  We discuss key observational signatures of Classical T Tauri stars (CTTSs)
accreting through Rayleigh-Taylor instability, which occurs at the interface
between an accretion disk and a stellar magnetosphere. In this study, the
results of global 3-D MHD simulations of accretion flows, in both stable and
unstable regimes, are used to predict the variability of hydrogen emission
lines and light curves associated with those two distinctive accretion flow
patterns. In the stable regime, a redshifted absorption component (RAC)
periodically appears in some hydrogen lines, but only during a fraction of a
stellar rotation period. In the unstable regime, the RAC is present rather
persistently during a whole stellar rotation period, and its strength varies
non-periodically. The latter is caused by multiple accreting streams, formed
randomly due to the instability, passing across the line of sight to an
observer during one stellar rotation. This results in the quasi-stationarity
appearance of the RAC because at least one of the accretion stream is almost
always in the line of sight to an observer. In the stable regime, two stable
hot spots produce a smooth and periodic light curve that shows only one or two
peaks per stellar rotation. In the unstable regime, multiple hot spots formed
on the surface of the star, produce the stochastic light curve with several
peaks per rotation period.
