Interannual variability in northern hemisphere mesospheric wintertime dynamics

Mesospheric temperature observations from SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) have highlighted the large inter-annual variations in this region. This study examines two January-February time periods: 2005 and 2006, that showed very different mesospheric temperatures. While 2005 showed some temperature fluctuations, overall the stratopause remained well defined. However, 2006 was marked by the disappearance of the stratopause, associated with the January 2006 stratospheric sudden warming (SSW) event, and the subsequent reformation of the stratopause at a very high (usually mesospheric) altitude, that persisted throughout the remaining January-February time period. Temperature changes associated with this SSW event extend from the mesopause to the surface.

To better study the dynamics of the January 2006 warming event we have run the NOGAPS-ALPHA (Navy Operational Global Atmospheric Prediction System- Advanced Level Physics High Altitude) GCM from initial conditions selected throughout January and February of 2005 and 2006. Results are presented from experiments run with different horizontal resolution (T79 and T239) to study the effects of resolved and unresolved gravity wave propagation and drag on the mesospheric winds and temperature. The effects the planetary waves are also examined in terms of EP-flux divergence, mean residual circulation and the momentum balance. While climate fields are used in the upper mesosphere initialization, the upward propagation of the planetary waves allows the mesosphere to develop in a dynamically consistent way in response to the analyzed initial conditions below. In addition to climate-based initialization, NOGAPS-ALPHA results are shown from an improved initialization based on SABER observations assimilated using a high altitude version of NAVDAS (NRL Atmospheric Variational Data Assimilation System). Results from experiments for 2005 and 2006 contrast the differences in the planetary and GW propagation, and in the residual circulations.