Modern research methods enable unfolding the structure of the water column with higher resolution than ever revealing the importance of submesoscale. Submesoscale processes have intermediate space and time scales of <5 km and a few days in the Baltic Sea. A glider mission was conducted in the Gulf of Finland in May 2018. During the formation of the seasonal thermocline characterized by the increase of the maximum temperature gradient from 0.5 to 3.1 °C dbar-1, the structure of the water column changed constantly. A horizontal buoyancy gradient was captured and the vertical flows at a lateral scale of a km were apparent in isotherms intersecting with isopycnals. We suggest that frontal submesoscale processes manifested as smaller-scale tracer patterns on the mesoscale background demonstrate an ageostrophic secondary circulation. While the front appeared because of the persisting NE–E winds, the decrease in wind stress promoted the non-forced dynamics that, in turn, appeared to enhance the development of the thermocline. The spatial spectra of isopycnal tracer variance revealed the slopes a bit gentler than -2 between the lateral scales 2–10 km in the upper part of the water column. The discrepancy from the deeper layers where the slope closed to -1 suggests the contribution of the ageostrophic frontal effects in the energy cascade.