The first Rossby Centre regional climate scenario for the Baltic Sea using a 3D coupled ice-ocean model

Type: Report
Series: RMK 95
Author: Meier, H.E.M


Temperature, salinity, sea ice and sea leve! in the Baltic Sea have been analyzed under different climate conditions using a 3D coupled ice-ocean mode!. As a reference, hindcast simulations for the period 1980-93 have been performed with observed three-hourly meteorological forcing fields and observed monthly river runoff. The observed Baltic Sea climate is well  reproduced by the mode!. Furthermore, two sets of 9-year time slice experiments have been performed using results of an atmospheric regional climate mode! as forcing, one representing pre-industrial climate conditions (control simulation), and the other one global waiming with a 150% increase of CO2 greenhouse gas concentration (scenario simulation). At the boundaries of the regional climate mode! results of the global atmosphere-ocean general circulation mode! HadCM2 (Hadley Centre) have been prescribed. To simulate river runoff, a large-scale hydrological mode! has been applied. As the time slices are too short to spin up initial stratification for future climate, salinity is treated as uncertainty. An extreme condition is obtained, integrating the Baltic Sea model for 100 years assuming that no salt water inflow occurs in future. The area averaged annual mean sea surface temperature change between scenario and control run is about 2.3 'C. Seasonal variability of the change is small compared to the corresponding 2 m air temperature change. The uncertainty due to unknown future initial conditions is relatively small (largest in summer with -0.5'C). The decrease of mean ice extent in the scenario compared to the control run is dramatic, from 210 • 109m2 to 82 • 109m2 (a relative change of 61 % ). However, in all years ice can still be found in the Bothnian Bay. The minimum ice extent is I 6 • 109m2 (for comparison: the area of the Bothnian Bay is about twice as !arge). The mean number of ice days decreases significantly, too. In the fast ice zone of the Bothnian Bay (Kemi) the mean ice season becomes 40 days shorter. The ice in the scenario run is thinner with less snow on top. In the·central Bothnian Bay mean maximum annual ice thickness is reduced by 25 cm from 54 to 29 cm. Mode! dependent uncertainties are discussed.