Stability Behind the Nonstationarity: The Case of the Recent Reversal in the ENSO–Stratospheric Polar Vortex Teleconnection

Teleconnections are crucial for regional climate variability and prediction. However, they often appear to be unstable over time, known as nonstationarity. The recent reversal of the well-established ENSO–stratospheric polar vortex (SPV) teleconnection illustrates this puzzle. As this teleconnection is a key pathway for ENSO to influence wintertime circulation in the mid-to-high latitudes, its apparent breakdown questions its reliability as a source of predictability. Here we demonstrate that this nonstationarity is more a statistical artifact than a dynamical shift. By distinguishing the underlying physical linkage from statistical association, we reveal that the observed reversal is driven by an extreme winter outlier and a persistent weakening trend. Much of this weakening can be attributed to the confounding influence of the Quasi-Biennial Oscillation (QBO), whose intermittent alignment with ENSO introduces spurious low-frequency fluctuations in the ENSO–SPV statistical relationship. A physically motivated toy model confirms that such apparent nonstationarity can arise even when the underlying ENSO–SPV linkage remains unchanged, emerging from chance alignment between ENSO and QBO. After accounting for these effects, the ENSO–SPV linkage is substantially more stable than suggested by the raw statistical relationship. Our findings suggest that caution is needed when interpreting time-varying fluctuations in short climate records as structural changes.