As part of the multidisciplinary Arctic Summer Cloud Ocean Study (ASCOS) in 2008, a comprehensive upper-ocean measurement program was performed during a 16 day long drifting ice station centred at 87.4° N and 6° W. During the drift, which lasted from 15 August to 1 September, high-resolution sampling of ocean stratification, turbulent mixing and exchange in the upper 500 m were made in addition to measurements of shortwave radiation at the ice surface and below the 1.8 m thick ice. Prior to the ASCOS drift, similar ocean turbulence measurements were conducted from the drifting ice base Barneo in April 2008. Based on the observations, we describe the evolution of the heat content in the upper ocean, comprising the mixed layer and the cold halocline, from end of winter (late April, maximum ice extent) to the end of summer (late August, minimum ice extent) and identify the mixing and exchange processes that are important for developing the mixed-layer stratification in the last phase of the summer season. During the first half of the ASCOS drift, the presence of melt ponds and open leads allows solar radiation to increase the ocean mixed-layer heat content by over 30%. With the onset of freezing and development of a layer of fresh snow and rime on the surface, energy input to the ocean mixed layer is limited to transmittance of solar radiation through the ice. This flux is small in magnitude and is found to be balanced by melting and heat conduction at the ice/ocean interface and heat transfer across the base of the mixed layer and into the upper cold halocline. The available amount of heat is thus redistributed by mixing internally. The heat flux across the pycnocline, inferred from dissipation rate measurements, accounts for the build-up of the observed heat in the upper cold halocline from the end of winter to the end of summer, suggesting that this heat is added locally.