High spatio-temporal velocity variations driven by water input at a Greenlandic tidewater glacier

Abstract. Ice flow controls the ice discharge at tidewater outlet glaciers and is, together with frontal ablation, a key contributor to the mass loss of the Greenland ice sheet. While annual glacier velocity variations of tidewater glaciers are well studied using satellite-derived data, research on their small-scale, short-term speed variations, ranging from sub-diurnal to multi-day scales, remains limited. We deployed a terrestrial radar interferometer, operating at a temporal resolution of one minute and a spatial resolution of a few meters, to investigate small-scale ice flow variations at the terminus of Eqalorutsit Kangilliit Sermiat, a tidewater outlet glacier in South Greenland. We observed clear diurnal and multi-day ice flow speed variations and link these to a high sensitivity of the glacier system to additional freshwater input. This water originates from different sources, such as enhanced surface melt during warm periods or sudden drainage events from subglacial or ice-marginal lakes. The amplitudes of diurnal velocity fluctuations remain remarkably consistent throughout the 6 km long terminus area, but their spatial evolution shows clear local variability. Spatio-temporal analysis of velocity map time-series revealed a general downstream propagation of diurnal velocity variations. However, on days characterized by particularly high ice flow speeds (multi-day speed-up events), these variations start at the terminus propagating upstream in a distinct block-wise pattern, connected to major rifts in the terminus area. Our high spatio-temporal resolution data underline the complex influence of water input and basal hydrology on the dynamics of tidewater glaciers.