Nodes of the Kuroshio Current system from multidecadal repeat observations along 137°E

The Kuroshio Current is a western boundary current in the northwestern Pacific, and its transport and path variability significantly affect air-sea interactions, thus modulating North Pacific climate, as well as ecosystems. The Japan Meteorological Agency (JMA) 137°E repeat hydrographic section, occupied every winter from 1967 to 2023 (57 years), provides a long and consistent benchmark for diagnosing the variability of the Kuroshio Current system. Here, we analyze these repeated occupations and derive the vertical structure of zonal geostrophic velocity and associated transport. Our analysis reveals that the Kuroshio Current system exhibits substantial variability and intrinsic asymmetry in its transport, axis position, and vertical hydrographic structure. To capture the asymmetric hydrographic patterns associated with these transport fluctuations, we extract leading variability in the vertical structure using empirical orthogonal functions and apply a 1×5 self-organizing map (SOM) to classify distinct circulation patterns. The SOM yields five physically interpretable nodes: two large-meander (LM) nodes (moderate and extreme) and three distinct non-LM nodes. The extreme LM node features a southward shift of the Kuroshio axis to around 30°N accompanied by a significant weakening of the recirculation gyre. Moderate LM events exhibit a less pronounced southward shift near 31°N. The non-LM nodes can be characterized by (i) strengthened recirculation with near-normal net transport, (ii) enhanced net eastward transport, and (iii) reduced net transport. The heaving of isopycnal lines mostly accounts for thermohaline anomalies throughout the nodes, whereas spicing plays a partial role only in the extreme LM node. This study argues that variation in the thickness of the Subtropical Mode Water (STMW) accounts for upper ocean heat content and consequently for volume transport, underpinning STMW thickness as a metric integrating variability across the Kuroshio Current system along the 137°E section.