Persistent Abyssal Warming but Emerging Salinity Shifts in the Southern Ocean

Recent observations reveal that Antarctic Bottom Water is thinning, warming, and spreading northward more slowly into the abyssal ocean. The causes and consequences of these changes remain uncertain due to limited observations. Using historical data (1985-2024) and model projections (2041-2050), we assess abyssal ocean changes in the past, present, and future. Between 1985-2024, isopycnals below 3000~m descended at -149±5 m decade^-1, and are replaced by warmer water, causing warming of 0.03±0.02 °C decade^-1. Freshening of -0.004±0.003 g kg^-1 decade^-1 occurred due to meltwater-driven changes in continental shelf waters forming bottom water. Projections suggest bottom water thinning will continue, doubling the abyssal ocean’s contribution to Southern Ocean sea level rise by 2050. However, freshening has slowed or reversed, indicating a salinity turning point. This transition occurs as shelf waters become too fresh and light to reach the abyssal ocean, with important implications for overturning circulation, nutrient cycling, and regional sea level rise.