"Shaken, not stirred" — it is widely known how James Bond prefers his martinis. In physics, stirring stretches a fluid into thin streaks, creating turbulence and mixing its properties. In the ocean, a similar process occurs as winds and other forces move seawater. When this happens horizontally over tens to hundreds of kilometres, it is called mesoscale horizontal stirring (MHS).
MHS plays a crucial role in redistributing heat, nutrients, and dissolved substances in the upper ocean, shaping plankton distribution and influencing the movement of fish eggs, larvae, and pollutants such as microplastics. However, studying small-scale ocean currents in polar regions has long been a challenge due to their remoteness and harsh conditions. Ship-based observations and satellite data provide limited detail, while most climate models lack the resolution needed to capture fine-scale turbulence and horizontal mixing accurately.
To address this gap, a team of researchers led by Professor June-Yi Lee, Mr. Gyuseok Yi, and Professor Axel Timmermann from the IBS Center for Climate Physics (ICCP) at Pusan National University, South Korea, conducted ultra-high-resolution simulations using the Community Earth System Model version 1.2.2 (CESM-UHR). These simulations, performed on the Aleph supercomputer at the Institute for Basic Science in Daejeon, enabled the team to examine how ocean stirring responds to greenhouse warming. Their findings, published in Nature Climate Change on November 5, 2025, show how this fully coupled model—integrating atmosphere, sea ice, and ocean components—captures the dynamic interactions that drive MHS under present-day, CO₂-doubling, and CO₂-quadrupling conditions.
"Our results indicate that mesoscale horizontal stirring will intensify considerably in the Arctic and Southern Oceans in a warming climate," said Mr. Yi.
The team found that this intensification is primarily driven by stronger ocean flow and turbulence resulting from sea ice loss. Using a diagnostic tool known as the finite-size Lyapunov exponent (FSLE), which measures how neighboring parcels of water drift apart, the researchers observed a clear increase in horizontal stirring across both polar oceans. In the Arctic, sea ice loss exposes open water to wind, stirring the water column more vigorously and increasing eddy activity. In Antarctic coastal regions, melting and freshening enhance density gradients, strengthening currents such as the Antarctic Slope Current.
As ocean turbulence intensifies, nutrient cycles, plankton distribution, and the movement of microplastics could change substantially. Prof. Lee 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."
Further research at ICCP will integrate biological models of plankton and fish into next-generation simulations. "Currently, at the IBS Center for Climate Physics in South Korea, we are developing a new generation of Earth system models that better integrate the interactions between climate and life," added Prof. Timmermann. "This will deepen our understanding of how polar ecosystems respond to global warming."
