A modelling study of the mixing of 137 Cs in the seas of the European Continental Shelf

Abstract

A two-dimensional numerical model is formulated to simulate long-term mixing in the coastal waters over the European Continental Shelf. By using both tidal and wind-driven components, the mean circulation over this region is computed and is shown to be in good agreement with earlier estimates from (i) modelling studies, (ii) observations of residual flow, and (iii) observations of mean sea level distribution. Long-term gradients of sea level in the adjacent oceanic region are found to be unimportant for shelf circulation. These residual flow components are used to develop a mixing model, observational data of the mixing of 137 Cs (released from Windscale over a 17 year period) are used to calibrate and evaluate the model. The model accurately simulates the transport routes of this material both spatially and temporally. This transport is shown to be dependent on both advection and dispersion. The model is used to determine various time constants for the shelf seas, namely: age, residence times and turn-over-times. Thus, the turn-over-time of the North Sea is calculated to be 530 d compared with a value of 328 d for the Irish Sea. The geographical distribution of turn-over-times differentiates regions of vigorous mixing from more stagnant regions and thereby indicates regions conducive to the formation of thermal fronts. The model also indicates that about one third of the material discharged from Windscale is lost to the ocean in the vicinity of the shelf edge to the north and west of Scotland.