To estimate the mean circulation, water exchange and water age in the Gulf of
Bothnia a ten-year simulation using a three-dimensional numerical model was
carried out. The results confirmed the early findings byWitting (1912) and Palm´en
(1930) that a mean cyclonic circulation takes place both in the Bothnian Sea and
in the Bothnian Bay. However, the modelling results showed clearly that there
exist meso-scale circulation features including coastal ‘jets’, not reported in the
Witting-Palm´en results. The simulated mean currents were also higher than those
found earlier, while the persistency of this circulation is typically between 20 and
60%, which is similar to the earlier results. There is a large difference between the
various model-based water-exchange estimates: these are strictly dependent on the
time-averaging used. Water age proved to reflect properties of the mean circulation
system, and the highest water age (of around 7.4 years) was found in the central
part of the Bothnian Bay. The water age was found to be rather high also in the
entire Gulf of Bothnia, which provides evidence of the rather slow water exchange
between the Gulf and the Baltic Sea. This leads to the conclusion that, from the
physical point of view, the Gulf of Bothnia is vulnerable to eutrophication