Assessing the impact of the North Atlantic baroclinic wave packets on the Mediterranean storm track in the extended winterof 2011- 2012

With regard to the importance of the Mediterranean region as a highly active region in the Northern Hemisphere winter, many studies have been devoted to the weather and climate of this region in general and the impact it receives from the North Atlantic storm track in particular. In 2014, Ahmadi-Givi et al. studied the interaction between the North Atlantic and Mediterranean storm tracks in winter 2005–2004 from the perspective of Rossby-wave propagation and introduced two different kinds of interaction which are referred to as the first and second kind of interaction in what follows. In this paper, based on the latter two kinds of interaction, we aim to identify and better understand the mechanisms of the impact of the North Atlantic storm track by investigating the way Rossby wave-packets are transferred to the Mediterranean region in the extended winter of 2012–2011. To this end, the Hovmoller diagrams, the wave envelope detection, wave activity diagnostics and the energetics are employed. For this extended winter, in total, five marked cases of wave-packet propagation from the North Atlantic to the Mediterranean region are detected. Results show that these five cases can be put into three categories. In the first category, there are two cases in which the wave packets reach Europe, in the upstream of the Mediterranean storm track and penetrate to lower latitudes and the southern branch mentioned in the work by Lee in 2000 and the first track mentioned in the work by Hoskins and Hodges in 2002. Therefore, these cases behave in the same manner of the first kind of interaction. In the second category, there are two cases in which the wave packets pass through the northern part of Europe, then propagate southeastward and join the southern branch along the second track introduced by Hoskins and Hodges in 2002, farther downstream from the first category. Therefore, the cases of the second category behave in the same manner of the second kind of interaction. In the third category, there is only one case in which the wave packet enters the upstream of the Mediterranean storm track and the southern branch in the west of Mediterranean region. However, wave activity diagnostics show that most of the wave activity propagates eastward in the Northern Europe along the second track with little penetration to the lower latitudes. Having signatures of both the first and second kind of interaction, the third category thus exhibits a mixed behavior. Detailed diagnostics are presented for two cases, one from each of the first and second categories. Overall, the fact that four of five cases are compatible with the two kinds of interaction adds to the credibility of this classification. It is also significant that on average, there is monthly one case of wave-packet propagation from the North Atlantic to the Mediterranean region. Finally, it is hoped that this work can be applied to the long-term data sets available, like that of NCEP/NCAR, in order to determine the statistical behavior of the wave packets and the interaction of the two storm tracks.