The influence of horizontal circulation on the supply and distribution of tracers

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Abstract
A theoretical analysis of the mixing associated with (horizontal) advection and dispersion is used to show that the larger scale, longer-term distribution of conservative tracers in the southern North Sea is primarily determined by the residual circulation pattern. Model estimates of this circulation have been substantiated from new year-long high-frequency radar monitoring of flow through the Dover Strait. Model simulations 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, using river and atmospheric inputs specified from the 1987 Quality Status Report. Computed concentrations are in reasonable agreement with measured values (except for lead where losses by rapid adsorption were not included in the model). The flushing time, F T , of the southern North Sea (to 56° N) is approximately 240 days. For any conservative tracer and this value of F T , the observed seasonal amplitude in concentration 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-conservative’ tracers can be almost entirely determined by localized exchange processes. Thus the spatial pattern of seasonal variability in temperature and dissolved oxygen is shown to be closely correlated with the inverse of water depth. For such tracers, the influence of advection is small and generally confined to coastal or stratified waters. For shorter term processes, this influence of advection is further reduced. The transport of suspended sediments is highly dependent on particle size. While the transport of fast-settling sediments is complicated by periodic episodes of settlement and re-suspension, transport of slow-settling sediments (the main agent for adsorbed contaminants) approximates that of a dissolved tracer. Thus, to first order, simple models incorporating residual advection are adequate to relate inputs to concentrations (averaged over appropriate time and length scales) for reasonably conservative dissolved tracers. Likewise point-models are often adequate to study processes fundamentally concerned with vertical exchange rates particularly for shorter-term processes in well-mixed waters away from the coastal zone.

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