Roles of surface forcing in the Southern Ocean temperature and salinity changes under increasing CO2: perspectives from model perturbation experiments and a theoretical framework

Abstract A rapid warming and freshening of the Southern Ocean have been observed over the past several decades and attributed to the anthropogenic climate change. In this study, ocean model perturbation experiments are conducted to separate roles of individual surface forcing in the Southern Ocean temperature and salinity changes. Model-based findings are compared with results from a theoretical framework including three idealized processes defined on the θ-S diagram. Under the future scenario of CO2 doubling, the heat flux forcing dominates the large-scale warming, deepening of isopycnals, and spiciness changes along isopycnals, which can be captured by an idealized pure warming process to represent the subduction of surface heat uptake. The poleward-intensifying westerly winds account for 24% of the enhanced warming between 35°–50°S and would have comparable contribution as the heat flux forcing after removing the global ocean warming effect. In contrast, the wide-spread freshening in the Southern Ocean driven by increased surface freshwater input is largely compensated by the wind-driven saltening. The response to freshwater forcing could not be approximated as a similar pure freshening process as the induced cooling and freshening have comparable effects on density. The wind-driven changes are primarily through the local heave of isopycnals, thus resembling an idealized pure heave process, but contain considerable spiciness signals especially in the mid-latitude Southern Ocean, resulting from anomalous northward transport and subduction of heat and salt which are largely density-compensating. These distinct signatures of individual surface forcing help to better understand observed and projected changes in the Southern Ocean.