Investigation of temperature and sea level changes in the North Sea for the period 1948-2010

Distribution and changes in sea water temperature and sea level are important items in the hydrographic conditions of the North Sea. The roles played by local air-sea exchange processes and oceanic advection for North Sea temperature variability are investigated in the present study. Furthermore, temperature contributions to sea level changes are evaluated under the influence of the North Atlantic Oscillation (NAO). Analyses are based on model results from the HAMSOM model for the entire North Sea system over the period 1948-2010. For regional ocean models, open boundary conditions (OBCs) are very important and still challenging, especially for long-term simulations. For OBCs, a nudging term added to radiative and/or advective conditions during the wave or slow propagation outward from the model domain of interest is widely used. However, nudging time scales are basically unknown, leading to many empirical selections. In this study, a method for objectively estimating nudging time scales during outward propagation is proposed, by using internal model dynamics near the boundary. We tested this method and other several commonly used OBCs for cases of both an idealized model domain and a realistic configuration, and model results demonstrated that the proposed method improves the model solutions. Many similarities are found between the nudging and mixing time scales, in magnitude, spatial and temporal variations, since the nudging mainly replaces the effect of the mixing terms in this study. However, the mixing time scale is not an intrinsic property of the nudging term because, in other studies, the nudging term might replace terms other than the mixing terms, and thus should reflect other characteristic time scales. From the investigation of the roles of local air-sea exchange processes and oceanic advection in North Sea temperature variability, we found that the seasonal average of surface heat flux (Qsur ) is larger than that of advective heat flux (Qadv ) over much of the North Sea, with an obvious exception in the region of the Norwegian Trench. On seasonal time scales, the temperature variation is determined by Qsur variability, which is consistent with the fact that the seasonal variability of Qsur is much larger than that of Qadv . On inter-annual time scales, Qsur and temperature changes are also significantly correlated for most parts of the North Sea, with the notable exception of the Norwegian Trench, and Qsur plays an important role in temperature inter-annual variability. On the other hand, along the main pathways of the North Sea circulation, Qadv also makes a considerable contribution to inter-annual temperature variability, especially in the northwestern inflow region, where North Atlantic water enters through the Fair Isle Passage and along the East-Shetland shelf. In the North Sea, the NAO strongly influences sea level and temperature variability and their linear trends in winter. With respect to the NAO influence, our analysis, consistent with previous studies, has shown that the sea level caused by the local atmospheric pressure effects and winds (wind+pressure) has smaller sensitivity to the NAO index, compared with the observed sea level. In order to explain this discrepancy, we investigated possible contributions, especially those associated with temperature and salinity effects. First, local steric contributions to sea level changes are calculated. It is found that the amplitude of local thermosteric sea level in winter is quite small in shallow areas of the North Sea, in particular off the coast of the Netherlands, Germany, and Denmark as well as the German Bight. Moreover, the sensitivity analysis (to the NAO index) has demonstrated that local steric contributions cannot explain the discrepancy between the wind+pressure modelled and the observed sea level. Furthermore, we estimated additional effects from the full 3D baroclinic model, which includes non-local steric contributions and large-scale wind field effects due to open boundary forcing by the global reanalysis data (GECCO2). As expected, the baroclinic modelled sea level is in much closer agreement to the observed sea level in the North Sea, compared to the results from the wind+pressure model. This clearly demonstrates that baroclinic processes play an important role as well with respect to sea level changes in the North Sea.