Springtime transition in upper mesospheric temperature in the northern hemisphere

Airglow emission observations by the wind imaging interferometer (WINDII) on the Upper Atmosphere Research Satellite and three optical ground-based stations previously revealed a “springtimetransition” in atomic oxygen. The transition is characterized by a rapid 2-day rise in the night-time oxygen nightglow emission rate by a factor of 2 to 3 followed by a subsequent decrease by a factor of 10 in the same period of time indicating a depletion of atomic oxygen that persists for days. The current study examines signatures in the upper mesosphere temperature field (70– height range), derived from the WINDII Rayleigh scattering observations, which may be associated with this springtime depletion of the atomic oxygen. Comparisons with ground-based OH airglow rotational temperatures, Na lidar and Rayleigh scattering lidar temperatures, and meteor radar temperatures at middle and high latitudes in the NorthernHemisphere are presented and discussed. Data from the northernspringtimes in 1992 and 1993 are reported upon in detail. It was found that all datasets used in the study agree well with each other taking account of the day/night time mean differences. A rapid temperature enhancement was observed at spring equinox at northern midlatitudes followed by a period of mean temperature colder than the one observed prior to the enhancement event, a pattern similar to that associated with the “springtime transition” observed in the oxygen emissions. The enhancement was also revealed in the average annual temperatures at , obtained by combining observations from 1992 to 1996, and in more recent temperature data from 1998 and 1999 at mid- and high northern latitudes. The results suggest that the temperature enhancement is associated with the last stratospheric warming event, observed at the end of March and early April.

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