The observed connection between the Quasi-Biennial Oscillation and the persistence of the North Atlantic Oscillation in boreal winter

This study reveals that the persistence of North Atlantic Oscillation (NAO) during boreal winter is closely linked to the phase of the Quasi-Biennial Oscillation (QBO) at 30 hPa for the period 1958-2020. Our results show that the early winter (i.e., November-December) NAO signal tends to persist into the subsequent January in the easterly phase of the QBO (EQBO). However, in the westerly phase of the QBO (WQBO) there is hardly connection between the early winter and the subsequent January NAO signals. Further analysis suggests that there is stronger stratosphere-troposphere connection and stronger amplitude of the NAO during the early winters of the EQBO compared to those of the WQBO. A stronger NAO in the EQBO tends to induce enhanced positive feedback of air-sea interaction, which may contribute to a prolonged signal of the NAO. Specifically, significant surface heat flux anomalies over the high-latitude and subtropical regions of North Atlantic induced by a stronger early winter NAO may trigger a tripolar pattern of anomalous sea surface temperature in the North Atlantic in the subsequent January. And this anomalous SST pattern may have a feedback on the atmosphere to strength the anomalous NAO by changing the atmospheric baroclinicity and synoptic eddy activities. In contrast, the NAO in the early winters of the WQBO tends to have a weaker amplitude and be confined to the troposphere. This weaker NAO is shown to induce a weaker air-sea interaction in the North Atlantic, and tends not to favour a prolonged NAO.