Isopycnal Eddy Stirring Dominates Thermohaline Mixing in the Upper Subpolar North Atlantic

• Published in: Journal of geophysical research. Oceans Vol. 129; no. 9
• Main Authors: Fernández Castro, Bieito, Fernández Román, Daniel, Ferron, Bruno, Fontela, Marcos, Lherminier, Pascale, Naveira Garabato, Alberto, Pérez, Fiz F., Spingys, Carl, Polzin, Kurt, Velo, Antón
• Format: Journal Article
• Language: English
• Published: 01.09.2024
• Subjects: diapycnal mixing; isopycnal stirring; meridional overturning circulation; microstructure turbulence; subpolar North Atlantic; thermohaline transformations
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2023JC020817

The Atlantic Meridional Overturning Circulation entails vigorous thermohaline transformations in the subpolar North Atlantic Ocean (SPNA). There, warm and saline waters originating in the (sub)tropics are converted into cooler and fresher waters by a combination of surface fluxes and sub‐surface mixing. Using microstructure measurements and a small‐scale variance conservation framework, we quantify the diapycnal and isopycnal contributions –by microscale turbulence and mesoscale eddies, respectively– to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for the majority of thermal (65%) and haline (84%) variance dissipation in the upper 400 m of the eastern SPNA. A simple dimensional analysis suggests that isopycnal stirring could account for O $\mathcal{O}$(5–10) Sv of diahaline volume flux, suggesting an important role of such stirring in regional water‐mass transformations. Our mixing measurements are thus consistent with recent indirect estimates in highlighting the importance of isopycnal stirring for North Atlantic overturning. Plain Language Summary The North Atlantic hosts an ocean circulation system called the Atlantic Meridional Overturning Circulation (AMOC). It is often likened to a giant conveyor belt in the ocean, moving warm, salty waters from south to north and transforming them into cold, fresh waters that flow back southward at greater depth. The AMOC is a crucial element of the Earth's climate, and if it were to slow down, it could lead to major climatic changes. For a long time, scientists thought that the AMOC was mainly driven by cooling in the North Atlantic. But recently, we have discovered that the mixing of different water masses is also important. In our study, we used small‐scale measurements of ocean properties to examine the processes behind this mixing. Our findings show that large swirling flows known as mesoscale eddies, which are tens to hundreds of kilometers wide and hundreds of meters deep, play a dominant role in mixing heat and salt in the North Atlantic. This discovery helps us to better understand the AMOC and its future behavior.