[A-OS13] Exploring Variability and Changes in Ocean Biogeochemical Cycles

[AOS13-P12] Reconsideration of the impact of temporal changes in winter mixed layer saturation of CFC/SF6 on water mass age estimation

*Ryohei Yamaguchi¹, Yuichiro Kumamoto¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:Water mass age estimation, Chemical tracer , Transit time distribution

CFCs (chlorofluorocarbons) and SF₆ (sulfur hexafluoride) are anthropogenic compounds that remain chemically inert in seawater. Due to their well-documented atmospheric histories, they are widely used as tracers to study ocean circulation. To estimate the age of water masses (i.e., the time elapsed since their last contact with the sea surface) from CFCs/SF₆ observations, the time series of the saturation state in the ocean surface mixed layer, which regulates the boundary condition in the estimation, is essential. However, global observations of the saturation state remain limited, primarily due to challenges in conducting shipboard measurements during winter, when water mass formation actually occurs. In this study, we first use the CFCs/SF₆ data archived in the GLODAPv2.2023 to investigate the temporal variability of the winter mixed layer saturation state on a global scale, increasing the number of samples by utilizing profiles measured even during non-winter months by reconstructing the winter saturation state from them. Our results reveal a widespread increasing trend in winter mixed layer saturation of CFCs/SF₆ across much of the global ocean since the 1980s. This trend likely reflects the time-lagged response of oceanic tracer concentrations to atmospheric levels. Conversely, decreases in saturation are observed in limited regions of the Southern Ocean and North Pacific. Next, we examine the impacts of the detected temporal changes in saturation on estimating the transit time distribution of water masses. It is shown that the use of constant saturation as a boundary condition leads to errors on the order of a few years to a few decades, especially for water mass age estimates based on recent observations, compared to the use of linearly increasing saturation. We demonstrate that the interannual variability in saturation at the time of water mass formation propagates downstream and affects the age of the water mass at the time of observation.