Fiammetta Straneo


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Atlantic Water Transformation in the Nordic Sea: The Lofoten Basin

PIs: Fiammetta Straneo (WHOI), Thomas Rossby (URI), Jonathan Lilly (ESR), Mike Spall (WHOI)

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.

We will conduct a combined observational and modeling study to 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 us to investigate 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.

Figure: Bathymetry and major geographic features of the Nordic Seas (the 2000 m and 3000 m depth contours are indicated by thin black and dashed lines). LB=Lofoten Basin; NS=Norwegian Sea; GS=Greenland Sea; MR=Mohn Ridge; KR=Knipovich Ridge; VP=Vøring Plateau. The warm Atlantic water pathways of the NwAC and of its inner and outer branches are drawn as black lines. The intermediate flow that feeds the overflows is indicated schematically as blue lines. Both redrawn from Hansen and Østerhus (2000).

This project is supported by the National Science Foundation Ocean Sciences and Office of Polar Programs Divisions.

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