Causes of the Abnormally Strong Easterly Phase of the Mesopause Semiannual Oscillation During the March Equinox of 2023 Revealed by a New Reanalysis Data Covering the Entire Middle Atmosphere

Abstract

During the March equinox of 2023, a strong easterly wind of ∼80 m s⁻¹ appeared at an altitude of ∼82 km in the equatorial upper mesosphere, which is regarded as an enhancement of the mesopause semi-annual oscillation. In this study, a new reanalysis data available up to 110 km was used to investigate its momentum budget. The strong easterly acceleration was due to a similar contribution from resolved waves and parameterized gravity waves, but largely counteracted by an upward advection of westerly momentum. The significant anomaly in the mean winds was not restricted to the 82 km height, but also included strong westerly winds (∼50 m s⁻¹) at 65 km and easterly winds (∼40 m s⁻¹) at 42 km. The stratospheric quasi-biennial oscillation was westerly. The mean wind intensification at each height is explained by the acceleration due to upward propagating waves, which do not suffer from critical filtering below.