Inverse Estimate of Atlantic Meridional Overturning Circulation During the Last Deglaciation

The most recent natural large-scale climatic transition – from the last Ice Age to the current epoch – occurred as a series of abrupt changes recorded in natural archives such as deep-sea sediment cores and polar ice cores. The estimated magnitude of these changes is truly impressive: Greenland ice core records suggest that the local air temperature varied by 5-14 degrees Celsius in just a few decades. Understanding the processes leading to these rapid changes has emerged as a pressing question for modern society. The observed climate changes during the last deglaciation could be explained by episodic releases of glacial meltwater from the disintegration of the large ice sheet that covered North America. Release of fresh water into the North Atlantic Ocean would change the density of water in the surface ocean, thereby altering density-driven circulation and the transfer of heat in the form of warmer waters from low to high latitudes. In this project, changes in circulation in the Atlantic Ocean during the last deglaciation will be estimated by combining (i) records preserved in deep-sea sediments and deep-sea corals, (ii) knowledge about the volume and rate of meltwater release into the North Atlantic, and (iii) a computer model of ocean circulation. The computer codes developed for this project will be written in the open-source language Julia. The codes and a manual will be publicly available so that scientists and students can use the codes for their own applications. A graduate student will contribute to the project and present results at scientific conferences and in the peer-reviewed literature. Three undergraduate students will work during summer months on sub-projects that are also essential components of this project. A web site will be created where the progress of the research activities to be undertaken for this project will be reported. This project will yield quantitative constraints on the changes in the Atlantic meridional circulation (AMOC) during the last deglaciation from the fit of an ocean circulation model to a variety of paleoceanographic datasets. Climate models that simulate deglacial AMOC changes as qualitatively inferred from paleo-data rely on surface forcing (glacial meltwater fluxes) that is inconsistent with the sea level record (the “meltwater paradox”). To resolve this paradox, researchers will apply an inverse model wherein the unknowns are deglacial changes in ocean circulation and surface forcing, and the observational constraints are (i) sediment-core and deep-sea coral records, (ii) glacial meltwater fluxes derived from ice sheet and drainage network reconstructions, and (iii) current knowledge about the deglacial evolution of the atmosphere. If proxy records cannot be reconciled with sea level records, researchers will explore whether shifts in North Atlantic/Southern Ocean winds can explain changes in AMOC. A numerical model with coarse spatial resolution and simplified dynamics will be required to fit an ocean circulation model to time series observations over a geological time scale (10,000 years). The inverse approach allows quantitative consideration of both data and model uncertainty. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.