Mechanisms and spatial variability of meso scale frontogenesis in the northwestern subpolar gyre

We combine 12-year long hydrographic data from a ship of opportunity with altimetric sea level data to infer the distribution and genesis of surface meso scale frontal structures in the Irminger Sea, detected on a salinity criterion. We find a complex spatial pattern in the occurrence of fronts: preferential locations are found both in the interior of the basin, and in the vicinity of the Irminger Current, a major current flowing along the western slope of the Mid-Atlantic Ridge, that separates contrasted surface water masses. These regions of high density of fronts do not systematically match regions of high eddy kinetic energy, but the occurrence of increased horizontal gradients is usually associated with confluence. We were able to reproduce reasonably well the spatial variability of fronts, by a simple model that passively advects the large scale salinity climatology with the altimetry-derived surface velocities. We find that stirring induced by meso scale turbulence as well as displacements of water masses are important contributors to the genesis of fronts. Thus, these fronts are often the result of a non-local process.