ALBERT

Description: The Filchner-Ronne Ice Shelf (FRIS), the largest (by volume) floating extension of the Antarctic Ice Sheet (AIS), fringes the southern Weddell Sea known to be the dominant source of the globally relevant Antarctic Bottom Water. As a link between ocean and ice sheet, this ice shelf plays an important role for the stability of the AIS and the preconditioning of water masses participating in the global thermohaline circulation. The dominant process serving this pivotal role of FRIS is the exchange of heat, salt and tracers at the base of the ice shelf. While the southern Weddell Sea has been considered as largely invulnerable to climate warming, recent projections point to a potential tipping of the ocean state from cold to warm by the end of this century. The lack of detailed knowledge about the ocean underneath FRIS and the possibility of dramatic changes in the near future brought together scientists from the UK, Norway, and Germany. In the framework of the Filchner Ice Shelf Project, they intensively investigate the southern Weddell Sea continental shelf, including the FRIS cavity, by means of ship-based observations, moorings in front of and beneath the ice shelf, sub-ice shelf water sampling, and numerical modeling. This presentation reviews the achievements of the Alfred Wegener Institute over the past 6 years focused on observation, modeling, and comprehensive understanding of on-shore flow, dense water formation, sub-ice shelf circulation, meltwater production, and Ice Shelf Water spreading on the southern Weddell Sea continental shelf. All together has an impact on the ice shelf mass balance and thus on the discharge of inland ice with consequences for global sea level rise.

Description: The Filchner Outflow System (FOS) in the southern Weddell Sea is one of the most important areas for deep water formation. Here the outflow of Ice Shelf Water (ISW) of the Filchner Ronne Ice Shelf interacts with Warm Deep Water (WDW) of the Weddell Gyre circulation, resulting in Weddell Sea Deep and Bottom Water production (WSDW, WSBW). The interaction around the sill of the Filchner Trough is thought to result in a physical oceanography "hotspot" that may also aggregate primary and secondary producers, leading to increased abundance of top predators. However, data on top predator abundance for the FOS are scarce. Two aerial surveys over ice-covered sea were carried out to estimate density gradients and the regional abundance of pack ice seals in the FOS. A digital imaging survey with fixed wing research aircraft Polar 6 in November 2013 preceded a helicopter survey from aboard RV Polarstern during January 2014. The Polar 6 survey comprised 11 transects summing to a survey effort of 1,148.44 km. 265 seals were counted on transect lines with a higher encounter rate on transects located further to the west. The helicopter survey allowed seal species to be identified. Twenty-five transects (1,367.61 km) were flown in the FOS region and another six transects (425.54 km) further south within the Filchner Trough. Only two seal species, the crabeater seal (Lobodon carcinophaga) (n = 754) and the Weddell seal (Leptonychotes weddellii) (n = 217), were observed. Distance sampling analysis suggested that the density of seals differed strongly between the two survey regions, with very few seals encountered on transects located in the more southerly Filchner Trough region. A longitudinal density gradient increasing from east to west within the FOS region during January 2014 supported the results obtained from the Polar 6 survey, but failed to support the idea of a top predator hotspot at the sill of the Filchner Trough at least for seals.

Description: The region of the Filchner Outflow System (FOS) in the southeastern Weddell Sea is characterized by intensive and complex interactions of different water masses. These hydrographic features convert the FOS in an oceanographic hotspot, which may also show enhanced biological productivity and corresponding aggregations of marine top predators. In this context, six adult Weddell seals (Leptonychotes weddellii) were instrumented with CTD- combined satellite-relay dive loggers during austral summer 2014 to investigate the influence of environmental conditions on the seals’ foraging behaviour over winter and identify potential foraging hotspots in the FOS. Weddell seals preferred foraging in shallow areas of the continental shelf (〈 700 m), where they presumably exploited the abundant bentho-pelagic fish fauna during both pelagic and demersal dives. Diurnal and seasonal variations in light availability also affected foraging activities. Furthermore, Eastern Shelf Water and modified Warm Deep Water were associated with increased hunting time and foraging effort. Moreover, two areas in the FOS were emphasized as potential foraging hotspots characterized by long residence times suggesting enhanced prey availability. However, the underlying biological principles contributing to these foraging hotspots still remain unclear. This highlights the importance of further ecological investigations as the FOS is already threatened by predicted climatic changes.

