Decadal reorganization of Subantarctic Mode Water

Subantarctic Mode Water (SAMW) is one of the most important water masses globally in taking up anthropogenic heat and carbon dioxide. However, its long-term changes in response to varying climatic conditions are not well understood. Here we use data from the ”Estimating the Circulation and Climate of the Ocean” (version 4, release 4, ECCOv4r4) state estimate to calculate SAMW volume budgets for the period 1992 to 2017. They reveal a SAMW volume reorganization on decadal timescales in the Indian and on multidecadal timescales in the Pacific Ocean. In the Pacific, this multidecadal variability exceeds the long-term trend and is governed by an accumulation of signals from the Interdecadal Pacific Oscillation. This implies that SAMW volume trends observed during the shorter Argo period largely arise from the multidecadal variability. In both ocean sectors, the SAMW reorganization exhibits a two-layer density structure, with nearly compensating volume changes of lighter and denser SAMW. They are caused by heat flux changes in the Indian Ocean, freshwater flux changes in the southeast Pacific, and both heat and freshwater flux changes in the central Pacific Ocean. Our results indicate that the recently observed SAMW changes have to be interpreted in the context of the strong long-term variability, which imposes challenges to detecting and attributing climate change signals in SAMW.