Evolution of the quasi 2‐day wave during January 2006

Abstract

We examine the evolution of the quasi 2‐day wave in the middle atmosphere during the period from 5 January to 5 February 2006 using global synoptic meteorological fields from the high‐altitude Navy Operational Global Atmospheric Prediction System Advanced Level Physics, High Altitude (NOGAPS‐ALPHA) forecast‐assimilation system. This period is characterized by a high level of planetary wave activity in the Northern Hemisphere (winter) extratropical stratosphere prior to a sudden stratospheric warming (SSW) on 20 January 2006. Space‐time spectral analysis of 6‐hourly NOGAPS‐ALPHA fields finds the largest quasi 2‐day wave amplitudes in the Southern Hemisphere (summer) extratropical upper mesosphere. Eliassen‐Palm flux diagnostics indicate that this extratropical quasi 2‐day wave is related to baroclinic instability along the equatorward flank of the summer easterly jet. The quasi 2‐day wave is also evident in NOGAPS‐ALPHA water vapor fields near the tropical stratopause and is related to barotropic instability. We find that the strong planetary wave activity leading up to the SSW produced an enhanced northward component of the residual meridional circulation that influenced the background zonal winds and, by extension, the quasi 2‐day wave forcing in both the tropical and extratropical regions. In the tropical region, the combination of enhanced horizontal momentum advection by the residual meridional circulation and inertially unstable circulations related to planetary wave breaking in the subtropics produced conditions favoring barotropic instability. In the extratropical region, the enhanced residual meridional circulation altered the zonal wind tendency through increased Coriolis torque.