  In climate modelling, the reality of simulated flows in the middle atmosphere
is largely affected by the model's representation of gravity wave processes
that are unresolved, while these processes are usually simplified to facilitate
computations. The simplification commonly applied in existing climate models is
to neglect wave propagation in horizontal direction and time. Here we use a
model that fully represents the propagation of unresolved waves in all
directions, thereby elucidating its dynamical effect upon the most important
climate mode in the tropical stratosphere, namely the quasi-biennial
oscillation. Our simulation shows that the waves at the equatorial
stratosphere, which are known to drive this climate mode, can originate far
away from the equator in the troposphere. The waves propagating obliquely
toward the equator are found to play a huge role in the phase progression of
the quasi-biennial oscillation as well as in its penetration into the lower
stratosphere. Such waves will require further attention, given that current
climate models are struggling to simulate the quasi-biennial oscillation down
to the lower stratosphere, which may be needed to reproduce its observed
impacts on the surface climate.
