High-resolution simulations show that summer-monsoon upwelling in the western Arabian Sea ignites a vigorous submesoscale vertical heat flux (SVHF), overturning the long-held belief that submesoscale activity peaks only in winter. Monsoon-sharpened fronts spawn submesoscale instabilities whose upward heat transport dwarfs that of co-existing mesoscale eddies. By steepening isopycnals and accelerating baroclinic eddy growth, the SVHF forges a positive mesoscale feedback loop that amplifies and sustains vertical heat exchange throughout the upwelling season. This multiscale coupling under strong monsoonal forcing and a forward kinetic-energy cascade enhances both the magnitude and longevity of heat redistribution. The results reveal a previously unrecognized pathway active in coastal upwelling systems and highlight the pivotal role of submesoscale–mesoscale interactions in setting the ocean’s vertical heat budget.