NZ scientists endured the dark polar winter to find what drives the dramatic growth of sea ice

A New Zealand scientific team has shed light on an important role of ice shelves around Antarctica. Initial results have shown for the first time that cold water melted from ice shelves enhances the dramatic growth of sea ice over winter. This mechanism potentially explains why Antarctic sea ice has not seen the same dramatic decline as Arctic sea ice.

The team from NIWA, Otago University, IRL, and Victoria University, was led in Antarctica by Dr Andy Mahoney (Otago University). They wintered at New Zealand's Scott Base from February - October, conducting surveys at two locations in McMurdo Sound.

They recently returned from their eight-month stint in Antarctica with prodigious amounts of scientific data.

Satellites can see the surface of the sea ice, but they can't tell us anything about the thickness or volume of the sea ice. Nor can they help us understand the complex relationship between ice shelves and sea ice, where melting from ice shelves helps sea ice grow. That process is becoming clearer as a result of this survey.

The scientists used three measuring techniques in their survey. Water temperature and salinity were measured to very high levels of accuracy from the surface down to the sea-floor, acoustics measurements of the currents and ice content of the water at different depths and ice cores were drilled to measure sea ice thickness. These gave the scientists important insights into what is driving sea ice formation.

They found that cold water melted from the floating part of the Antarctica's ice sheet (the "ice shelves") causes more sea ice to grow, especially in winter. This may explain why Antarctic sea ice has not declined as quickly as Arctic sea ice in response to global warming. In effect, the melting ice shelves may be buffering the sea ice.

This is just a year-long snapshot though. To really understand how sea ice and the oceans are changing in Antarctica, long-term sea ice monitoring is needed. "If you are measuring changes to the ice sheets, the bits on land, the timescales are hundreds to thousands of years. If you're looking at sea ice, then changes are going to be over years to decades," says NIWA's Dr Mike Williams.

Sea ice is a buffer between the ocean and the atmosphere and is a vital part of the Southern Hemisphere climate. It moderates atmospheric temperature changes by 'locking up' energy as it freezes. It also plays a very important role in a process that transports oxygen and carbon dioxide to the ocean bottom so that life can be supported even at great depths.

Antarctic sea ice is very dynamic. The sea ice is free to float northward into warmer waters rather than being land locked in as it is in the Arctic. As a result, almost all of the sea ice that forms during the Antarctic winter melts during the summer. During the winter, up to 18 million square kilometres of ocean is covered by sea ice, but by the end of summer, only about 3 million square kilometres of sea ice remains.

During this survey, scientists saw open ocean transformed into 2 metre thick ice in just a few months. For the first time one of the driving forces in this transformation was clearly observed and understood. The scientists' results show that it is the water from under the ice shelf that drives the dramatic sea ice growth.

Working in these extreme temperatures is a great challenge of endurance. It can take up to two hours in the morning to pack the skidoo before you can set off. The scientists stop every 150 meters to check the thickness of the ice they are travelling over - just to make sure they aren't going to fall through it. The scientists live in shipping containers during their stay. Tents are used infrequently - that's probably a good thing given the climate!

Last winter's research was funded by the Foundation for Research, Science and Technology as part of International Polar Year.