Mesoscale eddies with North Pacific subtropical mode water as observed by an autonomous underwater vehicle during April–June 2018

• North Pacific subtropical mode water (STMW) was observed by underwater glider. • STMW was thicker (thinner) in an anticyclonic (cyclonic) mesoscale eddy. • Very low potential vorticity waters were detected in multiple density layers. • Biogeochemical signals reflected meso- and submesoscale physical variability. • Meso-/submesoscale variations were linked with basin-scale STMW structure. Subtropical mode waters (STMWs) characterize the circulation of the subtropical North Pacific. Understanding the effects of mesoscale eddies on STMWs is essential for further advancing our understanding of water mass distributions and variability. We used an underwater glider to conduct detailed observations of mesoscale eddies in the open ocean region of the subtropical North Pacific, focusing on their relationship with STMW. The STMW layer was thicker in an anticyclonic eddy than in a cyclonic eddy, corresponding to depression and uplift, respectively, of the thermocline and suggesting a dominant contribution of the anticyclonic eddy to STMW volume. Intermittent observations of potential vorticities (PVs) much lower than the STMW threshold were recorded, indicating that the STMW core within the anticyclonic eddy had a multi-layered structure. A state-of-the-art high-resolution ocean reanalysis reproduced the mesoscale variations with the observed contrast between eddy polarities and also represented the submesoscale PV variations, although the PV values were not as low as those in the observations, and individual variations were not well captured. Biogeochemical observations also exhibited mesoscale and submesoscale variations corresponding to the physical STMW variations. For example, the chlorophyll- a concentration exhibited a relatively shallower and stronger core in the cyclonic eddy than in the anticyclonic eddy and was also enhanced by the lower PVs observed at submesoscale. These results are broadly consistent with previous studies and provide new insights into the linkage between the classical view of STMWs as basin-scale water masses in the mean field and their meso- and submesoscale variations.