Journal Article

Lagrangian Decomposition of the Atlantic Ocean Heat Transport at 26.5°N

Johnson, Helen L., Foukal, Nicholas P., Johns, William E., Wilson, Chris, Tooth, Oliver J.

2024

Overview

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the redistribution of heat, freshwater and carbon. At 26.5°N, the meridional heat transport has traditionally been partitioned geometrically into vertical and horizontal circulation cells; however, attributing these components to the AMOC and Subtropical Gyre (STG) flow structures remains widely debated. Using water parcel trajectories evaluated within an eddy‐rich ocean hindcast, we present the first Lagrangian decomposition of the meridional heat transport at 26.5°N. We find that water parcels recirculating within the STG account for 37% (0.36 PW) of the total heat transport across 26.5°N, more than twice that of the classical horizontal gyre component (15%). Our findings indicate that STG heat transport cannot be meaningfully distinguished from that of the basin‐scale overturning since water parcels cooled within the gyre subsequently feed the northward, subsurface limb of the AMOC. Plain Language Summary The Atlantic Meridional Overturning Circulation transports heat northward by converting warm, surface waters into cold waters returning at depth. In the subtropical North Atlantic, the heat transported by the overturning circulation has traditionally been separated from the wind‐driven gyre circulation by assuming that the gyre flows horizontally along constant depth levels. By tracing the pathways of virtual water parcels in a high‐resolution ocean model, we show that the heat transported by the subtropical gyre is larger than traditional estimates because water parcels spiral downwards across depth levels. Our results indicate that the subtropical gyre should not be considered separate from the overturning circulation, since the water parcels cooled within the gyre subsequently flow northwards to form cold, dense waters in the subpolar North Atlantic. Key Points Water parcels recirculating in the subtropical gyre account for 37% of the total heat transport at 26.5°N in an eddy‐rich ocean hindcast The heat transport of the subtropical gyre is associated with shallow vertical overturning rather than the horizontal circulation at 26.5°N Both horizontal and vertical circulation cells are fundamental components of the Atlantic Meridional Overturning Circulation

• Publisher: John Wiley & Sons, Inc,American Geophysical Union (AGU),Wiley
• Subject: Atlantic meridional overturning circulation / Atlantic Meridional Overturning Circulation (AMOC) / Circulation / Climate system / Cold flow / Cold water / Decomposition / Depth / Enthalpy / Flow structures / Freshwater / Global climate / Heat / Heat transport / Horizontal cells / Inland water environment / Lagrangian trajectories / Meridional heat transport / Ocean models / Oceans / RAPID 26.5N / subtropical gyre / Surface water / Trajectory analysis / Water depth