Multiple Drivers of Wintertime Zonally Symmetric (S0) Oscillations in the Middle Atmosphere

Abstract Zonally symmetric oscillations (zonal wavenumber‐0, S0) are atmospheric variations that are uniform in longitude, representing changes in the zonal‐mean state, and play a key role in modulating winter middle‐atmospheric circulation during Arctic sudden stratospheric warmings (SSWs). In a companion paper (Zhao et al., 2025, https://doi.org/10.1029/2025jd044024 ), we showed that 13 of the 31 S0 events detected in MERRA‐2 zonal wind data for 2004/2005–2023/2024 (42%) arise from nonlinear interactions between stationary planetary wave‐1 (SPW1) and traveling wavenumber‐1 waves (W1/E1). Here, we identify two additional mechanisms for 16 of the remaining events and document a third pathway in an additional case. (a) Quasi‐periodic oscillations in Eliassen–Palm flux divergence (EPFD) from SPW1–6, representing cumulative forcing from these dominant wave components, drive 12 cases (38.7%). (b) Residual EPFD, defined as total EPFD minus contributions from SPW1–6, excites 4 cases (12.9%). (c) Nonlinear sum‐frequency coupling between westward‐propagating (W1, 29.5‐day) and eastward‐propagating (E1, 24.3‐day) wavenumber‐1 waves generates one additional S0 event (13.4‐day period) in geopotential height during the 2011 SSW, consistent with theoretical predictions (1/(1/24.3 + 1/29.5) ≈ 13.4 days) and supported by nonlinear advective tendency analysis. The remaining 2 events (6.5%) are unattributed. Combined with Zhao et al. (2025), https://doi.org/10.1029/2025jd044024 , these mechanisms explain 93.5% of observed S0 events, revealing diverse generation pathways modulated by background atmospheric conditions during SSW periods.