Modelling the Impact of Baltic Sea Upwelling on the Atmospheric Boundary Layer

Coastal upwelling, with a strong sea-surface temperature signal, is extremely common in the Baltic Sea during the summer months. Although the spatial scale of upwelling is small, its high frequency of occurrence in the semi-enclosed basin may allow the SST signature to have a significant feedback onto the lower atmosphere. In this paper, we develop a method to remove the signature of upwelling from SST fields, and use these modified SST fields as the lower boundary condition of an atmospheric model, allowing us to evaluate the importance of coastal upwelling to the mean summer conditions over the Baltic. The contribution of upwelling is found to be small generally, although significant locally over areas where upwelling is most common. This includes surface temperature reductions of up to 2 °C, wind speed reductions of up to 0.25 m/s, and reductions in the mean boundary-layer height of up to 100 m. High resolution simulations with artificially enhanced upwelling suggest that upwelling may become increasingly important as winds slacken at the end of an upwelling period.