Climatology and decadal variations in multicore structure of the N orth P acific subtropical mode water

The pycnostad of the North Pacific subtropical mode water (STMW) often displays multiple vertical minima in the potential vorticity profile. Argo profile data from 2004 to 2015 are used to investigate interannual to decadal variations of the multicore structure. Climatologically, about 24% pycostads of STMW have multicore structure, and most of them distribute in the region west of 150°E. STMW cores are classified into three submodes—the cold, middle, and warm ones with potential temperatures of 16.0–17°C, 17–18°C, and 18–19.5°C, respectively. The Kuroshio Extension (KE) varies between stable and unstable states. The unstable KE with large meanders increases the subsurface stratification and shoals the winter mixed layer east of 150°E with warmer temperatures. There, the dominant STMW type varies from the cold single type in stable KE years (making up 72% of the profiles with STMW) to the middle single one (53%) in unstable years. The variation of the dominant STMW type in the region east of 150°E subsequently affects the multicore structure of STMW west of 150°E. In a broad region between 130°E and 180°E, profiles with STMW are fewer in unstable years but the proportion of multicore profiles increases among STMW profiles. This might be due to the split recirculation gyre with a chaotic KE. About 24% of the Argo profiles with STMW display multicore structures and mostly distribute in the region west of 150°E The core temperatures of the STMW, formed in the region east of 150°E, vary with the dynamic state of the Kuroshio Extension (KE) The variation of the dominant STMW type in the region east of 150°E subsequently affects the multicore structure of STMW west of 150°E