Influence of the QBO on tropical convection and its impact on tropical cyclone activity over the western North Pacific

On the basis of 70 hPa zonal wind (U70)-defined quasi-biennial oscillation (QBO) events, after removing the El Niño–Southern Oscillation (ENSO) signal, the present study investigates the process by which the QBO modulates tropospheric circulation and convection during summer (between July and October), when tropical cyclone (TC) activities enter their peak period. Concurrent with the western phase of the QBO (QBOW), significant tripole pattern modulations over the tropical Indo-Pacific Ocean are regressed onto the residual part of U70 after removing the ENSO signal, with enhanced convection observed over its central branch (0°–10°N, 120°E–180°) and the inactive convection branches located to both sides. A ventilation opening-like effect is exerted on the monsoon trough, which is shifted southward under the QBOW phase. According to the QBO-associated changes in the circulations over the tropical western North Pacific (WNP), equatorial environments (with low-level relative vorticity, high-level divergence, tropospheric vertical wind shear (VWS), and midlevel humidity) tend to be favorable for TC genesis. Consequently, the off-equatorial TC tracks show a significantly decreased occurrence frequency in the northern monsoon trough region. The present study provides a summary sketch showing the QBO-tropospheric circulation modulation process. Considering that the westerly or weak easterly is observed over the upper central Pacific (CP), QBOW phase-associated weak VWS can induce anomalous upward motion at equatorial latitudes, together with the upwelling between the equatorial westerly and off-equatorial easterly, leading to an enlarged convective window over the CP. Affected by the prevailing easterly within the tropics, the actual pumping shifts westward over the tropical WNP.