Despite this strong difference, large (100km) coherent eddies are observed in both areas, although more frequently to the North than to the south. There is no suggestion of systematic differences between the cyclonic and anticylonic eddies in the domain (Paillet, 1999). The role of these eddies in mode water formation and subduction will be investigated.
Scientific objectives
The prime objectives are to understand the process of Mode Water subduction at the mesoscale, to understand and quantify the processes responsible for the evolution of its bio-geochemical properties of the modal water and for the export of biogenic matter over a seasonal cycle.
More specifically, the physical oceanography will include the following objectives :
The late winter mixed layer. To map its distribution, and, in particular, the area of largest gradient between the usually deeper layers to the North and the shallower ones to the south; to understand how that distribution is shaped by the air-sea buoyancy fluxes and the mesoscale circulation, and to determine the pre-bloom distribution of bio-geochemical tracers.
Meso-scale: To understand what controls the physical fluxes (buoyancy, potential vorticity, mass, heat and salt) and the contribution of the meso-scale circulation to those fluxes. To identify and quantify the physical and biological processes controlling the bio-geochemical fluxes within eddies.
Fulfilling those objectives will contribute to the quantification of physical (buoyancy, potential vorticity, mass, heat and salt) fluxes over the amual cycle.
The Field experiment
The POMME field experiment will require specific measurements for the following topics:
･ The formation of the mode water: Air-sea heat and freshwater fluxes; the temperature and salinity evolution of the mixed layer; deepening of the mixed layer.
･ The subduction of the mode water: Two or three dimensional surveys of the circulation, the water masses, and the vertical stratification; tracer studies to investigate age and dispersion of the water.
A network of five tomography moorings, together with a wide variety of other instrumentation, will provide a detailed look at the time evolution of the three dimensional velocity, temperature, and salinity fields. Variability in the mixed layer depth, in the subducted water masses, and in the overall heat budget will be observed and modeled.
Statistics on the distribution of eddies and their size (roughly 100km), and on the interannual variability of the position of the transition zone, set the domain size to be at least 500km longitude by 750km latitude (Figure 2), which is the size retained for the experiment. The measurements will be obtained with a combination of fixed Eulerian and Lagrangian instruments, complemented by ship surveys, with the more extensive of these taking place in the late winter-early spring.
