The Influence of the Vertical Structure of QBO on the Summer NAM

Abstract This study investigates the linkage between the vertical structure of the quasi-biennial oscillation (QBO) and the high summer (JA) Northern Annular Mode (NAM) during 1979–2022 based on the ERA5 reanalysis dataset and further explores the underlying mechanisms. The singular value decomposition (SVD) analysis suggests that the time series of the leading mode of SVD (SVD1) of the QBO from March to JA exhibit a significant positive correlation with the JA NAM index. The zero line of the transition from easterly to westerly is located between 30 and 50 hPa in March descending with time and eventually transitioning to a pronounced deep easterly structure by JA, which tends to induce a positive NAM. The JA QBO modulates the JA NAM through the subtropical pathway. The deep easterly phase induces rising in the tropics and sinking in the subtropics in the upper troposphere and lower stratosphere (UTLS). The anomalous meridional circulation leads to positive meridional temperature gradient anomalies and easterly anomalies in the subtropical UTLS to maintain thermal wind balance, potentially weakening the subtropical tropospheric jet. The weakened subtropical jet could induce anomalous equatorward wave propagation and poleward eddy momentum flux anomalies, which may strengthen the polar front jet and promote the formation of the positive summer NAM. Model results successfully reproduce the relationship between the JA deep QBO and the JA NAM. This analysis applies to about half of the 44 summers that had deep descending easterlies or westerlies. These results can provide new insights for improving the seasonal forecast of the summer NAM. Significance Statement The relationship between the quasi-biennial oscillation (QBO) and the winter Northern Annular Mode (NAM) has been extensively studied. However, the influence of the QBO on the summer NAM has received much less attention. In this study, we diagnose the impact of the vertical structure of the QBO on the summer NAM. Both reanalysis and model simulations consistently show that a deep easterly QBO phase tends to induce a positive summer NAM through a subtropical pathway. This finding deepens our understanding of the summer NAM variability and offers potential insights for improving the summer NAM seasonal forecast.