Tropospheric forcing of the stratosphere: A comparative study of the two different major stratospheric warmings in 2009 and 2010

Abstract

In January 2009 and 2010, two major stratospheric warmings (MSWs) took place in the boreal polar stratosphere. Both MSWs were preceded by nearly the strongest injection of tropospheric wave activity on record since 1958 and their central date was almost coincident. However, the typical external factors that influence the occurrence of MSWs (the Quasi-Biennial Oscillation, sunspot cycle, or El Nio) were dissimilar in the two midwinters: favorable in 2010 but unfavorable in 2009. In this study, the driving mechanisms of these two different MSWs were investigated focusing on the amplification of upward wave activity injection into the stratosphere before the MSW onset. By decomposing the total wave flux injection into contributions from the climatological planetary waves and from deviations from the latter we found clear differences in this amplification between both MSWs. The pre-MSW period in 2009 was characterized by a peak in the 100 hPa eddy heat flux with a predominance of wave number 2 activity. This was due to strong anomalies associated with Rossby wave packets originating from a deep ridge over the eastern Pacific. In contrast, the amplification of the upward wave propagation prior to the 2010 MSW was equally due to Rossby wave packets and to the interaction between the latter and the climatological waves. This amplification enhanced wave number 1 stationary waves in January 2010, which seemed at least partially due to the 2009/2010 El Nio event. Our results show the relevance of the internal tropospheric variability in generating MSWs, particularly when the external factors do not play any role. © 2011 by the American Geophysical Union.