Eastern North Atlantic mode waters during POMME (September 2000-2001) : Subduction, water mass transformation, biochemical tracer distributions, and carbon cycle in the Northeast Atlantic Ocean at Mesoscale: The POMME Experiment

[i] Four hydrographic surveys in the northeast Atlantic (38°-45°N, 15°-21°W) carried out during the Programme Ocean Multidisciplinaire Meso Echelle (POMME) experiment (September 2000-2001) are used to investigate the water masses in the upper layers corresponding to Eastern North Atlantic Mode Waters. A large meridional gradient in isopycnal temperature/salinity is witnessed which has no counterpart in climatology and is associated with an anomalous eastward current near 41 °N. Waters tend to be warmer/more salty, somewhat more stratified, and less oxygenated on isopycnal surfaces in the southern part of the domain. In addition, there is an overlying zonal gradient of properties, in particular, north of the jet, consistent with earlier descriptions. However, these large-scale distributions also change significantly through the POMME year, in particular, between the February and early April surveys, which we attribute to (1) the arrival of a 2001 vintage of mode waters, in particular, in the northeastern part of the domain and to (2) stirring by the mesoscale motions, which contribute to down-gradient transport (equivalent diffusion coefficient of 2000 m2/s for σ = 27.10). On a smaller scale, there are noticeable changes in properties between surveys, some of which can be shown by quasi-Lagrangian diagnostics to result from advection by the horizontal circulation. In particular, in the southwest comer of the domain between the surveys P1 (February 2001) and P2 (April 2001), horizontal isopycnal advection is responsible for a large cooling (freshening) and oxygen increase, or in the northeast comer, where new oxygenated and freshly ventilated water (in particular for surface σ Θ = 27.05 kg/m 3 ) penetrates during that period. In other regions, or for a longer period, water masses are often difficult to track as a result of dispersion or stirring by the eddies and too small of a POMME domain.