Effects of Anticyclonic Eddies on the Multicore Structure of the North Pacific Subtropical Mode Water Based on Argo Observations

<jats:title>Abstract</jats:title><jats:p>The North Pacific subtropical mode water (STMW) often displays a “multicore structure” with more than one minimum in the vertical potential vorticity profile. Anticyclonic eddies (AEs) play an important role in STMW formation and transport, but their effect on the multicore structure of the STMW has not been fully studied. Here, we investigated the formation and evolution of STMW multicore structures in AEs using Argo profiles, which followed two AEs in the STMW formation region for approximately 1.5 years from 2013 to 2015. We found that after the two AEs formed east of 150°E, they trapped the local cold and dense STMW and migrated westward. Since sea surface temperatures increased toward the west, warmer and lighter STMWs were formed during the winter ventilation process as the two AEs moved westward. The newly formed STMW rode on the preexisting cold and dense STMW inside the eddy core, forming a multicore structure in the STMW. Given that water mass properties within AEs are much more conservative, the multicore structure of STMW is much more easily formed inside AEs than outside of eddies. There was a larger (smaller) proportion of profiles containing multiple STMW cores inside (outside) the AE cores, indicating that the AEs were “hot spots” for developing the multicore structure of STMW. The occurrence of the multicore structure of STMW in AEs was highest in spring (March–May) but decreased quickly as the season progressed.</jats:p>