Evaluating Southern Ocean Tides Using ICESat-2 over Ice Shelves

Abstract:

Most of the drainage from the Antarctic ice sheet is through its peripheral ice shelves, floating extensions of the land ice that cover 75% of the Antarctic coastline and 10% of the total ice-covered area. Large portions of the ice shelves outside the grounding zone are in hydrostatic equilibrium, subject to variations in ocean tides, circulation and sea level, as well as the inverse barometer effect induced by changes in atmospheric pressure. A major source of the vertical variability of the surface of ice shelves is due to the rise and fall of the oceanic tides. This vertical variability can be directly measured using measurements from the Ice Cloud and land Elevation Satellite-2 (ICESat-2), NASA's next generation laser altimeter. ICESat-2 has 1387 unique orbits that are repeated in an orbital cycle every 91 days. Over land ice in the polar regions, ICESat-2 is in repeat-track mode and resamples the ground tracks with high precision (less than 10 meters horizontal since orbital cycle 3). Here, we use repeat-track data from ICESat-2 to evaluate the tidal signature over ice shelves. While the tidal signal in a repeat altimeter is highly aliased and does not represent the full cycle of the tidal constituents, altimeter measurements do provide estimates of the spatial structure of tidal variation. Here, we calculate tide height estimates from both global and regional tidal models and compare with the elevation change derived from ICESat-2. We provide ice shelf specific estimates for the Circum-Antarctic Tidal Simulation (CAT2008), the Global Ocean Tide model (GOT4.10), the Finite Element Solution (FES2014) tide model, and the TOPEX/POSEIDON global tide model (TPXO9-atlas version 2). We also provide estimates of ice shelf flexure near the grounding zone, which will help inform the next generation of ocean tide models under ice shelves.