Submarine melting is an important contributor to the mass balance of tidewater glaciers in Greenland, and has been suggested as a trigger for their widespread acceleration. Our understanding of this process is limited, however. It generally relies on the simplified model of subglacial discharge in a homogeneous ocean, where the melting circulation consists of an entraining, buoyant plume at the ice edge, inflow of ocean water at depth, and outflow of a mixture of glacial meltwater and ocean water at the surface. Here, we use oceanographic data collected in August 2009 and March 2010 at the margins of Helheim Glacier, Greenland to show that the melting circulation is affected by seasonal runoff from the glacier and by the fjord's externally forced currents and stratification. The presence of light Arctic and dense Atlantic waters in the fjord, in particular, causes meltwater to be exported at depth, and influences the vertical distribution of heat along the ice margin. Our results indicate that the melting circulation is more complex than hypothesized and influenced by multiple external parameters. We conclude that the shape and stability of Greenland's glaciers may be strongly influenced by the layering of the Arctic and Atlantic waters in the fjord, as well as their variability.