Hartmut H. Hellmer, Tore Hattermann, Michael Schröder, Svenja Ryan and Gerd Rohardt
Antarctic ice sheet mass loss and, thus, global sea-level rise is related to reduced ice shelf buttressing and subsequent glacier acceleration. Ice shelf thinning, caused by increased basal melting, allows ice streams to move faster, grounding lines to retreat and, consequently, ice discharge to enhance. Basal melting is either fueled by ocean heat transported from the open ocean underneath ice shelves or by high-saline waters formed in polynyas off ice shelf fronts. A recent model study indicates that future atmospheric conditions in the southern Weddell Sea can switch the shelf circulation, formerly dominated by the formation of saline waters, to one influenced by open ocean waters with severe consequences for melting at the base of the Filchner-Ronne Ice Shelf.
Therefore, in 2015 AWI initiated in collaboration with the British Antarctic Survey (BAS) the Filchner Ice Shelf Project (FISP) for monitoring the present-day hydrographic conditions underneath the Filchner Ice Shelf (FIS) and retrieving basal melt rates and the ice shelf’s thermal regime. The project is aimed to foster our understanding of the ocean’s role in driving the evolution of Antarctica’s largest (by volume) ice shelf, floating in cold waters but threatened by warm waters of open ocean origin in a warmer climate.
After 20 years, FISP is AWI’s second attempt to deploy instruments within and underneath the Filchner-Ronne Ice Shelf (FRIS) – this time focused on the Filchner Ice Shelf (Fig. 1). The aim of the project is to monitor the thermal characteristics of and the hydrographic conditions beneath FIS (up to five years depending on battery reliability). FIS covers the Filchner Trough and, thus, (1) the main route of glacial melt from the whole FRIS and (2) the projected path of warm water intrusions from the Weddell Sea towards the grounding line. Glacial melt water contributes to Ice Shelf Water, the main ingredient of Weddell Sea Bottom Water formed at the continental shelf break.
The glaciological interest is focused on measurements, for the first time ever, of the FIS basal melt rates, retrieving in its southern part even the spatial distribution. A one-year daily record of basal melting will be obtained at one site, deploying a phase sensitive radar (pRES) of the newest generation. In addition, a GPS will be deployed for one year, allowing for the study of perturbations of the mean speed by grounding line processes and sliding of the ice stream due to tidal forcing.
The high-gain of this project is the combination of glaciological and sub-ice measurements, which will allow to relate changes in basal mass loss and ice stream dynamics to sub-ice ocean variability in a region affecting both the East and West Antarctic Ice Sheet.
After imense logistical efforts in the austral summers of 2013/14 and 2014/15, four holes were successfully drilled and equipped with instruments on both sides of the southern FIS in the austral summer season 2015/16 (Fig. 1). At the two oceanographic sites, the instruments record ocean currents, pressure, temperature, and salinity at six depth levels, covering the 450-m thick water column beneath the 850-m thick ice shelf (Fig. 2). Since all sites are equipped with a satellite link, data is transferred to AWI to be quality controlled and stored. First analysis shows that the southern FIS cavity is dominated by a two-layerd ocean with the thickness of both layers and the interface being strongly influenced by tides (Fig. 3).
An interview (in German) with one of the German scientists directly from the field camp on the southern Filchner Ice Shelf (Fig. 4) can be found at:
Latest updates about the present hot water drilling on the northwestern Filchner Ice Shelf (green squares in Fig. 1) are posted on Twitter:
Fig. 1: Bottom topography (blue = shallow, red = deep) of the southern Weddell Sea (see insert) with the locations of the instrumented bore holes, operated by the British Antarctic Survey, the Alfred Wegener Institute, and the University of Bergen. Arrows indicated the current pass of warm waters of open ocean origin along the continental slope (red), and into the Filchner Trough projected for a warming climate (yellow). The ice shelf fronts are marked as thin blue lines. (Photo: Tore Hattermann, AWI)
Fig. 3: Time series of ocean temperature (upper panel ) and velocity (lower panel) recorded so far at certain depth levels (see Fig. 2) beneath the southeastern Filchner Ice Shelf (Fig. 1). Solid lines represent the 5-day running mean velocity.
This link leads to the daily updated figure: http://apps3.awi.de/FISP/fse2_timeseries.png
Fig. 2: Schematic of the sub-ice shelf mooring arrangement. The instruments record temperature, salinity, and depth (yellow), and current speed and direction (green). (Photo: Tore Hattermann, AWI)
Fig. 4: Panorama view of the drill site on Fichner Ice Shelf. Important instruments are the hot water pressure hose (left), the aggregates for water heating (center), and the water reservoir with Pistenbulli (background right). (Photo: Jörg Brozek, AWI, Glaziologie)