The Southern Ocean absorbs most of the heat uptake in the climate system due to the anthropogenic warming. The fast ocean warming is found in the mid-latitude Southern Ocean accompanied by poleward expansion of the subtropical gyres. Using model experiments forced by the wind stress, heat flux, and freshwater flux perturbations for doubled CO₂ concentration provided by the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP), here we separate the roles of different types of air-sea fluxes in driving the Southern Ocean warming and water mass changes. Several key findings include: (1) The surface heat flux changes account for most of the ocean warming by adding heat into the ocean; (2) In terms of the enhancement of warming at middle latitudes (centred at ~45ºS), the wind-driven heat convergence and the accumulation of surface heat uptake by the background circulation have nearly equal contributions; (3) The poleward expansion of the subtropical gyres is primarily attributed to the wind forcing. The wind forcing also drives clockwise shift of the isopycnals in the Southern Ocean which resembles a spin-up of the meridional overturning circulation; (4) The surface heat flux changes dominate the spiciness changes on density surfaces including cooling and freshening within the Subantarctic Mode Water, whereas the surface freshwater flux changes and wind forcing contribute to a lesser extent.