Possible Sources Driving the Potential Vorticity Structure and Long-Wave Instability of Coastal Upwelling and Downwelling Currents

Abstract

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 long-wave instability by transforming the current's potential vorticity. A constant wind can likewise modify the potential vorticity. The resulting potential vorticity anomaly is always negative for both upwelling and downwelling currents, and this anomaly interacts with the outcropped front, destabilizing it. Examples are provided via numerical calculations using an idealized front. A wind stress is an effective means of inducing the negative PV necessary for instability; with wind, Kelvin?Helmholz instability, when present, merely modifies the instability characteristics. In addition, upwelling fronts are always less stable than comparable downwelling fronts.