Modelling GNSS-observed seasonal velocity changes of the Ross Ice Shelf, Antarctica, using the Ice-sheet and Sea-level System Model (ISSM)

Abstract. The flow speeds of floating ice shelves around the Antarctic Ice Sheet exhibit clear intra-annual variability. However, the drivers of this variability remain poorly understood. Here, we present three new velocity datasets from Global Navigation Satellite System (GNSS) stations on the Ross Ice Shelf collected between early 2020 and late 2021 and show that they have two distinct peaks observed in austral summer and austral winter. These measurements do not appear to be consistent with the yearly cycle of sea surface height, which has previously been identified as a possible driver. We investigate the potential role of basal melt variability in ice flow speed and use the Ross Ice Shelf as a test bed. First we identify the regions where changes in melt would have the largest influence on ice speed at our GNSS sites using automatic differentiation. We then apply idealized sinusoidal perturbations to modelled basal melt rates at these specific locations to identify what magnitude of variability is needed to match the GNSS-observed changes in ice speed. We show that, while very local perturbations in basal melt can have a significant impact on ice flow speed, the amplitude of the perturbation required to match observations is significantly higher than expected, which may indicate that these perturbations are not realistic. We suggest that a combination of external forcings and internal mechanics may be needed to reproduce the observed intra-annual velocity variation at all the GNSS sites.