Yayoi Harada

A major stratospheric sudden warming event in January 2009

Abstract The major stratospheric sudden warming (SSW) event of January 2009 is analyzed using the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS). This SSW event is characterized by the extraordinary predominance of the planetary-scale wave of zonal wavenumber 2 (wave 2). The total amount of the upward Eliassen–Palm (EP) flux for wave 2 was the strongest since the winter of 1978/79. It is found that the remarkable development of the upper troposphere ridge over Alaska played important roles in the SSW in January 2009. During the first development stage, the ridge excited wave packets upward as well as eastward over around Alaska. The eastward-propagating packets intensified a trough over eastern Siberia, which led to the development of the planetary wave over eastern Siberia during the second development stage. The results of this study indicate that the pronounced wave-2 pattern observed in the stratosphere was brought about by accumulative effects of rather localized propagati...

Extraordinary Features of the Planetary Wave Propagation During the Boreal Winter 2013/2014 and the Zonal Wave Number Two Predominance

Observational features of the winter 2013/2014 are investigated using of the Japanese 55-year Reanalysis (JRA-55) data. This winter can be characterized by the continuous predominance of planetary waves of zonal wavenumber two (WN2) that did not cause major sudden stratospheric warming (SSW) events. It is found that the vertical component of the Eliassen–Palm flux of WN2 for the winter 2013/2014 is almost equal to the highest value of the winter 2008/2009. The longitudinal distribution of vertical components of Plumb wave activity flux (WAFz) for this winter shows marked downward propagation around 100°W and upward propagation around 60°E, both of which are the strongest of their type among the 56 winters since 1958/1959. The convergence of wave packets propagating from around 60°E contributes to the development and continuance of the quasi-barotropic Aleutian High, which is associated with the extension of negative extended refractive index (Ks) region. The extension of negative Ks region is related to the convergence or reflection of the wave packets emanating from tropospheric blocking highs developing in the North Pacific Ocean; the development and continuance of the quasi-barotropic Aleutian High is considered to be one of plausible reasons for the lack of major SSWs in the winter 2013/2014. In addition to these results, we revealed the significant contribution of smaller scale waves (with a zonal wavenumber of three or more) to the structure of localized wave packet propagation in the stratosphere.