In September 2023, RV Meria S Merian ventured into the NW Atlantic to deploy nine oceanographic moorings across the continental slope north of Flemish Cap and south of the Grand Banks, as well as an array of compact pressure inverted echo sounders (PIES) south of Flemish Cap (see our Expedition MSM121 coverage here). The aim of EPOC’s work in this area is to measure the transport of the North Atlantic Deep Water (NADW) into and out of the Transition Zone (TZ) between the subtropical and the subpolar North Atlantic, where ocean models show a breakdown of the meridional (north-south) coherence of the AMOC.
Two years later, the German research vessel Meteor will return to the region for a month-long mission (30 July – 2 Sept 2025) to collect important new observational data from this complex and dynamic area of the Atlantic, as well as recover some of the instruments deployed two years ago. Existing observing systems implicitly rely on the global conveyor idea, that measuring the strength of the ocean circulation at an individual latitude provides some measure of the circulation as a whole – a concept that is being challenged in EPOC. Evidence suggests substantial discontinuities in the ‘conveyor belt’ at key latitudes in the Atlantic, often associated with sharp changes in topography.
These include locations where the deep western boundary current (DWBC) breaks away from the continental slope – such as at the Flemish Cap. In the area around Grand Banks and Flemish Cap the warm and salty North Atlantic Current (NAC) flows northward, close to the opposing Deep Western Boundary Current (DWBC) which is colder and fresher. Together these currents form an essential part of the AMOC and contribute significantly to the transports of heat and freshwater in the North Atlantic. Interactions between the two currents potentially influence the DWBC and affect its coherence along the path around Flemish Cap and the Grand Banks.
During the month-long mission on board RV Meteor, the EPOC team – led by University of Bremen’s Christian Mertens – will use a range of instruments to observe these interactions at high resolution. The data collected will add to that provided by the growing suite of EPOC instrumentation in the area, which includes current meter moorings deployed in October 2023 (and which will be recovered on the CROSSROADS2 mission later this year), and the inverted echo sounder array which will be recovered on this expedition.
Full-depth CTD (Conductivity-Temperature-Depth) and LADCP (lowered Acoustic Doppler Current Profiler) measurements will be taken at c.100 stations along different hydrographic sections around Flemish Cap and the Grand Banks (see maps). At stations with strong vertical mixing, these measurements will be complemented by microstructure profiles in the upper 1000 m of the water column using a free-falling tethered probe. The FC, PASS and GB sections are chosen to represent the upstream and downstream conditions; the 47N and A02 sections are located within TZ and will complement measurements made by the inverted echo sounder array.
Repeated CTD/LADCP profiles using a tow-yo will be made at two different locations on the continental slope with the ship moving very slowly across the bathymetry. Tow-yos are repeated casts over a defined depth range where the instruments are continuously lowered and raised while being towed by the ship at a speed of about 0.5 knots along a transect of about 20 km. The resulting horizontal resolution of a few hundred meters will allow the team to identify the mechanisms that maintain lateral and vertical mixing and current-topography interactions.
Finally, ocean gliders will be used to repeatedly transverse the western boundary current and the North Atlantic Current at 47° N between 41° 30’ W and 43° 30’ W. The transect will span about 150 km, with a single glider requiring roughly 10 days to complete its mission. Two gliders will be deployed for 20 days, which will allow them to collect high-resolution data from four transects across the front between the two water masses. This setup enables acquisition of length-scale diagnostics that specifically target cross-frontal exchange of properties such as heat, as well as investigation of energetic characteristics within the frontal system.
The M212 expedition departs Ponta Delgada in the Azores on 30 July, working in the Flemish Cap/Grand Banks region for 4 weeks before returning to port in St John’s, Newfoundland on 2 September.