Ar-Rehman

Melting polar ice offers unexpected powerful new ocean currents

The Atlantic Meridional Overturning Circulation (AMOC), a system of Atlantic Ocean currents which redistributes heat and nutrients between the tropics and the North Atlantic, is one of the planet's tipping points. This means there is a critical threshold which once crossed, could trigger abrupt, irreversible climate shifts. Polar ocean currents are entering a new phase of motion. As sea ice thins and retreats in both hemispheres, winds gain more leverage over open water, driving stronger sideways mixing which shifts where heat, nutrients and even pollutants end up near the surface. These changes can ripple through marine food webs and alter how much heat the Arctic and Southern Oceans send back to the atmosphere. According to predictions, melting of the Greenland Ice Sheet (GIS) could destabilize the AMOC. However, new research suggests that melt water from the West Antarctic Ice Sheet could prevent it from collapsing altogether.

To understand how this mixing is changing, a team led by Pusan National University doctoral researcher Gyuseok Yi, working with the Institute for Basic Science (IBS), used a high-resolution Earth system model. The model is capable of tracking ocean currents only a few miles across. Their simulations reveal how a warming climate can reorganize the very processes which stir polar waters. Scientists have known for some time that the GIS poses a significant threat to AMOC. When its melt water pours into the North Atlantic, it can slow down or stop the sinking of dense water which helps drive this ocean current system. But less well known was whether the West Antarctic Ice Sheet (WAIS), located far away in the Southern Ocean, could also influence AMOC. So scientists from Utrecht University in the Netherlands decided to model what would happen if both Greenland and Antarctica started melting rapidly. Within the simulations, coastal waters near Antarctica become lighter as melt water spreads along the margin. This change lets the slope current grab energy from gravity and winds. The stronger flow channels extra ocean heat along the continental slope. This pathway can influence how quickly ice shelves thin from below. Although the model used omits full ice sheet dynamics, it still produces a strengthening Antarctic Slope Current under warming.

The ice sheet dynamics describe how large ice bodies grow and move, and extra melt water from them would probably strengthen these simulated current changes. Scientists call one key process mesoscale horizontal stirring, the sideways stretching of water patches over tens to hundreds of miles. It moves heat, salt, nutrients, plankton and pollutants around the surface ocean, so changes in its strength can ripple through entire ecosystems. In their recent work, the team used the finite-size Lyapunov exponent (FSLE) to track how quickly neighbouring water parcels drift apart. This exponent measures how fast nearby water patches separate, so high values mark regions of especially strong stirring. The calculations ran inside the Community Earth System Model with an ocean grid spacing of about six miles. The resolution is sharp enough to partly resolve eddies, small, swirling currents which pinch off from larger flows.

The researchers used a complex computer model, CLIMBER-X (an Earth system model of intermediate complexity, or EMIC), to examine how the AMOC would respond to different speeds and timing of ice sheet collapse. The most significant finding was that the West Antarctic Ice Sheet's melt water didn't always increase the risk of an AMOC collapse. Under some conditions, such as when its melting was rapid and began to slow down as Greenland's melt peaked, it could prevent a total collapse. This happens because Antarctic melt water changes how layers of water behave in the Southern Ocean, which eventually sends slightly saltier water toward the North Atlantic. Over time, this helps the water stay dense enough to keep the AMOC moving, at least in the model. However, even with this stabilizing effect, the AMOC weakens by about 60% and would take about 3,000 years to recover. Arctic sea ice extent, the area of ocean covered by ice, has fallen for decades, according to Intergovernmental Panel on Climate Change reports. As that icy cover shrinks in the model world, more open water becomes exposed to wind. This exposure lets the air transfer more energy into the ocean. With more open water, the simulations show stronger mesoscale currents, flows that are tens-of-miles wide, spiralling across the central Arctic basin.

One result is that this strengthening appears when CO2 is doubled. Further increases then extend the season when these faster flows persist. In the model, the seasonal cycle, the regular pattern of change through each year, of stirring in the Arctic weakens as thick ice disappears. Instead of clear summer-winter contrasts, the simulations show more irregular mixing which varies from year to year. “Our results indicate that mesoscale horizontal stirring will intensify considerably in the Arctic and Southern Oceans in a warming climate,” said Yi. The finding confirmed similar results from previous, simpler conceptual models. "Our results clearly demonstrate that the AMOC stabilization driven by WAIS melt water fluxes is not only present in conceptual models, but can also be found in EMICs," wrote the researchers. Around Antarctica, a band of water known as the Antarctic Slope Current, a westward-flowing current hugging the continental margin, circles the continent just offshore from the sea ice zone. In the new simulations, the current becomes faster along much of the coast as sea ice coverage drops and surface waters freshen.

Studies show that freshening can sharpen density gradients and strengthen this current. These density gradients, differences in water heaviness from place to place, help steer shoreward and seaward flows that move heat toward the ice edge. When horizontal stirring speeds up, water parcels which would normally linger near a place instead move quickly across fronts, boundaries between water masses with different properties. Those shifting boundaries can change how nutrients reach phytoplankton near the surface and where young fish and zooplankton are carried. Microplastics, tiny plastic pieces smaller than an inch, already appear in Arctic waters and sea ice, based on shipboard surveys and laboratory counts. Studies show Arctic concentrations that rival or exceed levels seen in heavily populated ocean regions farther south. Recent global modelling efforts find that near surface currents can sweep buoyant plastic from midlatitude oceans toward polar regions, where it accumulates in subsurface layers. The subsurface layers sit several feet to tens of yards below the surface. In that zone, currents can trap plastic, letting it persist in waters used by many organisms. Assessments from the Arctic Report Card note that plastic in polar seas is transported by winds, currents, and sea ice drift as well as by local dumping.

Taken together with the new simulations, these lines of evidence suggest that the same stirring which organizes nutrients and heat can also reorganize human-made debris. Because the work focuses on physics, it does not yet include full models of plankton, fish or higher predators, so outcomes remain uncertain. Even so, stronger polar stirring implies climate models must better resolve structures to capture life–water motion feedbacks. “This study highlights important implications of global warming and associated ocean changes on the ocean ecosystem,” said June Yi Lee, a professor at IBS. She notes that such links between physical change, ecosystems and pollutants will matter for adaptation choices and climate policies at national and local levels. The key message from this research is that the physics beneath shrinking ice covers are not staying still. The way water moves can amplify or soften changes triggered by greenhouse gases, so events under polar ice matter for coasts and climates everywhere. However, if the timing is wrong, such as if the WAIS melt peaks too late or is too slow, this stabilizing effect vanishes. In fact, WAIS meltwater could even accelerate the AMOC's tipping point. Even though this would be catastrophic, the study's authors emphasize that WAIS melt itself is far worse. "Such a marked event is far too dangerous to bet on given its many severe consequences including, for example, a total contribution to global sea level rise of up to 4.3 m."