The Arctic Oscillation Response to the QBO in an EOF-Based QBO Phase Space

This study revisits the influence of the quasi-biennial oscillation (QBO) on the Northern Hemisphere surface climate during the boreal winter by using the QBO index based on the empirical orthogonal function (EOF) of stratospheric equatorial winds. When the QBO is defined with eight phases, the tropospheric anomalies significantly project onto the negative Arctic Oscillation (AO) in phase 1 and onto the positive AO in phases 5 and 6. The underlying mechanism can be partially explained by the Holton–Tan relationship and the associated planetary wave–mean flow interactions. Compared with the known QBO indices defined with the single-level winds, the EOF-based index better captures the AO-like response to the QBO. This may be because the vertical structure, particularly the depth of the QBO’s stratospheric easterly or westerly, is essential for the QBO’s extratropical influence, which is better described by the EOF-based metric. The results remain robust in phases 1 and 5 and become more significant in phase 6 when the El Niño–Southern Oscillation (ENSO) effects are linearly removed. This result suggests that the two-dimensional QBO index is more useful than the one-dimensional index for monitoring and predicting QBO-related tropospheric circulation changes.