The flow structure downstream of a confined buoyant round jet in cross flow discharging into a channel is investigated. Experimental visualizations in the rectangular channel cross-section revealed that the interface between the upper layer of light gas and the lower clean air layer is distorted, appearing in the form of ‘hollows' and ‘bulges'. Numerical simulations show that these flow features are due to vorticity transfers from the upper to the lower part of the channel. Applied to the tunnel fire issue, this study contributes to the understanding of the impact of a longitudinal ventilation on a fire plume in a tunnel that can affect the smoke natural stratification.