MIMO: Monitoring Melt where Ice Meets Ocean

Ice shelves play a crucial role in Antarctic ice dynamics. Ice shelves form when the ice reaches the ocean and starts to float according to the Archimedes principle. They spread under their own weight and typically extend hundreds of kilometers sewards before the ice eventually calves. Most ice shelves are fringed at their sides by ice islands and embayments and as long as this fringing remains, they slow down the inland ice flow. However, ocean and surface melting may destabilize ice shelves, hence leading to an upstream ice speed-up (such as they way you remove the cork on a bottle so that the liquid may pour out). This speed-up invokes an increased ice discharge into the ocean, hence sea level rise.

To determine the mass budget of ice shelves, one needs to carefully map its flow speed, which is generally done using satellite interferometry, calibrated with known velocities (for instance via GPS measurements). It is essential to know the mean flow speed, but also its variation over the course of a year to detect seasonal variability and the influences of tides.  Ice flux is then determined as the product of  ice thickness and flow-speed. Surface input (accumulation through snowfall) is determined from a combination of mass balance models and direct measurements in the field. The remaining unknown is the basal mass balance: at the ice-shelf bottom the ice can either melt or refreeze in form of marine ice. Due to the high salinity content of marine ice, radar measurements cannot detect the thickness of this marine ice layer . However, novel techniques, such as a phase sensitive radar (pRES) are capable of detecting the rate of change of basal layers in the bottom part of the ice-shelf ice, which enable to determine both accretion and melt rates.

One difficulty in studying Antarctic ice shelves arises from their unsteady nature. The ice geometry is evolving rapidly (years), ice is being advected at speeds >1 km/yr whilst the ocean beneath is expected to respond at sub-annual scales. In addition, there are many reasons to expect that the spatial pattern of melt is complex, where sub-shelf melting is concentrated in weaker zones of ice shelves, such as longitudinal bottom channels. Only a few studies have been focused on the spatial pattern, which requires high-resolution interferometrically-determined ice flow velocities and surface elevation and changes herein. Their application therefore remains limited to specific ice shelves or ice shelf sections for a particular snapshot in time, which is not useful for wider mapping, monitoring and impact assessment, which will be addressed in MIMO.

MIMO objectives

The main objective of MIMO is to quantify basal melt of ice shelves surrounding the Antarctic ice sheet at high spatial and temporal resolution (monitoring) to derive improved parametrizations for use in ice sheet modelling studies. The latter will also be implemented in a state-of-the-art ice-sheet model and compared to currently available datasets. Since basal melting under ice shelves is not only a great unknown, its spatial and temporal change is even more so.  Monitoring changes in ice shelves is considered of utmost importance in understanding and projecting future impacts of climate change on sea level.

The new fleet of high-resolution SAR satellites (ALOS-PALSAR2: COSMO-SkyMed constellation; TerraSAR-X; Sentinel) have an unprecedented temporal coverage, horizontal, and vertical accuracy (TanDEM-X elevation models are comparable to GNSS ground-truth at sub-meter level). Using state-of-the-art interferometric processing software, time series of surface elevation will be provided operationally. Because ice shelves are floating on the ocean, ice thickness can be derived from surface elevation considering hydrostatic equilibrium, but will require knowledge of density variations throughout the firn, which will be obtained from regional atmospheric models. Ice flow velocity on ice shelves is also obtained through satellite interferometry. A highly innovative point that will be addressed in the frame of ice surface velocity estimation concerns TOPSAR interferometry applied to Sentinel, based on the CSL interferometric processor. Finally sub-shelf melt rates will be determined by combining all data in an Eulerian framework.

Expected results

Besides a better knowledge of basal mass balance of ice shelves, the project will also deliver new high-resolution surface topography and ice thickness maps as well as surface velocity data, which are all beneficial to a wider scientific community. Basal melt rates will be employed to investigate new parametrizations of basal melt rate underneath ice shelves as a function of local geometry and ocean parameters.