We present the linear stability properties and nonlinear evolution of two-dimensional plane Couette flow for a statically stable Boussinesq three-layer fluid of total depth 2h between two horizontal plates driven at constant velocity +/-Delta U. initially the three layers have equal depth 2h/3 and densities rho 0 + Delta rho, rho 0, and rho 0 - Delta rho, such that rho 0 >= Delta rho. At finite Reynolds and Prandtl number, we demonstrate that this flow is susceptible to two distinct primary linear instabilities for sufficiently large bulk Richardson number Ri(B) =g Delta rho h/(rho 0 Delta U-2). For a given bulk Richardson number Rig, the zero phase speed 'Taylor' instability is always predicted to have the largest growth rate and to be an inherently two-dimensional instability. An inherently viscous instability, reminiscent, of the 'Holmboe' instability, is also predicted to have a non -zero growth rate. For flows with Prandtl number Pr = nu/k = 1, where v is the kinematic viscosity, and k is the diffusivity of the density distribution, we find that the most unstable Taylor instability, maximized across wavenumber and Rip, has a (linear) growth rate which is a non-monotonic function of Reynolds number Re = Delta Uh/nu, with a global maximum at Re = 700 over 50 % larger than the growth rate as Re -> infinity. In a fully nonlinear evolution of the flows with Re = 700 and Pr = 1, the two interfaces between the three density layers diffuse more rapidly than the underlying instabilities can grow from small amplitude. Therefore, we investigate numerically the nonlinear evolution of the flow at Re = 600 and Pr = 300, and at Re = 5000 and Pr = 70 in two-dimensional domains with streamwise extent equal to two wavelengths of the Taylor instability with the largest growth rate. At both sets of parameter values, the primary Taylor instability undergoes a period of identifiable exponential 'linear' growth. However, we demonstrate that, unlike the so-called 'Kelvin Helmholtz' instability that it superficially resembles, the Taylor instability's finite-amplitude state of an elliptical vortex in the middle layer appears not to saturate into a quasiequilibrium state, but is rapidly destroyed by the background shear. The decay process reveals Re-dependent secondary processes. For the Re = 600 simulation, this decay allows the development to finite amplitude of the co -existing primary 'viscous liolmboe wave instability', which has a substantially smaller linear growth rate. For the Re = 5000 simulation, the Taylor instability decay induces a non -trivial modification of the mean velocity and density distributions, which nonlinearly develops into more classical finite -amplitude Holmboe waves. In both cases, the saturated nonlinear Iloltnhoe waves are robust and long-lived in two-dimensional flow.
