The Baltic Sea ocean climate system memory and response to changes in the water and heat balance components

Abstract

The Baltic Sea climate system is analysed based on observation and mathematical modelling, and steady state and transient-response characteristics are derived and analysed. Some sensitivity experiments are also run based on observed forcing extracted from various Baltic Sea regions representing a range extending from sub-arctic to marine climatic conditions. We show that two important time scales should be considered: one is associated with the water balance (salinity) and the e-folding time is approximately 33 years; the other is associated with the heat balance and the e-folding time is approximately 1 year. Modelling demonstrates that current ocean conditions, starting from known as well as arbitrary initial conditions, can be realistically simulated. Our modelling indicates that salinity is non-linearly dependent on and strongly sensitive to changes in freshwater inflow, in accordance with the findings of other studies. The annual maximum ice extent is strongly sensitive to change in, the winter air temperature over the Baltic Sea. Calculations indicate that the sea will become almost completely ice covered or ice free at Baltic Sea winter air temperatures of −6 and 2 °C, respectively. Changes in the Baltic Sea annual mean water temperature are closely related to the air temperature above the sea surface. However, during climate warming the water and air temperatures may differ due to changes in the surface heat balance components.