  We study the oscillations modes of differential rotating remnants of binary
neutron star inspirals by modeling them as incompressible Riemann ellipsoids
parametrized by the ratio $f$ of their internal circulation to the rotation
frequency. The effects of viscosity and gravitational wave radiation on the
modes are studied and it is shown that these bodies exhibit generic
instabilities towards gravitational wave radiation akin to the
Chandrasekhar--Friedman--Schutz instabilities for uniformly rotating stars. The
odd-parity modes are unstable for all values of $f$ (except for the spherical
model) and deformations, whereas the even parity unstable modes appear only in
highly eccentric ellipsoids. We quantify the modification of the modes with
varying mass of the model and the magnitude of the viscosity. The modes are
weakly dependent on the range of the masses relevant to the binary neutron star
mergers. Large turbulent viscosity can lead to a suppression of the
gravitational wave instabilities, whereas kinematical viscosity has a
negligible influence on the modes and their damping timescales.
