<p>This study investigates the dynamic interactions and mixing behaviors of incompressible variable-density flows using a large-eddy simulation, focusing on the evolution of the flow momentum behavior and scalar mixing in relation to the Atwood number. To further understand how density differences influence flow dynamics and mixing, conditional sampling techniques were applied to analyze each stream and examine the associated flow structures and scalar transport mechanisms. The results indicate that increasing the Atwood number enhances scalar mixing, extending it deeper into regions dominated by lighter fluids. The changes in scalar mixing associated with the Atwood number originate from the inherent flow dynamics, a phenomenon further clarified through conditional sampling analysis. The conditional sampling analysis revealed that, in the case with the highest density difference (Atwood number = 0.5), the lighter stream contributes up to 59 % of the total vertical scalar flux, leading to enhanced scalar mixing. Increases in the Atwood number enhance the turbulent kinetic energy, particularly in lighter fluids, improving scalar mixing. Notably, these changes in the scalar field are accompanied by a hydrodynamic field, which is influenced by the formation of large and strong structures. These structures are essential for enhancing the mixing efficiency, particularly in lighter fluids, where more substantial and dynamic formations are observed.</p>