Work at sea
In addition to a sub-kilometric numerical simulation of the TZ region, the second methodological cornerstone of the CROSSROAD project is the realization of a deep-sea oceanographic cruise in the TZ area in 2024. The cruise will take place on the N/O Atalante (Ifremer).
(1) Hydrography survey.
Full-depth CTD (Conductivity-Temperature-Depth) – LADCP (Lowered Acoustic Doppler Current Profiler) profiles will be carried out at about 65 stations within the TZ (white dots in Figure). The measured properties will include temperature, salinity, dissolved oxygen, inorganic nutrient concentrations (at discrete levels), and horizontal velocities. A first set of stations will be distributed along four cross-slope sections in order to measure the along-slope transport of properties upstream, within, and downstream of the TZ. A second set of stations will be distributed along the 3000 m isobath at two "hot spots" of the TZ in order to identify key locations for slope-to-interior leakiness (i.e. loss of mass of the DWBC) and water mass property changes (due to lateral or vertical mixing, for instance). This specific isobath was chosen as it falls within the offshore edge of the so-called ”slope” velocity core of the DWBC, which is a well topographically-constrained vein of the DWBC (i.e. very limited lateral displacements). For each pair of stations, calibrated and gridded temperature and salinity fields will be used to obtain normal geostrophic velocities referenced to selected levels (thermal wind). The computation of the horizontal velocities at the reference levels and their associated errors will build on shipboard SADCP, LADCP, and altimetry data. A significant fractions (about half) of those CTD-LADCP stations will be complemented by on-station deployments of two Vertical Microstructure Profilers (VMP), which measure the vertical gradient of the turbulent fluctuations in velocity and temperature at centimeter scales, and hence provide a robust estimate of the rate of dissipation of turbulent kinetic energy (and of diffusivity consequently) over the whole water column.
(2) Moorings.
Mooring deployments will be caried out in the framework of the EPOC project (Explaining and Predicting the Ocean Conveyor: https://www.epoc-eu.org/home). Eight subsurface tall mooring lines equipped with ADCPs, current meters (CM) and CTD sensors will be deployed for 2 years : four at the northern (FC) section and four at the southern (GB) section in order to simultaneously monitor the DWBC variability upstream and downstream of the TZ (velocity and thermohaline properties). An additional mooring will be deployed in Flemish Pass to capture this distinct interior pathway for upper NADW. Four Pressure-Inverted-Echo-Sounder (PIES) will be deployed offshore to capture the variability of (integrated) interior flows (a note on PIES and associated methodology is available in a following document). Finally, an autonomous moored VMP – the Micro-Riyo – will be deployed at the tail of Flemish Cap. It will provide, for the first time, a time-continuous time series of turbulent kinetic energy dissipation in the TZ (a note on the Micro-Riyo is available in a following document). Alongside this mooring array, an array of 10 C-PIES 2 (PIES equipped with a current meter) will be deployed for 2 years at the tail of Flemish Cap along and perpendicular to the 4000 m isobath, which falls within the so-called ”rise” velocity core of the DWBC. The C-PIES array will be used to study the interactions between the NAC and that portion of the DWBC and the impact of those interactions on the meridional coherence of signal propagation in the TZ. The overall CROSSROAD mooring array will provide the large-scale to fine-scale monitoring of NADW transport and properties needed to describe and explain time-varying transport divergence and water mass transformation within the TZ.
(3) Deep- Argo floats.
A fleet of Deep-Argo autonomous profiling floats equipped with oxygen sensors will be deployed within the TZ throughout the duration of the project (12 via the presently requested cruise, and 12 during a subsequent AZMP cruise of opportunity). Floats will drift at 1250 m within UNADW (6 floats) and at 2500 m within LNADW (6 floats) and initially obey a fast-cycling scheme (e.g. 3-day) while in the TZ to maximize the sampling of the region. While contributing to the Deep-Argo array currently growing 2. Note that the number of C-PIES to be deployed will depend on the outcome of the EPOC Horizon Europe proposal. 5in the subpolar North Atlantic, the CROSSROAD Deep-Argo fleet will provide a set of Lagrangian trajectories and of along-trajectory anomalous properties (temperature, salinity, oxygen) in the vicinity of the TZ. It will furthermore be used to produce a spatially-homogeneous and high horizontal resolution (∼1/4 ◦ ) mapping of full-depth hydrography properties within the TZ.
(4) Tow-yos and microstructure survey.
Three areas of the continental slope will be sampled at fine spatial resolution using repeated ”tow-yos” CTD-ADCP profiles. Tow-yos are repeated CTD-ADCP casts over a defined depth range with the CTD package being uccessively and continuously lowered and raised in the water column while being towed by the ship at very slow speed over transects of a few tens of kilometers, thereby reaching nominal horizontal resolution of a few hundred meters. They will here yield an original assessment of boundary current dynamics and help deciphering the mechanisms sustaining lateral and vertical mixing (e.g. dominant instability at play) and current- topography interactions. The tow-yos will be carried out in region of likely intense boundary-to-interior leakiness (the tail of the Grand Banks and the tail of Flemish Cap) as well as in more quiescent flow regime where such exchanges are perhaps reduced (the concave portion of the Grand Banks continental slope). Each tow-yo section will be subsequently complemented by repeated deployments of the VMPs.
Note on observational array design. As part of the ongoing CROSSROAD-CESAR project (LEFE- GMMC 2021-2023), we will build on the GICATL3 simulation to refine the design of the CROSSROAD observational array and thus minimize experimental uncertainties. Key diagnostics (e.g. NADW transport variability) derived from the fully-resolved model simulation fields will be re-evaluated by considering observational constraints. We will quantify the impact of high frequency transient regimes (eddies, meanders, waves) on the reconstruction of observation-based transports by following the typical schedule and timing of the hydrography survey (i.e. near- synopticity of the CTD-ADCP measurements). Then, mooring and instrument configurations at the northern (FC) and southern (GB) sections will be tested in the model to set the optimal position of the lines as well as the optimal depth and spacing between instruments (currentmeters, CTD, ADCP).