Future evolution of thermodynamics and stratification in the North Sea, effects of North Atlantic Oscillation

Friday, 19 December 2014
Nikesh Narayan, Birgit A Klein and Holger Klein, BSH Federal Maritime and Hydrographic Agency, Hamburg, Germany
Two high resolution (~3km) future projections for North Sea, based on the IPCC A1B scenario was performed using the HAMburg Shelf Ocean Model (HAMSOM), one in a fully coupled ocean atmosphere setup and the other in an uncoupled mode. Both simulations show a steady warming trend in the North Sea, especially in the German Bight and central North Sea. A strong freshening of the North Sea is seen in both the projections towards the end of the 21st century, with enhanced fresh water input from the Baltic Sea. This could change the vertical stratification resulting in major changes in the nutrient distribution and ecology. Our results show that winter time SST in the North Sea is positively correlated with the North Atlantic Oscillation (NAO), which results in enhanced warming of the North Sea during the positive phase of NAO. The effect of NAO related changes in stratification and temperature is most prominent in the German bight region. A multi-decadal variability is observed in the singular spectrum trend of salinity, therefore an emphasis on disentangling the effect of climate change signal vis-à-vis natural variability on the physical parameters of North Sea is very important. There exist significant differences in simulated parameters between the coupled and uncoupled projections, with coupled model showing comparatively warmer and fresher North Sea in the future. Evaluation of the effect of coupling on simulating physical parameters and climate change will be also focussed in the study.