Weekly Reports Polarstern
The Expedition PS111 Cape Town - Punta Arenas
The cruise leg PS111 FROST, Filchner Ronne Outflow System Tomorrow, will start on 19th January 2018 in Cape Town (South Africa) and will end on 14th March 2018 in Punta Arenas (Chile). Polarstern will sail directly to the ice-covered Atka Bay to supply the German station Neumayer III. Due to a variety of field campaigns in the back-country of the station, the logistic requirements are at the limits for the ship so that the length of stay near Neumayer will take this in to account. At Neumayer one berth has to be exchanged so that during the whole cruise the ship is full to the limit. If the ice situation allows, we will have a short scientific programme within the coastal current to test the equipment of all working groups. Then Polarstern will adjourn to its main scientific region, the vast shelf areas in front of the Filchner-Ronne ice shelf. Here an extensive oceanographic and biological programme will proceed and complete the station grid, which already started during the PS82 FOS campaign (Filchner Outflow System) in 2013/2014 and the PS96 FROSN expedition (Filchner Outflow System Now) in 2015/2016
The wide southern shelf in front of the FRIS (Filchner Ronne Ice shelf) is characterized by the interaction of very cold ice shelf water (ISW) with the warm deep water (WDW) of the Weddell Sea. The mixture of both is able to build the Weddell Sea Deep Water (WSDW) as well as the Weddell Sea Bottom Water (WSBW) which act as the precursor of the deep waters of the global oceans and which are very important for their ventilation.
These hydrographical features mainly acting at the continental slope are supposed to be the primary cause converting this region into a biological "hotspot" as indicated by recent investigations.
As a supplement to the preceding expeditions the Ronne Trough is scheduled to measure the source area of the densest water mass, the High Salinity Shelf Water (HSSW). This water mass acts as the driver of the circulation in the ice shelf cavity and is therefore of great importance.
The formation of deep and bottom water (WSDW/WSBW) in the southern Weddell Sea is strongly influenced by flow of Ice Shelf Water (ISW) out of the Filchner-Ronne Ice cavity. Own hydrographic measurements along the Filchner Ice Front carried out with Polarstern in 1995, show that the breakout of three giant icebergs in 1986 and their grounding on the shallow Berkner Bank still modified the circulation and water mass formation in the Filchner Trough. Since then the iceberg A23A still exist and is responsible for the increased sea ice concentrations east and south of the iceberg’s position.
Even the adjacent sea areas further east show significant changes in the water mass characteristics and flow patterns compared to measurements from the early 1980s. New measurements from the summer seasons 2013-2014 (PS82) and 2015-2016 (PS96) provide a new picture showing the outflow of ISW on the eastern flank of the Filchner Trough. Recent model scenarios with the finite element model FESOM indicate that a redirection of the coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf during the twenty-first century would lead to increased inflow of warm MWDW waters (Modified Warm Deep Water) into the deep southern ice-shelf cavity. These changes will first effect the Filchner Trough and then the total circulation underneath the FRIS. If it is possible to detect also changes in the characteristics of the HSSW in the area of the Ronne trough compared to the last measurements in 1995 and 1998, the model predictions for the increased melting rates could be validated more precisely.
A higher shelf ice dynamic with higher numbers of iceberg scouring events and an increase in water temperature will significantly influence the biodiversity of the southern Weddell Sea.
In addition to the 4 ice shelf moorings, which continuously report the data since the FIS drilling programme in 2015 direct out of the Filcher ice stream cavity, the stations serviced by Polarstern in front of the ice shelf edge are necessary to interpret the changes of water masses even for the future.