August University of Galway researchers observe rare ocean mixing in Arctic waters

A research team from University of Galway has captured a rarely observed ocean mixing process during an expedition to the Greenland Sea, a finding that could improve our understanding of Arctic climate change.

The research team spent several weeks at sea during the summer of 2023 aboard the Marine Institute’s research vessel RV Celtic Explorer, carrying out surface ocean measurements in one of the most remote and climate sensitive parts of the world.

The team focused on a phenomenon known as cabbeling. This process involves the temperature and salinity (concentration of salt) in the ocean, which together make up the ocean density.

Cabbeling occurs when two water masses with different temperatures and salinities, but the same density, are mixed together. The result is a denser mixture than either of the original water masses, a consequence of the non-linear behaviour of seawater. This denser mixture then sinks, triggering turbulence and vertical mixing. Cabbeling has important implications for melting Arctic sea ice as it can increase the amount of heat from below to the ocean surface.

The study has been published in the Journal of Geophysical Research: Oceans.

To observe the cabbeling process, the team deployed a robotic instrument known as the Air-Sea Interaction Profiler (ASIP), which is a unique instrument specifically designed to study small-scale processes at the ocean surface. The ASIP is 2.8 metres in length, weighs about 90 kilograms, and is completely autonomous. Repeated dives and ascents by the robotic instrument carry its sensors through the upper 100 meters of the upper ocean, making fine-scale physical measurements including turbulence, temperature, and salinity.

The results have implications for improving scientists’ understanding of cabbeling and its potential role in models of sea surface warming and Arctic ice loss, particularly as climate patterns shift. The Greenland Sea is expected to experience increased freshwater outflow from melting ice in a warmer climate, altering the regional dynamics. Understanding and incorporating the effects of cabbeling will enhance the accuracy of predictions of ocean heat transport, especially in polar regions where warming is accelerating and sea ice is in decline.

The study was led by PhD candidate Kevin McGraw, Professor Audrey Morley and Professor Brian Ward from University of Galway, and took place along the East Greenland Polar Front, an area where cold, fresh Arctic water meets warmer, saltier Atlantic water.

            Kevin McGraw, PhD candidate at the School of Natural Sciences, University of Galway, said: “Cabbeling is rarely observed because it is sporadic and short-lived, with its intensity and reach varying across Polar Regions. Our underwater robotic platform, Air-Sea Interaction Profiler, is a unique instrument designed specifically to study the upper ocean without interference from the ship. Because it can capture rapid changes on the scale of seconds to minutes, it allowed the team to detect cabbeling in action which would have been nearly impossible with conventional methods.”

            Professor Audrey Morley, Professor of Marine Geology at the School of Geography and Archaeology, University of Galway, said: “The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that circulates water within the Atlantic Ocean, bringing warm water north and cold water south thereby distributing heat around the globe. Density gradients have been identified as a main driver of the AMOC, which may be altered by high-latitude cabbeling in a warming ocean. This suggests that the cabbeling effect needs to be considered to explain past and future AMOC variability.”

            Professor Brian Ward, Professor of Ocean Physics at the School of Natural Sciences, University of Galway, said: “These observations are a good example of the subtle processes associated with climate change and how small-scale processes can have broader implications. New methods of detection, such as the Air-Sea Interaction Profiler instrument, are required to fully understand the coupled ocean-atmosphere system.”

The research highlights the importance of combining innovative ocean technology with field observations to improve our understanding of fine scale mixing processes that shapes regional and global climate patterns.

The full study is available to read here: https://doi.org/10.1029/2025JC022567.

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