Despite a substantial body of literature dealing with the effects of freestream turbulence (FST) on $2D$ square cylinders, there remains some open questions regarding the influence of the bluff body shear layer development in a highly perturbed environment and the resulting impact on bluff body flow characteristics. Accordingly, flows with ambient and enhanced FST were studied at $Re_D=5.0\times 10^4$ using long duration Time Resolved Particle Image Velocimetry (TR-PIV). The data indicate a narrowing and lengthening of the mean wake and an accompanying rise in base pressure. Using triple decomposition, the underlying dynamics of the wake revealed a streamwise lengthening of the individual von-K\'{a}rm\'{a}n (VK) vortex structures, complementing the increase in mean wake length. Close inspection of the shear layer region, in the presence of FST, indicated a substantial increase in curvature towards the body but no pronounced increase in the growth rate. The loci of maximum turbulent kinetic energy and spanwise vorticity in the shear layer region further revealed that the most pronounced changes occurred during the very initial stages follow separation. Inspection of a series of instantaneous PIV fields of \emph{Q-Criterion} showed that the conventional transition pathway, via the formation and subsequent pairing of the Kelvin-Helmholtz (KH) vortices, was bypassed. The KH vortices are observed to immediately cluster and amalgamate before breaking into smaller random eddies. The bypass transition was followed by shear layer reattachment in some cases. This is considered a primary mechanism responsible for the reported changes in the global flow characteristics and the altered wake dynamics. Furthermore, a quantitative definition of the ``diffusion-length" was implemented to the square cylinder wake and its relationship with to Strouhal number and wake formation length was considered.
Physical Review Fluids, June 2016.