On the upstream spreading of bottom trapped plumes

MATANO, R. P.; PALMA, E. D.

Foz do Iguacu

Congreso; 2010 Meeting of the Americas; 2010

American Geophysical Union

It is well known that numerical simulations of freshwaterdischarges produce plumes that spread in the direction opposite tothat of the propagation of coastally trapped waves (the upstreamdirection). The lack of a theory explaining these motions in unforcedenvironments deemed the numerical results suspect. Thus, it becamea common practice in numerical studies to add a downstream meanflow to arrest the development of the upstream perturbation. Thisapproach is generally unjustified and it remains a matter of interestto determine if the upstream displacement produced by models is ageophysical phenomenon or a consequence of erroneousassumptions in the model set up. In this presentation we use theresults of highly idealized numerical experiments to investigate thesematters. We show that this phenomenon is associated with thegeostrophic adjustment of the discharge and that “upstream” motionis endemic to the baroclinic structure of bottom trapped plumes. Wealso show that “downstream” displacements are generated by thecross-shelf barotropic pressure gradient generated by thepropagation of coastally trapped waves. Sensitivity experimentsindicate that the speed of upstream propagation and densitystructure of the plume is affected by bottom friction, the slope of thebottom and the magnitude of the density anomaly. Bottom friction, inparticular, slows down the progression of the plume and changes itsdensity structure producing a more homogeneous downstream regionand a more stratified upstream region.