Main upwelling regions in the Baltic Sea — a statistical analysis based on three-dimensional modelling

In the Baltic Sea upwelling is an important process, especially in the coastal areas, causing vertical mixing and displacement of the water body. During thermal stratification, when the surface layer of the water is depleted of nutrients, upwelling plays a key role in replenishing the euphotic zone with the nutritional components necessary for biological productivity. Up to now, only a few comprehensive investigations have been carried out to study the locations of the main upwelling areas in the Baltic and how commonly these upwellings take place. Here, three-dimensional high-resolution modelling is used as a tool to statistically estimate an index reflecting the persistency of upwellings (downwellings) in various parts of the Baltic. This estimate is made for the summer season over the ten-year period from 1979 to 1988. The new idea in this paper is to use the persistency (stability) of the vertical velocity to define an upwelling index, instead of calculating the frequency of upwelling on the basis of changes in the sea-surface temperature, as is usually done. The model results were compared with measurements of surface temperature and salinity in the Gulf of Finland in 1988, where several strong upwelling cases were observed. The fit between the model results and measurements was found to be good. The upwelling indexes were compared with corresponding upwelling frequencies in the Swedish coastal area based on an analysis of long-term sea-surface temperature measurements. The results, based on these two different approaches, correspond well with each other. Consequently, we can expect that the three-dimensional model can be used to good effect as a tool to describe the overall statistics of the main upwelling areas in the Baltic. The results of the ten-year simulation show that coastal-type upwellings (downwellings) dominate, with values of the index as high as 30%–50% (–30% to –50%), being typically between 10% and 30% (–10% to –30%). The width scale of upwellings (perpendicular to the coast) is typically only 5–20 km. The length scale is somewhat more variable, being typically between 30 and 150 km (alongshore).