Thermodynamic variability and change in the North Sea (1948-2007) derived from a multidecadal hindcast

Using a 3D baroclinic shelf sea model for the North Sea a multidecadal (1948-2007) hindcast with a resolution of 20 km x 20 km is performed and analysed providing a description of long-terns thermodynamic variability and change in the North Sea region. The simulation is validated with short-term, long-term and cross-section temperature measurements. For diurnal sea surface temperature (SST) a correlation between 0.79 and 0.89 was obtained. Comparisons with observed sea surface temperatures from Helgoland Roads (1873-2007) in the German Bight indicate a moderate warm bias in the order of 0.5 degrees C; but relatively high correlation of 0.89 suggests that long-term variability and change are simulated reasonably. Biases at other stations appear to be negligible. Analysis of hindcast temperatures reveals that SSTs in the German Bight remained relatively constant until the mid 1980s with some inter-annual variability superimposed. Later a strong increase in SST was found reaching values of up to 0.05 degrees C-1 year between 1988 and 2007. A similar behaviour was identified in observations from Helgoland, the only station for which a long temperature record was available. The heat content for the entire North Sea region showed a comparable development, indicating that the surface warming observed over the last decades extends over a larger depth range. The variability of stratification is mainly dominated by year-to-year variability. A number of sensitivity experiments are performed, confirming existing hypotheses, that the local air-sea interactions are the main drivers behind the observed changes in North Sea heat content and SST and in particular for the strong warming emerging in the mid 1980s in the North Sea. (C) 2011 Elsevier B.V. All rights reserved.