Unraveling the Extensive Impact of Subthermocline Eddies on the Western Pacific Undercurrent System

Subthermocline eddies (SEs) influencing ocean circulation are progressively known, yet their extensive impact on the western Pacific undercurrent system remains largely unexplored and, in some regions, often underestimated. Okubo-Weiss parameter analysis reveals a distinctive meridional pattern of cyclonic and anticyclonic SE distribution in the interior western Pacific basin that aligns with zonally elongated mean flows. These westward-propagating SEs play a pivotal role in regulating the formation of zonal undercurrents, particularly off-equatorial regions, through the convergence of eddy potential vorticity flux. Along the Pacific western boundary region, anticyclonic SEs typically enhance (reverse) the velocity of boundary currents flowing northward (southward), primarily through barotropic energy conversion, while cyclonic SEs do the opposite. To summarize, we provide a schematic map of the circulation system in the western Pacific and emphasize the interconnected framework of undercurrents, particularly in relation to SEs.