INTERANNUAL VARIABILITY IN THE TEMPERATURE OF THE NORTH-SEA

A numerical model is formulated to study inter-annual variability in t he temperatures of the North Sea. Vertical diffusion is simulated usin g a simplified semi-analytical approach. Sensitivity tests examine the impact of changes in wind speed and air temperature, including specif ic anomalies observed during the 15-month survey period of the U.K. No rth Sea Project. In the North Sea, the sea surface temperature closely follows the air temperature, with a mean 1-2 degrees above the latter . Their seasonal amplitudes are closely similar in shallow water, but the sea surface amplitude is somewhat reduced in deeper water. Any inc rease in wind speed forces the sea surface temperature to converge eve n more closely towards the ambient air temperature. Beneath the surfac e, increasing depths both delay and attenuate surface variability-a pr ocess reinforced by thermal stratification that exists between March a nd October in deeper waters. Anomalies in observed sea surface tempera tures can generally be directly related to concurrent air temperature anomalies with an indirect influence of anomalous wind conditions. How ever, air temperature anomalies are reduced in amplitude in their impa ct on depth-averaged sea temperatures because of the attenuating effec t of water depth. Model simulations emphasise the essentially localise d nature of the air-sea thermal balance in the central North Sea, with only a secondary effect of horizontal advection and dispersion. Any c hanges in mean temperature along the Atlantic boundary will have littl e influence on the more enclosed regions of the North Sea. Moreover, t he effect of corresponding changes in the annual cycle will be reduced even further. Throughout this study, feedback effects of water temper ature onto ambient air temperatures have not been considered. Future s tudies will need to incorporate this mechanism. Copyright (C) 1996 Els evier Science Ltd.