Compounding sub-seasonal variations in Greenland outlet glacier dynamics revealed by high-resolution observations

Understanding the controls on seasonal velocity change for tidewater glaciers may provide insight into long-term retreat and acceleration. Leveraging recent high-resolution satellite data, we examine changes in surface elevation, velocity, and terminus position for four glaciers in Central Western Greenland over 2015–2021. Our approach uses a simplified force balance focused at the terminus to model the expected response in upstream velocity caused by the observed terminus changes. We find that seasonal velocities are strongly controlled by terminus advance/retreat for two glaciers. Residuals between modeled and observed velocities reveal two distinct signals: summertime pulses coincident with peak runoff and wintertime speedup extending several kilometers inland of the terminus. We evaluate the sensitivity of terminus-driven velocity to elevation change by incorporating observed seasonally varying surface topography and applying controlled modifications to the profile, specifically uniform vertical shifts and variations in surface slope. We find surface slope changes impact velocity response to terminus changes more than spatially uniform changes in elevation. Increased surface slope amplifies velocity response to terminus changes. While simplified, our model could be applied to other glaciers to assess the importance of terminus position change as a driver of seasonal velocity.