The Southern Ocean and the adjacent subtropical oceans generally exhibit deoxygenation trends. However, a significant oxygenation region exists in the intermediate waters of the South Pacific between 160°E–90°W and 20–55°S, and its cause remains unclear. Changes in dissolved oxygen at fixed depths may result either from the apparent redistribution caused by the vertical displacement of isopycnal surfaces, referred to as heave, or from changes in water-mass properties along isopycnal surfaces, referred to as spice. The relative contributions of these two components determine the physical nature of the observed oxygenation signal.

This study is based on the HanMY reconstructed dissolved oxygen product for 2005–2020. Neutral density is calculated using the internally consistent temperature and salinity fields provided by the product. The Bindoff–McDougall framework is then applied to decompose intermediate-water dissolved oxygen trends into heave and spice components. Attribution is conducted from multiple perspectives, including spatial distributions, area-weighted averages, and volume-integrated oxygen changes.

The results show that, within the oxygenating water body, the heave and spice components have the same sign and act together. However, the spice contribution is clearly dominant, accounting for approximately two-thirds of the dissolved oxygen increase. This indicates that the oxygenation is not an apparent signal caused by the displacement of isopycnal surfaces. At the same time, the isopycnal surfaces in the study region exhibit systematic deepening, and heave therefore represents a persistent background redistribution process. Notably, when the surrounding deoxygenating regions are also included, the net oxygen budget over the entire domain becomes dominated by heave. This result suggests that spice-driven oxygenation is a localized signal specific to the oxygenation region rather than a widespread process throughout the study domain.

On this basis, we  decomposed the spice component into an oxygen-solubility component and an apparent oxygen utilization component to determine whether the observed oxygenation is ventilation-driven or solubility-driven. We will also combine this analysis with water-mass analysis to characterize its distribution in neutral-density space, thereby providing observational constraints for understanding the mechanisms of oxygen-content changes in subtropical intermediate waters.