Five hundred kilometres east of Newfoundland, an underwater plateau rises up from the abyssal plain of the North Atlantic. This is the Flemish Cap, a place central to North Atlantic ocean circulation, and for centuries, it has attracted fishing vessels from across the world due to its bountiful waters. But its importance to the global ocean is only just becoming apparent.
The Flemish Cap is a place where some of the most important waters in the ocean change course, disperse and transform before continuing to journey south. That water is North Atlantic Deep Water (NADW). It forms at high latitudes in the Labrador, Irminger, and Nordic Seas, where surface water cools, becomes denser and sinks, travelling southward through the deep ocean, carrying carbon, oxygen, and nutrients as it goes. This southward flow is the deep return branch of the Atlantic Meridional Overturning Circulation (AMOC).
For years, the southward flow of NADW was imagined as a continuous current hugging the western margin of the Atlantic as it headed south, known as the Deep Western Boundary Current (DWBC). But that picture turned out to be incomplete.
When oceanographers began tracking individual water parcels using acoustically tracked RAFOS floats drifting at depth and anthropogenic tracers dispersing into the deep sea, a different reality emerged.[1] Much of the NADW that enters the DWBC at high latitudes never makes it cleanly to the south. A significant fraction “leaks” away from the DWBC, scattering into the interior of the ocean through eddies, recirculating gyres and dispersive interior pathways. The deep return branch of the AMOC is less a contained river and more like a hosepipe full of holes.
The evidence points consistently to one part of the ocean where most of this leakage occurs: the zone between the subpolar and subtropical North Atlantic, shaped by the topography of the Flemish Cap and the Grand Banks of Newfoundland. Here, the continental slope steepens and bends sharply before levelling out, transitioning to complex bathymetry that pushes parts of the DWBC offshore. The North Atlantic Current, carrying warm saline surface water northward, meets and interacts with the cold, deep DWBC flowing south via its meanders and intense eddies that contribute to mixing water masses vigorously.[2] Deep water that enters the region with one set of properties emerges, downstream, measurably altered.[3] As such, the Flemish Cap and the Grand Banks of Newfoundland are collectively called the subtropical-subpolar Transition Zone of the North Atlantic.
This transformation matters because the behaviour of the AMOC depends on two things: how much water sinks at high latitudes, and how reliably that signal travels southward.[4] If NADW is diverted or mixed before it reaches subtropical latitudes, the deep ocean’s ability to transmit change from the subpolar Atlantic to the wider global ocean is reduced.
As the Greenland Ice Sheet continues to melt, causing freshwater to enter the subpolar seas at increasing rates, the AMOC faces growing pressure. Whether those changes propagate globally, or are partly absorbed and damped within the Transition Zone, is a question with real consequences for how we project future climate. EPOC, in collaboration with the French CROSSROAD project, now provides novel observations and numerical tools to start answering that question.
Over four research expeditions between 2023 and 2025, we deployed an observational network around Flemish Cap and the Grand Banks designed to monitor the Transition Zone.
At the heart of the array are nine moorings equipped with current meters, temperature and salinity sensors, and Acoustic Doppler Current Profilers that measure water velocity at different depths. Four moorings sit north of Flemish Cap, four sit to the south at the Grand Banks, and one sits in Flemish Pass, a deeper channel to the west of Flemish Cap that provides a distinct interior pathway for upper NADW. Together, these moorings monitor the DWBC at the entrance and exit of the Transition Zone, providing a picture of how much water arrives in the region and how much leaves, and with which properties (temperature, salinity, density). An array of bottom-mounted instruments known as Pressure Inverted Echo Sounders (PIES) extended our coverage to the broader circulation around the tail of Flemish Cap, tracking the interactions between the North Atlantic Current and the deep boundary flow.
A fixed array of moorings can only tell us so much, however. To understand what is happening between the mooring lines, the 2024 expedition aboard the research vessel N/O Thalassa carried out a survey of water properties and velocities across the Transition Zone.[6] Where the slope was steepest, the ship conducted tow-yo surveys whereby sensors are repeatedly lowered and raised while the ship moves slowly forward, producing a very high-resolution “sawtooth” cross section of water mass structure and boundary current dynamics.
Complementing these surveys were measurements of turbulent mixing using a Vertical Microstructure Profiler. This instrument measures centimetre-scale fluctuations of velocity and temperature in the water column, providing a direct estimate of how vigorously and rapidly water is being stirred and kinetic energy dissipated. Mixing is where NADW transformation actually happens: not in large, smooth flows, but in localised turbulence where energetic currents and internal waves interact with steep and irregular underwater topography. The Flemish Cap region is one of the most energetically active mixing environments in the deep North Atlantic. Quantifying that mixing is essential for understanding how NADW is transformed as it passes through.
Beyond the ship-based work, a fleet of Deep Argo floats were deployed to drift freely at depth through the region.[5] Rather than measuring water properties at a fixed point, these autonomous instruments follow the actual trajectories of water parcels, capturing where deep water goes after it enters the Transition Zone. Most importantly, they collect full-depth temperature and salinity profiles every ten days, filling the gaps between ship-based sections and extending our monitoring of the region over longer timescales.
The moorings were recovered in September 2025, after two years on the seafloor. The data they carried are now in our hands, and analysis is underway. Combined with the hydrographic and microstructure surveys from 2024, the Transition Zone dataset represents a step change in what we know about this part of the ocean. It will allow us to quantify, from direct observations, how much NADW leaks from the DWBC within the Transition Zone, by what mechanisms, and with what consequences for water mass properties. It will allow us to test whether the Transition Zone acts as a damper on the southward propagation of AMOC signals, or whether dynamics here help transmit them downstream to the wider ocean.
Those answers matter beyond the Flemish Cap. The AMOC is a system whose future behaviour remains poorly constrained. Better observational constraints on how it operates at this critical junction will improve the models used for climate projection and sharpen our understanding of heat and carbon uptake by the deep ocean. The fisheries and coastal communities of the North Atlantic depend on the oceanic conditions that the AMOC helps maintain. Understanding what controls it, and where its critical junctions lie, is a problem that sits at the intersection of science and application.
Published July 2026 | By Damien Desbruyeres [1], Christian Mertens [2] and Russell Arnott [3]
[1] IFREMER; [2] University of Bremen; [3] Seascape Consultants