Ice cold in Commonwealth Bay
Commonwealth Bay in the East Antarctic will be forever associated with Sir Douglas Mawson’s pioneering scientific venture a century ago: the Australasian Antarctic Expedition. Mawson’s team had only intended to spend one year south, but it was not to be. A sudden turn of events led to tragedy on the ice, the deaths of two men, allegations of cannibalism and, with the return of winter sea ice, an extended stay in Antarctica. While the efforts of Mawson and his men laid the foundations for modern Antarctic science, the region has continued to experience dramatic change. And it doesn’t get any more dramatic than the arrival of iceberg B09B in 2011. The word ‘berg’ doesn’t really do it justice though – B09B is a monster. Weighing in at an estimated 500 billion tonnes, this block of ice is 97 miles long and over 20 kilometres wide. There’s enough freshwater in B09B to provide all of New York’s drinking needs for 300 years. And it started out even larger. In 1987, an iceberg the size of Bali broke free from the continent in the Ross Sea. It was one of the largest bergs ever seen. Riding the westward-flowing ocean currents, the Ross Sea berg smashed its way along the coast. By the time it reached Commonwealth Bay, the largest remaining part was labelled with the unassuming moniker B09B. Smashing a tongue of ice from the Mertz Glacier that extended 100 kilometres out to sea, the berg ground itself in Commonwealth Bay, trapping a vast amount of sea ice.
|Icebergs in Commonwealth Bay|
Normally, chilling Antarctic winds blow off the continent year round, freezing the surface of Commonwealth Bay, regardless of whether it’s winter or not: the sea ice is formed, blown offshore, a new open area of water known as a polynya is formed, and more seawater is frozen in its place. As sea ice is created offshore, a fundamental process takes place below the surface. The salt in ocean water is effectively squeezed out of the ice creating a dense mass of water. In most of the Southern Ocean, this cold, super-salty water just diffuses away. In Commonwealth Bay, however, the conditions are perfect for the formation of something known as Antarctic Bottom Water, a super cold water mass that forms a key part of the world’s ocean circulation system. Worryingly, some scientists have suggested the arrival of B09B may have completely stalled Antarctic Bottom Water production, with potentially global implications.
As part of the Australasian Antarctic Expedition 2013-2014 we wanted to find out just what impact the arrival of B09B has had on ocean circulation. Ably led by Chris Fogwill and Erik van Sebille, we have a new paper out in The Cryosphere journal: ‘Impacts of a developing polynya off Commonwealth Bay, East Antarctica, triggered by grounding of iceberg B09B’. Here we report temperature and salinity across the area and compare these to previous measurements – including those made by Mawson and his team – and some very exciting ocean modelling results headed up by Eva Cougnon. The headline discovery is the polynya appears to have moved out into Commonwealth Bay, and may have led to resumption of Antarctic Bottom Water formation, in part compensating for the reduction around the Mertz Glacier. This is a fascinating find. Further work is now needed to continue monitoring this sensitive region of our planet to see whether the recovery continues into the future.
You can read our research paper for free at http://www.the-cryosphere.net/10/2603/2016/.