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.
Highlights
The present and future environmental state of marine areas is a topical issue
The tracer particles released in the Aurajoki River estuary stayed mostly in the inner and middle archipelago areas during the 3-month simulation periods: 68– 90% of the released particles drifted to the middle archipelago and up to 16%, but typically less than 6%, reached the outer archipelago
We studied water age and transports in the Archipelago Sea using the COHERENS 3D hydrodynamic model and the OpenDrift Lagrangian particle model
Summary
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. The Archipelago Sea (Fig. 1) in the Baltic Sea, has thousands of small islands and islets. The geometry of the Archipelago Sea. The meteorological conditions in this area are variable. The surface salinity in this area varies from 4 g/kg in the inner archipelago to 6 g/kg in the outer archipelago. There is no salinity stratification, or it is weak, as the bottom salinity is on average only 0.1– 0.2 g/kg higher than the surface salinity. In some of the deeper channels, the bottom salinity can be higher than
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