Age and renewal time of water masses in a semi-enclosed basin – application to the Gulf of Finland

To estimate the water-renewal time of the Gulf of Finland, a numerical-model simulation comprising the years 1987–1992 was carried out for realistic meteorological forcing. The results concerning water age as well as renewal index proved to reflect properties of the mean circulation system. The highest water ages (of around 2 yr) were found in the south-eastern part of the Gulf. The water-age distribution after the 5-yr simulation proved to be almost identical to that obtained after 3 yr, indicating that the circulation in the Gulf may be rather stable. The analysis of the renewal-index results showed that it takes around 5 yr to renew 98% of the water masses of the Gulf. When local maxima of the renewal index are found in conjunction with high water ages, this may indicate that the region is particularly vulnerable to eutrophication.