High-Resolution Model Development to Quantify the Impact of Icebergs on the Stability of the Atlantic Meridional Overturning Circulation
Collaborative Institutional Lead(s):
In the present-day North Atlantic Ocean, relatively warm and salty water moves northwards from the tropics to the high latitudes, sinks, and returns southward towards the equator as North Atlantic Deep Water – the so called Atlantic Meridional Overturning Circulation (AMOC). Research over the last 30- years has identified that this circulation system is important for regulating the relatively warm and stable modern climate in much of the northern hemisphere, but that its strength is capable of rapidly weakening to trigger cooler conditions over a wide region. In particular, it has been found that its stability is non- linearly related to the freshwater budget of the North Atlantic. Despite this, a great deal of uncertainty surrounds the response of the AMOC (both in the past and the present-day) to changes in the input of freshwater to the North Atlantic. This large ambiguity arises due to the unrealistic manner in which freshwater is currently delivered from the ice sheets to the ocean in numerical models. Almost always there is no consideration given to the fact that a large fraction of freshwater discharge enters the ocean not as liquid, but as ice calving from marine glaciers (two-thirds in the case of Greenland). These icebergs can drift many thousands of miles before they finally melt.
To more accurately quantify AMOC sensitivity to changes in freshwater input, we propose the develop of a comprehensive iceberg model that, when coupled with a state-of-the-art, ultra-high resolution, eddy resolving, ocean-sea ice numerical model, will lead to groundbreaking advances in Earth System Modeling by improving the accuracy and skill of how models treat freshwater in the climate system to a level far beyond the existing suite of climate models currently available to the scientific community. The model will incorporate the latest developments in Operational Iceberg Forecast model technology to produce the highest level of accuracy possible, in terms of the current understanding of iceberg motion and decay. It will also include a sediment model to calculate the deposition of material entrained in icebergs to the ocean floor, as a way to reconstruct the pathways of massive iceberg episodes in the past, so called Heinrich events.
Through a combination of sensitivity studies we aim to understand the non-linearity of the AMOC to changes in iceberg discharge in the past (around ~16,000 yr BP), present-day and future (next 50-100 years). By examining changes in the AMOC to various iceberg discharge scenarios, we will be able to quantify the extent to which the current suite of IPCC AR5 ESMs has miscalculated the sensitivity of the AMOC to freshwater perturbations. Sensitivity studies will also enable us to isolate specific regions, or even individual ice streams, that have the most potential to disrupt, or weaken, the AMOC in the near-future. This information would be invaluable for guiding the deployment of a future observational monitoring system capable of recording changes in iceberg calving and freshwater discharge to the ocean. Such a monitoring program could act as an early warning system by notifying us when a volume of ice capable of disrupting the AMOC was discharged from, for example, the Greenland Ice Sheet.