The near-surface ocean regulates air–sea exchange of heat, momentum, and gases, while its fine-scale thermohaline structure remains poorly characterized, particularly under weak wind conditions in tidally influenced shelf seas. High-resolution Lagrangian observations of temperature and salinity in the upper two meters of the German Bight (North Sea) are presented, acquired during a period of weak winds and strong solar radiation. Two minimally invasive Lagrangian surface drifters equipped with a vertical sensor chain enabled continuous measurements within the same water mass, avoiding ship-induced disturbances and resolving the temporal evolution of near-surface stratification in a tidally energetic environment.</p> <p>During the calm period, a pronounced diurnal warm layer developed, with temperature differences of up to 2.5 °C over less than two meters. Concurrently, a distinct salinity anomaly emerged, characterized by higher salinity at 0.55 m compared to 1.75 m depth. Despite these pronounced thermohaline gradients, the water column remained statically stable throughout the observation period, as indicated by the density structure and consistently positive buoyancy frequencies.</p> <p>Temperature and salinity exhibited variability on timescales of seconds to minutes, indicating the presence of fine-scale processes such as shear-induced interleaving and intermittent vertical motions operating within an otherwise stable near-surface layer. Diagnostics based on the Turner angle and density ratio further suggest conditions favorable for salt-finger-type double-diffusive processes during the calm phase. A comparison with a one-dimensional water column turbulence model shows that while the model reproduces the bulk evolution of the diurnal warm layer, it does not capture the observed fine-scale thermohaline variability.</p> <p>These observations demonstrate that the near-surface layer in tidally influenced shelf seas can exhibit complex and rapidly evolving thermohaline structures under weak wind conditions and high solar radiation. The results underscore the importance of high-resolution Lagrangian measurements for characterizing near-surface processes and for improving the representation of air–sea exchange with fine-scale processes in coastal and shelf-sea environments.