On the diffusion of rhodamine B on both sides of a river plume front

Conclusions In spite of poor weather conditions, for which vertical mixing down to the bottom is expected in general, the rhodamine is still trapped between vertical and horizontal gradients 3 days after release. The turbulent flux through the front is so small that a significant transport through the front can probably only be effected by vortices occasionally occurring — as observed in the experiment of October '82.The front-parallel and cross-frontal spreading velocities have the same order of magnitude as in October '82. However, the decrease of rhodamine peak concentration runs much slower: in the first experiment, the spreading was hindered by only two boundaries, the front and the bottom. Now, the rhodamine is embedded in a field of gradients, allowing spreading in one dimension only, i.e. parallel to the isopycnicals. In this stage only processes reducing the gradients can intensify the exchange.To understand the dynamic of oceanic features such as fronts and vortices, it is essential to observe the movement of a marked water mass within the whole system. Again, the reconstructed distributions show clearly the ability of rhodamine to trace processes in the order of magnitude of several miles right down to very small scale processes, as the diffusion of the tracer through the front.