Photo: Mertz Glacier Polynya April 27 2010
In mid-February 2010, a massive iceberg designated B09B collided with the Mertz Glacier Tongue, the section of the glacier that protruded about 100 km out from the Antarctic coastline at about 145°E. The collision precipitated the calving of another massive iceberg from the tongue, C28, which has a length of 78 km and width between 33 and 39 km. This calving event removed 80% of the tongue, leaving only a 20 km long stub. The calving had been anticipated, as rifts cutting across the tongue have been developing over many years, but the timing was not.
The region about the Mertz Glacier plays an important role in the global ocean over-turning circulation. Polynyas in the region are a source of about 25% of Antarctic Bottom Water. The production of bottom water drives the deep over-turning circulation of the global ocean, carrying oxygen and nutrients to the ocean depths in all ocean basins.
The polynyas are areas of open water or low sea ice concentration. They recur and persist along the coast and adjacent to the Mertz Glacier Tongue and grounded icebergs. Here, new sea ice forms under the influence of persistent strong off-shore winds. These winds carry away heat lost from the water to the atmosphere, enhancing the freezing rate of the water. The winds also sweep away the newly formed sea ice, thus removing relatively fresh water, and re-exposing the surface of the water to the atmosphere. This process makes the area a very efficient sea ice "factory". The salt rejected during the freezing process creates cold dense water which sinks to the ocean bottom and ultimately forms bottom water.
The calving of the glacier tongue and the shift of the icebergs has changed the geography of the main polynya that was adjacent to the glacier. Iceberg C28 has moved very slowly westwards and for a time was sitting in what was the Mertz polynya. At the beginning of April, it had split into several still massive sections, and by the end of April had moved about 250 km from the glacier and is expected to continue outwards across the edge of the continental shelf. Iceberg B09B remains partly grounded to the north-east of the glacier. Satellite images show that the "ice factory" role of the polynya is still functioning. The Mertz polynya was temporarily split into several smaller sections by the presence of iceberg C28. With the exit of iceberg C28 the polynya appears to be now working as before. But further changes can be expected when iceberg B09B once again begins to move.
The Mertz polynya and glacier are the focus of research programs in ACE. Our partners in LEGOS (France) have GPS beacons on the glacier and iceberg C28 for a study of the glacier calving and iceberg evolution. Moorings have been deployed in the polynyas, and we have a voyage scheduled to go to the region at the end of 2010 to undertake oceanographic surveys. The body of work already undertaken and planned places us in a opportune position to use the area as a natural laboratory - to observe and monitor the impacts of changes in the glacier system and its surrounds on the ocean and climate system as well as the regional biology. Satellite remote sensing will continue to play a key role in this monitoring.
The image acquired by the Advanced Synthetic Aperture Radar on ENVISAT on 20 March 2010 shows the region about the Mertz Glacier (MG), the icebergs C28 and B09B, and the polynyas (P) within the sea ice. The Antarctic coastline is at the bottom of the image. The wind is blowing offshore and to the left of the image. The polynyas (P) are the dark areas in the lee of the massive icebergs and the lines of grounded small icebergs (GB), as well as the patterned areas along the coast to the west of the glacier. Image size is 500 km x 350 km.
The image acquired by the Advanced Synthetic Aperture Radar on ENVISAT on 27 April 2010 shows the region about the Mertz Glacier, the icebergs C28 (A&B) and B09B, and the polynya within the sea ice. The Antarctic coastline is at the bottom of the image. The wind is blowing offshore and to the left of the image. The polynyas are the relatively dark areas in the lee of B09B and the lines of grounded small icebergs in the right half of the image, as well as the patterned areas along the coast to the west of the glacier. The light band across the top of the image is pack ice moving with the coastal current along the outer edge of the continental shelf, from the east (left) to the west (right). Image size is 500 km x 350 km.
