Exploring currents and heat within the North Sea using a numerical model

Three-dimensional ecosystem models are important tools to examine ecosystem dynamics, and the single most important factor for reliable modeling of phytoplankton and nutrient distributions and transports is a proper physical model. In this paper we use the coupled physical, chemical and biological model system NORWECOM to simulate the North Sea circulation and hydrographic conditions during 1985–2007. The model is forced with atmospheric variables from the European Center for Medium-Range Weather Forecasts, four tidal constituents at the lateral boundaries and observed freshwater runoff. The model reproduces the observed hydrographic conditions at several locations and cross-sections, as well as both the inflows at the boundaries and the total heat content of the North Sea compared to observations or other model simulations. Based on our results, the North Sea can be divided into a northern and southern circulation regime. The inflows across several sections of the northern boundary are significantly correlated, and strongly connected to large-scale atmospheric forcing represented by the North Atlantic Oscillation (NAO). The circulation in the southern North Sea experiences a separate pattern and inflows at the southern boundary are only weakly related to the NAO. In the simulated period, the accumulated North Sea heat content is positive and corresponds to a warming of the North Sea of 0.62 °C. The interannual variation in maximum summer heat content is considerably lower than that of the winter minimum, and the oceanic heat inflow through the open boundaries has limited effect on the variability in heat content.