  We discover a new dynamical mechanism that significantly enhances the growth
of Rayleigh-Taylor fingers developed near the contact interface between the
supernova ejecta and swept-up ambient gas in young supernova remnants if the
supernova remnant expands into a clumpy (cloudy) circumstellar medium. Our
numerical simulation demonstrates that large Rayleigh-Taylor fingers can obtain
a sufficient terminal velocity to protrude through the forward shock front by
taking extra kinetic energy from vorticies generated by shock-cloud
interactions. We suggest this mechanism as a means to generate the aspherical
expansion of the supernova ejecta. Ambient magnetic fields are stretched and
amplified as the Rayleigh-Taylor fingers protrude, possibly leading to strongly
enhanced radio emission. The material in the protrusions originates from the
ejected stellar material with greatly enhanced heavy elements. Therefore, it
can be a strong X-ray emitter. The timescale for the Rayleigh-Taylor fingers to
reach the forward shock depends on the size, mass density and distribution of
clouds being engulfed by the supernova shock, although the details will require
further numerical investigation.
