  The Kelvin-Helmholtz (KH) instability occurring in a single shear flow
configuration that is embedded in a uniform flow-aligned magnetic field, is
revisited by means of high resolution two-dimensional (2D) magnetohydrodynamic
(MHD) simulations. First, the calculations extend previous studies of
magnetized shear flows to a higher compressibility regime. The nonlinear
evolution of an isolated KH billow emerging from the fastest growing linear
mode for a convective sonic Mach number $M_{cs}=0.7$ layer is in many respects
similar to its less compressible counterpart (Mach $M_{cs}=0.5$). In
particular, the disruptive regime where locally amplified, initially weak
magnetic fields, control the nonlinear saturation process is found for Alfv\'en
Mach numbers $4\simlt M_A \simlt 30$. The most notable difference between
$M_{cs}=0.7$ versus $M_{cs}=0.5$ layers is that higher density contrasts and
fast magnetosonic shocklet structures are observed. Second, the use of adaptive
mesh refinement allows to parametrically explore much larger computational
domains, including up to 22 wavelengths of the linearly dominant mode. A strong
process of large-scale coalescence is found, whatever the magnetic field
regime. It proceeds through continuous pairing/merging events between adjacent
vortices up to the point where the final large-scale vortical structure reaches
the domain dimensions. This pairing/merging process is attributed to the growth
of subharmonic modes and is mainly controlled by relative phase differences
between them. These grid-adaptive simulations demonstrate that even in very
weak magnetic field regimes ($M_A \simeq 30$), the large-scale KH coalescence
process can trigger tearing-type reconnection events previously identified in
cospatial current-vortex sheets.
