Long-Term Trends of the South Pacific–Subantarctic Mode Water Modulated by the Interdecadal Pacific Oscillation

The subantarctic mode water (SAMW) represents a large water mass in the Southern Ocean. This body of water forms through deep convection (subduction) in winter and contributes to the uptake and storage of anthropogenic heat. However, its long-term changes in subduction rate and volume in response to shifting climate conditions are unclear. In this study, we investigated the long-term trend of the subduction rate and volume of the South Pacific–SAMW (SPSAMW) using Simple Ocean Data Assimilation outputs during 1980–2017. The results show the overall increasing trend of the subduction rate of the SPSAMW. The increased subduction of the SPSAMW directly contributes to the volume variation in the SPSAMW. The increased subduction in the South Pacific reached (0.28±0.16) Sv⁻¹ per year, which explains nearly 68% of the volume increase in the SPSAMW. This variability in the SPSAMW reflects alterations in the overlying atmosphere. The positive to negative phase change of the Interdecadal Pacific Oscillation (IPO) in 1980–2017 deepened the Amundsen Sea Low (ASL) via atmospheric teleconnections over the South Pacific. Further analysis reveals that the increased westerly winds during the deepening of ASL resulted in more cold water transport from the south, which deepened the winter mixed layer and thus increased subduction and volume within the SPSAMW subduction region. This finding suggests the association of the long-term trends of SPSAMW subduction and volume with the phase change of the IPO.