Seasonal and latitudinal variations of gravity wave‐driven fluctuations in OH nightglow

Abstract

The seasonal and latitudinal variations of the gravity wave‐driven fluctuations in the OH nightglow are investigated theoretically using a model that accounts for emission from an extended OH layer and includes the effects of eddy diffusivities in the gravity wave dynamics. The mean (unperturbed) state is obtained from a two‐dimensional, nighttime model so that mean‐state number densities, temperatures and eddy diffusivities are all self‐consistent. Seasonal and latitudinal variations in the background OH nightglow emission and in the propagation and dissipation characteristics of the gravity waves influence how the OH nightglow modulations due to gravity waves depend on season and latitude. At intermediate gravity wave periods (i.e., periods between 0.5 and 3 hours for λx = 100 km; between 3 and 10 hours for λx = 500 km; and greater than about 4 hours for λx = 1000 km) possible trends in the behavior of <η> (equal to , where I is the airglow intensity, T is intensity‐weighted temperature, an overbar denotes the mean state, a prime denotes a perturbation about that mean state and the brackets indicate vertical integration over all emission levels) with latitude and season could be masked by interference effects. At long periods (i.e., periods greater than 3 hours and 10 hours for λx = 100 and 500 km, respectively) trends are complicated by the competing effects of eddy diffusivities, which directly modify both local values of and the altitudes of maximum wave amplitude, and the vertical distributions of the minor species. Only at periods of a few hours or less (depending on horizontal wavelength) are any seasonal trends seen in <η>, and these are more noticeable in the magnitude of <η> than in its phase. These seasonal variations in <η> are primarily due to seasonal variations in the undisturbed temperature which affect the temperature‐dependent chemical rate constants involved in the OH nightglow. A specific case showing latitudinal trends is presented, but the result is not representative of all of our results obtained involving latitudinal variations.