Evolution of water mass transformation and overturning in the western Nordic Seas in a warming climate

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The Nordic Seas are a key region for water mass transformation associated with the Atlantic Meridional Overturning Circulation (AMOC). The warming atmosphere has shifted the wintertime sea ice edge westward by hundreds of kilometers over the past fifty years. Greatly enhanced heat loss from the ocean along the western margin of the Nordic Seas and a reduction of heat loss in the interior of the Greenland and Iceland Seas has been coincident with this shift. To improve understanding of the consequences on water mass transformation, convection, the regional circulation, north-south heat transport and overturning circulation, this project will apply several complementary tools. These include theoretical development, the application of an idealized configuration of a high-resolution coupled ocean/atmosphere/sea ice model and the analysis of a recently acquired observational dataset in the region. The model simulations will represent the transition from the large ice extent of the mid 20th century to the reduced ice of present day and then project into the future under predicted increasing CO2 scenarios. The work is directly relevant to climate science and there are also educational broader impacts. The project will support a postdoc for two years and provide training in physical oceanography, numerical methods, applied mathematics, climate dynamics, and data analysis. Participation will also be sought from undergraduate students through the WHOI Summer Student Fellow Program (at no cost to this proposal). The PI will also co-advise and host a PhD student from the Univ. of Bergen for an extended period. The project uses idealized modeling which allows for control and isolated study of the influences of the various forcing mechanisms and ocean dynamics expected to be important. The primary tool to address the influences of sea ice retreat on water mass transformation is the SKRIPS fully coupled ocean/atmosphere/sea ice model, with components including the MITgcm with sea ice package for the ocean and ice and the Polar Weather Research and Forecasting model (WRF) for the atmosphere. The modeling studies are motivated by, and will be carried out in parallel with, simplified analytic models of the convective basin/boundary current system with new consideration of ice cover, Ekman transport, and a finite heat capacity atmosphere. In addition, PI Spall will work with collaborator Vage at the U. Bergen to analyze the Resilient Northern Overturning in a Warming Climate (ROVER) observational dataset, in light of the high resolution modeling and theoretical development of this project. ROVER included a two-year mooring array across the boundary current, summertime surveys, an unprecedented wintertime survey, glider data, wintertime deployment of ARGO floats directly in the boundary current, in addition to routinely collected glider transects across the boundary and a historical climatological data base developed by Vage.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.