A 2023 study https://www.sciencedaily.com/releases/2023/03/230330102327.h... observed slowdown in Antarctic overturning, in which cold water sinks down at the south pole and then spreads north in the deeper parts of the ocean.
The slowing of this process would cause deep ocean water to become warmer.
edit: the publication linked in the article https://www.pnas.org/doi/epub/10.1073/pnas.2500440122 makes this a bit clearer:
"In the polar Southern Ocean, cold, fresh surface waters overlay warmer, saltier deep waters (Fig. 2A). During winter, surface cooling and sea ice formation reduce stratification, allowing vertical mixing to transport heat upward, either melting sea ice from below or limiting its growth (8). However, decades of surface freshening strengthened stratification, trapping subsurface heat at depth, sustaining expanded sea ice coverage (7, 9) and limiting deep convection along with open-ocean polynyas (10). Here, we show that since 2015, these conditions have reversed: Surface salinity in the polar Southern Ocean has increased, upper-ocean stratification has weakened, sea ice has reached multiple record lows, and open-ocean polynyas have reemerged."