| Summary: | This paper proposes an algorithm named <i>EddyGraph</i> for tracking mesoscale eddy splitting and merging events. Twenty-seven years (January 1993–December 2019) of sea level anomaly (SLA) data are analyzed in the Northwest Pacific Ocean (105°E–165°W, 0°N–60°N). First, we propose a multilevel eddy identification method based on SLA to obtain an <i>eddytree</i> data set, representing a spatial topological tree structure of closed SLA contours with mononuclear eddies, multicore eddies and eddy seeds as the leaf nodes and <i>eddygroups</i> (reflecting the spatial topological relationship among eddies) as the intermediate nodes. The <i>EddyGraph</i> tracking algorithm is applied to the <i>eddytree</i> data set, which results in eddy-directed acyclic graphs (<i>Eddy-DAGs</i>). Only eddies contained within a common <i>eddygroup</i> are tracked as sources in merging events or sinks in splitting events. Furthermore, we extract typical splitting and merging events and composite the sea surface temperature anomalies (SSTAs) inside the <i>eddygroup</i>s and eddies during these events. The results confirm that merging eddies in the same <i>eddygroup</i> degenerate into a single eddy and that a splitting eddy evolves into eddies within the same parent <i>eddygroup</i>. Moreover, we match a merging event of cyclonic eddies with in situ data of both drifters and loopers in Lagrangian trajectories. Finally, we present <i>EddyGraph</i>, a data set of mesoscale eddy tracking in the Northwest Pacific Ocean (105°E–165°W, 0°N–60°N).
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