Summertime modification of surface fronts in the North Atlantic subpolar gyre

A 12-yearlong thermosalinograph data set from ships of opportunity was used to make an extensive study of meso-scale surface fronts in the western part of the North Atlantic subpolar gyre from 1997 to 2009. Fronts are identified on a sea-surface-salinity/sea surface temperature gradient criterion with a typical width of 20 to 60 km. The seasonal hydrographic properties of fronts are investigated. We find that, whereas salinity jumps across fronts are close to the larger scale variations, the temperature jumps across fronts are often smaller than the larger scale variations, in particular in summer-time. We also find in June vertical profiles that the relative weight of temperature over salinity in the density jump across fronts is smaller at the surface than at depth. A Lagrangian model based on altimetry data and in situ vertical hydrographic profiles indicates that surface stirring is able to create fronts, but not the contrast between temperature and salinity gradients. We suggest that air-sea fluxes, mostly heat fluxes, but possibly also differential vertical stirring, are responsible for damping the meso-scale horizontal surface temperature gradient. This is supported by an eddy-permitting numerical simulation of this region (DRAKKAR simulation ORCA-025-G70) which includes some heat flux feedbacks.