Multicore structure of the North Pacific subtropical mode water from enhanced Argo observations

Abstract Seventeen Argo profiling floats with enhanced vertical and temporal sampling were deployed in the Kuroshio recirculation gyre in the western North Pacific in late March 2014. The Subtropical Mode Water (STMW) observed in many profiles displays a “multicore structure” with more than one minima in potential vorticity (PV), corroborated by vertical covariations in apparent oxygen utilization (AOU). These cores are classified into four submodes according to density and AOU. The submode waters are typically 100 m thick, in which PV varies by 1 × 10−10 m−1 s−1 and AOU by 10 µmole/kg. The STMW multicore structure is most frequently observed in spring, gradually taken over by single‐core profiles into summer. The seasonal evolution is suggestive of vertical mixing, especially in STMW of lower density.