Ocean2Ice: Processes and variability of ocean heat transport toward ice shelves in the Amundsen Sea Embayment

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

Imagine that the ocean is like a large gin and tonic. When you add ice to the drink, the level in the glass goes up. When the lump of ice melts, the level in the glass doesn't change, because the ice is floating. When ice that is currently resting on land in Antarctica goes into the sea, either as an iceberg or as meltwater, the sea level all over the world goes up. It used to be thought that the same amount of water went back to the Antarctic as snowfall, to compensate for the icebergs and meltwater, so the whole system was in balance. But some glaciers in the Antarctic (and Greenland) seem to be melting at a faster rate than they are being replaced. So the total amount of ice is getting smaller, because more of that water is in the ocean, adding to sea level rise. This is worrying, because we don't really know why this is happening, and if we can't understand why, it's difficult to predict whether future sea level will carry on increasing at a faster and faster rate, or whether it will slow down or go back to equilibrium. Governments planning sea level defences in low-lying areas for the next decades need to have a more certain prediction of likely levels. That means that the big computer models that they use to forecast future climates need to have even better and more complex physics than they do already.

So, what can scientists do to find out why the ice is melting? When the glaciers finally reach the sea, they float on the seawater, as an ice shelf. One suggestion is that the ocean is providing more heat to melt the ice than it used to do. Even though the ocean isn't that warm in the Antarctic, it is a few degrees above freezing, and if it washes underneath the ice shelves it can give up a lot of heat. What we plan to do in this project is to go to one of the fastest melting glaciers, the Pine Island Glacier in the Amundsen Sea, Antarctica. This is one of the most remote parts of our planet - imagine going to the Pacific Ocean and then heading south until you meet Antarctica. We will put some instruments in the water near the ice shelf, to see how and why the warm ocean water gets close to the ice. Is it the wind that forces the water there? Is it waves going round the Antarctic continent? Does the water get channelled up troughs in the sea floor gouged by glaciers thousands of years ago?

We plan to use some novel equipment in the Antarctic, such as gluing tiny sensors onto elephant seals' fur. The seals will remain in the area over winter, long after we've gone back home. Their sensors will send back information about the seals' habitat - for example the temperature and the saltiness. This is useful for us because we can't get observations in the wintertime any other way because the area is covered in sea ice. And it's good for the seals because it will help our biologist colleagues to better understand how vulnerable the elephant seals might be to climate change. We'll also put in the water a mechanical version of a seal, called a Seaglider. This goes up and down in the water making measurements as it goes, and much like the seal sensors, it will communicate when it's at the surface using mobile phone. While we're there with the ship, we'll make lots of measurements of the temperature and saltiness of the water, how fast it's going, and how mixed up it is. Looking at all these data sets together should give us a better understanding of how the heat is getting to the glacier.

One of the important tools will be a variety of computer models. These will range from all-singing, all-dancing climate models, that try to include ice, ocean and atmosphere all interacting, to much simpler models that test our understanding of the physics at play. The final result of the work we plan to do should be better climate models to predict future sea levels.

Planned Impact

The most immediate beneficiaries will be global operational ocean and atmosphere forecasters, since our seal, Seaglider and radiosonde data will be freely available in real time for assimilation into operational forecasts.

The scientific impact of the project will primarily benefit climate modellers, in particular the Hadley Centre at the UK Met Office. They will gain insight from our observations and process models of the processes that lead to warm water melting the Amundsen Sea ice shelves. We will assess the performance of many of the Coupled Model Intercomparison Project (CMIP-5) models, and also the various NEMO-based Met Office models such as FOAM and the new HadGEM3. The results of the project will aid the eventual parameterisation of the physical processes for use in climate models. The major results of Ocean2ice will inform government policy with regard to the uncertainties in sea level rise predictions.

Those designing the global ocean (and climate) observing systems will benefit from Ocean2ice. It is likely that the Amundsen Sea embayment is a key location for monitoring future climate. Our observations will allow us to feed into the Southern Ocean Observing System (SOOS) design in order to locate long-term measuring systems in the most useful and efficient locations in a cost-effective way. We will assess the value of seal tags and Seagliders in such a system. We will engage with the SOOS community through the Scientific Committee for Antarctic Research (SCAR).

Ocean2ice will employ several early career researchers, who will be nurtured and trained within the project, learning a variety of skills to equip them for a productive career. More widely, we will bring the Ocean2ice observing techniques to the UK Polar Network of early career polar scientists through a dedicated workshop.

The general public and young people in particular will gain from the exhibits, displays and outreach efforts. We hope to interest more young people in science and in higher education, raise awareness of global change and polar processes, and attract more people to careers in scientific research.

Publications

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Description Thinning of the ice sheet that drains into the Amundsen Sea is currently the major contribution of Antarctica to sea level rise. Acceleration of the outflow glaciers and inland thinning of the ice are now well-documented, but our understanding of the underlying causes remains rudimentary. The observations that thinning is most rapid at the coast and propagates inland suggest that changes in ocean-forced melt of the ice shelves are responsible. However, a critical question that remains unanswered is whether current changes are a continuing response to an earlier trigger, or are driven by current ocean variability.
It has been argued that atmospheric circulation changes associated with sea surface temperature variability over the central tropical Pacific Ocean could drive changes in ocean conditions along the Amundsen Sea coast. That conclusion was based on a coarse-resolution model, some results of which now appear questionable. However, observational evidence is supportive of such a link with the tropical Pacific, a pronounced cooling of the Amundsen Sea in 2012 being linked with cool La Nina conditions in the central Pacific.
We have re-evaluated the evidence for such a link with large-scale atmospheric forcing using a database of historical observations from the Amundsen Sea continental shelf. The record is characterised by decadal-scale variability that fits well with the frequency and phase of tropical forcing. An important result is that there is no observable trend in the ocean conditions that drive melting of the ice shelves, so much of documented change in the ice sheet is more readily explained a response to an earlier trigger. Furthermore, there was a period of exceptional warmth in the central tropical Pacific that had a major impact on West Antarctic atmospheric circulation in the early 1940s, and this anomaly has now been shown to be responsible for triggering the current ice sheet changes.
Another key result of the earlier modelling study was that the decadal variability on the shelf was a caused by the impact of the winds on ocean currents at the continental shelf edge that are the sources of the warm water on the shelf. While results from this project show that other processes and short term variability of limited spatial extent, the longer-term larger scale variability is indeed causes by variability of shelf edge currents that appears to be associated with the winds.
Exploitation Route Understanding the drivers of change in West Antarctica is one of the most important outstanding problems in climate change science. We now know that ocean variability is the key to driving those changes, and the shelf-edge processes that we need to understand in order to improve projections of future sea level rise.
Sectors Environment