Mechanisms of the QBO Influence on the Tropical Troposphere: Modulation by ENSO Conditions

Abstract

The Quasi‐Biennial Oscillation (QBO) is the dominant mode of tropical stratospheric variability that can influence tropospheric circulation and convection. El Niño‐Southern Oscillation (ENSO), which is the primary source of interannual variability in the tropical troposphere, can modulate both the QBO and its impact on the tropical troposphere. However, separating the tropospheric QBO teleconnection from the dominant influence of ENSO remains challenging. In a companion article, we used an atmosphere‐only experiment under climatological conditions to isolate the QBO impact and found that the QBO modifies convection over the Maritime Continent, particularly during summer and autumn. Here, we use two additional atmosphere‐only experiments under strong El Niño and La Niña conditions to explore how ENSO modulates the QBO impact. The results show that the QBO affects summer tropical convection in all experiments. For La Niña, the QBO impact resembles that under climatological conditions, but for El Niño, this impact shifts equatorward in early summer following the ENSO‐driven change in mean convection. Our findings also show that the QBO period is shorter (longer) under El Niño (La Niña) conditions. The faster downward phase propagation during El Niño leads to a sign reversal of the tropospheric QBO impact from early to late summer, evident across several dynamical and thermodynamical fields, including temperature, lapse rate, vertical motion and the divergent circulation. The QBO influence on tropical convection is therefore modulated by ENSO through shifts in the location of deep ascending motion and changes in the QBO downward‐propagating characteristics.