We use an ice-flow model to demonstrate how flow variations initiated in the marginal zone of an ice sheet affect flow farther inland through longitudinal (along-flow) coupling. Our findings allow for an alternate interpretation of seasonal accelerations observed near the equilibrium line of the Greenland ice sheet (Zwally and others, 2002). We demonstrate that these observations can be explained by accelerations initiated up to 12 km closer to the margin where the ice is ∼40% thinner, is heavily crevassed, experiences a seasonal doubling of velocity, and where the ablation rate, surface meltwater flux and ice temperature are likely higher. Our modeling and observations suggest that conditions and processes normally found near ice-sheet margins are adequate for explaining the observations of Zwally and others (2002). This and considerations of the likely subglacial hydrology in the marginal zone lead us to suggest that seasonal accelerations may have limited impact on ice-sheet mass balance even in the face of climate warming.