Water exchange between the inner and outer archipelago areas of the Finnish Archipelago Sea in the Baltic Sea
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Water exchange between the inner and outer archipelago areas of the Finnish Archipelago Sea in the Baltic Sea Elina Miettunen 1
&
Laura Tuomi 2 & Kai Myrberg 1,3
Received: 30 March 2020 / Accepted: 3 September 2020 / Published online: 26 September 2020 # The Author(s) 2020
Abstract We studied the water age and transport of passive tracers in the Archipelago Sea, Baltic Sea, using the COHERENS 3D hydrodynamic model and the OpenDrift Lagrangian particle model. The mean water age, which was calculated with COHERENS over a period of 6 years, varied between 1 and 3 months in the outer archipelago and between 3 and 6 months in the middle archipelago. The water age was highest in the inner archipelago, up to 7 months. As the density stratification is weak in large parts of this area, except for the seasonal thermocline, significant differences in the water age between the surface and bottom layers were seen only in the river mouths and in the deep channels of the middle archipelago. The Lagrangian particle simulations showed that the middle archipelago is more open towards the north than south. From the northern boundary, the Bothnian Sea, the largest transport to the middle archipelago occurred with NW winds. Due to the geometry and density of the islands in the area, the prevailing wind direction, SW, alone is not optimal for transporting tracer particles to the middle archipelago. From the southern boundary, the Baltic Proper, transport to the middle archipelago occurred mainly with SE winds and during events when the wind direction shifted from SW to SE or vice versa. The transport further into the inner archipelago was limited to only a few cases, indicating that the inner archipelago is fairly sheltered from transport from the outer archipelago. Keywords Baltic Sea . Coastal archipelago . High-resolution 3D modelling . Lagrangian tracers . Water age
1 Introduction The present and future environmental state of marine areas is a topical issue. In particular, the coastal shallow seas, such as the Baltic Sea, are subject to enhanced anthropogenic pressures, and sheltered archipelago areas, with strongly limited water exchange with the open ocean, are extremely vulnerable to loads of nutrients or toxic substances. Our research area, the Archipelago Sea (Fig. 1) in the Baltic Sea, has thousands of small islands and islets. The mean depth of the area is only 19 m, but the channels that cross the area are partly deeper than 50 m. The geometry of the Archipelago Sea
Responsible Editor: Eric Deleersnijder * Elina Miettunen [email protected] 1
Marine Research Centre, Finnish Environment Institute, Helsinki, Finland
2
Marine Research, Finnish Meteorological Institute, Helsinki, Finland
3
Marine Research Institute, Klaipeda University, Klaipeda, Lithuania
is complicated. Shallow and narrow channels and clusters of islands restrict the connections between the various sub-basins. For the EU Water Framework Directive (WFD), the area has been divided into sub-basins that represent the outer archipelago, the midd
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