A regime shift in Ross Sea shelf-slope circulation and abyssal ventilation under future extreme CO2 forcing conditions

The Ross Sea is a critical source region for Antarctic Bottom Water (AABW), driven by the export of Dense Shelf Water (DSW) through major submarine troughs. Under a warming climate, reduced sea-ice production and enhanced surface freshening are projected to weaken buoyancy loss on Antarctic shelves. However, how the Ross Sea shelf–slope circulation reorganizes under such significantly altered surface forcing remains poorly understood. Using high-resolution (0.1°) CESM simulations, we examine the response of this circulation to progressive warming and freshening under present-day, doubled (2xCO₂), and quadrupled (4xCO₂) CO₂ conditions. Our results show that as DSW formation declines, shelf waters become increasingly buoyant, with the most pronounced changes occurring on the western shelf. This asymmetric freshening reshapes the cross-shelf density structure and eventually reverses the horizontal density gradient. In the Joides Trough, the traditional two-layer overturning pattern disappears under 4xCO₂ forcing; instead, warm Circumpolar Deep Water (CDW) enters along the bottom, establishing a new deep pathway from the slope onto the shelf. Oceanic instability metrics indicate that strengthened lateral density gradients become comparable to, and locally exceed, the stabilizing effect of vertical stratification along the continental slope. We suggest that conditions favorable to symmetric instability may facilitate vertical exchange and support the emergence of this deep inflow, even as the Antarctic Slope Current intensifies. Rather than providing a single deterministic outcome, these findings illustrate a physically consistent scenario for the regime shift in Antarctic shelf-slope exchange, with profound implications for future abyssal ventilation and global ocean heat uptake.