Future warming of Circumpolar Deep Water linked to surface buoyancy forcing

Circumpolar Deep Water (CDW) is the warmest water mass in the Southern Ocean and its transport onto the Antarctic continental shelf is the main contributor of heat to the region. Observations suggest that CDW has warmed and shifted poleward in recent decades, potentially contributing to increased poleward heat transport and the accelerated melting of Antarctic ice shelves. However, due to the sparsity of observations and the complex dynamics of CDW, the precise processes driving its warming in the Southern Ocean remain unclear. In this study, we employ a high-resolution global ocean-sea ice model to investigate the mechanisms of CDW change, focusing on future change. We conduct separate perturbation experiments to isolate the effects of projected surface buoyancy forcing and changes in wind stress. Our findings reveal a warming signal of CDW driven by surface buoyancy forcing, contrasted by a reduced warming signal when wind stress changes are also included. A heat budget analysis shows that the warming of upper CDW is due to reduced winter cooling from mixing with warmed waters around the thermocline. These changes in mixing processes include weaker convective mixing due to increased stratification, and less heat loss by vertical diffusion because of the warmer ocean surface. In addition to changes in the mixing processes, the warming of lower CDW is partly driven by a reduction in Antarctic Bottom Water formation, which causes isopycnal descent and therefore enhances the convergence of advective heat flux.