Recent evidence points to a decadal modulation of the quasi-biennial oscillation (QBO), one that varies with the 11-yr cycle of UV irradiance and ozone heating in the upper stratosphere. Interaction between the QBO and the Hadley circulation is considered here through an analysis that accounts for cyclic variations in their relationship, which may cancel and, hence, be invisible in the long-term average. The analysis reveals coherent changes in the tropical stratosphere and troposphere. Involving periods shorter than 5 yr, their relationship manifests itself in major properties associated with the QBO and the Hadley circulation. Like the QBO's relationship to the polar stratosphere, its relationship to the Hadley circulation reverses on the time scale of a decade. The systematic swing in their relationship leads to two important implications: 1) Interannual changes of one circulation operate coherently with changes of the other, reflecting their interaction. 2) At least one is influenced by a decadal variation. The latter is interpreted in light of the cyclic variation of ozone heating in the upper stratosphere, where the phase of the QBO is set.