Polar Ocean Mixing Intensifies as Sea Ice Retreats — Major Risks for Ecosystems and Climate

As polar sea ice continues to decline, small-scale churning and lateral mixing in the Arctic and Antarctic oceans are growing stronger — a change researchers say could have wide-reaching impacts on marine ecosystems and the global climate.

In a study published in Nature Climate Change, scientists at the Institute for Basic Science used ultra-high-resolution simulations with the Community Earth System Model v1.2.2 to examine future changes in mesoscale horizontal stirring (MHS). MHS refers to the swirling and mixing of seawater at finer scales that redistribute heat, nutrients, carbon, and pollutants across the ocean.

Most present-day climate and ocean models lack the spatial resolution needed to fully capture these fine-scale processes. By running higher-resolution simulations, the research team could reveal changes that coarser models miss and show that horizontal stirring is likely to intensify as polar sea ice retreats under continued warming.

Different Drivers at Each Pole

The study finds that the mechanisms driving increased mixing differ between the Arctic and Antarctic. In the Arctic, stronger MHS is primarily linked to winds blowing over newly exposed open water as sea ice retreats, which enhances turbulence and lateral mixing. Around Antarctica, increased mixing is driven mainly by surface freshening from meltwater: reduced surface density creates stronger vertical and lateral gradients that amplify stirring.

Kyuseok Lee, lead author, said: "The contrasting geographical structures — the land-locked Arctic Ocean and the coastal Antarctic waters — lead to fundamental differences in the dynamic processes that determine changes in horizontal seawater mixing. Nevertheless, if global warming continues, horizontal mixing is expected to increase significantly in both regions."

Ecological and Climatic Implications

Stronger horizontal stirring could significantly alter the distribution of nutrients that sustain marine food webs, potentially reshaping ecosystems. Intensified mixing and altered currents may carry fish larvae and other plankton to less suitable habitats, harming recruitment and biodiversity. The researchers also warn that changes in stirring could affect the transport and dispersal of pollutants, including microplastics.

June-Yi Lee, co-author, noted: "This study highlights important implications of global warming and associated ocean changes on the ocean ecosystem and the dispersal of pollutants such as microplastics. This type of research will be crucial for developing climate policies, including adaptation measures."

Beyond ecological effects, rising MHS interacts with broader climate processes: reduced ice cover and melting glaciers both feed back into mixing dynamics, while warming increases the risk of more frequent and intense extreme weather events.

What This Means For Research and Policy

The authors emphasize the need to improve model resolution and to incorporate fine-scale ocean dynamics into climate projections and ecosystem assessments. Accurate representation of mesoscale stirring is essential for reliable forecasts of regional ecosystem change, pollutant dispersal, and the ocean’s role in global climate regulation. Policymakers should consider these findings when designing conservation strategies, fisheries management, and pollution mitigation plans.

While uncertainties remain, the study provides a clear signal: as polar ice retreats, small-scale ocean mixing is likely to increase — with important consequences for life in the oceans and for humanity’s efforts to adapt to a warming world.