On the creation and maintenance of stratification in the Gulf of Riga

Factors affecting the creation and maintenance of stratification under simple conditions in a boreal estuary (Gulf of Riga, Baltic Sea) are studied in terms of buoyancy forcing. It is found that the ratio determines if the stratification is created by freshwater or heat fluxes. Here β and α are the haline contraction and thermal expansion coefficients of sea water, s is salinity and Cp is specific heat. It is discovered that stratification in this gulf, as well as in the Baltic Sea in general, is created by dilution of salinity due to freshwater fluxes, the thermal contribution being negligible. Data from four cruises in three consecutive years are used to illustrate this effect. An almost ideal river plume bulge and an associated coastal current is found in the gulf, and their spreading rate compared with recent eddy parameterizations. A restratification time scale for the gulf is estimated as 5 weeks and found to be in reasonable agreement with available observations.