Downward wave coupling occurs when an upward propagating planetary wave from the troposphere decelerates the flow in the upper stratosphere, and forms a downward reflecting surface that redirects waves back to the troposphere. To test this mechanism and potential factors influencing the downward wave coupling, three 145-year sensitivity simulations with NCAR’s Community Earth System Model (CESM-WACCM), a state-of-the-art high-top chemistry-climate model, are analyzed. The results show that the QBO and SST variability significantly impact downward wave coupling. Without the QBO, the occurrence of downward wave coupling is significantly suppressed. In contrast, stronger and more persistent downward wave coupling occurs when SST variability is excluded.
The above influence on the occurrence of downward wave coupling is mostly due to a direct influence of the QBO and SST variability on stratospheric planetary wave source and propagation. The strengths of the tropospheric circulation and surface responses to a given downward wave coupling event, however, behave differently. The surface anomaly is significantly weaker (stronger) in the experiment with fixed SSTs (without QBO), even though the statistical signal of downward coupling is strongest (weakest) in this experiment. This apparent mismatch is explained by the differences in the strength of the synoptic-scale eddy-mean flow feedback and the possible contribution of SST anomalies in the North Atlantic during DWC event. The weaker synoptic-scale eddy-mean flow feedback, and the absence of the positive NAO-related SST-tripole pattern in the fixed SST experiment are consistent with a weaker tropospheric response in this experiment. The results highlight the importance of synoptic-scale eddies in setting the tropospheric response to downward wave coupling.