Lagrangian Perspective on the Origins of Denmark Strait Overflow

The Denmark Strait Overflow (DSO) is an important contributor to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). Determining DSO formation and its pathways is not only important for local oceanography but also critical to estimating the state and variability of the AMOC. Despite prior attempts to understand the DSO sources, the upstream pathways and circulation remain uncertain due to short term (3-5 days) variability, mixing and watermass transformation. This makes it challenging to study the DSO from observations. Given this complexity, this study maps the upstream pathways and along-pathway changes in its water properties, using Lagrangian backtracking of the DSO sources in a realistic numerical ocean simulation. The model is compared with Eulerian and Lagrangian observations. Eulerian fields are compared with historical mooring and CTD sections, and particle pathways are compared with RAFOS float data. The Lagrangian pathways confirm that several branches contribute to the DSO from the North, namely, the East Greenland Current, the separated EGC, and the North Icelandic Jet. Moreover, the results reveal an additional pathway from south of Iceland along the Iceland shelf, which supplies 26% of the DSO during winter of 2008 when the NAO index is positive. The southern contribution is as high as 34% by the end of March. This southern pathway presents a short circuit for the North Atlantic Deep Water and needs to be explored further, especially in observations. Finally, an updated schematic DSO circulation diagram is proposed.