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Abstract

Climatological features of the intraseasonal oscillation (ISO) in the equatorial mesosphere and lower thermosphere were analyzed using temperature data from the Aura Microwave Limb Sounder (MLS) and global analysis data from a data assimilation using satellite observations, including the MLS, over 15 years. Based on spectral analyses, fluctuations with a period of 15–90 days were extracted as the ISO component. First, an empirical orthogonal function (EOF) analysis was performed for the ISO components of the zonal mean temperature from the MLS in the region of 10°S–10°N, 68–85 km, where the ISO amplitudes are found to be maximized. The global structure of the ISO anomalies of temperature and zonal winds estimated assuming a gradient wind balance was examined by a regression analysis using the first and second principal EOFs. It is shown that the ISO phases propagate downward with a vertical wavelength of 25–30 km above a height of 60 km and that the ISO signal synchronized with the equatorial ISO is observed even in the middle and high latitude regions of both hemispheres. Second, the global analysis data were used to examine the driving mechanism of the ISO. It was confirmed that the time series of the equatorial ISO zonal mean zonal wind in the analysis agreed well with radar observations, which were not used for assimilation. Results suggest small-scale phenomena, such as gravity waves not necessarily represented in the parameterization and the meridional advection of angular momentum, mainly contribute to drive the ISO.