Response of Antarctic ocean circulation to increased meltwater

Abstract

The implications of ocean freshening from accelerating Antarctic land-ice loss are poorly understood, due to the scarcity of observations near the Antarctic coast, and the high spatial and temporal resolution required to resolve Antarctic continental shelf processes in ocean models. Here, a high-resolution global ocean--sea-ice model is used to investigate the response of Antarctic continental shelf circulation to increasing meltwater. Two freshwater perturbation experiments are conducted, using projected Antarctic ice-loss rates under RCP 4.5 and RCP 8.5 emissions scenarios at 2100.We find that surface freshening near the Antarctic coast increases stratification and reduces the formation of cold, dense waters on the Antarctic continental shelf that, in the current climate, drive abyssal ocean circulation and ventilation. In our simulations, the connection between the abyssal ocean and the cold Antarctic shelf collapses within 10 years following the application of projected 2100 meltwater forcing, as downwelling surface waters on the continental shelf are freshened by glacial runoff, leaving them too buoyant to sink to the abyssal ocean. Around Antarctica, coastal freshening increases lateral density gradients between the cool, fresh shelf and the warm, saline open ocean, strengthening frontal structures that separate the adjacent water-masses, and accelerating geostrophic currents that flow westward along the coast and along the continental shelf break. This process acts to homogenise shelf waters and increasingly isolate the cool continental shelf from the warmer open ocean, leading to a net cooling on the continental shelf. Acceleration of the circumpolar coastal current results in remote temperature feedbacks unique to these experiments; most notable being a strong cooling signal on the West Antarctic shelf, an historically warm region associated with high rates of ice shelf melt, generated by the advection of cold Weddell Sea shelf waters around the Antarctic Peninsula by the strengthening coastal current. However, shelf cooling is not a circumpolar response to freshening. Deep warm anomalies arise in the Ross Sea, Adelie Coast, and Prydz Bay continental shelf dense water source regions under enhanced meltwater forcing, as full depth convention collapses and surface cooling fluxes diminish. Warming is strongest in the Ross Sea, where the absence of strong frontal structures and zonal currents at the shelf break grants warm, open ocean waters uninhibited access to the shelf. On the Prydz Bay and Adelie Coast shelf, warming due to convective shutdown is counterbalanced by decreased shoreward advective heat transport, due to strengthening frontal structures at the shelf break. As such, we find that coastal freshening induces both warm and cool anomalies at different locations along the Antarctic continental shelf, suggesting that shelf freshening by meltwater can both accelerate ice shelf melt (a positive feedback) and inhibit melt (a negative feedback) depending on regional factors. These findings improve our understanding of the complex suite of climate effects associated with the melting of Antarctica's land-ice. Both the disruption of abyssal overturning and shifts in heat transport to ice shelves have implications for sea-level rise; as reduced deep ocean ventilation may increase oceanic heat content (effecting steric sea-level), and accelerating land-ice loss, in regions of warming, adds mass to the oceans (effecting eustatic sea-level).