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

Abstract. Water exchange through the Aland Sea, 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 Aland 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 details. Notably, no high-resolution modelling studies of water exchange in the Aland Sea area have been published. In this study, we present a configuration of the NEMO 3D hydrodynamic model for the Aland Sea-Archipelago Sea area at around 500 m horizontal resolution. We then use it to study the water exchange in the Aland Sea. 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 Aland Sea. These calculations provided new detail of water transport in the area. Time series of monthly mean volume transports showed a 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 Lagskar Deep are more complex than has been previously understood. While Lagskar Deep is the primary route of deep water exchange, still a significant volume of deep water enters the Aland Sea through the depression west of the Lagskar Deep. Better spatial and temporal coverage of current measurements is needed to further refine the understanding of water exchange in the area.