Antarctica – Changing Icescapes

Australia: The Land Where Time Began

Antarctica – Changing Icescapes

The Mertz Glacier Tongue in East Antarctica was struck by a massive iceberg in 2010 which caused it to calve. The icescape along the coast was changed by this event with important implications for the physical and biological processes that occur there.

Sea ice surrounds the land mass of Antarctica, and in winter this sea ice can extend to the northwest for hundreds of kilometres, though the ice pack is not a continuous frozen icescape. Strong winds blowing downhill from the Antarctic Plateau onto the shoreline drives sea ice that forms along the coast of Antarctica, which results in open water known as a coastal polynya. The polynyas are extremely efficient factories of sea ice, newly formed ice being pushed away from the coast.

These persistent open water patches play a key role in physical and biological processes in Antarctica. Polynyas are important feeding areas of penguin colonies, as in early spring they can sustain plankton blooms. When see ice forms salt brines (see brinicles) is expelled into the underlying seawater, a process that increases the density of the surrounding water, so they sink to the ocean floor. The lower limb of the ocean conveyor belt is comprised of Antarctic bottom waters. They are therefore a key component of the global climate system.

In East Antarctica, the Mertz Polynya, located on the lee side of the glacier tongue, is the 3^rd most productive polynya in Antarctica. A vast iceberg collided with the glacier in 2010 resulting in massive calving of the tongue. The size of the Mertz Polynya was reduced significantly by this event, as well as a reduction of formation of dense bottom waters. The iceberg became grounded in Commonwealth Bay just west of the Mertz Glacier after this event, forming a barrier that blocked the transport offshore of newly formed ice. The coastal polynya is now no longer present and the area landward of the iceberg is now covered with thick ice. In situ and satellite oceanographic observations from 2013 and high resolution model simulations have been used to determine the effect of change in the icescape of the oceanographic regime in Commonwealth Bay (The Cryosphere 10, 2603-2609; 2016). The study found there had been a shift in the water properties in the lee of the grounded iceberg in the northwest part of Commonwealth Bay, the water of the entire water column is now saltier and colder. Dense bottom water formation is indicated by this adjustment of water properties, which is consistent with estimates of sea ice production from satellite observations, and a new polynya may be forming on the leeside of the iceberg. Also, it is suggested by model simulations that the transient state that the system is in at present as it adjusts to the calving event could possibly lead to a new stable state in which an active polynya, that is capable of producing bottom water, becomes a permanent feature of the northwest of Commonwealth Bay.

The location of the formation of Antarctic bottom water has shifted from the Mertz and southern Commonwealth Bay to the northwest side of the Bay. It remains to be quantified what the effect of this new arrangement is on the ocean overturning circulation, though the development of a new polynya may partially compensate for the reduction of the formation of dense bottom water in the weakened Mertz Polynya. It is also suggested that marked biological changes can be expected. The benthic communities in the south of Commonwealth Bay are believed to have been severely affected by the presence of ice cover throughout the year. At the same time, a new area of marine productivity in the northern polynya that is emerging could provide food for the penguin colonies in the region.

This natural event is believed to occur about every 100 years, as well as highlighting the sensitivity of the regional ocean circulation and biology to icescape changes, it also allows a glimpse on how the ocean and ecosystem might be affected by increased global warming.

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