The Changing Icescape About the Mertz Glacier Polynya, East Antarctica, and its Impacts
ACE CRC, AUSTRALIA
The calving of the Mertz Glacier Tongue [MGT] in early 2010 brought about major changes to the icescape in the region of the Mertz Glacier. Removal of a 75 km length of the ice tongue, and the re-location of the massive iceberg B09B that was grounded nearby have changed the way that sea ice forms, and where it can move along the coast. This change has impacted the spatial distribution of sea ice, the location of polynyas in the region, and the processes involved in the production of sea ice and cold dense saline water that leads to the formation of Antarctic Bottom Water [AABW]. We have used data from a variety of satellite-borne sensors to monitor the region over the years prior to the calving and since that event. These sensors include: ASAR and MERIS on ENVISAT, AMSR-E on Aqua and AMSR-2 on GCOM-W1, MODIS on Terra and Aqua, GLAS on ICESat, SIRAL on CryoSat2. Prior to the calving, the approximately 90 km length of the MGT that protruded from the coast formed a physical barrier against the westwards transport of sea ice through this region. A large active polynya formed in the protected area on the west side of the tongue where strong winds blowing across the coastline repeatedly swept the sea surface clear of new sea ice. Heat was lost from the ocean to the atmosphere at a very high rate, and cold, dense saline water filled a deep basin under this very active "ice factory". This large polynya together with smaller adjacent polynyas formed a distributed system that contributed between 15 % and 25 % of total annual AABW formation. In conjunction with the calving a large expanse of thick multi-year / multi-decadal sea ice residing to the east of the MGT broke out. Intense phyto-plankton blooms occurred in association with the break-up and melting of this sea ice and its very thick snow cover. Iceberg B09B initially moved and re-grounded next to the remaining stub of the MGT, and then moved further west into Commonwealth Bay and close to the coast. Polynyas still form within the sea ice zone, but with smaller areas, and are often filled with new sea ice. This inhibits their activity, and so the sea ice and dense saline water production rates have decreased. Oceanographic observations made in 2010-11 and 2011-12 showed that the water column over the continental shelf and in the deep basin had markedly freshened. That work showed that AABW production had declined considerably in 2010-11 and further again in 2011-12. Bottom water flows to the depths of the ocean basins and drives the deep ocean over-turning circulation. Freshening of the water column over the continental shelf is also expected to impact on the Antarctic continental ice by enhancing the basal melt rate beneath the MGT which will further reinforce the freshening processes and could lead a change in the ice dynamics.