Description: The region of the Filchner Outflow System (FOS) in the southeastern Weddell Sea is characterized by intensive and complex interactions of different water masses. These hydrographic features convert the FOS in an oceanographic hotspot, which may also show enhanced biological productivity and corresponding aggregations of marine top predators. In this context, six adult Weddell seals (Leptonychotes weddellii) were instrumented with CTD- combined satellite-relay dive loggers during austral summer 2014 to investigate the influence of environmental conditions on the seals’ foraging behaviour over winter and identify potential foraging hotspots in the FOS. Weddell seals preferred foraging in shallow areas of the continental shelf (〈 700 m), where they presumably exploited the abundant bentho-pelagic fish fauna during both pelagic and demersal dives. Diurnal and seasonal variations in light availability also affected foraging activities. Furthermore, Eastern Shelf Water and modified Warm Deep Water were associated with increased hunting time and foraging effort. Moreover, two areas in the FOS were emphasized as potential foraging hotspots characterized by long residence times suggesting enhanced prey availability. However, the underlying biological principles contributing to these foraging hotspots still remain unclear. This highlights the importance of further ecological investigations as the FOS is already threatened by predicted climatic changes.

Description: The Weddell Sea plays an important role for the global oceans and climate by being one of the biggest production and export areas of Antarctic Bottom Water (AABW). Circumpolar Deep Water (CDW) enters the Weddell Gyre (WG) at its eastern boundary. Then called Warm Deep Water (WDW), it is a major contributor to the formation of deep and bottom waters due to ocean-ice shelf interactions in the southern and soutwestern Weddell Sea. Hydrographic data collected between 0 and 30°E on the RV Polarstern cruise ANT XX/2 reveals a two-core structure for the eastern inflow of warm water at roughly 20°E but not further downstream at the Greenwich meridian (GM). Model results and climatological fields suggest that the two cores represent two separate modes of warm inflow. One mode is driven by eddy mixing in the northeastern corner of the WG and the other one is an advective mode, forming the southern branch of the inflow which extends beyond 30°E before turning westward. Both pathways are likely to carry waters from different origins within the Antarctic Circumpolar Current (ACC), where more ventilated CDW is found at the Southern Boundary (SB) compared to the centre. The southern route shows considerable interannual variability in the model. A variable inflow of two types of CDW together with admixed recirculated and cooler waters from the Weddell Sea can potentially contribute to the observed variability and warming trend of WDW over the last decade at the GM.

Description: The Filchner Ronne Ice Shelf (FRIS), located in the southern Weddell Sea, plays a key role in the dense water formation, which is a precursor of AABW in the world ocean. Today, the ice shelf is protected by a large cold continental shelf. Model studies, however, have suggested the potential for an increased flow of Modified Warm Deep Water (MWDW) toward and under FRIS via the Filchner Trough, causing a substantial increase in basal melt rates by the end of this century. Historic data in the region suffer from a strong summer bias. New two-year long mooring time series from 2014 to 2016 reveal a distinct seasonal cycle in hydrography along the eastern flank of the Filchner Trough, with a southward flow of MWDW only in summer, connected to a seasonal shoaling of the Antarctic Slope Front (ASF). With the goal of analysing the seasonal and interannual variability of this warm inflow, we set up a new global configuration for the Finite Element Sea-ice Ocean Model (FESOM) with increased resolution over the Weddell Sea and eddy-permitting resolution over the continental shelves. We find a strong sensitivity of the on-shelf flow of MWDW to the representation of the general Weddell Gyre circulation and in particular the ASF properties upstream of the Filchner Trough. Both improve significantly with the new high-resolution grid. In order to ensure a correct representation of the water masses encountering the Filchner Trough, which is necessary to study the inflow dynamics and variability, we perform an experiment with restoring the upstream temperature and salinity field over the continental slope. For this, a monthly climatology at17°W compiled from historic data was incorporated into the World Ocean Data Atlas.