Crucial Role of Obliquely Propagating Gravity Waves in the Quasi-Biennial Oscillation Dynamics

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.