Collaborative Research: Mode Water Formation in the Lofoten Basin: A Key Element in the Meridional Overturning Circulation

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The traditionally central role played by the deep convective regions of the North Atlantic - the Labrador Sea and the Greenland Sea - in driving the Atlantic Meridional Overturning Circulation (AMOC) and its associated poleward heat transport has been severely questioned over the last decade. In particular, it is now known that relatively little sinking and only a limited surface buoyancy loss occurs in these regions and, furthermore, that the dense water masses feeding the AMOC are not a direct product of these regions. In the case of the Nordic Seas, the Greenland-Scotland overflow waters comprising the deepest branch of the AMOC contain only small amounts of Greenland Sea Water, and are instead largely the product of the progressive transformation of the circulating warm, salty Atlantic Waters. Yet the pathways and processes through which this transformation occurs are largely unknown. Emerging as playing a central role in this transformation is the Lofoten Basin, which sits adjacent to the Greenland Sea between the two main branches of the Atlantic inflow. The warm, salty Atlantic waters are strongly modified as they transit through this basin, which alone accounts for a substantial fraction of the surface buoyancy loss over the Nordic Seas. This leads to the formation of an intermediate convected product, the Lofoten Basin Mode Water. Unlike Labrador Sea Water and Greenland Sea Water, the Lofoten Basin Mode Water can be rapidly exported from the basin, and is a direct source for the overflows waters. Despite its importance, however, the basic dynamical features of this basin and of its intermediate water mass formation have yet to be addressed. Intellectual MeritThe principal intellectual merit of this project is its importance for elucidating a key facet of the AMOC, namely, mode water formation in the Lofoten Basin and its role in the Nordic Seas transformation pathway. The project is timely since there is growing evidence that the Greenland Sea's deep convection by itself has a limited impact on the AMOC and, at the same time, our understanding of intermediate/dense transformation pathways has greatly improved. By using the Labrador Sea as a reference point, the investigators anticipate contributing to a more universal understanding of convective transformation in the presence of varied topography and an unstable boundary current. The combined observational and modeling study will investigate mode water formation in the Lofoten Basin and its eventual export. A profiling mooring together with RAFOS floats deployed directly within the basin will clarify the annual cycle of transformation, restratification, and dispersal. Analysis of existing hydrographic, float, and remote sensing data will help to guide the field work, and will enable the investigation of larger-scale and longer-term variability. A variety of models will be employed to study the dynamics of the transformation process in the Lofoten Basin and elsewhere in the Nordic Seas. The net result will be a new understanding of the interaction between air-sea fluxes and ocean dynamics along this crucial branch of the AMOC. Broader ImpactsThis project seeks to increase our understanding of a key process in the Earth's climate system, the Nordic Seas transformation pathway, which affects not only the AMOC but, also, the warm inflow into the Arctic Ocean, a region currently undergoing rapid change. The project has been carefully planned in collaboration with several Norwegian and other European groups so as to optimize resources and foster international collaboration. It includes two workshops, as well as exchange of data, ideas, and personnel throughout the project. Finally, it brings together a group of diverse PIs, who will bring complementary skills to this problem, and includes training of a post-doc who will greatly benefit from the diverse and international scientific environment of this project.