An Indian Ocean-sector Pathway of Downward QBO Influence in Boreal Summer Revealed by an Adaptive Signal Isolation Hindcast

The stratospheric quasi-biennial oscillation (QBO) is predictable many months in advance, making it a potential source of seasonal predictability. However, identifying its impacts on the troposphere remains challenging because QBO-related signals are often masked by the stronger influence of the El Niño–Southern Oscillation (ENSO). Statistical detection is difficult because of QBO–ENSO aliasing, whereas idealized numerical model experiments often lack direct observational support. To address this issue, we present a novel framework, termed the Adaptive Signal Isolation Hindcast (ASH). For each forecast case, two hindcast experiments are performed with either the observed QBO phase or the opposite phase imposed during model integration. By differencing the resulting forecasts, the QBO response can be separated from non-QBO variability while allowing direct comparison with observations for each forecast case. Using this framework, we identify an Indian Ocean-sector pathway through which the QBO influences the Asian summer monsoon and the Walker circulation during boreal midsummer. During the westerly QBO phase, enhanced shear instability associated with the monsoon easterly jet allows warm lower-stratospheric anomalies to extend downward over the Indian Ocean–Maritime Continent region. The resulting upper-tropospheric warming is redistributed toward the off-equatorial regions through a Matsuno–Gill-type response, increasing atmospheric stability and weakening the large-scale monsoon circulation. In regions influenced by this mechanism, forecasts with the observed QBO phase exhibit improved agreement with the reanalysis. These findings highlight the importance of improving QBO representation in numerical models and leveraging QBO information in seasonal forecasting, particularly when atmospheric predictability driven by sea surface temperature anomalies is limited.