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, its upstream pathways and circulation remain uncertain due to short-term (3–5 days) variability. 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 Lagrangian pathways confirm that several branches contribute to the DSO from the north such as the East Greenland Current (EGC), the separated EGC (sEGC), and the North Icelandic Jet (NIJ). Moreover, the model results reveal additional pathways from south of Iceland, which supplied over 16% of the DSO annually and over 25% of the DSO during winter of 2008, when the NAO index was positive. The southern contribution is about 34% by the end of March. The southern pathways mark a more direct route from the near-surface subpolar North Atlantic to the North Atlantic Deep Water (NADW), and needs to be explored further, with in situ observations.@en

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.@en

OceanSpy is an open-source and user-friendly Python package that enables scientists and inter- ested amateurs to analyze and visualize oceanographic data sets. OceanSpy builds on software packages developed by the Pangeo community, in particular Xarray (Hoyer & Hamman, 2017), Dask (Dask Development Team, 2016), and Xgcm (“Xgcm,” n.d.). The integration of Dask facilitates scalability, which is important for the petabyte-scale simulations that are becoming available. OceanSpy can be used as a standalone package for analysis of local circulation model output, or it can be run on a remote data-analysis cluster, such as the Johns Hopkins University SciServer system (Medvedev, Lemson, & Rippin, 2016), which hosts several sim- ulations and is publicly available. OceanSpy enables extraction, processing, and visualization of model data to (i) compare with oceanographic observations, and (ii) portray the kinematic and dynamic space-time properties of the circulation.@en