The Upwelling of Downwelling Currents: Analytical and Numerical Results Compared

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

Foz do Iguacu

Congreso; 2010 Meeting of the Americas; 2010

American Geophysical Union

Alongshore flow in the direction of the coastal trapped waves can result in upwelling at the shelf break. The intensity of this upwelling can be comparable in magnitude to coastal upwelling, with its associated ecological features. Recently, Matano and Palma (2008) performed a series of numerical experiments to investigate this phenomenon. The fundamental physical basis for understanding upwelling at the shelf break under the proper circumstances is the arrested topographic wave Csanady(1978). The spreading of the inflow jet results in an alongshore gradient in sea surface height, which, in turn, results in cross-shore flow that is partly geostrophically balanced. Mass balance is maintained by cross-shore Ekman transport in the bottom boundary layer. Rapid change in the Ekman transport at the shelf break results in upwelling. Here we exhibit a family of analytic solutions to the equations of the arrested topographic wave, and we calculate the resulting horizontal velocities and upwelling velocities. This relatively simple result agrees remarkably well with the numerical results obtained by Matano and Palma. Most of the features of the output of the model of the fully stratified, nonlinear ocean can be reproduced quantitatively with this relatively simple analytical calculation. Our analytic solution exhibits dynamic balances similar to those found in the numerical simulation, and predicts the general features of the numerical solution, including spreading of the meridional current, and concentration of upwelling just offshore of the shelf. As with all analytical solutions, our solution admits simple investigation of fundamental physical questions, and, in conjunction with numerical simulation, facilitates systematic evaluation of the relative importance of different physical influences.