Projected Forecast of Hydrodynamic Conditions in the North Sea for the 21st Century

A projected forecast of hydrodynamic conditions in the North Sea is carried out for the 21st century by means of a dynamical downscaling with the shelf ocean model HAMSOM (University of Hamburg). The investigation is based on the IPCC (International Panel of Climate Change) SRES A1B scenario. In order to force the HAMSOM model, results from the global ocean model MPIOM (Max Planck Institute for Meteorology, Hamburg) are used at the open lateral boundaries of the HAMSOM model domain and results from the regional atmosphere model REMO (Max Planck Institute for Meteorology, Hamburg) are used for the meteorological forcing at the air-sea interface. A bias correction method relative to ERA40 reanalysis data (European Centre for Medium-Range Weather Forecasts) and WOA-2001 climatology (National Oceanographic Data Center) is applied to the forcing variables, respectively, in order to reproduce reasonable hydrodynamic present-day conditions in the North Sea. The entire simulation period is subdivided into the control period 1951-2000, to be used as reference, and the scenario period 2001-2099. In the analysis of the HAMSOM model results particular attention is drawn on regional issues in order to provide a better understanding of potential climate change impacts on the North Sea. The analysis generally focuses on changes of the atmospheric forcing variables, the general circulation in the North Sea, heat fluxes and water temperature, thermocline parameters, fresh water fluxes and salinity, and changes of the sea surface elevation. Thereby, it comprehends trend analysis of time series of the various variables to address change signals as well as EOF analysis to address regional anomalies in the spatial distribution of the variables. According to the model results, in the 21st century more frequent westerly winds over the North Sea enhance its general circulation in spring with an increase of northerly inflow by about +21%. Slightly decreasing wind speeds in the other seasons, however, entail a slight weakening of the general circulation in summer, autumn, and winter. The increase of annual mean air temperature by about +2.11 °C, with a distinctly stronger warming trend in winter than in summer, is reflected by the water temperature of the North Sea with an increase of annual mean SST by about +1.78 °C. The decomposition of the total surface heat flux reveals increasing cloud cover in spring and summer to cause significant decrease of ingoing short wave radiation in the warm seasons, while converging air-sea temperature differences in autumn and winter to cause significant decrease of sensible heat flux in the cold seasons. The strong warming in the cold seasons finally turns out to be the main driver for the total warming of the North Sea in the 21st century. In accordance to the changing wind speeds and the strong winter warming, the mean depth of the thermocline is slightly decreasing, i.e. the thermocline is raising, and the intensity of the mean vertical temperature gradient is weakened, respectively. Decreasing salinities of the inflowing Atlantic waters, increasing Baltic discharge, and higher positive surface fresh water fluxes give rise to a drop of the North Sea salinity by about -0.59 psu. The estimated global mean sea level rise for the 21st century amounts to about +0.46 m, while the maximum sea level change in the North Sea of about +0.6 m is observed in the eastern part because of steric effects and more frequent westerly winds.