  Context. Disk accretion onto weakly magnetized stars leads to the formation
of a boundary layer (BL) where the gas loses its excess kinetic energy and
settles onto the star. There are still many open questions concerning the BL,
for instance the transport of angular momentum (AM) or the vertical structure.
Aims. It is the aim of this work to investigate the AM transport in the BL
where the magneto-rotational instability (MRI) is not operating owing to the
increasing angular velocity $\Omega(r)$ with radius. We will therefore search
for an appropriate mechanism and examine its efficiency and implications.
Methods. We perform 2D numerical hydrodynamical simulations in a cylindrical
coordinate system $(r, \varphi)$ for a thin, vertically inte- grated accretion
disk around a young star. We employ a realistic equation of state and include
both cooling from the disk surfaces and radiation transport in radial and
azimuthal direction. The viscosity in the disk is treated by the
{\alpha}-model; in the BL there is no viscosity term included. Results. We find
that our setup is unstable to the sonic instability which sets in shortly after
the simulations have been started. Acoustic waves are generated and traverse
the domain, developing weak shocks in the vicinity of the BL. Furthermore, the
system undergoes recurrent outbursts where the activity in the disk increases
strongly. The instability and the waves do not die out for over 2000 orbits.
Conclusions. There is indeed a purely hydrodynamical mechanism that enables AM
transport in the BL. It is efficient and wave mediated; however, this renders
it a non-local transport method, which means that models of a effective local
viscosity like the {\alpha}-viscosity are probably not applicable in the BL. A
variety of further implications of the non-local AM transport are discussed.
