A study published in the journal Nature Climate Change by an international team of scientists, from the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea, presents new evidence that ocean turbulence and a process known as "horizontal stirring" will increase dramatically in the Arctic and Southern Oceans due to human-induced Global Warming and decreasing sea ice coverage.
"Shaken, not stirred" - it is widely known how James Bond prefers his Martinis. Stirring works by stretching a fluid into thin streaks, which eventually helps to create turbulence, contributing to the mixing of water properties. In the ocean, a similar stirring process of seawater occurs due to the action of winds and other sources of energy. When it happens horizontally over scales of tens to hundreds of kilometers, it is referred to as mesoscale horizontal stirring (MHS). MHS transports and redistributes heat and nutrients, thereby determining the distribution of plankton in the upper ocean. Moreover, the stretching, rotation, and spatial separation of nearby fluid parcels over time also control the dispersal of fish eggs and larvae, but also of pollutants, such as microplastics.
Due to the remoteness of polar regions, it has remained challenging to study the impact of global warming on small-scale ocean currents and marine ecosystems using ship-based observations and satellite data. Instead, climate scientists have relied heavily on climate computer models. However, the current generation of such models lacks the spatial resolution to resolve small-scale ocean processes relevant to MHS and the production of turbulence and horizontal mixing.
To overcome this shortcoming, the South Korean research team analyzed results from ultra-high-resolution simulations conducted with the Community Earth System Model version 1.2.2 (CESM-UHR), on the Aleph supercomputer at the Institute for Basic Science in Daejeon. This fully-coupled model integrates atmosphere, sea ice, and ocean components to realistically represent their interactions within the climate system, using a horizontal resolution of 0.25° for the atmosphere and 0.1° for the ocean. The team focused on simulations under present-day (PD), CO2 doubling (2xCO2), and quadrupling (4xCO2) conditions to investigate how MHS responds to human-induced warming.
To characterize the stretching of fluids into elongated filament-like structures (Fig. 1), the research team employed a technique known as finite-size Lyapunov exponents (FSLE), which tracks how quickly neighboring fluid parcels separate over time due to mesoscale ocean eddies (swirling currents with scales of tens to hundreds of kilometers), meandering flows, and ocean fronts. Using daily data from 10 years of simulation, the computationally demanding FSLE calculations show a pronounced future intensification of MHS across the Arctic Ocean and along the Antarctic coastal region (Fig. 1, 2), which can be attributed primarily to the dramatic decline in sea ice in a warming world. The researchers found that the mechanisms linking sea ice loss to enhanced MHS differed between the two regions.
In the Arctic Ocean, the disappearance of sea ice increases the mechanical energy input into the ocean. Uninterrupted by sea ice, a clockwise wind forcing can strengthen both the mean ocean flow and enhance the generation of upper ocean eddies, ultimately leading to intensified MHS (Fig. 1) and turbulence.
By contrast, in the Antarctic coastal region, the projected future strengthening of MHS around Antarctica arises from near-shore freshening due to sea ice decline, which enhances the north-south density gradient. This, in turn, reinforces the mean ocean currents, such as the Antarctic Slope Current, enhancing eddy activity and MHS (Fig. 2). Given that such intensification of MHS is expected to induce major changes in ocean ecosystems as well as in the dispersal of marine pollutants, further research is urgently needed.
"The contrast between the Arctic Ocean, which is enclosed by surrounding continents, and the Southern Ocean, where the continent is encircled by ocean, creates different physical conditions for ocean stirring. But the outcome for ocean stirring under warming is quite similar," said lead author YI Gyuseok, a doctoral researcher at the ICCP and Pusan National University.
"Horizontal stirring is a crucial factor for fish larval transport across the ocean. For moderate values, this process connects populations and habitats geographically, increasing their genetic exchange. However, for increasing stirring in the future, larvae can be transported into unsuitable areas where they may not survive," remarks Prof. LEE June-Yi from the ICCP and co-corresponding author of the study.
Understanding the ecological implications of the author's main findings requires additional earth system modeling experiments at high spatial resolution, including computer models of plankton and fish.
"Currently, at the IBS Center for Climate Physics in South Korea, we are developing a new generation of earth system models that better integrates the interactions between climate and life. This will deepen our understanding of how polar ecosystems respond to Global Warming", said Prof. Axel TIMMERMANN, co-author of the study and Director of the ICCP.
