A destabilizing gradient of one density-affecting component maintained by a linear combination of the flux and fixed-value boundary conditions is considered with stable stratification imposed by the difference in the values of another component. It is emphasized that, even if the diffusivities of the components are equal, there exists a band of directly unstable finite wavelengths in the region of the stable net stratification irrespective of instability of the waves in the vicinity of zero wave number. The steady instability is related to the unequal vertical diffusion gradients formed in the perturbed state due to the different boundary conditions. Implications of this insight for interpretation of previous theories are also discussed. (C) 1997 American Institute of Physics.
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