The Influence of the Tropical Middle Atmosphere on the Northern Annular Mode in Seasonal Forecasts using a High-Vertical Resolution Version of the NOGAPS-ALPHA Model

The quasi-biennial oscillation (QBO) in tropical stratospheric winds can affect the dynamics of the Northern Hemisphere winter polar stratosphere by modulating the vertical and meridional propagation of extratropical planetary waves that are generated at the surface. This modulation is often manifested as a phase reversal of the teleconnection pattern known as the Northern Annular Mode (NAM). Sudden reversals of the stratospheric NAM pattern from positive (stronger, colder polar vortex) to negative (warmer, weaker polar vortex) are often accompanied by strong surface cold air outbreaks across the continental United States and Western Europe. A new version of the global spectral forecast model component of the Navy Global Atmospheric Prediction System with Advanced Level Physics-High Altitude (NOGAPS-ALPHA) has recently been developed that generates an internal QBO through a combination of resolved and parameterized gravity wave drag. The vertical domain of this new model consists of 139 vertical levels (L139) on a hybrid coordinate system that smoothly transitions from terrain-following at the surface to isobaric at a top pressure level of 0.005 hPa (~85 km altitude), with a vertical resolution of ~0.5 km throughout the stratosphere. Multi-year free-running L139 NOGAPS-ALPHA simulations show realistic QBO features such as alternating regimes of easterly and westerly flow in the equatorial lower stratosphere with a mean period of just over 2 years, longer easterly phases above ~50 hPa, and longer westerly phases below 50 hPa. Ensembles of 120-day L139 seasonal simulations beginning in early December 2007 have been carried out both with and without the internally generated QBO. The simulations with the QBO capture the observed transition of the QBO from easterly to westerly phase, while simulations without the QBO exhibit persistent easterly flow in the lower stratosphere, contrary to observations. Ensemble means of the temperature, zonal wind, and mean sea level pressure fields from these sets of simulations show statistically significant differences in the NAM teleconnection pattern extratropical troposphere and stratosphere. The implications of these findings for improving seasonal forecasts will be discussed.