Variability of surface transport pathways and how they affect Arctic basin-wide connectivity
Authors: Aksenov Yevgeny , Wilson Chris , Rynders Stefanie , Kelly Stephen , Krumpen Thomas , Coward Andrew C. 
Published: March 2022

&lt;p&gt;The Arctic Ocean is of central importance for the global climate and&amp;#160;ecosystems. It is undergoing rapid climate change, with a dramatic&amp;#160;decrease in sea ice cover over recent decades. Surface advective pathways&amp;#160;connect the transport of nutrients, freshwater, carbon and contaminants&amp;#160;with their sources and sinks. Pathways of drifting material are deformed&amp;#160;under velocity strain, due to atmosphere-ocean-ice coupling. Deformation&amp;#160;is largest at fine space- and time-scales and is associated with a loss of&amp;#160;potential predictability, analogous to weather often becoming unpredictable&amp;#160;as synoptic-scale eddies interact and deform. However, neither satellite&amp;#160;observations nor climate model projections resolve fine-scale ocean&amp;#160;velocity structure. Here, we use a high-resolution ocean model hindcast&amp;#160;and coarser satellite-derived ice velocities, to show: that ensemble-mean&amp;#160;pathways within the Transpolar Drift during 2004&amp;#8211;14 have large&amp;#160;interannual variability and that both saddle-like flow structures and the&amp;#160;presence of fine-scale velocity gradients are important for basin-wide&amp;#160;connectivity and crossing time, pathway bifurcation, and also for&amp;#160;predictability and dispersion (the latter are covered in an associated paper&amp;#160;[1].&lt;/p&gt;&lt;p&gt;The saddle-points in the flow and their neighbouring streamlines define&amp;#160;flow separatrices, which partition the surface Arctic into separate regions of&amp;#160;connected transport properties. The separatrix streamlines vary&amp;#160;interannually and identify periods when the East Siberian Arctic Shelf, an&amp;#160;important source of terragenic minerals, carbon and nutrients, is either&amp;#160;connected or disconnected with Fram Strait and the North Atlantic. We&amp;#160;explore the implications of this transport connectivity, with our new metric&amp;#160;- the Separatrix Curvature Index &amp;#8211; which in this context is arguably more&amp;#160;informative than either the Arctic Oscillation or Arctic Ocean Oscillation&amp;#160;indices.&lt;/p&gt;&lt;p&gt;This work resulted from the Advective Pathways of nutrients and key&amp;#160;Ecological sub- stances in the Arctic (APEAR) project (NE/R012865/1,&amp;#160;NE/R012865/2, #03V01461), part of the Changing Arctic Ocean&amp;#160;programme, jointly funded by the UKRI Natural Environment Research&amp;#160;Council (NERC) and the German Federal Ministry of Education and&amp;#160;Research (BMBF). This work has received funding from the European&amp;#160;Union's Horizon 2020 research and innovation programme under grant&amp;#160;agreement no. 820989 (project COMFORT). The work reflects only the&amp;#160;authors' view; the European Commission and their executive agency are&amp;#160;not responsible for any use that may be made of the information the work&amp;#160;contains. This work also used the ARCHER UK National Supercomputing&amp;#160;Service and JASMIN, the UK collaborative data analysis facility. Satellitebased&amp;#160;sea ice tracking was carried out as part of the Russian-German&amp;#160;Research Cooperation QUARCCS funded by the German Ministry for&amp;#160;Education and Research (BMBF) under grant 03F0777A. This study was&amp;#160;carried out as part of the international Multidisciplinary drifting&amp;#160;Observatory for the Study of the Arctic Climate (MOSAiC) with the tag&amp;#160;MOSAiC20192020 (AWI_PS122_1 and AF-MOSAiC-1_00) and the&amp;#160;NERC Project &amp;#8220;PRE-MELT&amp;#8221; (Grant NE/T000546/1). We also&amp;#160;acknowledge funding support received from the NERC National Capability&amp;#160;programmes LTS-M ACSIS (North Atlantic climate system integrated&amp;#160;study, grant NE/N018044/1) and LTS-S CLASS (Climate&amp;#8211;Linked Atlantic&amp;#160;Sector Science, grant NE/R015953/1). The authors would like to&amp;#160;acknowledge the contribution of Maria Luneva to the discussions about the&amp;#160;initial idea of the study and for highlighting the historical importance of&amp;#160;observations from the Russian North Pole drifting stations. Sadly, Maria&amp;#160;passed away suddenly in 2020 before the draft of the reported paper was written.&lt;/p&gt;&lt;p&gt;[1] Wilson, C., Aksenov, Y., Rynders, S. et al. Significant variability of&amp;#160;structure and predictability of Arctic Ocean surface pathways affects basinwide&amp;#160;connectivity. Commun. Earth. Environ. 2, 164 (2021).&amp;#160;https://doi.org/10.1038/s43247-021-00237-0.&lt;/p&gt;