Downward propagation of the weak stratospheric polar vortex events: the role of the surface arctic oscillation and the quasi-biennial oscillation

Based on the JRA-55 reanalysis data, the persistent downward impacts of the weak stratospheric polar vortex (WPV) events are examined. The WPV events with persistent tropospheric impacts are selected and classified into two groups according to whether there is a phase switch of the surface Arctic Oscillation (AO) around the onset day of the cases. The results suggested a one-month longer duration of the tropospheric impacts for the cases with preexisting negative AO anomalies in the troposphere (i.e., the −AO-pre-WPV events), than the cases with a phase switch of the surface AO from positive to negative (i.e., the +AO-pre-WPV events). The connections between the precursory surface AO anomalies and the following downward propagation are found to be closely related to the different events background state of the two types of events. A strong signal of the east quasi-biennial oscillation (QBO) at 30 hPa is observed during the −AO-pre-WPV events, which induces an anomalous poleward propagation of the planetary waves and convergence anomalies of the wave flux in the lower stratosphere. This slow-changing QBO signal exerts continuous wave forcing in the polar region of the lower stratosphere during the −AO-pre-WPV events and delays the recovery of the polar vortex, leading to the long-persistent tropospheric impacts. Moreover, after the background state of each independent case is removed, the preexisting negative surface AO for the −AO-pre-WPV events also disappears. However, the downward propagation during the +AO-pre-WPV events has not been affected with the events background removed, probably because the QBO signal is quite weak. The results reveal the possible role of the precursory surface AO and the background QBO on the downward propagation of the WPV events, which may offer useful information for the subseasonal prediction.