The Greenland Ice Sheet is the primary cryospheric source of global sea-level rise, and at least half of its mass loss acceleration since the 1990s originates from surface melt. Previous studies using melt extent focus on the role of longwave radiation, enhanced by the increasing liquid-containing clouds. Using surface mass loss measured by automatic weather stations (AWS) and simulated by the Regional Atmospheric Climate Model (RACMO), we find that the seasonal cycle of surface melt is dominated by shortwave radiation, determined by solar zenith angle and albedo feedbacks, while the sub-seasonal (i.e., daily) variability is dominated by sensible heat exchange, enhanced by the increasing vertical mixing during katabatic winds. This dominance of sensible heating on the daily timescale is robust throughout the study period from 1979-2017 and in most drainage basins, including the southwestern basin which contributes the most to surface mass loss. Using melt extent to study melt mechanisms tends to emphasize surface melt in the accumulation zone during massive melt events since surface melt occurs constantly in the ablation zone throughout melt seasons. Therefore, when investigating the leading contributors to the total surface mass loss and thus global sea-level rise, surface melt flux is more appropriate than melt extent.