Latitudinal double-peak structure of stationary planetary wave 1 in the Austral winter middle atmosphere and its possible generation mechanism

We report a hitherto unknown latitudinal double-peak structure in the amplitude of stationary planetary wave 1 (SPW1) geopotential height in the stratosphere and lower mesosphere during austral winter. The primary peak is located at 60-70 degrees S and 30-40 km, and the secondary peak is at 30-50 degrees S and 40-60 km. According to 36 years (1981-2016) of the Modern-Era Retrospective analysis for Research and Applications, version 2, reanalysis data, the double-peak structure occurs with frequencies of similar to 61%, 97%, 53%, and 25% in May, June, July and August, respectively, while it rarely exists in other seasons. Significant downward Eliassen-Palm fluxes suggestive of downward-propagating waves are often found above the secondary peak, and phase progressions show opposite directions on its two sides. From the free-running Whole Atmosphere Community Climate Model and linear mechanistic model simulations, the secondary peak is likely generated by the interference of primary upward-and secondary downward-propagating SPW1 excited in situ by gravity wave forcing in the upper mesosphere. The strengths of the primary and secondary waves need to be comparable to form an effective interference pattern, which may explain the missing double-peak feature in boreal winter as the primary waves are too dominant. Summer-to-winter interhemispheric wave coupling is identified in the austral midwinter and appears to originate from the secondary SPW1 generated in the summer hemisphere. Since the double-peak structure of SPW1 is sensitive to the mean wind, wave-mean flow, and wave-wave interactions, this study provides a reference for general circulation and mechanistic models to simulate the middle atmosphere wave dynamics in austral winter.