Transport dynamics in a complex coastal archipelago

Abstract. The Archipelago Sea (in the Baltic Sea) is characterised by the complex geometry of thousands of islands and steep gradients of the bottom topography. Together with the much deeper Åland Sea, the Archipelago Sea acts as pathway to the water exchange between the neighbouring basins, Baltic proper and Bothnian Sea. We studied circulation and water transports in the Archipelago Sea using a new high-resolution NEMO configuration that covers the Åland Sea–Archipelago Sea area with horizontal resolution of around 500 m. The results show that currents in the area are steered by the geometry of the islands and straits and the bottom topography. Currents are strongest and strongly aligned in the narrow channels in the northern part of the area, the directions alternating between south and north. In more open areas, the currents are weaker with wider directional distribution. During our study period of 2013–2017, southward currents were more frequent in the surface layer. In the bottom layer in areas deeper than 25 m, northward currents dominated in the southern part of the Archipelago Sea, while in the northern part, southward and northward currents were more evenly represented. Due to the variation in current directions, both northward and southward transports occur. During our study period, the net transport in the upper 20 m layer was southward. Below 20 m depth, the net transport was southward at the northern edge and northward at the southern edge of the Archipelago Sea. There were seasonal and inter-annual variation in the transport volumes and directions in the upper layer. Southward transport was usually largest in spring and summer months and northward transport was largest in autumn and winter months. Our results demonstrate the complexity of the transport dynamics in the Archipelago Sea. No single transect can be chosen to represent water transport through the whole area. Further studies on the water exchange processes between the Baltic proper and the Bothnian Sea through the Archipelago Sea would benefit from using a two-way nested model setup for the area.