Sources and upstream pathways of the densest overflow in the Nordic Seas

The overflows from the Nordic Seas feed the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), yet questions remain as to where they are ventilated and their upstream pathways. Here we use historical hydrographic data from 2005 to 2015, together with satellite altimeter data, to elucidate the source regions of the Denmark Strait and Faroe Bank Channel overflows and the pathways by which the dense water reach the respective sills. We use a metric known as the σ-π distance to calculate how close two different water parcels are to each other in terms of physical properties, where σ is potential density and π is potential spicity. Specifically, we calculate the σ-π distance between the densest overflow water at the sills and the water upstream in the Nordic Seas. With regard to late-winter mixed layers, the smallest σ-π distances are found in the Greenland Sea, suggesting that this is the primary source region for the densest portion of both overflows. Two potential pathways emanating from the Greenland Sea are found in the spatial distribution of σ-π distance: (1) at 250-300m, the overflow water from the Greenland Sea flows along Mohn ridge, crosses the West Jan Mayen ridge into Iceland Sea, then progresses southward east of the Kolbeinsey Ridge to ultimately feed the Denmark Strait overflow via the North Icelandic Jet; (2) at 600-650m, the overflow water from Greenland Sea and Mohn ridge instead flows southward east of the Jan Mayen ridge to supply the Faroe Bank Channel overflow. These suggested pathways are confirmed kinematically by considering the absolute geostrophic velocity field, referenced using the satellite surface geostrophic velocity data. Long-term variations of salinity and temperature in the overflows imply that the sources of the overflows were different prior to 2000.