Variability of surface transport pathways and how they affect Arctic basin-wide connectivity

The Arctic Ocean is of central importance for the global climate and ecosystems. It is undergoing rapid climate change, with a dramatic decrease in sea ice cover over recent decades. Surface advective pathways connect the transport of nutrients, freshwater, carbon and contaminants with their sources and sinks. Pathways of drifting material are deformed under velocity strain, due to atmosphere-ocean-ice coupling. Deformation is largest at fine space- and time-scales and is associated with a loss of potential predictability, analogous to weather often becoming unpredictable as synoptic-scale eddies interact and deform. However, neither satellite observations nor climate model projections resolve fine-scale ocean velocity structure. Here, we use a high-resolution ocean model hindcast and coarser satellite-derived ice velocities, to show: that ensemble-mean pathways within the Transpolar Drift during 2004–14 have large interannual variability and that both saddle-like flow structures and the presence of fine-scale velocity gradients are important for basin-wide connectivity and crossing time, pathway bifurcation, and also for predictability and dispersion (the latter are covered in an associated paper [1].

The saddle-points in the flow and their neighbouring streamlines define flow separatrices, which partition the surface Arctic into separate regions of connected transport properties. The separatrix streamlines vary interannually and identify periods when the East Siberian Arctic Shelf, an important source of terragenic minerals, carbon and nutrients, is either connected or disconnected with Fram Strait and the North Atlantic. We explore the implications of this transport connectivity, with our new metric - the Separatrix Curvature Index – which in this context is arguably more informative than either the Arctic Oscillation or Arctic Ocean Oscillation indices.

This work resulted from the Advective Pathways of nutrients and key Ecological sub- stances in the Arctic (APEAR) project (NE/R012865/1, NE/R012865/2, #03V01461), part of the Changing Arctic Ocean programme, jointly funded by the UKRI Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF). This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 820989 (project COMFORT). The work reflects only the authors' view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains. This work also used the ARCHER UK National Supercomputing Service and JASMIN, the UK collaborative data analysis facility. Satellitebased sea ice tracking was carried out as part of the Russian-German Research Cooperation QUARCCS funded by the German Ministry for Education and Research (BMBF) under grant 03F0777A. This study was carried out as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020 (AWI_PS122_1 and AF-MOSAiC-1_00) and the NERC Project “PRE-MELT” (Grant NE/T000546/1). We also acknowledge funding support received from the NERC National Capability programmes LTS-M ACSIS (North Atlantic climate system integrated study, grant NE/N018044/1) and LTS-S CLASS (Climate–Linked Atlantic Sector Science, grant NE/R015953/1). The authors would like to acknowledge the contribution of Maria Luneva to the discussions about the initial idea of the study and for highlighting the historical importance of observations from the Russian North Pole drifting stations. Sadly, Maria passed away suddenly in 2020 before the draft of the reported paper was written.

[1] Wilson, C., Aksenov, Y., Rynders, S. et al. Significant variability of structure and predictability of Arctic Ocean surface pathways affects basinwide connectivity. Commun. Earth. Environ. 2, 164 (2021). https://doi.org/10.1038/s43247-021-00237-0.