Quantification of simulated ISOW spreading pathways

To quantify the simulated ISOW branches, we categorize the floats into five groups based on their spreading pathways and surface locations. The first group follows the west-northwestward pathway and ends up in the western subpolar gyre (west of the Reykjanes Ridge and north of 52°N). The second group travels southward with the floats’ final locations south of 52°N and west of the MAR. The third group is carried eastward into the eastern subpolar gyre and the fourth group circulates locally near the CGFZ (within [38°W, 32°W], [52°N, 54°N]). An example of these four groups is shown in Supplementary Fig. ³.

The last group includes the floats that follow the historical boundary current schematic. That is to say, the group includes floats that follow the deep boundary current continuously from the CGFZ northward along the western flank of the Reykjanes Ridge, which is characterized as the path between 2000 and 3000 m isobaths. The trajectories of this group are shown in Supplementary Fig. ⁶.

Of all 3593 simulated floats, a subsample of 21 floats is randomly selected and their trajectories are categorized based on the above criteria. [Note that the subsample size is chosen to match the number of the RAFOS floats.] This process is repeated for 10,000 times to derive the mean and the uncertainty of each group’s percentage.