Local buoyancy supply associated with the pulsed horizontal advection of water of lower salinity at the sea surface can initiate the development of a shallow thermocline in spring. This process has been studied with an analysis of historical hydrographic data from the central parts of the Baltic proper and numerical experiments with a seasonal pycnocline model. It is shown that this situation occurs in the Baltic Sea when the temperature of the surface water is below the temperature of maximum water density and the buoyancy flux due to heating at the sea surface is negative. ¿ 1997 American Geophysical Union