Generation of a Quasi-Biennial Oscillation in an NWP Model and its Impact on the Northern Annular Mode

The top boundaries of many operational numerical weather prediction (NWP) systems now extend well into the stratosphere, and these systems are beginning to be used to generate forecasts beyond conventional 5-10 day periods out to seasonal time scales. Past observational and modeling studies have shown that the quasi-biennial oscillation (QBO) in equatorial stratospheric winds can play an important role in stratosphere-troposphere dynamical coupling over these longer time scales, and so stratosphere-resolving NWP models used to generate seasonal forecasts should contain the necessary physics to generate and maintain the QBO. We describe several key modifications that were necessary in order to produce a QBO with realistic amplitude and period in a high-altitude version of the NOGAPS NWP model. These modifications include an increase in model vertical resolution, implementation of a computationally efficient stochastic gravity wave drag parameterization, and significant reductions in the amount of horizontal and vertical diffusion in the stratosphere. Ensembles of 120-day free-running simulations over the December-March period show that the transition of the QBO from easterly to westerly phase can affect surface weather patterns over the Northern Hemisphere by reinforcing the positive phase of the Northern Annular Mode.