Linear trends of temperature, salinity, and oxygen in the North Pacific based on GOBAI-O₂ over 2 decades and their controlling factors

Abstract. Oxygen concentrations in the ocean are believed to be declining due to global warming. However, our understanding of its variability remains limited compared to physical parameters such as temperature and salinity, because oxygen is difficult to observe with high spatial and temporal resolution. In this study, we analyzed linear trends in potential temperature, salinity, and dissolved oxygen in the North Pacific over the past two decades (2004–2023), using the GOBAI-O₂ data. We then compared oxygen trends with physical parameters to investigate the spatial pattern of linear changes across the region. The oxygen trends derived from GOBAI-O₂ were consistent with those observed along hydrographic lines that have been relatively frequently and continuously surveyed by ship-based observations. Although an overall declining trend in dissolved oxygen was evident, localized increases were observed in certain density layers. By examining the associated physical conditions, we found that the spatial heterogeneity of the oxygen trends can be attributed to known oceanographic processes, such as the southward retreat of the oxygen minimum layer and the northward migration of a recently identified front separating the subtropical and subarctic gyres. Our findings highlight the utility of GOBAI-O₂ data in linking physical changes to previously unrecognized biological and biogeochemical patterns in the ocean.