Rapid water parcel transport across the Kuroshio Extension in the lower thermocline from dissolved oxygen measurements by Seaglider and BGC Argo floats

With regard to water mass across the Kuroshio Extension, clarifying not only their physical mechanisms but also their spatial and time scales are crucial to understand heat/freshwater and materials budget and variability in the basin. Especially in the northwestern North Pacific Ocean, deep mixed layer is actively formed by strong winter cooling, and major water masses (e.g., STMW, CMW, and NPIW) are broadly produced. Under those circumstances, we observed water mass characteristics and their changes with autonomous in-situ measurement tools. In 2011-2012, over 30 BGC-Argo floats with dissolved oxygen sensors had been deployed around 30N,145E within a framework of INBOX project in JAMSTEC, which aimed to clarify small scale physical and geochemical processes of a mesoscale eddy. In addition, a Seaglider had launched near the area in 2014 to investigate water mass formation and modification process, continuously operating for 4 months. From above observations, we obtained detailed temperature, salinity and oxygen data with dense spatial and temporal resolutions in the Kuroshio Extension. The analysis of the Seaglider data shows that small water parcels with low salinity and/or low dissolved oxygen were observed in the lower thermocline (26.0- 26.8σθ) during middle of April to early May, whose scales were ~ a few ten kilometers for horizontal width and ~ a few ten meters for vertical thickness. Based on apparent oxygen utility (AOU) analysis, the small water parcels had characteristics of less amount of oxygen consumption, suggesting that the water parcels took shorter time after contacting atmosphere than surrounded water. Also, the water parcels subducted from subpolar northwestern North Pacific region to the north of the Kuroshio Extension. Similar small water parcels with the same characteristics had been observed by the BGC Argo floats operating in the INBOX project. We will discuss some comparisons with a high-resolution numerical model.