Negotiating the ice – How Polarstern breaks through the Arctic

Quelle: blogs.helmholtz.de/polarstern/en/

8 June 2015
Fokje Schaafsma, Hauke Flores and Anna Nikolopoulos

At first it feels like flying over the ice. Gently the bow of Polarstern hovers forward over the ice floe at the speed of racing horses. Then a scream and a crack, as the keel hits the ice with its bladelike front edge, sometimes creating extra noise in our cabins and labs, books in the shelves falling forward by sudden deceleration. At this moment, tension on the bridge reaches its maximum. Will the floe break, or will we get stopped, and be forced to dwell backwards, taking yet another leap in the attempt to push our way through the frozen ocean? Then, the dark sound of muffling snow and crushing ice takes over, as the bow of Polarstern sinks through the ice with its full weight, making way for another half ship length.

In fact, the expression ‘icebreaker’ seems misleading for a vessel of the Polarstern type. Rather, the way the ship makes its way through the ice is a constant negotiation between the ice, the weather and the ship. Sea ice of the same thickness can easily be worked through by our 20,000 horsepower engines on one day, but just as well can keep us stuck for 12 hours on the other. When the wind presses the ice together, the ice pressure on the ship acts like a hydraulic brake. Snow can increase the friction during ice-breaking; inhibiting the forward progression of the ship like glue. As a rule of thumb, the resistance of a 10 cm snow layer on the ice equals one extra meter of ice thickness.

Sometimes it is a contest between the bridge officers, the one off-shift teasing the officer on shift with jokes as to how he would never make mileage. Then, bowed over the latest satellite image and over the sea ice radar of the ship, the perfect track between the kilometer-sized ice floes is discussed. Depending on the ambient conditions, finding the best way of breaking the ice is a delicate art. In good conditions, young ice of up to 1.5 m thickness can be penetrated just by the forward propulsion of the vessel. Often, the challenge is to find the way along the edges of ice floes or through open leads. In heavy, ridged ice, another approach is taken: instead of ramming straight forward into the ice floe, the ice is ‘sliced’ away by hitting it from a side angle, alternating between portside and starboard side.

In heavy Arctic springtime ice as we find it here, the negotiations between ice, weather and ship may not always result in the course desired by the ship. Sometimes the drift of the ice is directed against the intended course and faster than the progression of the ship. Effectively, this may result in moving in the opposite direction than intended. Our cruise track of two days ago gives good proof of how difficult these negotiations were, as we were trying to make our way further east.

Standing at the reeling on the working deck, one can observe huge broken chunks of sea ice passing by. Glowing blueish in the light from the low-standing sun, they tell their story to the observant visitor. Some have distinct layers of differing structure, recapitulating the succession of freezing and melting. Some have brown enclosures of earth, brought from the shallow sea floor of the Siberian shelf, where the ice floes once formed. Some are stained with a brownish-green layer: algae growing in the numerous channels and cavities inside the ice. Every now and then, one can observe a fish wriggling on an over-turned ice floe: Polar cod, a welcome treat to the birds following the ship.

The crushing, the muffling and ramming has stopped. Time for a detailed look at the ice, its associated lifeforms, and the stories they can tell us. Time for an ice station.