Climatology and decadal variations in multicore structure of the North Pacific subtropical mode water

Abstract

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.

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

“…Journal of Geophysical Research: Oceans potential temperatures in the range of 18-19.5, 17-18, and 16-17°C, respectively. Following Liu et al (2017), we defined the STMW by its potential temperature in the range of 18-19.5, 17-18, and 16-17°C as the warm, middle, and cold STMWs, respectively.…”

“…Previous studies suggest two possible formation mechanisms for the STMW multicore structure. First, in the STMW formation season, the newly formed warmer STMW core overlies the older and colder STMW core, which has not been ventilated in the current winter due to insufficient buoyancy loss (“insufficient ventilation mechanism”; Liu et al, ). Second, due to the background circulation, STMWs with different properties eventually interleave and stack up vertically, forming the STMW multicore structure there (“interleaving effect”; Oka et al, ).…”

“…However, high vertical resolution vertical profiles show that the STMW pycnostad often has two, three, or even more vertical minima in the PV (Figure ) (Gao et al, ; Oka et al, ; Taneda et al, ). The multicore structure of STMW is frequently observed in the spring and gradually dissipates into a single broad core after the summer (Gao et al, ; Liu et al, ).…”

“…Second, due to the background circulation, STMWs with different properties eventually interleave and stack up vertically, forming the STMW multicore structure there (“interleaving effect”; Oka et al, ). In particular, during the KE's unstable state, the associated stronger eddy activities may lead to a more frequent occurrence of the STMW multicore structure (Liu et al, ; Oka et al, ). Although previous studies have realized the mesoscale eddy effects on the STMW multicore structure, the detailed physical processes remain unclear, due to the lack of long temporal eddy‐tracking subsurface observations.…”

“…Because of atmospheric cooling, colder and denser STMW tends to form toward the eastern part of the formation region (Bingham, ; Oka et al, ; Suga & Hanawa, ). Typically, STMWs with potential temperatures of 16.0–17.0, 17.0–18.0, and 18.0–19.5 °C, which define cold, middle, and warm cores, are formed east of 150°E, between 140° and 150°E, and west of 140°E, respectively (Liu et al, ; Oka et al, ). Our previous work found that most of the STMW multicore structure is located west of 150°E (Liu et al, ), which may result from the interleaving of STMW with different potential temperatures and densities by the background westward flow.…”