apparent modulation by the stratospheric quasi-biennial oscillation (QBO). The first half of the MSAO cycle (beginning with a cold temperature anomaly in the northern winter) appears stronger at the equator both for the temperature and the airglow emissions compared to the second half (beginning with a cold temperature anomaly in the southern hemisphere). In the airglow emissions the MSAO appears as alternating cells of enhanced and decreased emission rate of the O ( 1 S ) and OH emissions, corresponding to cold and warm temperature anomalies. The QBO effect is observed as a temperature decrease and O ( 1 S ) and OH emission rate enhancements during the easterly phase of the wind MSAO with peaks at the equator in March/April 1993 and 1995. Away from the equator the O ( 1 S ) MSAO/QBO coupling appears 180 ∘ out of phase with respect to that at the equator with emission rate enhancements in March/April 1992 and 1994. The observed temperature and airglow signatures correlate strongly with the QBO and easterly phase of the zonal wind MSAO. At the 5 ∘ S – 15 ∘ S latitude band the wind MSAO deteriorates into prevailing easterly wind possibly resulting from downward motion of the stratospheric SAO westerlies and small-scale gravity wave activity.