  This paper is the continuation of a program, initiated in Grenier-Nguyen
[8,9], to derive pointwise estimates on the Green function of Orr Sommerfeld
equations. In this paper we focus on long wavelength perturbations, more
precisely horizontal wavenumbers $\alpha$ of order $\nu^{1/4}$, which
correspond to the lower boundary of the instability area for monotonic
profiles.
