  The nature of angular momentum transport in the boundary layers of accretion
disks has been one of the central and long-standing issues of accretion disk
theory. In this work we demonstrate that acoustic waves excited by supersonic
shear in the boundary layer serve as an efficient mechanism of mass, momentum
and energy transport at the interface between the disk and the accreting
object. We develop the theory of angular momentum transport by acoustic modes
in the boundary layer, and support our findings with 3D hydrodynamical
simulations, using an isothermal equation of state. Our first major result is
the identification of three types of global modes in the boundary layer. We
derive dispersion relations for each of these modes that accurately capture the
pattern speeds observed in simulations to within a few percent. Second, we show
that angular momentum transport in the boundary layer is intrinsically
nonlocal, and is driven by radiation of angular momentum away from the boundary
layer into both the star and the disk. The picture of angular momentum
transport in the boundary layer by waves that can travel large distances before
dissipating and redistributing angular momentum and energy to the disk and star
is incompatible with the conventional notion of local transport by turbulent
stresses. Our results have important implications for semianalytical models
that describe the spectral emission from boundary layers.
