Fine-scale Thermohaline Stratification in the Near-Surface Layer Under Weak Wind Conditions with Indications of Salt Fingering

Abstract. 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.

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.

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.

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.