Modeling the age of Baltic Seawater masses: Quantification and steady state sensitivity experiments

Abstract

Ages of Baltic Seawater masses for the period 1903–1998 were quantified using a three‐dimensional (3‐D) coupled ice‐ocean model. Therefore an additional Eulerian tracer for the age of seawater was embedded. The age is the time elapsed since a water particle left the sea surface. Median ages of the bottom water between 1 year in the Bornholm Basin and 7 years in the northwestern Gotland Basin were found. During 1903–1998 the oldest bottom water of about 11 years appeared at Landsort Deep. In the halocline of the deeper basins a secondary age maximum was calculated. In the eastern Gotland Basin 3 stagnation periods (in the 1920/1930s, 1950/1960s, and 1980/1990s) with ages exceeding 8 years were found. Further, the sensitivities of modeled salinity and age on freshwater supply, wind speed, and amplitude of the sea level in Kattegat were investigated. In steady state the average salinity of the Baltic is most sensitive to perturbations of freshwater inflow. Increased freshwater inflow and wind speed result both in decreased salinity whereas increased amplitude of the Kattegat sea level results in increased salinity. The average age is most sensitive to perturbations of the wind speed. Especially, decreased wind speed causes significantly increased age of the deep water. On the other hand, the impact of changing freshwater or sea level in Kattegat on the average age is comparatively small, suggesting invariance of stability and ventilation in steady state approximately. A simple conceptual model for the Baltic deep water ventilation was applied to explain the 3‐D model results.