Tides at the ice shelf edge

Antarctica +++ 70° 31’ S, 8° 45’ W +++ --‐2°C, cloudy, snow showers +++

11 January 2015
(by Peter Lemke, translation by Ina Lefering)

It was important to find a good mooring site for Polarstern to supply Neumayer station with fuel (380,000 litres Arctic Diesel and 60,000 litres kerosene). Polarstern had to stay absolutely still for 24 hours during the pumping process, because the hose for the fuel had to be free from any tension. We chose a bay in the ice shelf edge, which was sheltered by an iceberg that grounded on the seafloor north of the bay. Strong easterly winds came over the iceberg and ice shelf and pushed ice floes out of the bay, so that a 2 nm-wide lee polynya was created and stayed open.

Now, that we were reasonably well protected from drifting sea ice, Polarstern’s operations were still affected by the tides. Calculations with a tidal model showed that the difference between high and low water was about 1m at our spot. This rise in seawater level, however, does not affect the tension on the hose, because the ice shelf floats on the sea and moves up and down with it, and the ship. Hence, the distance of 30 m between ship and ice shelf edge stays constant. The tidal current – the incoming/outflowing water during ebb and flood – moves with 10 cm per second. But our main propeller was able to compensate this flow with the support of the two sideways-oriented thrusters, one in the front and one in back.

Once we had finished pumping fuel off the ship, we determined the effect of tides on temperature and salinity in the polynya by running hourly CTD casts. We observed that during ebb tide cold water flows out of the area underneath the ice shelf and is pushed back out of the polynya during flood tide. If the temperature of the outflowing water is low enough (super-cooled) platelet ice, which we found on several occasions under the sea ice, forms.