Precipitation and melting Antarctic ice are cooling the Southern Ocean

Most of the world’s oceans are warming at a dizzying pace, but the Southern Ocean is governed by different laws – its surface has shown surprising cooling for decades. A team of Stanford University scientists, led by Earle Wilson, has discovered that the key to this phenomenon is melting Antarctic ice and increased precipitation, still underestimated in climate models. Published March 27, 2025 in Geophysical Research Letters, the findings show how global warming can lead to a local decline in temperatures.

Why do the models differ from reality?

Between 1979 and 2015, when the planet was experiencing intense warming, the surface of the Southern Ocean behaved differently: the temperature dropped and the sea ice around Antarctica – instead of melting – grew. The latest climate models, part of the sixth generation of the CMIP6 project, predict an increase in sea surface temperature(SST) for this region. However, observational data show the opposite.

Scientists confirm that a process called freshening is responsible for the discrepancy. It involves the influx of freshwater into the surface layers of the ocean, which increases density differences and impedes heat exchange between the cool surface and warmer deep waters. As Earle Wilson points out: what happens at the surface can effectively cut off access to heat stored deeper, changing the thermal and circulation dynamics of the entire region.

Underestimated sources of fresh water

The researchers identified two main sources of freshwater inflows causing the snowmelt effect that are either overlooked or insufficiently accounted for in the models:

• Increased precipitation over the Southern Ocean, driven by global warming, which intensifies the circulation of water in the atmosphere. Changes in wind circulation, especially the strengthening of westerly winds, are increasing the amount of rain in the region;
• melting of Antarctic ice caps and shelves, which provides huge amounts of fresh water, especially along the continent’s coasts.

To study their impact, the researchers worked with the faf-water and antwater projects. In two series of experiments (faf-water – simulating the impact of widespread freshening by precipitation; antwater – investigating the effects of melting Antarctic ice), controlled portions of fresh water were added to CMIP6 models, simulating its inflow under different scenarios. The results revealed that the sea surface temperature response is much stronger when fresh water hits the Antarctic coast than when it dissipates into the open ocean. It turned out that location matters a lot in this case.

Where fresh water enters the ocean has proven to be critical. Concentrated meltwater along Antarctica’s shores is more effective at lowering surface temperatures than precipitation distributed evenly over a larger area. Proximity to land enhances sea ice formation, which in turn amplifies the cooling effect through changes in its seasonal cycle, the study authors explain. As Wilson adds in the Stanford announcement, melting ice shelves, such as those in the Weddell Sea, can have a domino effect, stabilizing cool conditions at the surface for decades.

This mechanism partly explains why the Southern Ocean remains cooler, even though deeper water layers – such as Antarctic bottom water – are gradually warming. It is the cutoff of the surface from these reservoirs that is driving the observed trend.

60% difference explained

Analyzing data from 1990 to 2021, the team estimated that freshwater flows not accounted for in the models could account for up to 60 percent of the discrepancy between simulations and reality. A key tool in this analysis was climate response functions (CRFs), which allowed them to accurately examine how the ocean responds to sudden changes in water inflows. The research showed that underestimating both precipitation and ice melt significantly disturbs the models’ ability to reproduce historical temperature trends.