Qeqertarsuup Tunua (Disko Bay), located in West Greenland, is a highly productive ecosystem and a major site of glacier-ocean interaction with the Kangia (Ilulissat Icefjord), yet its circulation and the mechanisms governing its seasonal variability have not been previously investigated. An empirical orthogonal function (EOF) analysis of upper-ocean velocity from the GLORYS12 reanalysis (1993–2026) provides the first quantitative description of the bay's dominant circulation mode, EOF1, which explains 55.8 % of the variance and is coherent through the upper ~130 m. EOF1 reveals a seasonal reversal between a cyclonic regime, in which an intensified West Greenland Current establishes cyclonic circulation in the bay, and an anticyclonic regime, in which the flow within the bay reverses. The seasonal cycle accounts for 49.5 % of the mode's variance, with the cyclonic regime prevailing from approximately May–June to November–December (with a peak in August), and the anticyclonic regime dominating in January–April. The alternation is consistent with a reversal of the along-shelf ocean surface stress forcing, strongly correlated with the circulation mode. The bay's circulation is, in turn, coherent with the West Greenland Current system at Davis Strait, where observed near-surface along-strait velocities flowing into Baffin Bay approximately double during the cyclonic regime and fall to near zero during the anticyclonic regime. Temperature transport towards the bay along the West Greenland shelf is an order of magnitude larger during the cyclonic regime than during the anticyclonic regime (+7.6 against +0.7 TW). Independent evidence from seven drifting-iceberg trajectories (June–July 2025) and an Argo float (June–October 2022) is qualitatively in line with these regimes. By governing the seasonal delivery of the warmest, fastest-flowing West Greenland shelf water into the bay, this circulation mode affects the bay's heat budget and the near-surface transport of freshwater, nutrients, and larvae that support the region's fisheries.