This study investigates multiscale oceanic interactions over complex deep-sea topography, with a particular focus on deep-ocean submesoscale processes in the Drake Passage of the Southern Ocean. By combining high-resolution numerical simulations, analysis of in situ observations, and theoretical investigation, we systematically examine the dynamical characteristics of deep-ocean submesoscale motions and their influence on turbulent mixing. Particular attention is given to the generation, evolution, and energy-transfer pathways of submesoscale processes arising from the coupled interactions among waves, background currents, and complex topography. We further clarify the mechanisms through which these processes induce flow instabilities, internal-wave radiation and breaking, and ultimately trigger enhanced turbulent mixing. On this basis, the overall contribution of deep-ocean submesoscale processes to turbulent dissipation and deep mixing in the Drake Passage is quantitatively assessed, and the dynamical causes of the discrepancies between current in situ estimates of turbulent dissipation and theoretical predictions are explored. These results advance the understanding of deep-ocean submesoscale dynamics and multiscale energy transfer over complex topography, and provide a theoretical basis for interpreting the regulation of deep mixing in the Southern Ocean and for improving related parameterizations in ocean models.