Inter-annual variability in the temperature of the North Sea

A numerical model is formulated to study inter-annual variability in the temperatures of the North Sea. Vertical diffusion is simulated using a simplified semi-analytical approach. Sensitivity tests examine the impact of changes in wind speed and air temperature, including specific anomalies observed during the 15-month survey period of the U.K. North Sea Project. In the North Sea, the sea surface temperature closely follows the air temperature, with a mean 1-2°C above the latter. Their seasonal amplitudes are closely similar in shallow water, but the sea surface amplitude is somewhat reduced in deeper water. Any increase in wind speed forces the sea surface temperature to converge even more closely towards the ambient air temperature. Beneath the surface, increasing depths both delay and attenuate surface variability—a process reinforced by thermal stratification that exists between March and October in deeper waters. Anomalies in observed sea surface temperatures can generally be directly related to concurrent air temperature anomalies with an indirect influence of anomalous wind conditions. However, air temperature anomalies are reduced in amplitude in their impact on depth-averaged sea temperatures because of the attenuating effect of water depth. Model simulations emphasise the essentially localised nature of the air-sea thermal balance in the central North Sea, with only a secondary effect of horizontal advection and dispersion. Any changes in mean temperature along the Atlantic boundary will have little influence on the more enclosed regions of the North Sea. Moreover, the effect of corresponding changes in the annual cycle will be reduced even further. Throughout this study, feedback effects of water temperature onto ambient air temperatures have not been considered. Future studies will need to incorporate this mechanism.