Modulation of El Niño by the Quasi-Biennial Oscillation

The Quasi-Biennial Oscillation (QBO) of equatorial zonal winds is the leading mode of lower-stratospheric variability. Numerous studies have explored its connection with the troposphere, including its sensitivity to tropical convection and the El Niño-Southern Oscillation (ENSO). In particular, the upward ENSO impact on the QBO is known: observational evidence suggests that during El Niño the QBO propagates faster (shorter period). However, the potential downward QBO influence on ENSO has not been thoroughly assessed and needs better understanding. Here, we focus on the strongest ENSO events, dubbed super El Niños, characterized by extreme sea surface temperature (SST) anomalies in the eastern Pacific. Super El Niños are exceptional due to self-limiting ENSO dynamics in the tropical Pacific and seasonal SST cooling during summer and autumn, which prevents strong eastern Pacific SST warming and convective anomalies from developing. Their existence requires one or more factors external to the tropical Pacific to aid the Bjerknes feedback in building an El Niño event. In both observations and models (EC-EARTH), super El Niño events seem to require the westerly QBO phase to coincide with a growing El Niño, i.e. in boreal summer and fall. We thus propose a novel element that contributes to the generation of extreme El Niño events that involve the QBO and its modulation of the Walker circulation. While an El Niño event typically leads to a weaker Walker circulation, the weakening becomes more pronounced if the QBO is in its westerly phase. Consequently, the low-level trade wind anomalies over the equatorial Pacific are intensified, which reinforces the Bjerknes feedback and enhances the warm anomalies over the cold tongue region. Our results suggest that the QBO state could be considered to improve El Niño predictions, especially for extreme events.