An approach developed recently to study the dynamics of vorticity defects i
n homogeneous shear flow extends naturally to the case of baroclinic, quasi
-geostrophic flow. It is shown that an inviscid geostrophic flow with unifo
rm vertical shear may be destabilized by introducing a 'potential vorticity
defect', an arbitrarily small but sufficiently sharp and horizontally unif
orm change in stratification or vertical shear. The linear baroclinic probl
em is nearly identical to the linear homogeneous problem, with differences
arising only from the boundary conditions. The nonlinear baroclinic problem
differs substantially from the nonlinear homogeneous problem, as the leadi
ng-order baroclinic nonlinearity is the Jacobian of the 'inner' streamfunct
ion and potential vorticity in the horizontal plane aligned with the defect
. An example of the linear instability is described.