Unusual Features of the Planetary Wave Predominance of Zonal Wavenumber Two in the Boreal Winter 2013/2014

Abstract:

The boreal winter 2013/2014 can be characterized by continuous strong activity of planetary waves of zonal wavenumber 2 (wave 2) in the stratosphere almost throughout the season, in spite of no major stratospheric sudden warming (SSW) occurrence. The purpose of this study is to compare features of the planetary wave propagation in boreal winters and discuss causation mechanisms of the predominance of wave 2 in the winter 2013/2014 using three reanalysis data (JRA-55, MERRA and ERA-Interim) along with satellite data retrieved from the Aura-MLS observations. We first investigate interannual changes of the vertical component of Eliassen-Palm (EP) flux averaged over 30-90N at 100 hPa integrated during each boreal winter since 1958/1959. As a result, we find that the upward EP flux of wave 2 during the winter of 2013/14 was almost equal to the highest value during the winter 2008/2009. In addition, the ratio of the wave 2 contribution to the sum of wave 1 and wave 2 largely surpassed 0.75 in only these two winters. Note that no major SSWs occurred in the winter 2013/14 while a remarkable major SSW occurred in the winter 2008/09. However, on the basis of wave activity flux analyses, both the upward propagation over Western Russia (around 60E) and the downward propagation over Northern Canada (around 120W) of planetary wave packets were the strongest in recent 34 winters since 1980/1981; similar structural features could not be seen in the winter 2008/09.