| It is well known that upwelling and downwelling currents are unstable to perturbations. Less is, however, known about the physical mechanism responsible for the observed and modeled instabilities. It is shown that the origin of the long-wave barotropic/ baroclinic instability observed on upwelling currents has to be sought among diabatic or thermobaric mechanisms. In particular, the role of mixing associated with Kelvin Helmholtz instability and of wind forcing is investigated. Low Richardson numbers occur when the pycnocline outcrops at the sea surface. The criterion for instability ( Ri <= 1/ 4) can be reached in a narrow region close to the upwelling front, permitting Kelvin - Helmholtz instability and mixing. This can precondition the current for... |
