Coupled Evolution of Ice Shelf and Ocean in the Amundsen Sea Sector of Antarctica
Lead Research Organisation:
Northumbria University
Department Name: Fac of Engineering and Environment
Abstract
As our planet warms the ice cover shrinks, a process that transfers water from land to ocean and thereby raises sea level. The result, which could ultimately raise global sea level by 10s of metres, seems intuitively obvious. However, in the case of the Antarctic Ice Sheet, the processes at work are less than obvious. The atmosphere over the ice sheet is too cold to drive significant melting, so all the snow that falls in the interior is returned to the ocean as ice that only melts once it is afloat. The cold atmosphere creates cold surface waters, so most of the heat that melts the ice comes from deep within the ocean's interior. As it melts the floating ice from underneath, the thinning of the so-called ice shelves allows ice to flow off the land more rapidly, hence raising sea level.
So, the underlying process is clear, but why should it drive a loss of ice from Antarctica as the climate warms? The waters that melt the ice are too deep in the ocean to feel atmospheric warming. However, as the atmosphere warms the circulation patterns change, influencing the winds that drive the ocean currents, and that delivers more of the deep warm water to the ice. Understanding how the processes work has been challenging. It is not immediately obvious why a change in the winds should deliver more, rather than less, warm water to the ice. Nevertheless, observation and modelling give us a consistent answer and our understanding of the processes grows as we focus our research on key unknowns.
However, there is another puzzle that has received much less attention to date. More warm water leads to more rapid melting of the ice shelves, they thin and the flow of ice off the land accelerates. That acceleration of the flow delivers more ice to the ice shelves, and they should therefore start to grow, or at least thin less rapidly, unless the ocean heat delivery continues to grow. Until recently it was assumed that that is exactly what was happening, but as our record of ocean observations has lengthened, we have seen decadal cycles of warming and cooling. Why then should the ice shelves continue to thin?
The answer must lie in the way in which the thinning of the ice shelves themselves affects the melt rate. Again, it is not immediately clear why the change in the ice should increase rather than decrease the melt. However, in this case observation of the key processes is exceptionally difficult because they take place beneath 100s or even 1000s of metres of ice.
That is the challenge we will address with this project, by sending an autonomous submarine beneath the ice to make the critical measurements of the ocean, including the temperature of the water and the currents. Those direct observations of the ocean beneath the ice will allow us to verify that the ocean models we use to simulate the processes are correct, or to improve them if they are not.
This will not be the first time such measurements have been made, but the new observations will differ in two important respects from the very few that have been made in the past. Some will be repeats of earlier measurements, so we will have observations from before and after a significant change in the extent of the ice shelf. Thus, we can directly answer the question of what change in the ocean circulation accompanied the change in shape of the ice cover. Other observations will target regions where the ice was grounded until recently. Because radar signals penetrate ice, but not seawater, we are able to map the topography only when the ice rests on the land and not when it is afloat. Thus, we paradoxically know the geometry of newly formed ocean cavities with much greater accuracy than we do the cavities that have been there since humans first explored the south polar regions. Our ability to understand the links between cavity geometry and ocean circulation is therefore enhanced in the newly opened cavities that are among the targets of our field campaign.
So, the underlying process is clear, but why should it drive a loss of ice from Antarctica as the climate warms? The waters that melt the ice are too deep in the ocean to feel atmospheric warming. However, as the atmosphere warms the circulation patterns change, influencing the winds that drive the ocean currents, and that delivers more of the deep warm water to the ice. Understanding how the processes work has been challenging. It is not immediately obvious why a change in the winds should deliver more, rather than less, warm water to the ice. Nevertheless, observation and modelling give us a consistent answer and our understanding of the processes grows as we focus our research on key unknowns.
However, there is another puzzle that has received much less attention to date. More warm water leads to more rapid melting of the ice shelves, they thin and the flow of ice off the land accelerates. That acceleration of the flow delivers more ice to the ice shelves, and they should therefore start to grow, or at least thin less rapidly, unless the ocean heat delivery continues to grow. Until recently it was assumed that that is exactly what was happening, but as our record of ocean observations has lengthened, we have seen decadal cycles of warming and cooling. Why then should the ice shelves continue to thin?
The answer must lie in the way in which the thinning of the ice shelves themselves affects the melt rate. Again, it is not immediately clear why the change in the ice should increase rather than decrease the melt. However, in this case observation of the key processes is exceptionally difficult because they take place beneath 100s or even 1000s of metres of ice.
That is the challenge we will address with this project, by sending an autonomous submarine beneath the ice to make the critical measurements of the ocean, including the temperature of the water and the currents. Those direct observations of the ocean beneath the ice will allow us to verify that the ocean models we use to simulate the processes are correct, or to improve them if they are not.
This will not be the first time such measurements have been made, but the new observations will differ in two important respects from the very few that have been made in the past. Some will be repeats of earlier measurements, so we will have observations from before and after a significant change in the extent of the ice shelf. Thus, we can directly answer the question of what change in the ocean circulation accompanied the change in shape of the ice cover. Other observations will target regions where the ice was grounded until recently. Because radar signals penetrate ice, but not seawater, we are able to map the topography only when the ice rests on the land and not when it is afloat. Thus, we paradoxically know the geometry of newly formed ocean cavities with much greater accuracy than we do the cavities that have been there since humans first explored the south polar regions. Our ability to understand the links between cavity geometry and ocean circulation is therefore enhanced in the newly opened cavities that are among the targets of our field campaign.
