Refined estimates of water transport through the Åland Sea in the Baltic Sea

Abstract

Water exchange through the Åland Sea (in the Baltic Sea) greatly affects the environmental conditions in the neighbouring Gulf of Bothnia. Recently observed changes in the eutrophication status of the Gulf of Bothnia may be connected to changing nutrient fluxes through the Åland Sea. Pathways and variability of sub-halocline northward-bound flows towards the Bothnian Sea are important for these studies. While the general nature of the water exchange is known, that knowledge is based on only a few studies that are somewhat limited in detail. Notably, no high-resolution modelling studies of water exchange in the Åland Sea area have been published. In this study, we present a configuration of the NEMO 3D hydrodynamic model for the Åland Sea–Archipelago Sea area at around 500 m horizontal resolution. We then use it to study the water exchange in the Åland Sea and volume transports through the area. We first ran the model for the years 2013–2017 and validated the results, with a focus on the simulated current fields. We found that the model reproduced current direction distributions and layered structure of currents in the water column with reasonably good accuracy. Next, we used the model to calculate volume transports across several transects in the Åland Sea. These calculations provided new details about water transport in the area. Time series of monthly mean volume transports showed consistent northward transport in the deep layer. In the surface layer there was more variability: while net transport was towards the south, in several years some months in late summer or early autumn showed net transport to the north. Furthermore, based on our model calculations, it seems that dynamics in the Lågskär Deep are more complex than has been previously understood. While Lågskär Deep is the primary route of deep-water exchange, a significant volume of deep water still enters the Åland Sea through the depression west of the Lågskär Deep. Better spatial and temporal coverage of current measurements is needed to further refine the understanding of water exchange in the area. Future studies of transport and nutrient dynamics will eventually enable a deeper understanding of eutrophication changes in the Gulf of Bothnia.