THE INFLUENCE OF HORIZONTAL CIRCULATION ON THE SUPPLY AND DISTRIBUTION OF TRACERS

A theoretical analysis of the mixing associated with (horizontal) adve ction and dispersion is used to show that the larger scale, longer-ter m distribution of conservative tracers in the southern North Sea is pr imarily determined by the residual circulation pattern. Model estimate s of this circulation have been substantiated from new year-long high- frequency radar monitoring of flow through the Dover Strait. Model sim ulations of the saline balance and for the dissolved metals Cd, Cu, Pb , Zn and Ni are made for the North Sea Project (NSP) survey period, us ing river and atmospheric inputs specified from the 1987 Quality Statu s Report. Computed concentrations are in reasonable agreement with mea sured values (except for lead where losses by rapid adsorption were no t included in the model). The flushing time, F(T), of the southern Nor th Sea (to 56-degrees-N) is approximately 240 days. For any conservati ve tracer and this value of F(T), the observed seasonal amplitude in c oncentration corresponding to a seasonal inflow I cos wt is only 0.23 of the steady concentration corresponding to a continuous inflow I. By contrast with the above responses for 'conservative' tracers entering from rivers or adjacent seas, concentrations of certain 'non-conserva tive' tracers can be almost entirely determined by localized exchange processes. Thus the spatial pattern of seasonal variability in tempera ture and dissolved oxygen is shown to be closely correlated with the i nverse of water depth. For such tracers, the influence of advection is small and generally confined to coastal or stratified waters. For sho rter term processes, this influence of advection is further reduced. T he transport of suspended sediments is highly dependent on particle si ze. While the transport of fast-settling sediments is complicated by p eriodic episodes of settlement and re-suspension, transport of slow-se ttling sediments (the main agent for adsorbed contaminants) approximat es that of a dissolved tracer. Thus, to first order, simple models inc orporating residual advection are adequate to relate inputs to concent rations (averaged over appropriate time and length scales) for reasona bly conservative dissolved tracers. Likewise point-models are often ad equate to study processes fundamentally concerned with vertical exchan ge rates particularly for shorter-term processes in well-mixed waters away from the coastal zone.