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°S and 30–40 km, and the secondary peak is at 30–50°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 ~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. Plain Language Summary: The winter‐hemisphere stationary planetary waves are the most dominant perturbations in the lower and middle atmosphere of the Earth. They make significant contribution to the dynamics, chemistry, transport, and transition of the stratosphere and lower mesosphere. It is therefore important to understand their characteristics, forming mechanisms, and impacts. In this paper, we newly identify an intriguing latitudinal double‐peak feature of stationary planetary waves in austral winter months. Its statistical significance has been proved by using long‐term satellite observations and reanalysis data. The underlying mechanisms suggested by modeling work not only rely on the structure of the polar vortex but also involve the secondary planetary wave generation and wave‐wave interactions. This work uses a combination of satellite measurements, reanalysis product, whole atmosphere model, and mechanistic model, which provides a powerful tool to digest the important atmospheric phenomenon. Key Points: We report a novel finding of double‐peak structure in geopotential field of stationary planetary wave 1 in austral winter middle atmosphereDownward SPW1 generated by in situ GW forcing interferes with upward SPW1 which generates the secondary peak near midlatitude stratopauseSummer‐to‐winter interhemispheric wave coupling is identified at 70‐80 km and opposite in direction from winter‐to‐summer ducting above it [ABSTRACT FROM AUTHOR]