Submarine glacier melt rates of the Greenland Ice Sheet remain a major uncertainty in climate model projections of future sea level rise. Development of submarine melt parameterizations have to a high degree relied on ocean circulation modelling of glacial fjords, designed to quantify effects such as ocean thermal forcing and fjordglacier geometry. Greenlandic fjords are relatively narrow, and it is frequently assumed that across-fjord flow variations are small enough to allow marine melt to be quantified with two-dimensional ocean–circulation models. Here, we present three–dimensional model simulations showing that the interplay between fjord–glacier geometry, side wall friction, and Earth’s rotation makes the circulation in ice–shelf cavities three–dimensional even in narrow fjords. Remarkably, we find that Earth’s rotation changes the flow pattern in the cavity below the ice shelf leading to a decrease in the marine melt on a 10 km wide ice shelf by a factor of five compared to a non–rotating simulation. Our study prompts using three–dimensional model configurations of Greenlandic fjords.