Decadal Reorganization of Subantarctic Mode Water

Abstract

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. We use an ocean state estimate to analyze SAMW volume budgets for the period 1992 to 2017. They reveal a decadal SAMW volume reorganization comparable to the long-term trend in Indian Ocean, and a multi-decadal volume reorganization exceeding the long-term trend in the Pacific. In both sectors, the SAMW reorganization exhibits a two-layer density structure, with compensating volume changes of lighter and denser SAMW, driven by heat flux changes in the Indian sector and central Pacific and freshwater flux changes in the southeast Pacific. This variability is governed by a cumulative effect of surface flux anomalies associated with the Interdecadal Pacific Oscillation. Shorter-term trends observed during the Argo period are largely explained by this variability.