The future climate of our planet depends on the way heat and carbon dioxide are absorbed by the ocean. The top 50-200 metres, the ocean surface layer, is well mixed by turbulence from wind, waves and convective circulations. This mixed layer mediates the transfers between the atmosphere and the deep ocean. It is spatially heterogeneous at scales ranging from 1 to 100 km. The MEDLEY project will map this heterogeneity to understand its origin and its consequences on climate processes, using observations and numerical models.